Several interesting features in the study of stabilities ofbased mainly upon the work of RobertsAusten(1), but wasphases, and in phase transformations, are discussed. It is

put into question later by many authors(2)(4) when the struc-

proposed that symmetry considerations related to thetures of the a, b and d phases were determined by Xraypresence of magnetism in iron suggests that the respective

diffraction to be all BCC(5).

phases, BCC alpha and FCC gamma, have in fact lower sym-

The difference between the a and b phases involves themetries than cubic. A proposal is made that the symbol betachange in magnetism: a phase is ferromagnetic below the Cu-

used in the past for the designation of the paramagnetic BCC

rie temperature, and b phase is paramagnetic above. In theiron should perhaps be returned as a feature in phase dia-late 1920's and 1930's the b phase was abandoned in pub-grams. It is proposed that since the thermal activation is alished phase diagrams mainly because Xray diffraction

major feature in the behavior of isothermal martensites, aseemed unable to reveal any change of structure. However,more suitable designation for these types of phase transfor-one may correctly argue that there is a change of symmetry,mations might be ``thermally activated martensites'', orfrom tetragonal to cubic, and that this constitutes a phase

TAMs. Massive transformations are discussed briefly and itchange. Recently, D.E. Laughlin and I have argued this pointis emphasized that they present a specific example of an idio-

in a contribution to the Ricardo Ferro Symposium(6). We

morphic transformation process, not requiring the need for

proposed to adopt a definition of a phase that includes orderorientation relationships (ORs) between the parent andparameters specifying additional properties to be considered

product phases.

in addition to the composition and structure; such parameters
as atomic order, magnetic order, electronic configuration,1.
1.1

Phase Diagrams and Phase Stability

Symmetry and the structure of phasesShould wereturn the b phase to the Fe phase diagram?

etc. This enlarged definition of a phase can already be foundearlier in the writings of Landau(7), or Christian(8), andothers.
As illustrated in Fig. 1, the overall symmetry is lowered

In terms of conventional crystallography, the structure of awhen the motif of ordered magnetic moments is included.

phase may be defined as a result of positioning of a ``motif'' ofThe repetition of lattice (points) and the motif of the atoms

atoms (or groups of atoms) upon a ``lattice'' of points inwith aligned magnetic moments reduce a plane group sym-

space. Difficulties arise when we begin to apply this concept

metry of 4mm to a plane group symmetry of 2mm only by ato motifs that are more complicated than just neutral single

change in the motif, not the lattice. As a consequence, if all

atoms. I will illustrate this with reference to the elements ironsuch moments are aligned in one direction on a bcc lattice,(Fe) and plutonium (Pu), which exhibit allotropic structure

say along the [001] direction, they create an overall direc-

changes depending on temperature. The sequence of al-tional motif that changes the structure. Some people would

lotropic phases in Fe includes the phase b, as was considered

say that even if the cubic lattice retains the symmetry of theto be the case in the late 1800's and early 1900's. This wascrystal parameters (a＝b＝c), the total spatial symmetry of2010年 3 月28日，筑波大学筑波キャンパスにおける第146回本会春期大会において講演
Emeritus Professor, CarnegieMellon University; Thaddeus B. Massalski(Materials Science and Engineering, and Physics, Carnegie
Mellon University, Pittsburgh, PA, USA)
Keywords: phase diagrams, phase transformations, bcc Fe, martensites, massive transformation
Received: January 18, 2010.
Email: massalski＠andrew.cmu.edu

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Fig. 1The BCC crystal structure, including magnetic
and symmetry information(9).

4
m′
,
m
due to the aligned moments. Arguments of this kind havethe structure is nevertheless lowered from Im3̃m to I

been used for several years now by Laughlin et al.(9) andothers(10) to discuss the symmetry of crystals that have magnetic moments associated with the atoms. So, the motif canchange the lattice. That is a tricky statement, but there areplenty of comments on this now in the literature. Some people may say, I will accept this concept if the presence of the
BCT structure is demonstrated experimentally, even if it cannot be demonstrated by Xray diffraction. Perhaps measurements of magnetostriction can accomplish this(11). Numerous
features of magnetic materials, such as magnetic domains,
domain interactions, magnetocrystaline anisotropy, and magnetostriction, can be addressed more accurately by startingwith the full symmetry aspects of the magnetic state, i.e. thestructure with reduced symmetry due to the magnetic ordering rather than the original cubic lattice. Electronic structure

Fig. 2

Two stacked fcc unit cells with the central atom
showing the 12 nearest neighbours. In the caseof plutonium, the 12 bonds with the nearest
neighbours widely vary with strength and can beseparated into six pairs: blue (3.3), black (3.5
3.7), red (3.73.9), pink (3.94.1), green (4.5
4.7) and brown (4.75.3). When the fcc lattice
is combined with the motif of these bondstrengths, the resultant space group is monoclinic Cm(14).

calculations for such materials produce more consistent
results when the symmetry is reduced due to the orderedmagnetic state. Accurate orbital magnetic moments of BCC
iron can only be obtained from calculations that introduce a

netic ordering. This implies that the space group changes

lowering of the symmetry, from 48 (cubic) to 16 (tetragonal)

from P6/mmm in the paramagnetic state to P6/mm′
m′in theoperations, resulting from the preferred orientation of theferromagnetic state. Equally interesting are the calculations

magnetic spin moments along the [001] easy axis. Cubic

by Widom and Mihalkovic(13), regarding the crystal structure

symmetry does not support ferromagnetism because theof boron. Experimental work suggests that the brhombo-aligned moments always favor a special direction. So, as

hedral (black) form is stable over all temperatures from 0 K

more ``ground state'' calculations are becoming involved in

to melting. However, early calculations indicate that its

assessment of phase stability, perhaps the time has come toenergy is larger than the energy of the arhombohedral (red)accept these subtle features of crystal structure.

form, implying that the b phase cannot be stable at low tem-

Although not cubic, PrCo5 has yielded experimental evi-peratures. Furthermore, the b form exhibits partially occu-

dence of symmetry reduction upon magnetic ordering. Shenpied sites, seemingly in conflict with the thermodynamic re-and Laughlin(12) have shown by convergent beam electron

quirement that entropy vanishes at the low temperatures (thediffraction that the projected point group symmetry along theThird Law). Using electronic density functional methods,[0001] direction of PrCo5 is reduced from 6mm to 6 on mag-Widom and Mihalkovic conclude that this unique, energy

ま て り あ 第49巻 第 5 号(2010)Materia Japanminimizing pattern of occupied and vacant sites can be stable
at low temperatures, but this seems to break the brhombo-

1.2

Competition for stability between the allotropic
phases in Fe: Is the occurrence of antiferromag-

hedral symmetry.

netism at very low temperatures in the gamma

Returning to the general question regarding the sequencephase of Fe responsible for the observed BCC＞

of phases in Fe, as depicted at present in phase diagrams and

FCC＞BCC transition?

accepted in books(2)(4), should we restore the b symbol to thephase diagram? My personal preference is yes, but perhaps

As is well known, the role of magnetism in deciding theanother solution would be to call the present a phase as a′,competition for stability between the observed allotropic

and the phase above the Curie temperature as a, similarly tochanges with temperature in Fe can be considered in terms of

the situation with the atomic order transition ( b′to b) in thethe usual thermodynamic Gibbs free energies:

case of the CuZn b brass.
Later on I shall remind you that at very low temperaturesthe (metastable) FCC gamma phase of Fe may also be

G i＝H i－TS i

where i refers to either phase a or phase g. Similarly, both enthalpy and entropy are expressed asTregarded as tetragonal. When gamma becomes antiferromag-f

H i＝H i0＋

netic (AF) below the Neel temperature (TN), gamma AF
may in fact be regarded as no longer cubic, but FCT in this(1)

C iP dT

(2)

T0

and
T

new phase nomenclature context. One can askis the fer-f

S i＝S i0＋romagnetic FCC Ni also actually FCT? Laughlin suggests

C iP
dT,
T T

(3)

0

that it is actually rhombohedral, because the magnetic mo-where C iP represents the specific heat at constant pressure forments are aligned in the [111] direction.

the phase i, T0 is 0 K and H i0 is the enthalpy at 0 K. Thus, eq.

Perhaps I should also mention that in the case of other ele-

(1) is explicitly rewritten asTments that exhibit numerous allotropic changes, there aresimilar arguments about symmetry, namely, that the accep-

C iP
dT.
T T
T

f

G i＝H i0＋f

C iP dT－T

T0

(4)

0

tance of the existence of additional ``order parameters''

Keeping in mind the Third Law(15), the entropy S0 of a pure(magnetic, electronic, etc.) affecting the motif in the defini-element is zero at 0 K. Thus, we see that the Gibbs freetion of a ``phase'' tends to reduce the crystal structure sym-energy can be expressed in terms of the specific heats CP,

metry. We can start with a ``cubic lattice'', but if we add to it

which can be measured experimentally, or evaluated from aa motif (for example a nonsymmetric distribution of thesuitable model(16)(18)(19):electronic bonding forces which show directional features)the resulting ``structure'' can be regarded as having a ``lower

CP ＝ 3R(UT ) (e e －1) ＋aT＋bT ＋C ,E

2

U E/ TU E/ T

4

2

mag
P

(5)

symmetry''. For example, in Pu the sequence of phases is a→where the first term represents the Einstein specific heat→e→liquid. The Xray studies show that the db → g → d→ d ′with the Einstein temperature UE, the second term the elec-

structure is FCC. It can be argued, however(14), that Pu

tronic specific heat, the third term anharmonic lattice specificatoms in the Pu d phase are not all electronically the same,heat and the last term magnetic specific heat.

and may exhibit ``lattice position dependence'' indicated by

In the case of Fe, it is still a relatively little known fact thatthe fact that the calculated bonding forces between them vary

at very low temperatures the FCC form of iron, i.e., the g,

strikingly and are highly unisotropic, depending on the location on the FCC lattice. There is still an additional difficulty
here, namely the fact that first principles calculations are``ground state'' calculations (at 0 K) and the actual d structure exists at much higher temperatures.
The picture of bonding forces between atoms in Pu, as illustrated in Fig. 2, suggests that Pu is totally different from aclassical FCC metal, like Al, where all forces between atoms
are the same(14), and where the structure is indisputably A2.
So, here again, the total symmetry of the Pu d phase may beconsidered to be reduced from FCC to ccentered monoclinic,
if the arguments based on the first principles calculations for
0 K are accepted.

Fig. 3Temperature dependence of specific heats of
FCC and BCC forms of iron(17).

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Fig. 4

Fig. 5Temperature dependence of experimentally measured specific heats of iron(18).

Modeled height and shape changes of the Neelpeak in the specific heats of gamma iron at low
temperatures, and their effect on the related
Gibbs free energies. The arrow on the left of thepeak indicates height changes, and the one on
the right indicates shape changes of the peak.
The insert indicates the resulting changes in the
Gibbs free energy difference DGa→g as a functionof temperature between the a and g forms ofFe(18).

Fig. 6

Modeled changes in the Gibbs free energy
difference DGa→g between the a and g forms of
Fe as a function of small temperature displacements of the Neel peak in the specific heat trendof FCC g Fe(18).

The Neel temperature is indicated as a small peak, but it has
not been studied in detail experimentally. A recent
calculation(16) places the TN at 67 K. The actual CP data collected together for both phases is shown in Fig. 4(18).

metastable, but retained artificially(2)(17) is antiferromagnetic

Recently, we have modeled the competition for phase(AF). This is also confirmed by the observed 1/x trend withstability as a function of temperature between the a and gtemperature in the g phase which, when extrapolated to 0 K,

forms of Fe by examining the effect of the magnetic peaks on

cuts the temperature axis at a negative value. So, we may ex-

the respective free energies of these phases and the resulting

pect a Neel peak at low temperatures in the corresponding

difference in the Gibbs free energy, DG a→g. For the Neelspecific heat trend. Similarly, a Curie peak due to the fer-

peak in g we introduced small changes in both the positionromagnetic/paramagnetic transition at higher temperatures

and the shape of the peak, and for the Curie peak in a, wherein the BCC (i.e. the a) form of Fe at 770°
C (1043 K) is a well

the temperature of the peak is well established, we in-documented feature in the measured specific heats. These

troduced only small changes of shape compatible with the ob-trends are shown in Fig. 3, which is from Haasen's book(17).

served experimental data. Figures 5 and 6 show the results

ま て り あ 第49巻 第 5 号(2010)Materia Japan

of such modeling(18).

transformation group there is a subgroup known as the ``mas-

The situation may be considered as follows: Starting at 0sive transformations'' (which I happen to have named more

K, when the respective entropies of both phases may be as-than 50 years ago(20)(21)). So, I will make a few observations

sumed to be zero, the enthaply Ha＜Hg because the magnetic

on both of these small subgroups.

interaction in the ferromagnetic a is stronger than the interaction in the antiferromagnetic g. As already mentioned, as
temperature rises, the free energy of each phase is related tothe specific heat (Fig. 3). A little above TN, the paramagnet-

2.1

Isothermal

Martensitic

Transformations:Isthermally activated martensite (TAM) a better
term than isothermal martensite (IM)?

ic g phase will be even less stable compared to the (still fer-

There is a very large literature on the isothermal martensitic

romagnetic) a, but at higher temperatures the situation be-transformations, including much work in Japan. Experimental

gins to change when the negative entropy terms (TS ), eachwork in this field typically involves kinetic studies, or struc-related to the respective specific heat, increasingly come intotural studies, during cooling and reheating, with only a few

play in the free energy of each phase. The modeling confirms

experiments actually performed ``at constant temperature''. I

that at the well documented phase transition temperature ofwill comment only on the terminology, the kinetics and the911°
C (1184 K), the DG a→g value becomes negative, as ex-

activation features of isothermal martensites. Recently, there

pected, but only by a very small amount. This indicates that it

was a symposium in the USA on phase transformations, justmust be the Gibbs energy reduction in the gamma phase due

published this year(22).

the Neel peak at low temperatures, and the relatedTS term,

The kinetics of martensitic phase transformations arethat tips the balance. At still higher temperatures the BCC

usually designated as being either athermal or isothermal.

structure returns in the form of the delta phase at 1393°
C

Athermal implies that the transformation is not (hence the A)(1666 K), when the vibrational entropy effect in the `more

thermally activated, i.e., in an athermal transformation there is

open' BCC structure imparts to it a comparable advantage.

no thermal activation necessary for the transformation to pro-Thus, it can be argued that if the FCC g were not AF at low

ceed. Athermal transformations therefore do not depend on

temperatures, there would be insufficient free energy reduc-time but depend on the change in temperature. The wordtion for it to be stable at high temperatures when it isisothermal (the same heat ) is used as an adjective for a trans-

paramagnetic. Perhaps there is also an additional contribu-

formation that occurs at a constant (same) temperature.

tion to the specific heats of gamma iron from lattice harmon-

When used in the context of martensitic transformations,

ics, as argued some years ago by Zener(19). We should also be

isothermal refers to those transformations that proceed withaware that TN is likely to be a function of experimental details

time and are therefore contrasted with athermal transforma-(dispersed particle sizes, stresses, matrix influences etc.), so

tions which require cooling or heating to proceed, and are notuntil the magnitude of the Neel peak, its shape and form, as

thermally activated(23)(25). It is helpful to realize that thewell as the exact temperature TN, are well established ex-term thermal describes different concepts in the words

perimentally, or the whole low temperature range modeled in

isothermal and athermal. In isothermal, thermal implies ``tem-still more detail, the above conclusion involves a certain

perature'', while in athermal, thermal is short for ``not ther-

degree of speculation. Our modeling indicates that raising themally activated''(23).

TN by just a few degrees higher than 67 K eliminates the

gamma phase altogether.

Although the literature contrasts isothermal with athermal,
not all nonathermal transformations are necessarily thermal.
There may be thermally activated processes which occur on

2.

Phase Transformations

continuous cooling. These have sometimes been calledanisothermal(26) but the term is rarely used. In martensite

The area of Phase Transformations has been of great in-which forms athermally, thermal energy is insufficient to in-terest at different times in my research career. The field of

itiate the transformation. Whatever the mechanism of the in-phase transformations is also very large, like the field of

itiation of the transformation is, it cannot proceed by thermal-

phase diagrams, and many attempts have been made to clas-

ly influenced fluctuations. The athermal transformation is in-sify transformations. Professor Jack Christian, my predeces-

itiated at specific sites only after a large enough chemicalsor JIM gold medal winner, has made well known contribu-

driving force is generated by cooling to a large enough degree

tions here. Today, I would like to comment on two specificbelow the equilibrium phase transformation temperature.

types of transformations. In the displacive (nondiffusional)

This thermodynamic driving force must overcome the elastic

phase transformations group, there is a subgroup sometimes(plastic) energy which is in opposition to the initiation of thereferred to as ``isothermal martensites,'' and in the diffusionaltransformation at specific sites and at below Ms. Thermal ac-

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tivation implies a statistical probability, meaning the same

thermally activated martensitic transformations (see Fig. 8)site will not always be repeatedly the first one to initiate thecannot be explained in the same way as precipitation trans-

process. This is unlike in some of the thermoelastic martensites

formations because there is little or no activation barrier towhere the same site has been shown to repeatedly initiate thegrowth in thermally activated martensitic transformations.

process(27).

Of course all thermally activated processes must cease at 0 K,

Since the isothermal martensites involve time as a major

but the TTT curves for the thermally activated martensitic

parameter, the TTT (transformationtimetemperature)transformations bend back well above 0 K. Thus, it appears

diagrams have been used to depict the progress of a giventhat the barrier to nucleation must also have the shape of a Ctransformation. The well known Ccurve shape for the

curve if it controls the transformation kinetics. Since this bar-isothermal thermal activated precipitation reactions can berier has within it the elastic energy of the transformation,plotted on a TTT diagram as shown in Fig. 7. The time tosuch an increase in the barrier can arise because of an in-form the new phase initially decreases as the temperature iscrease in the elastic stiffness of the matrix at lower tempera-

lowered below the equilibrium transformation temperature,tures. This has been discussed by Lobodyuk and Estrin(28).

due to the decrease in the barrier to nucleation, DG , asLaughlin et al.(23) have considered some possibilities

shown in Fig. 7. This decrease in the nucleation barrier oc-

regarding the role of thermal activation and summarize themcurs because the thermodynamic driving force increases asin the form of Table 1, where I have also included the ther-

the material is cooled to lower temperatures. However the

moelastic martensites for comparison. We suggested that thetime to form the new phase begins to increase at lower tem-

isothermal martensites could be conveniently renamed asperatures due to the lack of thermal energy necessary forthermally activated martensites (or TAMs)(23). The TAMs

diffusion to take place. In terms of the thermally activatedhave thermal activation only at the initiation stage, because

processes, nucleation controls the upper region of the Cthe subsequent growth stage normally occurs rapidly, and in

curve while diffusion controls the lower region of the curve.

a martensitic mode, without requiring thermal activation. Of

The Ccurve behavior on TTT diagrams depicting thecourse there could be TAMs which have thermal activation in

Fig. 7

Schematic of (a) a typical TTT curve for adiffusion controlled precipitation transformationand (b) a plot of DG, the barrier to nucleation,
vs. T for precipitation(23).

Table 1

Fig. 8

Schematic of (a) a typical TTT curve for a thermally activated martensitic transformation and(b) a plot of the smallest DGfor the given undercooling vs. T for a thermally activated martensitic transformation(23).

Thermal activation of phase transformation processes.
Thermal Activation
for Initiation

Thermal Activation
for Growth

I

Classical Precipitation

Yes

Yes

II

Thermally Activated Martensite (TAM) (on cooling, or at const. temp.)

Yes

No
Yes?

III

Thermally Activated Martensite (Requiring Thermal Activation During lnitiation
and Growth)

Yes

Yes

IV

Athermal Martensite (AM) (substantially below T0)

No

No

V

Thermoelastic Martensite (close to T0)

No

No
Yes?

VI

Diffusion Controlled Growth from Preexisting Nuclei

No

Yes

ま て り あ 第49巻 第 5 号(2010)Materia Japantheir initiation process, as well as in their growth process.

contrast to this, curves 2 and 3 represent transformations

These also would show a Ccurve behavior and would be

which proceed from athermal to anisothermal to isothermal.

difficult to distinguish from the normally observed ``isother-

This is because the isothermal hold temperature was notreached before the material passed through the TTT curve,

mal martensites''.
In contrast, athermal martensitic transformations exhibit

allowing for some thermally activated process to occur during

no activated processes and they do not need waiting time (in-the continuous cooling stage before the isothermal martensite

cubation) to proceed, only a sufficient thermodynamic driv-

begins to form(28).

ing force obtained by lowering the temperature. A fifth pos-Figure 9(b) shows a TTT curve in which the Ms tempera-

sibility is also included in the table, namely one in which theture is below the knee of the TTT curve(23). Cooling curve 4

initiation stage does not require thermal activation, but the

represents an overall transformation which starts as athermal

growth stage does. This would also be the case for diffusion

one and after holding at a temperature below the Ms, and be-controlled precipitation reactions that grow from preexist-

low the knee of the curve becomes isothermal in character.

ing nuclei. As pointed out by Laughlin et al.(23) and also by

Curve 5 shows a transformation which starts as an anisother-

Lin et al.(29), it is of interest to note that there could occur se-

mal one, becomes athermal below Ms, but when held at a con-quences of martensitic transformations as shown in Table 2.

stant temperature below Ms and below the C curve knee, it

These sequences can also be illustrated on the TTT dia-

becomes isothermal in character. Curve 6 represents a trans-

grams.

formation which begins anisothermally and becomes isother-

As is well known, the majority of TAMs are studied during

mal on holding at a constant temperature above Ms. Finally,

continuous cooling, or during cooling and subsequent holding

curve 7 represents an isothermal martenstitic transformation

at temperature. In Fig. 8 are shown progressions of different

at a constant temperature. After holding at this temperature

types of possible martensitic transformations. The relative

an athermal reaction could develop on further cooling below

position of the Ms and the nose of the TAM Ccurve, and theMs, depending on how far the TAM has progressed.

rate of cooling allows for several possible sequences of trans-

So, since the isothermal martensite is a thermally activatedformations. In Fig. 9(a), the Ms temperature for athermalmartensite, perhaps the term isothermal martensite should bemartensite has been placed above the knee of the TTT curve,

replaced with the more descriptive and more accurate term

but below the highest temperature at which TAM can form,

TAM. One can consider further possibilities if the CcurveMsi(23)(30). Cooling curve 1 represents the sequence of an

for TAMs is displaced more to the left, eventually intersect-athermal martensitic transformation followed by a TAM. In

ing the temperature (T ) axis. The transformation will thenappear to be an athermal one since it occurs rapidly without

Table 2

Possible martensitic transformation sequences.
Isothermal
Athermal→Anisothermal→

any apparent incubation time. Such a transformation could becalled a ``pseudoathermal'' transformation. One way to distinguish this kind of transformation from a truly athermal

Isothermaltransformation is to hold the sample at the temperature atwhich the transformation was first observed. If it does not

Isothermal

Athermal →Anisothermal →Isothermal

proceed with time it is truly an athermal transformation. If,

Anisothermal→

Athermal →Isothermal

however, the transformation continues over an extended``Mixed Athermal and Anisothermal''→

Isothermal

period of time it is seen to be thermally activated and hence it
started as an anisothermal transformation. For more details
see Refs. (22) and (23).
2.2

Massive Transformations: Can totally idiomorphic nucleation and growth occur in diffusionaltransformations?

Massive transformations have been studied since the late1950's(20), and the actual definition of this type of transformation is still a matter of some discussion. However, in my briefcomments today I would like to address mainly the crystalloFig. 9

Schematic of the possible sequence of different
martensitic transformations with (a) Ms aboveand (b) below the nose of the TTT Curve(23).

graphic features and not the kinetics of this transformation
process. One of the main questions regarding massive transformations has been whether or not there is a crystallograph-

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ic orientation relationship between the parent phase and thethe studies now possible with the HRTEM techniques show

product phase as the massive grains nucleate and grow. Suchthat the idiomorphic massive (product) interface often in-orientation relationships are almost always observed involves very fine multiple crystal layers, referred to as facets,

diffusional transformations, and they are expected on the ba-

whose habit is that of the massive phase itself and not of thesis of activation energy arguments in the theory of transfor-

matrix as shown in Fig. 12(a) and (b). Many years ago,mations.

Prof. C. S. Smith(34) has suggested a parent/product inter-

Based on the research information developed in the 1960's

face possibility as shown in Fig. 13, and also in the schematicand 1970's, Christian included the massive transformation in

picture in Fig. 10. Recent HRTEM pictures confirm this pos-

his well known classification of transformations in metals andsibility when the nucleation appears to take place at the par-alloys in 1986(31). There, he concluded that massive transfor-

ent phase boundary (Fig. 14(a) and (b)). However, duringmations are ``orientation free''. In 2000, there was a Symposi-

subsequent growth the idiomorphic feature is clearly seen inum on Massive Transformations (with 30 pages of discus-

the HRTEM micrographs, confirming the existence of thesion, interesting reading!(32) and I proposed there a definition

type3 (incoherent and irrational) interfaces(35). Faceting is

for a massive transformation which seemed to cover all ob-

frequently observed along such incoherent interfaces (forserved experimental features. In a later paper regarding thedetailed references see Refs. (32) and (33)). In view of theselack of orientation relationships between the parent (i.e. therecent symposia and discussions the most appropriate defini-matrix) and the growing massive phase in it, Laughlin, Soffation of a typical massive transformation seems to be best de-

and I have concluded that there are indisputably clear cases

fined as follows: ``a compositioninvariant, interfacecontrolled

where the growth of the product phase (i.e., the massive

diffusional phase transformation, involving a characteristicphase) is taking place as if it were occurring at a ``free sur-patchy microstructure and frequent faceting and ledges, but not

face'' of the growing grain, seemingly uninfluenced by thepresence of the matrix phase ahead of it(33). We describe thistype of growth as idiomorphic, and it implies a lack of crystallographic communication between the parent and productphases. This is illustrated in the schematic Fig. 10(33). A
good example of an early Xray demonstration of a complete
lack of orientation relationships in CuZn alloys is reproducedin Fig. 11(33). Subsequently, elegant electron microscopy
studieshave

investigations

confirmed(32)(33)

thisfeatureinnumerous. Some people have argued that this situ-

ation may be only occurring in alloys where there may be achange of chemistry (i.e. of bonding forces) taking place between the two phases at the interphase boundary, but idiomorphic growth has been so universally observed in typicallymetallic systems, like CuGa, CuZn or AgAl that this explanation seems unwarranted. However, at the atomic level,

Fig. 11Lack of orientation relationships (ORs) between matrix and massive grains in a partial
massive transformation in a CuZn alloy, as
demonstrated by X ray selective area
diffraction(21).

Fig. 10

ま て り あ 第49巻 第 5 号(2010)Materia Japan

Schematic types of various typesof nucleation and growth that canoccur
indiffusional
phasetransformations(21).



Fig. 12Fig. 13(a) HREM image of acurved section of an et interface and (b) higher magnification of selected area
showing composite (020)and {111} nanofacets/terraces/ledges(33).

(a) Classical critical nucleus model for singly
faceted grain boundary nucleus which is incoherent with respect to grain a2 but coherent orsemicoherent with respect to a1. (b) Grainboundary nucleus involving faceting of the incoherent boundary segment(33).

necessarily involving lattice orientation relationships.''(21). A

suitable name for the idiomorphic massive phase product has
not been established.
So, these various forms of massive transformations continue being studied with increasingly sophisticated techniques and present a real challenge to researchers, also inJapan where the massive transformation has been studied
very little. The overall conclusion at present is that isomorphic massive transformations are a specific unique feature andthat there can be diffusional transformations that show no
orientation relationships, i.e. no communication between parent and product phases, both in the nucleation and growthstages.

Fig. 14(a) Grain boundary nucleation and growth of
L1o tphase in MnAl(C) alloy during massive transformation e (hcp)→L1o. (b) Faceted
grain boundary idiomorphs with Burgers ORwith respect to one grain and apparent incoherent boundary with respect to the other. Growth
is restricted along the semicoherent boundary
segment with the Burgers OR, but occurs readily into adjacent grain(33).

Acknowledgments
touched on just a few subjects that may be currently of inMy friends in Japan know well that in my long researchterest, and with which I have had a personal connection. This

career I have been involved in many diverse topics in thealso enables me to mention a few scientific dilemmas and con-wide field of Materials Science. In this presentation, I havetroversies. Since this is an invited lecture I have written it in




学

会

賞

`first person' style.
I wish to express many thanks to my colleagues Professor
Uichiro Mizutani in Japan and Professor David Laughlin in
the USA for many stimulating discussions and for their help
in the preparation of this manuscript.
I feel deeply honored and moved by the award of the Japan
Institute of Metals gold medal. I also feel very proud to join
your Institute as Honorary Fellow. Thank you very much forselecting me for both of these special honors. Your invitation
to present an invited lecture in this connection is a special
challenge. I have covered a lot of topics in this presentation
and I hope that I have provided some of you with food forthought and for future research on some of the fundamentals
aspects and on terminology in materials science. If this happens I shall be richly rewarded both by the JIM medal andyour kind attention today.

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★★★★★★★★★★★★★★★★★★★★★

Thaddeus B. Massalski

1952: University of Birmingham, B.Sc., (Physical andTheoretical Metallurgy, Class I. Honors); 1954,
Ph.D.; 1964, D.Sc.; 1973, Dr.h.c. (Warsaw)
1954
56: Post Doctoral Fellow, University of Chicago,
IL, USA
1956
59: Lecturer and Senior Lecturer, University ofBirmingham, England
1959
62: Senior Fellow and Head of Metal Physics,
Mellon Institute, Pittsburgh, PA, USA
1962
72: Director, Staff Fellow and Member of
President's Advisory Committee, Mellon Institute, Pittsburgh, PA, USA
1966
2001: Institute Professor of Metal Physics, Mellon Institute of Science, Carnegie Mellon
University, Pittsburgh, PA, USA
2001～Emeritus Professor of Materials Science andEngineering, and Physics, Carnegie
MellonUniversity, Pittsburgh, PA, USA★★★★★★★★★★★★★★★★★★★★★

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