Microstructural Effects of Martensitic Transformation Cycling of a Cu-Zn-AI Alloy: Vestigial Structures in the Parent Phase

JEFF PERKINS and PAUL BOBOWIEC

Martensitic transformation kinetics and microstructural effects are correlated in a study of trans- formation cycling in a Cu-Zn-A1 alloy. During the first few cycles of martensitic transformation and reversion, vestigial features develop in the parent phase. During these same cycles, the parent-to- martensite transformation temperature range shifts upward by several degrees and the martensite-to- parent reversion temperature range shifts down by about a degree, in effect reducing the transformation hysteresis. The vestigial ridge features are associated particularly with burst martensite phenomena, and this and other manifestations of imperfect thermoelastic martensite behavior during initial cycling lead to the kinetics changes. These changes virtually stabilize after 20 to 25 transformation cycles as the martensitic transformation achieves a completely reproducible pattern, both in terms of micro- structure and kinetics.

I. INTRODUCTION

RECENTLY, several papers have reported on the effects of cyclic transformation in martensitic alloys. 1-9 Among the various effects reported have been kinetic changes, such as shifts in M , and substructural changes, such as the appear- ance of dislocation 'debris'. The present work attempts to correlate and explain these various effects by means of a combined kinetic and microstructural study of transfor- mation cycling in a Cu-Zn-A1 alloy.

II. EXPERIMENTAL

The alloy material used in this work was provided by Delta Metals Research, Ltd., Ipswich, Suffolk, England, with a nominal composition 66.2 at. pet Cu-24.8 at. pet Zn-9.0 at. pet AI and a measured Ms temperature of 268 K. This material, originally in the form of 1 cm diameter hot- worked bar, was machined into rods 3 mm in diameter which were sealed in evacuated quartz tubes, solution treated for 15 minutes at 1173 K, and quenched into ice water. The rods were then wafered into 0.25 mm thick discs using a low- speed diamond saw. Transformation cycling was carried out, prior to electropolishing to thin foils, in a Perkin-Elmer Model DSC-2 differential scanning calorimeter, at a scan rate of 10 K per minute between 230 K and 300 K, which spans the Mf to Pf range for this alloy. Three-to-five discs were cycled as a group to enhance the DSC sensitivity and to ensure that at least one good thin foil could be ob- tained when the discs were subsequently electropolished. Several groups of samples were cycled more than one hun- dred times. Thin foils were then prepared with a Struers TENUPOL unit, using a 3 pet perchloric acid-methanol solution at a temperature of 240 K and a voltage of 70 to 75 VDC. The thin foils were examined in a JEM 100-CX II transmission electron microscope.

JEFF PERKINS, Professor of Materials Science, Department of Me- chanical Engineering, and PAUL BOBOWIEC, Graduate Student, are with Naval Postgraduate School, Monterey, CA 93943.

Manuscript submitted April 5, 1984.

III. RESULTS AND DISCUSSION: TRANSFORMATION KINETICS

The sample groups were thermally cycled more than 100 times while continuously monitoring the transformation in the DSC. Based on experience in earlier work, 3 the tem- peratures corresponding to the maxima of the DSC peaks were taken as sufficient to assess any cyclic effects. These temperatures are designated as MMAX, corresponding to the temperature of the maximum rate of heat evolution during the exothermic parent-to-martensite (cooling) transforma- tion, and PMAX, corresponding to the temperature of the maximum rate of heat absorption during the endothermic martensite-to-parent (heating) transformation. The tem- perature MMAX is roughly halfway between M, and Mr, while PMAX is roughly halfway between Ps and Pc. 3 Shifts of the transformations with cycling are reflected in Figures 1 and 2 by plotting the difference between MMAX on the initial cycle with MMAX on subsequent cycles, and similarly for PMAX. The initial cycle MMAX was 256.5 K for Figure 1, and the initial cycle PMAX was 273.4 for Figure 2. Third order polynomial curve fits were applied separately to data points representing cycles 1 to 5 and cycles 6 to 100. After consid- erable trial and error, we have chosen the method of data analysis described above as the most reliable. In particular it is very difficult to obtain systematic data for the onset and completion of these transformations, i .e . , for M , Mf, Ps, and Pf. These temperatures have been determined as well, 3 but the MMAX and PMAX values are more consistent.

The main features of the data may be summarized as follows: (1) MMAX rises sharply, by several degrees, in the first few cycles, continues to rise gradually until perhaps 20 to 25 cycles, then virtually levels off; (2) PraAX follows a similar pattern except that the temperature shift is in the opposite sense, that is, PMAX decreases with cycling, and the magnitude of the shift is much less, amounting to less than a degree overall. In the last regard it should be noted that the temperature scale in Figure 2 is expanded to twice that in Figure 1 in order to reveal the very slight change in PM~X as compared to MMAX.

METALLURGICAL TRANSACTIONS A VOLUME 17A, FEBRUARY 1986-- 195

1 6

9 . 5 9

^ 6 . 5 I 6

Y 7 . 5

Ld L p 6 . 5 6

~5.5 t) 5

~ 4.5

~ 4 I - - 3 . 5 n~ 3 ~ 2 . 5

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I

. 5

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§ +

" ~ " 4 ~XI ~ IT) ~ ~" ~ " U') I t ) tO G) i%. IX- t o t o 0 ) ~D

NUMBER OF CYCLES ->

Fig. 1 --Variation of M~,,x with martensitic transformation cycling. Mu^x is the temperature corresponding to the maximum rate of heat evolution during the exothermic parent-to-martensite transformation on cooling. The figure plots the increase of MM^x over its value on the initial cycle.

This pattern has been found to be reproducible and clearly contradicts the report of Li and Anseil 2 who, using intermit- tent measurements with a resistivity apparatus, suggested that M~ was still increasing after 300 cycles. Our results using continuous monitoring confirms the earlier low-cycle work of Perkins and Muesing, 3 and although our data show virtually no significant change in the transformation beyond (conservatively) 20 to 25 cycles, we extended some runs to beyond 300 cycles and can confidently state that there is no

systematic change in either Ms o r MMAX over the range (say) 25 to 300 cycles.

The initial (first 20 cycles) increase of MMAX accompanied by a decrease in PMAX reflects an overall narrowing of the transformation hysteresis, as previously demonstrated by Perkins and Muesing 3 and consistent with Kajiwara's 9 sug- gestion that complete reproducibility of transformation is not obtained in Cu-Zn alloys until 20 cycles have been completed.

2 . 5

2 . 2 5 2

^ 1 . 7 5

I 1 . 5

2~ 1 . 2 5

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- 2 . 5 ~

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§ 2 4 7 § § § §

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NUMBER OF CYCLES ->

Fig. 2--Varia t ion of PM,,x with martensitic transformation cycling. PMAX is the temperature corresponding to the maximum rate of heat absorption during the endothermic martensite-to-parent transformation on heating. The figure plots the slight decrease of PuAx below its value on the first cycle.

196--VOLUME 17A, FEBRUARY 1986 METALLURGICAL TRANSACTIONS A

IV. RESULTS AND DISCUSSION: MICROSTRUCTURE

All samples were observed in the TEM at room tem- perature, which is approximately 15 K above our measured Pf, the temperature where the parent phase is measured as finishing its reversion from the martensitic state on heating. Therefore, any martensite plates or vestigial features seen at this temperature are: (a) truly residual, that is, debris derived from the normal bulk transformation, (b) features resulting from surface transformation, (c) thin foil artifacts, or (d) the result of stress-induction in the foil.

Throughout this work, careful attention has been paid to eliminate the possibility of these last three cases, which would not reflect bulk behavior. We were interested only in characterizing truly residual features, i.e., features derived from the fully developed bulk martensitic microstructure that had existed prior to reversion.

A. Vestigial Ridges

Linear vestigial ridges develop after even a single thermal transformation cycle (Figure 3). Features of this sort are never seen unless the sample has at least partially traversed the Ms to M f range and then reverted to the parent phase. Care was taken to assure that even a partial pre-cycle had not occurred during sample preparation, such as during the quench after parent phase solutionizing, or during the electro-thinning operation. Also, care was taken that no samples were ever transformation cycled as thin foils (with the exception of a few purposeful trials to see what sort of structure this induced; these experiments, by the way, typi- cally resulted in slight remnant surface upheavals in the thin foils and made the images difficult to interpret). Selected area diffraction patterns show only reflections from the par- ent/3 phase in the vicinity of the vestigial ridges.

Structures similar to these vestigial ridges have been re- ported by other workers. Schroeder and Wayman 1~ observed faint "ghosts" above Ms on the surface of superelastically cycled Cu-Zn alloys. Kajiwara 11 closely associated similar ridges in Cu-Zn alloys, seen along directions parallel to

martensite plates and increasing in number with cycling, with the reproducible formation of martensite plates. Kajiwara 12 also observed a similar structure in Fe-Ni alloys, as elongated "islands" parallel to directions in which surface upheavals had occurred on the reverse transformation, pro- duced as a result of stress self-accommodation among revert- ing plates. The remnants of self-accommodating plates are also seen in Figure 3, with the thin double needle vestigial features perpendicular to the main plate. These vestiges may be associated with stress-induced martensite similar to that described by Schroeder and Wayman 13 as forming above Ms in Cu-Zn alloys. Tanner, Pelton, and Gronsky 14 proposed that such ridges are due to fluctuations in atomic order, concentration, or lattice strain. Bricknell and Melton,~S ob- serving Ni-Ti-Cu alloys in a TEM with a heating and cool- ing stage, found that "ghost" martensite images, associated with image mottling especially near the Bragg condition, ran parallel to prior martensite plates, and that this martensite reformed in a burst manner. This observation is consistent with the observation by Perkins and Sponholz 4 that vestigial features are not left by every single martensite plate, but rather only by certain plates in the overall microstructure, and that these are those which are least favored by the local stress field i.e., those that would exhibit "burst" kinetics.

As cycling increases (see Figure 4, at 5 cycles), the linear vestigial features become more numerous. Figure 4 displays several orientation variants, as well as the fork type mor- phology with nearly parallel shape strains, and spears with two variants forming pairwise bisected by a common sym- metry plane, such as described by Schroeder and Wayman.13 Another morphology seen here (and also reflected in figures to be discussed later) is similar to the Type A butterfly martensite found in ferrous alloys, such as reported by Umemoto and Tamura. 16 This morphology is composed of two plates with an irregular junction and smooth interface with the remaining parent phase, forming as a result of im- perfect self-accommodation due to the similarity of shape strain directions. The interpretation given of this I6 is that the twins on the outer surface act as autocatalytic nucleation

Fig. 3--Vest igial structures observed in the parent phase after a single martensitic transformation cycle, i .e. , after an excursion to below Me and then return to above Pc at room temperature. In this alloy, room temperature is about 15 K above Pc.

Fig. 4--Vest igial structures observed in the parent phase become more numerous and more sharply defined as the number of martensitic trans- formation cycles increases, as seen here after 5 cycles. This area includes linear vestigial ridges corresponding to several different martensite plate variants.

METALLURGICAL TRANSACTIONS A VOLUME 17A, FEBRUARY 1986-- t97

sites, with the initially transformed plate generating slip which nucleates the second plate of the same variant. While accommodation problems are not as severe in the present alloy system, this is the sort of situation which, if obtained locally, would be most likely to leave vestigial features. Residual areas with internally twinned and banded struc- tures have been occasionally observed in this work.

The inconsistent nature of vestige formation from grain to grain, and even within a given grain, is illustrated by Figure 5, where an intersection of three grains is seen in a sample cycled 10 times. Two distinct grain boundary- vestige relationships are evident, with a set of parallel mark- ings terminating at the grain boundary in the two grains to the left, as contrasted to the cluttered arrangement of ves- tiges in the region of the boundary for the right grain. As described by Dvorak and Hawbolt, 17 the formation of thermoelastic martensite is accompanied by shape changes that are restricted in part by grain restraint. When the matrix is constrained, as at the triple point, a higher driving force is needed to form the martensite. By an extension of this same reasoning, incompatible plate groups in the same grain may also interfere with one another. Notice that an array of parallel vestiges is evident in the area farther to the right in the right grain of Figure 5.

With increased cycling, the ridges become more distinct, as exemplified by Figure 6. In addition to the distinct linear features a wavelike pattern is seen that may be associated with the more continuously forming and reverting thermo- elastic martensite, as opposed to burst plates. The increased definition of the vestigial ridges appears to assist in the cycling by enabling the parent phase to "anticipate" some of the microstructural characteristics of the martensitic state; this is also consistent with the observed increase in M s .3 An initial activation barrier to transformation is clearly over- come in the very first few transformation cycles, which is when the greatest proportion of the increase in Ms is re- corded, and which is when the essential features of the vestigial microstructure are developed.

Another example of the various interactions of different martensite orientations with a grain boundary is shown in Figure 7. As discussed by Takezawa et al.,~8 the martensitic

Fig. 6 - - H i g h e r magnification view on the linear vestigial ridge markings seen in the parent phase after 5 transformation cycles.

(a)

(b)

Fig. 5--I l lustrat ion of the non-uniform distribution of vestigial features from one grain to another and even within a given grain in this poly- crystalline material, after 5 transformation cycles.

Fig. 7 - - ( a ) Interactions of vestigial markings with a parent phase grain boundary in a sample cycled 10 times; (b) higher magnification view on some of the spearlike arrangements of vestigial markings in this area.

198--VOLUME 17A, FEBRUARY 1986 METALLURGICAL TRANSACTIONS A

transformation does not necessarily take place indepen- dently in the grains, but may proceed with a strong influence across the boundary caused by the necessity for com- patibility of net transformation strains at the boundary. The spearlike morphology in the upper grain of Figure 7(a) is similar to that found in Cu-A1 alloys by Villasenor et al., 19 which is the twinned 3# type. As exhibited by the closeup of one of the present spears in Figure 7(b), the mottled, stepped structure of the interface between the vestigial spear and the parent phase is quite different from the sharp inter- face seen in the Cu-A1 alloys mentioned above and in the fully developed martensitic microstructure in the present alloy system.

The variety of orientations and relief associated with an array of vestigial plates is seen in Figure 7, while areas with just one distinct residual plate allow us to distinguish be- tween these and the vestigial ridges, as shown in two exam- pies in Figure 8. The slight surface relief of the ridges, from which we derive their name, can be compared to a similar phenomenon in Fe-Ni alloys, as discussed by Kajiwara, 2~

(a)

(b)

Fig. 8 - (a) Distinction between a bona fide residual martensite plate (at top) and nearby vestigial ridge features is seen in this area, after 10 trans- formation cycles; (b) another example of this comparison; here the residual plate is in the center of the view and the vestigial markings are approxi- mately parallel to its habit trace, also after 10 transformation cycles.

with upheavals occurring along the habit plane and finer striations due to detwinning of transformation twins. At high magnification under dark-field imaging conditions the sub- structure of the present ridges can sometimes be revealed and consists of a stepped midrib intersected by bands of parallel twins. 2~

B. Dislocation Substructure

Another parent phase substructural feature that changes with transformation cycling is the dislocation density. The structures generated are represented by Figure 9, where distinct orientation relationships with the vestigial ridges are seen. As discussed by Kajiwara 9 and by Beyer, 2z the in- creased dislocation density with cycling may be related to enhanced reversibility of transformation, with martensite formation being specified by the particular dislocation ar- rangement and (at least for the first few cycles) new disloca- tions being generated by reverse transformation. According to Jara et al. 6 the dislocations found in the /3 phase of thermally cycled Cu-Zn-A1 alloys have (111)/3 line direc- tions and occur inrows in the (110)/3 direction; {110}/3 is considered to be the slip plane, containing paired dis- locations with (100) type Burgers vector. The concept of growth accidents has been proposed by Gleiter, 23 with inter- face movement generating lattice dislocations with spacing dependent on the rate of migration of the boundary. Thus the lowest resultant dislocation density will be in the region where the boundary just begins to migrate.

A low-angle boundary is seen at the top of Figure 10. Some structure is definitely confined by the boundary, while other structures continue across the boundary relatively un- disturbed. None of the dislocation structures passes through the boundary. In the left center, an indistinct vestigial struc- ture appears in an orientation perpendicular to the dis- locations. The equidistantly spaced and aligned arrangement of the dislocations is seen in the central region of Figure 11. Kajiwara's results with Cu-Zn alloys L2 present similar aligned structures, with the spacing of the alignment bands equivalent to the martensite plate width. The Burgers vec- tors of dislocations in neighboring bands were found to be different, but were the same for alternate bands, thus dis- allowing their possible origin as a result of the release of accommodation strain around plates during BCC---) 9R transformation. In the reverse transformation to the parent phase, the lattice invariant deformation within the individual martensite plates must reverse on effectively different /3 phase slip systems for neighboring plates in the same grain. Thus these are transformation dislocations and not accom- modation dislocations. As with the vestigial ridges, we may note that the dislocation structures are not uniformly distributed.

C. Debris from Burst and Slow-Growth Martensite

It seems probable that the vestigial ridges are primarily debris associated with martensite which formed by the "burst" mode. As described by Pops and Massalski, 24 as temperature is lowered below Ms, initial martensite for- mation is primarily by slow growth of parallel bloc and wedge-shaped volumes, but at a temperature Mb, somewhat lower than Ms, some quantities of martensite form in rapid bursts, and revert with similar speed on heating. The typical morphology of burst martensite consists of V-shaped groups

METALLURGICAL TRANSACTIONS A VOLUME 17A, FEBRUARY 1986-- 199

Fig. 9--Dislocation substructure left in the parent phase as a result of martensitic transformation cycling, after 15 transforma- tion cycles.

or parallelpipeds delineated by two habit plane variants. Burst formation is associated with mechanical (strain) cou- pling between several habit-plane-variant plates which are nearly parallel to the plane on which initial martensite forms, with the bursts relieving the stored elastic energy of the parent lattice. Since burst martensite formation is based on the buildup of such stored elastic energy, then it seems reasonable to speculate that burst martensite will cease if some other mechanism is developed to relieve these strains.

The results of the present work associate the stabilization of the martensite transformation kinetics, after 20 to 40 cycles, with the gradual cessation of burst martensite formation, with the alternative microstructural process being the gradual formation of the vestigial ridges. The ridges represent a more subtle means of elastic energy relief in the microstructure. Warlimont et al. 25 suggest that the cessation of burst martensite formation occurs when the free enthalpy acting as the thermal component of the driving energy for the plate growth drops below a critical value. As noted by Li and Ansell, 2 burst martensite ceased at about 60 cycles (they did not monitor transformation kinetics on every cycle) and was replaced by continuously growing

martensite, whose growth rate they observed to be about 300/zm/min. The propagation of burst martensite at nearly sonic velocity is about 10 s greater in magnitude. Trans- formation and reversion at this rate may either shock-induce irreversible structure and/or not allow sufficient time for elimination of all the substructure of the martensite, re- spectively. The increased general mottling (tweed structure) would seem to be a reflection of the subtle alteration of the parent phase as a result of the reversion of slow-growth martensite, while the distinct vestigial ridges may be associ- ated with incompletely reverted burst martensite. Clearly, as cycling increases, there is continued enhancement of the strain contrast and surface relief associated with the vestigial ridges.

D. Reproducibility of the Transformation

The formation of the vestigial ridges serves to decrease the hysteresis of the transformation, increasing MMAX by serving as nucleation sites and decreasing PMAX by acting as residual martensite structures. The transformation hysteresis, PMAX -- MM~, is therefore decreased by cycling; the trans- formation becomes more perfectly reversible and more truly

- 1 . 0 u

Fig. 10--Interaction of dislocation substructure with a low-angle grain boundary, in a sample cycled 15 times. Fig. l l - - A l i g n e d dislocation substructure in a sample cycled 15 times.

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Fig. 12--Vestigial markings with diamond morphology, seen in a sample given 15 transformation cycles.

thermoelastic. With the increase in the volume occupied by the ridges with cycling, there is an effective reduction in the amount of martensite actually reverting to the parent phase, so that the measured temperature of the reversion peak natu- rally decreases. At about 20 to 40 cycles, the stabilization of M~AX and PMAX is associated with the cessation of burst martensite as an accommodation mechanism and its replace- ment with vestigial structures and dislocation debris. The diamond-shaped parallelpiped vestigial morphology of a burst-type group is seen in Figure 12. Similar structures have been reported by Perkins and Sponholz in trained two- way memory alloys. 4 The diamond morphology as de- scribed by Schroeder and Wayman 13 contains 4 martensite variants twin-related in pair combinations with the parent. Relative to the parent phase, the long and short bisectors of the diamond are reflection planes; this is confirmed for these vestigial features. Saburi and Wayman 26 have presented de- tailed stereographic analysis indicating how the diamond morphology may be derived for 18R (9R) for 24 martensite crystal variants.

As the number of transformation cycles advances, the accumulation and increased interaction of vestigial ridge features may actually lead to a slight decrease in MMAX. As pointed out by Oshima, 27 this may occur by the buildup of internal stress which serves to limit twin formation in the martensite; in Figure 13, the interactions of spear and wedge morphologies with a grain boundary are shown. A higher magnification view of the distinctive stepped structure is similar to that seen for the 3" (2H structure) martensite in Cu-A1-Ni-Fe-Mn alloys as reported by Hasan et al. 28 In the range where/3 '-type and "y'-type martensite may be formed concurrently, small local changes in internal stress are suf- ficient to transform the parent phase into either of the two structures.

The butterfly martensite morphology becomes more com- mon and distinct with cycling (Figure 14), because its for- mation is aided by the increased presence of dislocations which contain growth of martensite on one side and assist nucleation on the other. As proposed by Kajiwara, ~~ part of the driving force for the transformation is supplied by re- sidual internal stresses that are not completely relieved by self-accommodating plate groups. Therefore, with mor- phologies such as butterfly forming as an alternative to self-accommodating groups, there is a demand for a lower M~ to provide greater driving force. As cycling increases the intersection of groups of plates having different orientations are evermore effective in mutually interfering and reducing plate growth, and at the same time stimulate reversion, as demonstrated by Yong et al. 29 This may account for slight decreases in PMAX-

Another aspect of reversion debris is the retention of specific plate substructure, such as indicated in Figure 15 for samples cycled 77 times. Here, the interaction of martensitic substructure is seen in a prior plate area traversed by a set of parallel stacking faults in the parent phase; note the stack- ing fault displacements by the remnant plate substructure. Andrade et al. 30 noted similar structures in a Cu-Zn-A1 alloy wherein the substructural defect spacing was about 0.1 to 1.0/xm. The defect planes were determined to be {128}-type /3'1 planes, distinct from the {i28} twinning_planes which separate self-accommodating variants. The (128) and (128) planes are both derived from {110}-type parent/3 planes, but

Fig. 1 3 - Vestigial markings, seen in the parent phase of a sample cycled 100 times.

Fig. 14--Vestigial markings seen in the parent phase of a sample cycled 100 times.

METALLURGICAL TRANSACTIONS A VOLUME 17A, FEBRUARY 1986--201

(a) (b) Fig. 15 - - Retention of specific forms of lattice irtvariant deformation of martensite plates in the parent phase above P~ (a) slip deformation; (b) stacking faults; both views from samples cycled 77 times.

while (]-28) is parallel to {110}/3, (]28) is rotated 10 deg. The (]'28) faults always terminate on other faults in the basal plane of the martensite. A variation in the density of stack- ing faults has been explained by Ahlers, 31 in the context of a concentration-dependent contribution to the free energy of the martensite, as variations in the lattice parameter change the amount of secondary shear required to obtain an un- distorted habit plane. The formation of stacking faults end- ing within a plate, with partial dislocations at the edges, is described by Gotthardt 32 and allows for accommodation of small changes in stress distribution. Where self-accommo- dating variants exist there is a lower net transformation strain field and so less tendency for fault formation. The non-appearance of such faults in the majority of samples is due to the dislocation structures in the martensite acting in a similar manner to that of partial dislocations at stacking fault edges. The subject of stacking faulting within the 18R martensite will be discussed in detail in a separate paper. 33

V. SUMMARY

1. Vestigial ridges, with a definite martensitic substructure, develop in the parent phase after even a single thermal transformation cycle, and increase in number and in- tensity up to about 40 cycles.

2. The formation of vestigial ridges is tentatively attributed to the incomplete reversion of burst martensite; the burst transformation phenomenon gradually diminishes with cycling as the lattice strain is accommodated by alter- native mechanisms including dislocation generation, stacking faulting, and by the creation of alternate ar- rangements of martensite plate variants.

3. The presence of vestigial martensite features, along with dislocation debris, stacking faults, and evidence of mar- tensite plate variant arrangements such as the butterfly morphology, are all indications of the locally imperfect nature of the thermoelastic martensite microstructure during initial cycling.

4. During the same initial transformation cycles when the microstructure is developing vestigial features, MMAX

.

rises by several degrees and PMAX decreases by about a degree, so narrowing the transformation hysteresis. The kinetics changes are most pronounced in the very first few cycles and virtually level off after 20 to 25 cycles. It is considered that the vestigial ridges formed during initial cycles of martensite reversion act as nucleation sites for subsequent martensite formation and so promote an increase in M~aAx. At the same time, these features act to slightly decrease PMAX by effectively acting as residual martensite structure that does not revert to the parent phase above Pf.

ACKNOWLEDGMENTS

This work was sponsored by the Division of Mate- rials Research of the National Science Foundation through grant DMR-81-08407. Particular thanks are due to Dr. J. J. Rayment for the insights gained from nearly three years of close collaboration.

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:all. Trans. A German-85 59 Galley 80 of 85 51 12-6-85 tr, hv A mr (sd)

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