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Solid state phase transformation study on a series of PuGa alloys

containing 0.180.63 wt.% Ga

Michael Ling , Roderick F.E. Jenkins, Nigel Park

AWE, Aldermaston, Reading RG7 4PR, UK

a r t i c l e i n f o

Article history:

Received 11 April 2013

Received in revised form 11 October 2013

Accepted 15 October 2013

Available online 28 October 2013

Keywords:

PuGa alloy

aPhasedPhase

Phase transformation

Annealing

Density

TMA

a b s t r a c t

The effects of Ga concentration and starting condition of PuGa alloys on their solid-state phase transfor-

mation behaviours in a set of PuGa alloys containing 0.180.63 wt.% Ga were studied under non-

equilibrium conditions with a heating/cooling rate of 10 C/min in the temperature range of 25560 C

by a dilatometry method. The characteristic abrupt slope changes shown in the dilatometry tracks during

heating and cooling are ascribed to the known phase transformations in the PuGa alloys. The rationale

for such an ascription is discussed. The study showed that, as expected, Ga concentration and the starting

condition of the PuGa alloy have significant effects, demonstrated by the variety of the onset and com-

pletion temperatures of a phase transformation. We find that the nature of as cast a-phase is independentof the nominal Ga content. The e? d transformation happens through two mechanisms whenGa < 0.63 wt.%. Furthermore, the increase of thermal expansion coefficient of d-phase with increasing

Ga is also confirmed.

Crown Copyright 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction

Plutonium is probably the most complex element in the peri-

odic table. It sits near the middle of the actinide series, which

marks the emergence of the 5f electrons in the valence shells of

the element[1]. It is the nature of the 5f electrons that governs

the physical and chemical behaviour of this element. The fact that

right at plutonium the 5f electrons transition from participating in

bonding to being localised leads to its notorious instability[1,2].

Consequently, Pu readily changes its crystal structure with temper-

ature, pressure, chemistry and time[1,2]. For example, when pure

Pu is heated from room temperature to its liquidus temperature

under ambient pressure, the metal undergoes 5 solid state phase

transformations in the following sequence: a? b, b? c, c? d,

d?

d0

andd0?

eprior to thee?

liquid transformation. The occur-rence of each of the solid state phase transformations of a Pu metal

is always accompanied by a significant specific volume change,

leading to dimensional variations. It is therefore possible to de-

tect/probe the phase transformations by measuring the dimen-

sional changes during heating and/or cooling of the material

sample.

Dilatometry is a widely used technique for solid state phase

transformation study. In this method, the relative length changes

of the sample with temperature are recorded as dilatometry traces.

Any abrupt slope change may indicate the occurrence of a phasetransformation and the slope of the dilation curves indicates either

expansion or contraction of a specific phase(s). This technique has

been applied to the study of both unalloyed Pu and d-stabilised Pu

alloys[1,35]. For clarity, a d-stabilised Pu alloy means Pu alloyed

with sufficientd-phase stabilising element e.g. Ga to retain the fcc-

structured-phase at room temperature which is actually thermo-

dynamically metastable.

Many investigations have been carried out to explore phase

equilibria, phase transformation behaviours and structure/prop-

erty relations previously. However, there still remain many out-

standing problems, for example, transformation mechanisms are

still not well understood in both the pure Pu metal and PuGa al-

loys[6]. The similarities and differences for the same type of phase

transformation occurring in pure Pu metal and ind-stabilised Pualloy, effect of the concentration of the d-phase stabiliser alloying

element and effect of prior thermal history of the Pu metal and

the d-stabilised Pu alloy on the transformations etc. are still not

fully understood. Understanding these effects on the phase trans-

formation behaviour in various stages is obviously desirable.

It is well-known[1] that thed-phase in the pure Pu metal, un-

like most other metallic materials, has a negative thermal expan-

sion coefficient while the d-phase in the Ga-stabilised Pu alloys

containing 0.891.26 wt.% Ga has a positive thermal expansion

coefficient (Fig. 1), indicating the complexity of the Gas role in

the PuGa alloy, e.g. not only retaining the engineering preferred

d phase down to room temperature but also changing its

0925-8388/$ - see front matter Crown Copyright 2013 Published by Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.10.112

Corresponding author. Tel.: +44 1189825467; fax: +44 1189815320.

E-mail address:[email protected](M. Ling).

Journal of Alloys and Compounds 586 (2014) 709717

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m
http://dx.doi.org/10.1016/j.jallcom.2013.10.112mailto:[email protected]://dx.doi.org/10.1016/j.jallcom.2013.10.112http://www.sciencedirect.com/science/journal/09258388http://www.elsevier.com/locate/jalcomhttp://www.elsevier.com/locate/jalcomhttp://www.sciencedirect.com/science/journal/09258388http://dx.doi.org/10.1016/j.jallcom.2013.10.112mailto:[email protected]://dx.doi.org/10.1016/j.jallcom.2013.10.112http://-/?-http://-/?-http://-/?-http://-/?-http://crossmark.crossref.org/dialog/?doi=10.1016/j.jallcom.2013.10.112&domain=pdfhttp://-/?-


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thermo-physical properties. It is logical to infer that there is a crit-ical point of Ga concentration in PuGa alloys where the thermal

expansion coefficient of thed-phase changes its sign from negative

to positive. In fact, as early as in 1960, it was reported[7,8]that the

addition of the alloying element Al (another d-phase stabiliser)

gradually drives the thermal expansion toward a positive value.

The results from[7,8] show positive expansion for PuAl alloys

containing greater than 0.35 wt.% Al in the whole temperature

range from room temperature to 500C, whereas ad-Pu containing

0.26 wt.% Al showed a positive expansion below about 300C,

which decreased with increasing temperature and finally became

negative above 400C. For PuGa alloys, Lawson et al. [9,10] re-

ported that the 0.59 wt.% Ga is the critical concentration point at

which the thermal expansion coefficient of the d-phase changes

its sign.Two primary objectives of this investigation were: (1) to study

the effects of Ga concentration and prior thermal history of PuGa

alloys on their solid-state phase transformation behaviours and (2)

to experimentally identify (approach to) the critical point of Ga

content where the thermal expansion coefficient of the d-phase

changes its sign from negative to positive.

In this paper, we report results on the PuGa alloys having Ga

concentrations of 0.18, 0.42 and 0.63 wt.%. Prior to each solid state

phase transformation test, the alloy sample was characterised by a

set of techniques to determine its starting phase constituents. The

solid state phase transformation study was conducted by measur-

ing the alloys thermal expansion characteristics using a Perkin

Elmer TMA-7 Thermal Mechanical Analyser (TMA) in the tempera-

ture range of 25560

C. The temperature range was so chosen thatall potential solid state phase transformations in those alloys could

be studied. The thermal expansion curves with temperature were

recorded during both heating and cooling periods. For each Pu

Ga alloy, the measurements were performed on both its as-cast

and annealed conditions. The influence of Ga concentration and

the starting condition of the alloys on the observed phase transfor-

mation behaviour are discussed. The thermo-physical behaviour

for thed phase in those alloys is described.

2. Experimental

2.1. The alloy preparation

The PuGa alloys were prepared by successively diluting a PuGa alloy with Ga

content of 0.63 wt.% with electro refined pure Pu to achieve the required amount ofGa. The alloy samples were first melted at 850 C in a resistance heated Ta crucible

and then cast into a flat disc mould pre-heated to 640 C. The mould consists of six

pockets surrounding a central hub and is made of fine grain graphite to ensure

smooth casting surface finish. The pockets are about 52 mm diameter by 5 mm

thick and incorporate a mould release taper. Each disc is fed by a short runner from

the central hub. The hub acts as a header to feed the disc castings.

The samples were cut from the disc by hand using a hacksaw. Some samples

were annealed by holding at 450C for 250 h and then cooling down to room tem-

perature at a natural cooling rate of about 3 C/min. in a heat treatment furnace un-

der an argon atmosphere. Both as-cast and annealed samples were subjected to a

set of analytical techniques for the characterizations prior to the solid state phasetransformation study by TMA.

2.2. The analytical techniques and specimen preparation

Chemical analysis for each of the casts was performed using AWEs standard

analysis methods, e.g. for the analysis of Ga and U in Pu by Wavelength Dispersive

X-ray Fluorescence (WDXRF). A Siemens D5000 diffractometer was adopted to car-

ry out the X-ray diffraction (XRD) analysis to identify the phase(s) in the alloy sam-

ples. A Mettler AT200 balance with a standard density measurement set was used

to carry out the density measurements. An immersion density measurement tech-

nique was used, with Perklone as liquid medium.

The specimens for metallographic examination were mounted in epoxy resin

before being ground and polished using successively finer abrasives, with the final

stage being performed using a 1lm diamond paste. To ensure removal of any pos-sible mechanically-induced phase(s) during grinding and polishing, the specimens

were electro-polished. They were then electrolytically etched for 5 s in a solution

of 10% nitric acid/ethylene glycol to reveal the grain structure. The microstructureof the alloy materials were examined using optical microscopy. The representative

microstructure images are shown inFig. 2.

Thermal expansion experiments were conducted in a Perkin Elmer TMA-7 Ther-

mal Mechanical Analyser under high purity helium gas atmosphere (with a flow

rate of 20 ml/min.) to minimise surface oxidation and to ensure good thermal con-

trol. These measurements were made on samples with a dimension of approxi-

mately 5 mm square by 5 mm thick. The dilatometry samples were taken such

that the measurement was made across the flat disc surfaces which had not been

machined/polished during the sample preparation procedure. In consideration of

the possible detrimental effect of the sample preparation method, we note the work

described by Los Alamos [11]. The primary deformation mode of the hacksaw is

shearing the material, with some localised tensile stresses generated. The amount

of compressive stress applied to the sample is small, and therefore the expected de-

gree of transformation which may be observed is also expected to be negligible.

Furthermore, in a study of the effect of metallographic polishing of a PuAl alloy

by[12] (which would include some small compressive stress), it was found that

the transformed layer was less than 40 lm thick. Lastly, during sample preparationsmall burrs are removed with a file, which serves to ensure that the sample sitscleanly on the instrument platen. During a test, the specimen is placed inside a

fused silica tube, which is mounted vertically within the furnace, and a probe, also

made from fused silica, rests on its upper surface with a static force of 110 mN. The

probe is connected to a linear variable displacement transducer (LVDT), which mea-

sures any change to the specimen length during the experiment. The change to the

specimen length was continuously monitored by the LVDT throughout the tests. All

specimens were heated from 25C to 560C, held at 560C for 15 min (note: for

Pu0.63 wt.% Ga alloys, 5 min holding were applied to ensure the full completion of

thed? etransformation) and then cooled to 25 C at a heating and cooling rate of10C/min. respectively.

3. Results

3.1. The characterisation results of the alloy samples prior to the

dilatometry tests

The chemical analysis result for each of the alloy made are sum-

marised inTable 1.

Micrographs of the as-cast alloys are shown inFig. 2(a)(c) and

of the annealed alloys inFig. 2(d)(f) respectively. Examination of

Fig. 2(a)(c) shows that the phase composition of the as-cast alloys

is a mixture ofa-phase plus coredd-phase, seen as darkerd grainssurrounded by a lightera phase matrix; no other phase(s) are vis-ible. The amount ofa phase decreases as the Ga content increases.For the annealed alloys containing 0.42 wt.% Ga and 0.63 wt.% Ga,

single d-phase was clearly shown in Fig. 2(d) and (f). For the

annealed Pu0.18 wt.% Ga alloy (Fig. 2(d)), it is hard to distinguish

the phase constituents based purely on the metallography image.

However, with the help of other complementary techniques suchas XRD analysis (which showed that only diffraction peaks

Fig. 1. Length change of unalloyed Pu compared to Pu3.0 at.% Ga and 4.5 at.%Ga

(Pu0.89 wt.%Ga and Pu1.36 wt.%Ga) alloys (re-printed from[1]).

710 M. Ling et al. / Journal of Alloys and Compounds 586 (2014) 709717


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corresponding to both d anda-phases were detected), the phasecomposition of the annealed Pu0.18 wt.% Ga was identified to

be a mixture ofa + d [13].The mean values of measured densities for the PuGa alloys in

their as-cast and annealed conditions are summarised in column 3inTable 2.

If the density values of thea-Pu and the Ga-stabilisedd-Pu wereknown, together with the measured density values shown in

Table 2, the proportion of thea-phase in these alloys could be esti-mated by applying a simple rule of mixtures calculation. Although

the theoretical density value of 19.86 g/cm3 for thea-Pu has been

determined[14], the actual density value measured on un-alloyeda-Pu is usually lower than that due to impurities, voids andpossible micro-cracks etc. For example, Farrow et al. recently

100m

(a)

m100

(a)

100m

(b)

100m

(b)

100m

(c)

100m

(c)

100m

(d)

100m

(d)

100m

(e)

100m

(e)

100m

(f)

100m

(f)

Fig. 2. Optical microstructure images of the as-cast PuGa alloys having Ga content of 0.18 wt.% (a), 0.42 wt.% (b) and 0.63 wt.% (c), and the annealed PuGa alloys having Ga

content of 0.18 wt.% (d), 0.42 wt.% (e) and 0.63 wt.% (f).

Table 1

The results of chemical analysis for the set of PuGa alloys made in this study (note:

the uncertainty for Pu, Ga and other elements are 0.05%, 1.00% and 10.00%

respectively).

Name of alloy A1 A2 A3

Pu + Ga + Am (w/o) 100.06 100.05 100.02Ga (w/o) 0.18 0.42 0.63

Al (wt. ppm) 85 70 90

C (wt. ppm) 29 55 80

Fe (wt. ppm) 57 90 80

Si (wt. ppm) 18 70 55

Am (wt. ppm) 550 800 980

U (wt. ppm) 130 240 320

Np (wt ppm) 20 34 42

Ni (wt. ppm) 3 8 13

Table 2

The results of measured densities for the PuGa alloys in their as-cast and post-

annealing conditions.

As-cast Ga concentration (wt.%) Density (g/cm3) a-phase%

A1 0.18 17.90 56

A2 0.42 16.64 23

A3 0.63 16.08 8

Annealed

A1H 0.18 18.13 62

A2H 0.42 15.77 0

A3H 0.63 15.77 0

M. Ling et al. / Journal of Alloys and Compounds 586 (2014) 709717 711
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[15] measured the density of a-Pu to be between 19.45 and19.65 g/cm3. It is also known that the density value of the Ga-sta-

bilisedd-Pu varies[16]with Ga concentration as a consequence of

lattice parameter shrinkage caused by the substitutional effect of

Ga in thed-Pu lattice. It is very likely that even for a single d-phase

grain in as-cast alloys the actual density value would be higher in

these Ga-rich parts of the grain than those parts where Ga is

depleted. Thus, there is no generally applicable density value ofd-Pu in the as-cast alloys available to perform the calculation.

However, since only d-phase grains were identified for the an-

nealed alloys containing 0.42 wt.% Ga and 0.63 wt.% Ga (Fig. 2(e)

and (f)), the measured density value of 15.77 g/cm3 can thus be

viewed as the true density of thed-phase in these alloys. This value

is slightly less than the literature [14] value of 15.92 g/cm3. The

discrepancy could be caused by the following factors: (1) different

manufacturing route for the alloys; (2) different mean grain size;

(3) impurity level difference.

It is clear that there is an uncertainty when selecting the density

values ofa-Pu and d-Pu to carry out the calculation, which willunavoidably lead to an uncertainty in the estimateda-phase pro-portion value. Despite the uncertainty in the estimateda:d ratio,such an estimation would still be useful to observe trends in micro-

structure evolution. For our purposes, the middle point density va-

lue [13] of 19.55 g/cm3 for the a-Pu and the density value of15.77 g/cm3 for the Ga-stabilised d-Pu were adopted. The esti-

mateda-phase percentages in those alloys are listed in column 4ofTable 2.

3.2. The results of the dilatometry traces of the alloy samples

Fig. 3 shows the complete dilatometry dL/L0 vs temperature

curves (whereLis the sample length during testing andL0the sam-

ple length at room temperature at the beginning of the test) for the

as-cast PuGa alloys containing 0.18 wt.% Ga (a), 0.42 wt.% Ga (b)

and 0.63 wt.% Ga (c) respectively.Fig. 4shows the complete dila-

tometry dL/L0vs temperature curves for the annealed PuGa alloys

containing 0.18 wt.% Ga (a), 0.42 wt.% Ga (b) and 0.63 wt.% Ga (c)

respectively.

3.3. The thermal expansion coefficient values for the single d phase of

the PuGa alloys in their existing temperature ranges

Using a simple linear fitting method, the thermal expansion

coefficient values of the single d phase for every individual alloy

under different conditions are derived from the dilatometry traces

shown inFigs. 3 and 4 and are summarised inTable 3.

4. Discussions

Without applying other more direct analytical techniques (i.e.in situ XRD) for phase identification, it is never easy to unambigu-

ously ascribe all the observed abrupt slope changes to the known

phase transformations in the PuGa alloys based purely on the

dilatometry tracks. Since most of the possible phase transforma-

tions in this study occur in thed-phase matrix, depending on the

relative amount of the d-phase, it is possible that some character-

istic features of phase transformation clearly shown in pure Pu

may be prevented from displaying in the PuGa alloys; Further-

more, all the phase transformations take place at non-equilibrium

conditions (with a heating or cooling rate of 10C/min.) which

would shift the onset and completion temperatures of a known

phase transformation from the equilibrium points shown in the

PuGa phase diagram, making it difficult to assign. Nevertheless,

majority of the solid state phase transformations have been de-duced by careful consideration of the experimental data.

4.1. The phase transformations for the as-cast PuGa alloys containing

0.18 wt.% Ga,0.42 wt.% Ga and 0.63 wt.% Ga and the annealed PuGa

alloy containing 0.18 wt.% Ga during heating

All the above alloy samples have the same starting mixed phase

structure ofa plus d prior to the thermal expansion experiments.They all display similar dilatometry track behaviours during heat-

ing, primarily marked by the 4 distinctive abrupt slope changes

(Figs. 3(a)(c) and4(a)). Recalling the idealised characteristic dila-

tometry track in the temperature range between RT to 560C for

pure Pu (Fig. 1), in which the 5 abrupt slope changes observed inthat temperature region have been attributed to the sequential

Fig. 3. Dilatometry heating and cooling curves for as-cast PuGa alloy samples

containing 0.18 wt.% Ga (a), 0.42 wt.% Ga (b) and 0.63 wt.% Ga (c) heated from RT to560C (e-phase region) and cooled back to RT.



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phase transformations of a? b, b? c, c? d, d? d0 and d0? eupon heating, the observed 4 slope changes in Figs. 4(a)(c) and

5(a) during the heating period are similarly associated with the

a? b, b? c,c? dand d? ephase transformations respectively.This ascription is rational because unlike in the pure Pu, the

phase transformations of d? d0 and d0? e upon heating will nothappen in the PuGa alloys studied here. Instead d-phase is be-

lieved to transform directly intoe-phase. This is due to the factsthat (1) the maximum Ga solubility in d0 is 0.073 wt.% [17] and

the formation of d0 phase is readily depressed by increasing the

amount of impurity (including Ga) in Pu metal; (2) all the PuGa

alloys described here contain much higher Ga concentration than

0.073 wt.%, thed? d0 phase change will thus be depressed in those

alloys.

The phase structure evolution upon heating for those PuGa

alloys is now qualitatively described in more detail.

When a PuGa alloy having the phase structure ofa-phase plusd-phase is heated, initially there is no phase transformation untilthe temperature increases to about 118C. The phase composition

of the alloy in the temperature range of RT to 118 C is the same as

its starting phase structure. One distinctive characteristic in this

temperature range is the gradual elongation of the sample alloy

with temperature, indicating that the alloy has an overall positive

thermal coefficient. Using the simple linear fitting method, the

thermal expansion coefficient value is derived. They are

30 106 K1, 20 106 K1, 9 106 K1 and 30 106 K1 for

the as-cast Pu0.18 wt.% Ga alloy, as-cast Pu0.42 wt.% Ga alloy,

as-cast Pu0.63 wt.% Ga alloy and the annealed Pu0.18 wt.% Ga

alloy respectively. It is noted that the thermal expansion coefficient

decreases with increasing Ga concentration in the PuGa alloys.

The phenomenon may be explained by considering the following:

for a pure Pu metal, its a-phase has a positive thermal expansioncoefficient. It is not unreasonable to assume that thea-phase pres-ent in the mixed structure of the PuGa alloys also has a positive

thermal expansion coefficient. For the d-phase in those as-cast

alloys, the situation is more complicated than that for pure Pu.

Two types ofd-phase may be categorised: one (that contains high

enough Ga) with a positive thermal expansion coefficient and the

other (with low Ga content) having a negative thermal expansion

coefficient. Taking into account its cored structure, different parts

within an individual d-phase grain may display different thermal

behaviour, either expansion (in those parts ofd-phase grains where

the Ga content is higher than some threshold value) or contraction

(in those parts of d-phase grains where the Ga content is lower

than that threshold value) with increase of temperature. However,

for the PuGa alloys considered here, due to their relatively lowoverall Ga contents, it can be assumed that most parts of the

d-phase grains in those alloys would have a negative thermal

expansion coefficient. Since the thermal expansion coefficient val-

ues for those alloys are the combined result of the positive values

ofa-phase and the negative value of low Ga contentd, increasingthe portion of thea-phase in the phase mixture and/or decreasingthe low Ga content d-phase will lead to the higher value of the

positive thermal coefficient of the alloys. Under the same casting

condition, decreasing the Ga concentration in the PuGa alloys

would just have the effect of increasing the a:d ratio (as showninTable 2), resulting in a more positive thermal coefficient value.

It is also noted that the same thermal expansion coefficient value

of 30 106 K1 for both as-cast Pu0.18 wt.% Ga alloy and an-

nealed Pu0.18 wt.% Ga alloy are derived. This is interesting aswe would expect a slightly higher thermal expansion coefficient

value in the annealed alloy than that of its as-cast counterpart

due to its highera proportion in the mixed phase structure (refertoTable 2). This may be qualitatively explained as: since the as-

cast Pu0.18 wt.% Ga alloy has severely coredd phase grains, part

of thed grains may display positive thermal expansion. For the an-

nealed 0.18 alloy, after 250 h annealing at 450 C the Ga in the d

grain should distribute more evenly. Therefore, all parts of the d

grain (containing 0.18 wt.% Ga) would have negative thermal coef-

ficient. It is thus possible that in comparison to the relatively big-

gera-phase contribution from the annealed Pu0.18 wt.% Ga alloy,the relatively smaller contribution to the thermal expansion coef-

ficient from thea-phase in the as-cast Pu0.18 wt.% Ga alloy is just

compensated by the contribution from those parts of thed grainshaving positive thermal expansion value.

Fig. 4. Dilatometry heating and cooling curves for annealed PuGa alloy samples

containing 0.18 wt.% Ga (a), 0.42 wt.% Ga (b) and 0.63 wt.% Ga (c) heated from RT to

560C (e-phase region) and cooled back to RT.

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When the temperature of the alloy sample reaches to about

118C, thea-phase begins to transform intob-phase while the d-phase remains unchanged. After completion of the a? b phasetransformation at about 154C, the alloy sample now has the

phase composition ofb + d with an overall positive thermal coeffi-

cient. The overall thermal expansion coefficient values are

20 106 K1, 6 106 K1, 3 106 K1 and 30 106 K1 for

the as-cast Pu0.18 wt.% Ga alloy, as-cast Pu0.42 wt.% Ga alloy,

as-cast Pu0.63 wt.% Ga alloy and the annealed Pu0.18 wt.% Ga

alloy respectively. As expected, the thermal expansion coefficient

value decreases with the increase of Ga concentration in the Pu

Ga alloys.

Further heating the sample to about 218C results in the

b-phase transforming intoc-phase while the d-phase still remainsunchanged. After completion of theb? cphase transformation atabout 240C, the alloy sample now has the phase composition of

c+ d with an overall positive thermal coefficient. The overall ther-mal expansion coefficient values are 30 106 K1, 7 106 K1,

6 106 K1 and 40 106 K1 for the as-cast Pu0.18 wt.% Ga al-

loy, as-cast Pu0.42 wt.% Ga alloy, as-cast Pu0.63 wt.% Ga alloy

and the annealed Pu0.18 wt.% Ga alloy respectively. As expected,

the thermal expansion coefficient value decreases with the in-

crease of Ga concentration in the PuGa alloys.

Further increasing the temperature to about 290C, all thec-phase will transform into d-phase while the extant d-phase re-mains unchanged. After completion of the c? dphase transforma-tion at 343C, the alloy sample now contains just single d-phase

that has an overall negative thermal expansion coefficient. The

thermal expansion coefficient values have previously been sum-

marised for each individual alloy inTable 2. In general, the thermal

expansion coefficient value increases with increasing Ga content in

these alloys.

Further increasing the temperature of the alloy sample leads to

the single d-phase transforming into thee-phase. It is noted thatfor the alloys containing 0.18 and 0.42 wt.% Ga, the completion of

the d? e phase transformation is certain, demonstrated by thevisible positive thermal coefficient of the sample materials imme-

diately after the abrupt slope change due to thed? etransforma-tion. However, for the Pu0.63 wt.% Ga alloy it is not certain

whether the d? e phase transformation has been completedimmediately after the temperature reaches to 560C,since the sin-

glee-phase region has not been clearly probed. It is therefore pos-sible that, the Pu0.63 wt.% Ga alloys (both as-cast and annealed)

would have the phase composition ofe plus a small amount ofdat that time. However, after holding at 560C for 5 min, the

remainingd is believed to have completely transformed to e. ThePu0.63 wt.% Ga alloys, like other lower Ga content alloys, are

composed of single e phase (as indicated by the initial section ofcooling curves inFigs. 3(c) and4(c)) prior to cooling down to room

temperature. The onset temperatures of thed? eare identified tobe 497C, 512 C, 526 C and 496 C for the as-cast Pu0.18 wt.%

Ga alloy, as-cast Pu0.42 wt.% Ga alloy, as-cast Pu0.63 wt.% Gaalloy and the annealed Pu0.18 wt.% Ga alloy respectively. The

completion temperatures of the d? e are 527 C, 551 C, 560 Cand 527 C for the as-cast Pu0.18 wt.% Ga alloy, as-cast Pu

0.42 wt.% Ga alloy, as-cast Pu0.63 wt.% Ga alloy and the annealed

Pu0.18 wt.% Ga alloy respectively. It is clear that both onset and

completion temperatures ford ? e phase transformation increasewith Ga concentration of the PuGa alloys, which is in agreement

with the published PuGa phase diagrams.

It is noted that for the 3 as-cast PuGa alloys despite their

significant variation of Ga content they have the same onset and

completion temperatures fora? b, b? c and c? dphase trans-formations upon heating respectively. These features are high-

lighted in Fig. 5. This is a very interesting and important

observation as it may suggest that the nature ofa-phase formedduring casting processing is independent of the nominal Ga con-

centration of the PuGa alloys. The initiala-phases in those alloysin their as-cast condition are currently believed by the authors to

be virtually the same chemically and physically. The formation of

thea-phase in the as-cast alloys is discussed below.From other studies[1,18,19] we know that subject to a cold

treatment at sub-zero temperatures, so called alpha-prime (a0)-phase can be formed from d-stabilised PuGa alloys. Thea0-phasehas the same monoclinic crystal structure as that of normal a-phase (that has zero Ga solubility) but contains the same amount

of Ga as its parent d-phase. Upon heating, depending on the Gaconcentration, the behaviour of its reverse phase transformation

can be very different. For example, the a0-phase formed from aPu0.59 wt.% Ga alloy will directly transform back to d, but the

a0-phase formed from a Pu0.29 wt.% Ga alloy will partially trans-form back to d via b and thenc indirectly[17]. Furthermore, theonset temperature of the reverse phase transformation ofa0-phaseis also dependant on the Ga content. In general, with decreasing Ga

content in a0-phase, the onset temperature increases. The abovestudies strongly suggest that the Ga concentration in a monoclinic

type phase (a or a0) plays a vital role in determining its reversephase transformation behaviour. If two monoclinic type phases

in PuGa alloys have the same reverse phase transformation

behaviour, it is not unreasonable to assume that they have the

same Ga contents.Unlike the formation of the a0-phase, the formation of the

as-cast a-phase in the PuGa alloys during casting is through acompletely different path. The former occurs at sub-zero tempera-

tures through the d? a0 martensitic type phase transformationand consequently the Ga element in thed-phase lattice does not

have sufficient mobility and time to eject due to very slow diffu-

sion at low temperatures during the transformation processing,

resulting in the Ga being trapped inside the monoclinic crystal

structure. However, the latter is believed by the authors to be in-

volved with Ga ejection from the newly formed phase from the

parent d-phase at relatively elevated temperatures. A description

of the as-casta -phase formation is considered below:Upon the completion ofe? dtransformation during the casting

processing, the PuGa alloy is composed of single d phase grainswith varied Ga contents within a grain and between individual

Table 3

Linear coefficient of thermal expansion (LCTE) of the d-phase derived from a simple linear fitting under various conditions. (LCTE is defined here as relative length change within

the temperature range where only single d-phase exists, thus LCTE = (DL/L0)/DT).

As-cast Pu

0.18 wt.% Ga

As-cast Pu

0.42 wt.% Ga

As-cast Pu

0.63 wt.% Ga

Annealed Pu

0.18 wt.% Ga

Annealed Pu

0.42 wt.% Ga

Annealed Pu

0.63 wt.% Ga

LCTE measured during heating/K1 22.50 106 15.50106 7.12106 20.40106 8.72106 2.33106

Singled existent temperature range

during heating/C

348475 346469 356478 327491 45511 45523

LCTE measured during cooling/K

1 9.34 10

6 5.52 10

6 2.95 10

6 8.47 10

6 4.85 10

6 2.04 10

6

Singled existent temperature range

during cooling/C

216445 170442 148440 212457 178453 136451



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grains. For thed-phase containing insufficient Ga content to retain

its fcc structure down to room temperature as the cooling contin-

uously proceeds, it is designated in this paper asdlow-Ga. For thed-

phase containing sufficient Ga content to retain its fcc structure

down to room temperature, it is designated as dhigh-Ga. From the

above description, it can be seen that a critical Ga content for the

d-phase exists. Only when thed-phase contains the critical Ga con-

tent or more, will it retain its fcc structure down to room temper-

ature. The critical Ga content for the d-phase is defined as the

boundary value of the Ga to differentiatedlow-Ga and dhigh-Ga. It is

important to bear in mind that the concept of critical Ga here is

a non-equilibrium term which varies with cooling rate. When the

cooling rate between the d-phase existent temperature and RT is

fixed as in the case of this study, the critical Ga content is

determined.The as-casta-phase formation begins by firstly the d low-Ga? c

phase transformation. Due to the zero-Ga solubility of c-phase,the small amount of Ga originally contained in thedlow Gawill nat-

urally eject from the newly formed c-phase to the surroundingcored-dhigh Ga-phase. Whether the ejection of the Ga from the

c-phase can be completed is dependent on the cooling rate inthe temperature range of that transformation. After completion

of the dlow Ga? c, the formed c-phase contains zero Ga. Uponfurther cooling, thectransforms intob-phase (also contains zeroGa). Thea-phase obtained in the as-cast alloy is eventually formedthroughb?aphase transformation, which also contains no Ga. Asregard to the transformation mechanisms for each of the above

phase transformations, they are currently believed by the authors

to be similar to those for pure Pu. The major difference is that allof them take place in thedhigh-Ga-phase matrix. Those phase trans-

formations will be discussed further in Section 4.3.

It is possible that some Ga may still be retained in the c-phaseafter the completion of thedlow-Ga? cwhen the cooling rate is sohigh that the ejection of Ga cannot be completed due to insufficient

time. In this case, the remaining Ga inc may be further ejectedduringc? b as it is still at relatively high temperature. After thecompletion of thec? btransformation if there still remains a littleGa in b, the Ga will be retained in the final a-phase which is laterformed throughb? a transformation, since below 130C (for theb? atransformation to begin), the Ga diffusion is extremely slow[20]. The total remaining Ga in the finala-phase is determined bythe cooling rate in the temperature range between the onset

temperature ofd?

c to the onset temperature ofb?

a. The crit-ical cooling rate can only be determined by further experiments.

Nevertheless, once the cooling rate is fixed, the Ga content in the

final a-phase of a PuGa alloy would be the same irrespective toits nominal Ga concentration. Further work needs to be done to

clarify the above hypothesis.

The gradual decrease of gradient values for the transformation

slope and the smaller transition fora? b (b? c and c? d) withincreasing Ga in the PuGa alloys is indicative of a reduction in

the amount ofa-phase in thed

-phase matrix with increasing Gacontent. This is understandable since in a binary phase system, if

one phase transforms while the other remain unchanged, the total

relative dimension change due to a particular phase transforma-

tion is mainly determined by the proportion of that changeable

phase in the system. Once the proportion of that phase is fixed,

the gradient values of that transformation slope fora ? b shouldalso be fixed.

4.2. The phase transformations for the annealed PuGa alloys

containing 0.42 wt.% Ga and 0.63 wt.% Ga during heating

The two alloy samples have the same starting single phase

structure of d; they show similar dilatometry track behaviours

during heating, reflected mainly by a single abrupt slope change

(negative) as shown inFig. 4(b) and (c). Taking into account both

the temperature range where the slope change occurs and the

characteristic dilatometry track behaviour (a positive linear ther-

mal expansion) immediately after the abrupt change, the observed

sudden slope change can be confidently ascribed to d? e phasetransformation. The onset temperatures of d? e phase transfor-mation for Pu0.42 wt.% Ga alloy and Pu0.63 wt.% Ga are identi-

fied to be 512C and 552C respectively; the completion

temperatures of d? e phase transformation for Pu0.42 wt.% Gaalloy and Pu0.63 wt.% Ga alloy are 531 C and 560C respectively.

It is clear that both onset and completion temperatures for d ? ephase transformation increase with Ga concentration of the Pu

Ga alloys, which is qualitatively in agreement with the published

PuGa phase diagrams.

4.3. The phase transformations for both the as-cast alloys and

annealed alloys during cooling

It is believed that all the above alloy samples have the same

structure of single e-phase(as discussed in paragraph 7 Section 4.1)prior to cooling. Their cooling dilatometry curves all look similar,

primarily marked by 3 distinctive slope changes (Figs. 3(a)(c)

and4(a)(c)), indicating 3 solid-state phase transformation events.

A more detailed analysis reveals that the first cooling dilation

jump starting from the e field is clearly discernible to be two-step-like for annealed Pu0.18 wt.% Ga alloy and Pu0.42 wt.% Ga

alloy (Fig. 4(a) and (b)) and as cast Pu0.42 wt.% Ga alloy

(Fig. 3(b)). This feature seems to have vanished for Pu0.63 wt.%

Ga alloys.Since this two-step dilation jump is observed between the eas-

ily identifiable e-phase and d-phase regions in the dilatometrycurves (through their temperature ranges and their characteristic

thermal expansion slope in those temperature ranges), it is reason-

able to consider that this two-step dilation jump actually indicates

the occurrence of two solid-state transformations frome? d.It is known[1,23]that for a pure Pu metal, the phase transfor-

mation frome ? d is a two-step process. When it is cooled frome-phase region, itse-phase firstly transforms into the d 0, followedbyd 0 to d phase transformation (Fig. 1). Although the phase trans-

formation from e? d in the low Ga (60.42 wt.%) content PuGaalloys is also a two-step process, the transformation route is, how-

ever, completely different from that of pure Pu.

Because the Ga diffusion rates in the e-phase with open bccstructure is very high (the diffusion coefficient of Ga in e-phase

Fig. 5. Dilatometry heating curves for as-cast Pu0.18 wt.% Ga alloy, Pu0.42 wt.%

Ga alloy, Pu0.63 wt.% Ga alloy heated from RT to 560C (e-phase region).

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measured by M.Harvey et al. [21]is in the range of 107cm2/sec,

which is nearly the same as that in Pu liquids), after completion

of the d? e phase transformation by heating the PuGa alloysup to 560C, the Ga concentration distribution in the single

e-phase grains is fairly uniform. Upon cooling, the single e phasewill firstly transform intod. It is known[22]that thee? dtrans-formation in PuGa alloy is diffusive. In any diffusive transforma-

tion, the diffusion of atoms through the materials plays animportant role in both nucleation and growth of the new phase

from the original phase. In Pu-rich PuGa alloys, the Ga diffusion

rate in thee phase is approximately 104 times faster than that inthe d phase[20]. Therefore, the diffusion of Ga through d phase is

the controlling kinetic factor during thee? dtransformation. Fora PuGa alloy, when the temperature falls below thee/(d+ e) tran-sus, the first d nuclei to be formed will have a Ga concentration

higher than its nominal Ga content. These d nuclei then grow as

cooling proceeds, with a tendency of reducing their average Ga

concentration down to its nominal Ga content, which requires

the migration of Ga atoms from the d grain centre where there is

higher Ga concentration to the grain edges that form on the initial

nucleus with continuous cooling and have lower Ga concentration.

However, under a cooling rate of 10 C/min, due to the slow Ga

diffusion rate in thed phase, the re-distribution of Ga will not reach

the stage at which the Ga distribution profile within a d grain is

constant after the completion of e? d transformation. Conse-quently, after the completion ofe ? d transformation, the net re-sult is that within a d phase grain, Ga concentrates in the grain

centre and depletes in the grain edge.

In addition to the formation of coreddgrains through the direct

e? dphase transformation described above, a portion of the finalremaining part ofephase will have so low Ga concentration that itwill initially transform to d0 grains and then transform to d. The

two-step like dilation jump feature detected in those PuGa alloys

containing 60.42 wt.% Ga demonstrated that they all had gone

through two types ofe? d phase transformation, namely (1) directe? dand (2) indirecte? d0? dtransformation. For Pu0.63 wt.%

Ga alloys, due to their higher Ga content, the indirect transforma-tion ofe? d0? ddoes not occur and all the e-phase grains trans-form intod directly.

The onset temperatures of thee? d(inclusive both direct andindirect) transformation for the as-cast Pu0.18 wt.% Ga, Pu

0.42 wt.% Ga and Pu0.63 wt.% Ga alloys are identified to be

508C, 530C and 550 C respectively, and the completion temper-

atures 452C, 450 C and 445C. The onset temperatures of the

e? d transformation for the annealed Pu0.18 wt.% Ga, Pu0.42 wt.% Ga and Pu0.63 wt.% Ga alloys are 508 C, 530 C and

550C respectively, and the completion temperatures are 460C,

456C and 450 C. It can be see that the onset temperature of

e? d increases with the increase of Ga concentration in thePuGa alloy, which is qualitatively in a good agreement with the

indication from PuGa phase diagram. However, the completiontemperature of e? d seems to decrease with increasing the Gaconcentration in the PuGa alloys studied. This observation seems

to contradict the indication from published PuGa phase diagrams

[1,20,23,24]. The most likely cause for this behaviour is the effect of

the high cooling rate adopted in this study.

After the completion of thee? dphase transformation, the al-loy is composed of single d phase grains with varied Ga contents

within a grain and between individual grains. The single d phase

structure in the alloys, which has an overall negative thermal

expansion coefficient value, will remain unchanged with further

cooling down until the temperature drops to the point where an

abrupt slope change having a bursting feature begins, indicating

the occurrence of another phase transformation. It is known[25]

that for pure Pu the d?

c transformation is a martensitic typeand a bursting mode dilation trace is a typical feature of this type

of transformation. At present, we believe that the basic character-

istics of thed? c phase transformation in the PuGa alloy wouldbe similar to that of pure Pu, it is therefore reasonable to conclude

that the phase transformation here asd? c. The onset tempera-ture point is thus the non-equilibriumd/(d+c) transus under thecooling rate of 10 C/min. It should be pointed out that here only

the dlow-Ga-phase will transform into c-phase, coupled with the

Ga ejection from c as discussed earlier. The d

high-Ga-phase willretain its fcc structure down to room temperature. The burst fea-

ture due to thedlow-Ga? c phase transformation is clearly visiblein the as cast Pu0.18 wt.% Ga alloy (Fig. 3(a)) and Pu0.42 wt.%

Ga alloy (Fig. 3(b)), annealed Pu0.18 wt.% Ga alloy (Fig. 4(a))

and Pu0.42 wt.% Ga alloy (Fig. 4(b)). The disappearance of the

burst feature in the Pu0.63 wt.% Ga alloys (Figs. 3(c) and4(c)) is

due to the larger portion ofdhigh-Ga-phase in the material system

that hinders the burst feature. After completion of thedlow-Ga? cphase transformation, the alloy sample now has the phase compo-

sition ofc+dhigh-Ga. It needs to be noted that the dhigh-Ga-phasenow contains slightly higher Ga than it did previously due to the

Ga ejection from thec.The onset temperatures of thed ? c transformation for the as-

cast Pu0.18 wt.% Ga, Pu0.42 wt.% Ga and Pu0.63 wt.% Ga alloys

are identified to be 225C, 170C and 156 C respectively, and the

completion temperatures 130C, 110 C and 60 C. The onset

temperatures of the d? c transformation for the annealed Pu0.18 wt.% Ga, Pu0.42 wt.% Ga and Pu0.63 wt.% Ga alloys are

210C, 181 C and 156 C respectively, and the completion temper-

atures are 131C, 111 C and 58C. It is evident that the onset tem-

perature ofd ? c decreases with the increase of Ga concentrationin the PuGa alloy, which is qualitatively in agreement with the

published PuGa phase diagram.

Now the alloy consisting of thec + dhigh-Gafrom the completiontemperature of the d? c, upon further cooling will remainunchanged until the temperature falls to the point where thecwilltransform tob. Note that thedhigh-Gawill remain unchanged. This

transformation is clearly seen in the dilatometry traces. It is very

unlikely that thec will transform directly intoa by skipping theb phase region when the c in the PuGa has virtually the samenature (zero Ga solubility) as its counterpartc in pure Pu, whichwill normally follow the phase transformation sequence of

e? d0? d? c? b? a as shown inFig. 1.The onset temperatures of the c? b transformation for both

the as-cast and post-annealed Pu0.18 wt.% Ga, Pu0.42 wt.% Ga

and Pu0.63 wt.% Ga alloys are identified to be 63C, 54C and

47C respectively, and the completion temperatures 44C, 40 C

and 40 C.

Further cooling to room temperature, will not produce further

phase transformation due to the sluggish nature of the b? atransformation[26]. At the time when the experimental test is ter-

minated, the alloy is composed ofd + b phases.

4.4. The thermal expansion coefficient property of the single d-phase as

a function of the Ga concentration in the temperature range in which

just single d-phase exists

FromTable 3, it can be seen that there is a general trend that the

singledphase in its existent temperature range gradually increases

its thermal expansion coefficient value with increasing Ga content

in the PuGa alloys. For example, the thermal expansion coefficient

values during heating are 22.5 106 K1, 15.5 106 K1 and

7.12 106 K1 for the as-cast Pu0.18 wt.% Ga alloy, Pu

0.42 wt.% Ga alloy and Pu0.63 wt.% Ga alloy respectively. The

thermal expansion coefficient values during heating for the an-

nealed Pu0.42 wt.% Ga alloy and Pu0.63 wt.% Ga alloy are

8.72 106

K1

and 2.33 106

K1

respectively. The resultsare expected and encouraging as they demonstrate that by varying



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the Ga content in a PuGa alloy it is possible to experimentally

identify the critical point of Ga content where the thermal expan-

sion coefficient of thed-phase changes its sign from negative to po-

sitive. When the Ga content increases to 0.63 wt.% (refer to

Fig. 4(c)). the value of LCTE is very close to zero, confirming the

observation from[9,10]. The slight discrepancy may be caused by

different methodologies used and the relatively high heating rate

in this study.To explain the unusual negative thermal expansion behaviour

of pure Pu in its d-stabilising region, a mixed-level model was

developed by Erikssonet. al.[27]in 1999. For PuGa alloys, Lawson

et al. [9] proposed an Invar effect model in which Pu atoms in

d-phase PuGa alloys are assumed to be in two atomic states: a

lower-energy higher-volume state d1, and a lower-volume high-

er-energy stated2. Furthermore, the two statesd1and d2 are sepa-

rated by a small energyDE. The addition of Ga forces a stabilisation

of the d2 and suppresses the negative thermal expansion.

It needs to be pointed out that for the annealed Pu0.63 wt.% Ga

alloy, although at first glance, the dilation track of the sample upon

heating looks to be almost parallel to the X-axis with a general

trend of slightly decreasing from room temperature until an abrupt

slope change at about 531 C, a more careful observation shows

that initially there is actually a slight increase of the relative length

change in the temperature range of 25189C (which implies a po-

sitive thermal expansion coefficient in that temperature range),

then followed by an almost linear decrease from 189C up to

531C until the d? e transformation begins (Fig. 4(c)). A similarphenomenon is also seen for the annealed Pu0.42 wt.% Ga alloy

where a slight increase of the relative length change in the temper-

ature range of 25110C was recorded, followed by an almost

linear decrease from 110 C up to 512C. These phenomena are

similar to what was reported[7,8]in Pu0.26 wt.% Al alloy. These

observations reflect the complexity of the thermal expansion vari-

ation with temperature in those alloys.

5. Conclusions

The effects of Ga concentration and prior thermal history of

PuGa alloys on their solid-state phase transformation behaviours

in a set of PuGa alloys containing 0.180.63 wt.% Ga were studied

under non-equilibrium conditions with a heating/cooling rate of

10C/min. in the temperature range of 25560C by a dilatometry

method.

The characteristic abrupt slope changes shown in the dilatome-

try tracks during heating and cooling are ascribed to the known

phase transformations in the PuGa alloys. The rationale for such

an ascription is discussed.

For all three as-cast PuGa alloys and the annealed Pu

0.18 wt.% Ga, the same subsequent phase transformations of

a? b,b? c,c? dand d? eupon heating to 560 C are observedand the observation is attributed to the same starting phase com-

position ofa+ d. Furthermore, with increasing Ga concentration inthe PuGa alloy, less a-phase is formed in its as-cast phase struc-ture, leading to gradually decreasing gradient values of the trans-

formation slopes froma? b, b? c and c? d and a decrease inthe measurable phase transitions as highlighted inFig. 5.

The observation that all as-cast PuGa alloys, despite their

significant variation in Ga content have the same onset and com-

pletion temperatures fora? b,b? c and c? d phase transforma-tions upon heating strongly suggests that the nature ofa-phase

formed during the casting process is independent of the nominal

Ga concentration of the PuGa alloys.

For the annealed Pu0.42 wt.% Ga and Pu0.63 wt.% Ga alloys,

only d? e is observed upon heating due to their single d-phasestructure. Both onset and completion temperatures ford? ephasetransformation increase with Ga concentration of the PuGa alloys

as expected.

Upon cooling from 560

C to RT, all the alloys display thee? d

/d0, d? c andc ? b phase transformations. For PuGa alloys con-taining

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