Phase transformations in metastable β′ CuZn alloys—I. Martensitic transformations

PHASE

TRANSFORMATIONS MARTENSITIC I. CORNELISt

IN METASTABLE f3’ CuZn TRANSFORMATIONS*

ALLOYS-I.

and C. M. WAYXAXP

The shape strain, substructure, crystal structure, and orientation relationship of martensite formed in metastabie @’ Cuzn alloys has been investigated by optical and electron microscopy and electron diffraction. “Bulk” martensite plates in a Cu-38.4 wt .yb Zn alloy were found to have a slightly distorted CuAu 1 structure: for such plates twinning is the mode of inhomogeneous shear. The morphologically different ‘needle-like” martensite plates possess a 3R structure. The phenomenological crystallography theory of martensite formation has been applied, and the predicted orientation relationship, habit plane and shape ebange mere found to be in good agreement with the esperimental observations. TR_~SFOR~L~TIO~S

DE PBXSES DAXS LES ALLIAGES CuZn B’ XETASTABLES-I. TRAXSFORXATIOI XARTESSITIQUE Le changement de forme, fa surstructure, Is structure ccistalline et la relation d’orientation de la martensite form&e dans les alliages Cuzn B’ m&astables ont 6th Studies par microscopic optique et Plectronique ainsi que par diffraction Blectronique. Dans un alliage Cu-33,&O,& Zn en masse, les auteurs ont mis en evidence des plaquettes de martensite importantes dont la structure Cu.& I prkaente une certaine distorsion; pour des plaquettes de ce type, le mode de cisaillement h&&rogeneest le maclane. Les plaquettes de marten&e de morphologie differente, en forme d’aiguilles, qui ont et& obser&a egaiement, presentent une structure 3R. ~applioation de la theorie ph~nom~nolog~qu~de la cristallographie de la formation de la martensite donne une relation d’oriantation, un plan d’aecolement et un changement de forme qui sont en t&s bon accord avec les observations esperimentales.

PHdsE~~~I~~~_~sDLUSGES Ix ~IETASTABILEN

~.C~ZS-LEGIERU~GEA-. I: x4RTEs. SITISCHE L~IWASDLUSG Gestalta&nderung,Substruktur, Kriatallstruktur uncl Orientierungsbeziehung des in metastabilem fi’-CuZn gebildeten Xartensits wurden mit Hilfe der Licht- und Elektronenmikroskopie soaie der ~lektronenbeu~g untersucht. “Xasaive” ~Iartensitplatten in einer Cu-3S,4 Gew.:; Zn-Legierung haben eine leiaht verzerrte Chkul-Struktur. Fiir diese Platten wirkt die Z~ili~gsbild~g ah inhomogener Schermeehanismus. Die morphologisch verschiedenen “naclelfbrmigen” Yartensitplatten beritzen eine 3RStruktur. Die phiinomenologische Theorie der &lartensitbildung wurde angewandt. Die vorhergesagten Werte der Orientierungsbeziehung, Habitusebene und Geataltarinderungsind in gurer Ubereinstimmung mit experimentellen Ergebnissen. INTRODUCTION

The metastable ordered p-phase, obtained by the high temperature stable /?-phase, quenching transforms martensitically on cooling below J1s(l*e) or by deformation below il[d.(3) The X, temperature was found to change from -10°C for a 38.55 wt. 0A Zn alloy to -2ti”C for a 41.97 wt.% Zn al10y.‘~) Although the crystallographic characterisitics of the martensite have been studied extensively in the past, a considerable amount of confusion still exists. The earlier S-ray diffraction studies(*-g) were in general unsuccessful and resulted in a wide range of proposed structures. Hosever, sig~ficant advances have been made by transmission electron microscopy and selected area electron diffraction studies of the /?-Cu alloys in general, e.g. CU-AI,(‘~~~) Cu-Sn,(15) Cu-Zn,Ua*U) Cu-Zn-Si,ClsJ@, Cu-Zn-CaUQ-211, etc. In all these systems the martensite structure has been described as a stacking variant of a close-packed structure (disregarding small distortions) such as lR(= ABC), 3R(= ABCBCACAB), 2H(= AB), llH(= ABCBCACABAB) or a lamellar mixture of these stacking variants. * Received May 29, 1973; revised July 30, 1973. t Dept. of Metallurgy and Mining Engineering and Materials Research Laboratory, University of Illinois, Urbana-Champaign, Urbana, Illinois 61801, U.S.A. ACTA ~IETALLURGICA, VOL. “I, JL%RCH 197.4 291

A main difficulty in the study of martensite in binary Cu-Zn alloys is the sub-zero .Jfs temperatures of these alloys. Although t.he ..JJ# can be raised by small additions of a third element such as Si or &1,(‘s) the extrapolation of the data obtained from the ternary alloys to binary Cu-Zn alloys is not without danger. The characterist,i~s of the martensite (such as morphology, substructure, ordering, etc.) may be affected considerably by minor additions of a third element. The results obtained by the use of a cooling stage inside the electron microscope should also be interpreted with great care, since the martensite plates are subjected to essentially Tao-dimensional constraints during formation in the thin foils and are therefore not representative of the martensite plates formed in “bulk” specimens. Sato and Takezawaui) investigated the structure of the martensite plates obtained by quenching a 39 wt. % Zn binary alloy and proposed a 3R structure. However, the martensite plates they obtained formed well above the known X8 temperature”*“) and were explained as being caused by quenching stresses. The needle-like appearance of these plates (see Fig. 1, Ref. 17) closely resembles the mo~hology of the “t~hermoelastic” plates reported by Pops and Massalski(2) and is very distinct from the clusters of acute-angled 7” shaped pairs of plates obtained by

lCT_4

342

MET_iLLURGIC_i,

further cooling.(g’ These closed groups of plates, previously called “burst-type” martensite,‘“’ will receive primary consideration in the present work and will be referred to as “bulk” martensite. This name has been chosen to distinguish the martensite formed in bulk specimens from “thin foil” martensite. _1n attempt was made in the present work to raise the -11, temperature above O’C, by lowering the Zn content in the binary CuZn alloy to .~38.1 wt.% Zn. The main problem which had to be overcome was the formation of extensive amounts of massive a during quenching from 895% (i.e. the extreme upper left hand corner of the p-region in the CuZn diagram).@)

VOL.

07,

1971

martensite. In some untransformed regions, thin parallel “needle-like” transformation products were occasionally found. Foils for transmission electron microscopy lvere prepared by chemical- and electro-polishing (window technique). TKO different electropolishing solutions were used; (a) orthophosphoric acid saturated w-ith CrO, (stainless steel cathode, 50”-80% and 6-10 V); or (b) a 2: 1 methanol/nitric acid solution (stainless steel cathode, and B-12 V). -30°C to -6O”C, Hitachi HU-11B and HU-12 electron microscopes (equipped with a &40” tilting and 360” rotation stage) were used for transmission electron microscopy. RESULTS

EXPERIMENTAL

AND

DISCUSSION

PROCEDURE

Twelve copper-zinc alloys were prepared from electrolytic copper (99.99 + %) and “Horse Head Special” zinc (99.99 f ‘A), so that the composition range 38.4-41.0 n-t. ‘A Zn n-as covered in intervals of 0.5%. The weighed materials were encapsulated in quartz tubes which were evacuated to ~10-~ Torr, held at 1lOO’C (i.e. -20°C above the melting point of Cu) for ~45 min, and shaken vigorously at regular intervals to ensure complete mixing of the elements. The tubes were taken out of the furnace, cooled to -950°C in a vertical position, and water quenched. The ingots were resealed in evacuated vycor tubes and annealed at ~800°C for a minimum of 48 hr. The total loss of weight, assumed to be Zn, was of the order of 0.02-0.04 g for 100 g ingots. This small zinc loss was confirmed by chemical analysis of three alloys, using the electrolytic deposition method. The homogeneity of several alloys was confirmed by microprobe analysis. All ingots were hot rolled (750°C) to a thickness of -0.0-C in. in B-10 passes, using intermediate heating between passes when necessary. The cut specimens (1 x 0.5 x 0.01 in.3) were solution treated in the $-field using a vertical tube furnace and free-fall quench in an iced 10% XaOH solution. The formation of massive r was avoided completely by this quenching procedure for specimens with a zinc content larger than 39 u-t. %. However, an extensive amount of massive a formed in the 38.4 wt.% Zn Massive z formation in these specimens specimens. could be reduced to small amounts at the /?-grain boundaries by using a two step quenching technique which consisted of a short annealing (~45 set) at 895’C and quenching in 10% XaOH, followed by a second annealing at 895’C for ~80 set and quenching again. The 38.4 wt. % Zn specimens, obtained by this double quenching, contained X-90 per cent “bulk”

Surface relief and martewite

morphology

The needle-like transformation products, preriously called thermoelastic martensite,“) have also been observed prior to the formation of “bulk” martensite on cooling the 39 wt. % Zn specimens. However, some “needle-like” martensite plates were often present in as-quenched specimens, and showed basically the same morphological characteristics as the “thermoelastic” needle-like product. It was also found that the needle-like products formed on cooling were not always completely reversible: thin needlelike products remained present in the 39 wt. % Zn specimens up to temperatures as high as 100‘72, even after only one cooling cycle, Thus the terminology “thermoelastic martensite” is therefore believed to be inaccurate for this needle-like transformation product. This paper will be concerned only with “bulk” martensite plates unless stated otherwise. The large size of the martensite plates in the present alloys (5-25,~ in width) allowed the measurement of scratch displacements without difficulty with conventional light microscopy. In making the measurements, prepolished and scratched @’ specimens were transformed using a cooling stage adapted to a metallograph. Figure 1 shows a typical prescratched specimen which has been completely transformed. As espected, the displacement of scratches occurs by an invariant plane strain deformation. Angular measurements using a number of specimens showed scratch displacement angles ranging from 0 to 11.5’ (&0.2”). Such a range is expected. The maximum displacement, as seen in the plane of polish, would be observed when the direction of displacement of the invariant plane strain lies in the surface; when the displacement direction is normal to the surface scratch displacements mill not be observed.

FIG. 2. Transmission eIectron rnicrogtq.4~ of a typical gcoup of mettensite p&es formed in 8 f&-3.&-L wt.~& ih &UOJ-.

Frt,. 1. Surfsee relief micrograph of 8 prepolished and scratched Cu-39 nt.oo Za alloy cooled to .- -150% illustrating the scratch displacements. typic81 of an invwiant plane strsitl, as we11as the morphology of ‘-bulk” msrten.Gte. Figure 1 also show it typical morphologicrt.1 feature of the “‘bulk” martensite. The plates in gewral form acute angled Y-shaped pairs initially, which become closed groups upon complete trcinsformation.

As mentioned in the ~r~troduction, a SR-stacking sequence (i.e. .-lECEC’.4CdB) has been proposed for the “needle-like” murtenaite formed by quenching ,z 39 wt. % Zn allor.~li~ The same authors claimed a. a11 identical structure for the plates formed below _Ic,.(17) Their eridence resulted, hoverer, from selected area diffraction studies of marten&e plates retained in thin foils! after immersing these foils (while in the electron microscope specimen holder) in liquid nitrogen. It is likely that neither of these transformation product. s can be regarded as being representative of the martensite formed b:- cooling bulk specimens. In the present work. electron microscopy studies were made on martensite formed in a 35.4 n-t *A Zn alloy, n-ith an J1, temperature of ~1.5”C. Figure 2 shows a typical transmission electron micrograph

of a group of martensite plates formed in this alloy. This arrangement of plates in closed groups is typical for the bulk martensite, OccaGonaIlv, isolated martensite **bands” were observed in the untransformed jsl’-matrix. These “aeedte-like” martensite platen: which are induced by quenching stresses or formed on cooling prior to the “bulk” martensite we easily recognized by their distinct morphology. Figure 3 shows it typical example of both kinds of martensite plates; plate A is a %eedle-like” martensite plate, &ile 3 is a typical “bulk” martensite plate. It should be emphasized, hoverer, that small plates like A in Fi,0. 3 were infrequently found. and 95% of the martensite plates ob.serred were that large plates occurring in closed groups as illustrated in Fig. 1. Figure 4 &ox-s a selected area diffraction pattern obtained from the area encircled in the small plate A of Fig. 3. This is a typical diffractiou pattern obtained from the 3R-stacking sequence. when the stacking plane is pitralle1 to the electron beam. In the ease of the ‘*bufk” martensite plates, B different structure is found. Figures 5 (a, b) show a bright field micro,araph and corresponding diffraction pattern of such a plate> again oriented so that the stacking plane is parallel to the beam direction. This difkwtion pattern can be indexed as a tkmed f.c.c. lattice with a ,110j zone axis and fill) twinning plane. An identification of all intensity masima is given in Fig. 6. The relative intensity of the twin dpots varies, depending on the particular martensite plate or area. in a @-en plate from which the diffraction pattern

FIG. 3. Tranamiswion electron micrograph ilhxstrating ctifferenee in morphology between the (quenching) stress-induced marten&e {e.g. plate A) and the martensite obtained by cooling (e.g. plate 3). Era. 4. Selected area electron diffraction pattern obtairled from p&e A in Fig. 3. The indexing of this dif. fraction pattern. vhich is t:-pical for a 3R &ructurc, ia illustrated in Fig. 13 of Part II.

sphere. It is therefore extremely important to carefully tilt specimens into the esact orientation xx-here the Streak are normal to the electron beam.

is obtained

and results from the relative size of the twin-related regions in a certain plate. In addition to the twin and matrix reflections, streaking in a direction normal to the close-packed plane (i.e. (111) direction in Fig. 6) is also present. These relrods are due primarily to the thinness of the win lamellae and consequent relasation of the Laue condition. The (111) relrods make the interpretation of diffraction patterns not containing this direction more diEcult since apparent diffraction maxima are obtained where the streaks intersect the Ex~ald

Although the selected area diffraction patterns from the martenaite were successfully indexed to a first approximation on the basis of a twinned f.c.c. lattice: significant deviations from the ideal f.c.c. position were found. After specimens were ver>carefull!- orientated so the relrodj associated with the (111) twins were perpendicular to the electron beam and the cliffrartion patterns photographed, angles were mea.sured hetxeen the (111) tvk.ning plane and

CORSELIS

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I?;

YETASTABLE

$’

CuZn

295

Considering the b.c.c.-f.c.c. Bain correspondence, it might be expected that the martensite formed from the C&l @‘-phase would be tetragonal. The situation is similar to that found in the ordered CuXu I structure, where tetragonality is caused by a difference in atomic sizes between Cu and Au. Although the size difference is not large comparing Cu and Zn atoms, approximate calculations using the observed angles as given in Table 1 indicate clearly that the martensite is not f.c.t. The displacement of certain diEraction maxima from the ideal f.c.c. positions indicates that the martensite is orthorhombic, the difference between a, b and c axes being not more than 1-2 per cent. Such distortions can therefore be regarded as ‘iperturbations” on the basic f.c.c. structure (or f.c.t. with e/a. 210.98) and result in no significant differences in predictions (i.e. habit plane) when the phenomenological crystallographic theory is applied. Lattice orientatiorr relationship

(144, 600 TWINNED ZONE

t42z1

5ii

422

FCC AXIS

TWINNING FIG.

(335)

6. Identification

3?l3

0 MAIN

LATTICE

X TWIN

RELATED

of intensity Fig. 5(b).

maxima

= [Oii] PLANE

(5%

LATTICE

= (ill)

seen

in

(Iii), (200) and (022). These angles are designated a, /? and y respectively as shown in Fig. 6, and the results from eleven different specimens are given in Table 1. The scatter in the data may be expected in that the martensite plates are surrounded by others which may set up additional strains. These strains could induce perturbations in a basically f.c.c. (or f.c.t.) structure but are unlikely to change the basic structure. A regular distribution of faults would be necessary to alter the structure and it is doubt’ful that accommodation effects would be so regular. TSBLE

1. Angle

Angles (degrees) f.c.e.

measurements in martensite deviation from cubic&y Y. 70.53

B j&i4

indicating

Y 35.26

Measured values Specimen

1 f 3 d 5

s 7

: 10 11

50.8

69.6 69.2 71.5 50.3 73.“ 73.8 iO.-I i9.5 73.0 69.9

55.1 X.6 31.-i a&I5 34.6 53.7 53.6 X.6 5.k.7 53.3 55.0

31.5 34.8 31.6 3&33 36.33 36.3 37.0 33.4 36.1 36.5 35.0

The orientation relationship was determined by selected area electron diffraction from adjacent regions of martensite plates and the @‘-matrix. Only plates having their twinning plane parallel to the beam were considered, since other orientations lead to false interpretations due to relaxed Laue conditions. Once the specimens were tilted so the plates had the required orientation it was noted that most of the /Y-grains examined had a (111) zone axis orientation while a few showed a (100) orientation. The orientation relationship measured on a series of diffraction patterns which show a (111) b.c.c. zone axis orientation can be summarized as: Wl),.,.,.

(= twinning

plane) 5.5’ (&O,Y)

wf.,.,.

5.5’

(LOO),,.,.

II \F-ith (ilo),.,.,..

from (&0.2”) from (Oll)b.c.c.

(lol)b.c.c.

In arriving at this result the beam direction is assumed to be parallel to both the zone axis of the plates and B’-matrix; i.e. 23.D. [I [Oil],.,.,. 11[l-I’ll,.,.,.. This orientation relationship is identical to that found for al-plates (322 structure) formed isothermally from the /Y-phase as will be discussed in Part II.

A computer program,(z4) has been used for computing the geometrical features of marten&tic trans. formations. The algorithm follosvs the theoretical formulation of Bowles and Mackenzie,@@ and makes it possible to compute all crystallographic features from the lattice parameters of the product and parent

_4CT_4. NET_kLLCRG

296

phases: the lattice correspondence between the two phases and a system (plane and direction) of inhomogeneous shear. For convenience all calculations are made in an orthonormal coordinate system of the parent crystal structure. The Bain correspondence between the parent b.c.c. lattice and the product f.c.c. (or f.c.t.) lattice was assumed to hold. Three correspondences are possible, depending on which axis in the b.c.c. lattice is chosen as the c-axis of the f.c.t. unit eelI, the latter defined in four original b.c.c, unit cells. The first Bain correspondence {indicated by I) is schematically shown in Fig. 7 and has [OOl] as the “Bain ask.” The second (II) and third (III) correspondences are chosen to correspond with [loo] and [OlO] as Bain axes respectively. As mentioned before, a b.c.c. -+ is more likely due to the f.c.t. (c/a ?250.98) transition &Au ordering obtained after the Bain distortion of the CsCl lattice. The relevant principal distortions for such a case are: % = r.2 --

-% \/“a$

(
and

t

q3 = $ f>l)*

BAIN

AXIS

BCC

LATTICE F =ag

The lattice parameters of the parent phase (n,J were extrapolated from previously reported results in the higher zinc concentration region.(c6) The lattice parameters of the martensite were calculated from the values of a@and the interplanar distances measured on superimposed diffraction patterns containing both matrix and transformation product intensity maxima. This method is not accurate since it is limited by astigmatism, streaking of intensity maxima and the limitations in measuring accuracy. Therefore, measurements were made on “sharp” diffraction patterns which showed only small deviations. To eliminate the error due to astigmatism, only d&., values of both lattices measured in nearly the same direction in the diffraction pattern were comparecl. The lattice parameters determined this way correspond well with the values calculated from as, assuming that there is no change in volume involved during the martensitic transformation; i.e. a% = Zags for the b.c,c. -+ f.c.t. transition. Besides the lattice correspondence and lattice parameters of the product and parent phases, a system of inhomogeneous shear must be employed to compute the crystallographic features of the transformation. In the present work, the twinning plane was deduced from electron diffraction, and corresponds to a (111) L.c.c4 plane. The only possible shear direction in (111) which does not disturb ordering corresponds to only one of the three (112) directions in each (111) plane. This is illustrated in Fig. 8 for the particular (111) plane. Only a shear in a the [i-E?] direction does not disturb the ordering;

w’+cc

4

BAIN

= a ond

a

Cu

ATOMS

IN PLANE

a

Zn ATOMS

IN PLANE

A

0

Cu ATOMS

IN PLANE

a

0

Zn ATOMS

IN

8

PLANE

A

DISTORTIOI’~

c

FIG. 5. Schematic representation of the Cu_Iu I structure obtained from the C&l structure by the first B&n correspondence (i.e. [OOI] as B&n axis).

FIG. 8. Schematic representation of the three potential shear directions in a (lll)c.~.~. plane. Sate that only one of the three shear directions does not disturb the ordered arrangement of atoms.

CORSELIS

_&SD

MARTESSITIC

~~_-iY>~AS:

TRASSFORMATIOS

shear in the other two (11’3) directions causes neighbor The (111) [ii?] f.c.c. shear system violations. corresponds to a (110) (110) shear system in the b.c.c. matrix lattice, while the other txo shear systems correspond to a (i.e. (111) [?ll] and (111) [lfl]) {llO> (311) system in the b.c.c. lattice. These latter shear systems in the b.c.c. lattice are highly uncommon. It is nox possible to compute the crystallographic features of the martensite transformation. The results for the (111) [n2] shear system (i.e. Bain correspondence I) and assuming a b.c.c. -+f.c.c. transition in a 38.45 wt. % Zn alloy will be discussed first. The lattice parameters for this alloy are up = 2.935 A and a, = 3.709 -4 (= latt.ice parameter of the product phase), so that ql = q2 = 0.891894 and distortions. rs = 1.261329 are the principal The printed output data for this particular example are given in Fig. 9. Four solutions are obtained for the set of input data. Of the four solutions (or variants) obtained, solution (X,, Jrl) is crystallographically similar to (Xs, .iis) and variant (X,, N,) is similar to (X,, MJ. The first two variants can, however, be disregarded since the values of m2 (magnitude of inhomogeneous shear) and 19(orientation rotation) are much larger than the respective values obtained for the last two variants. The shear angles, co~esponding to mst are equal to 23.38O and 13.18” for the fkst two and last two variants respectively. The first two solutions are therefore disregarded. The two remaining solutions will be referred to as solutions A and B in further discussion. The direction cosines of the predicted habit planes are (0.175937, 0.669606, -0.730733),, and (0.175927, -0.659606, -0.730733),, for solutions A and B respectively. The position of these poles relative to (2, 11, 12),, is shown in Fig. 4 of Part II. The orientation relationship between the parent and product phases from solution B is shown in Fig. 10. The angles betn-een planes were measured on a 5Ocm stereographic net and the following results were ob mined : (lllh.c.c,

0.o”

from

(111)~,.,.

do

from.

m-%.c.c.

1” from

(Tro)b.o.c.

wW,.,.,.

1’

Ph.,.,.

wh.,,,.

10” from

from

(lol)b,c.c. (oll)b.c.c.

(ooi),.,.,. (iloh.,.,.6”from(010)b.e.e.

By comparing these angles with the experimentally determined ones, it is seen that excellent agreement exists. The orientation relationship according to solution A is shown in Fig. 11. Comparing Figs. 10

IS

METASTABLE

8’

CuZo

997

and 11 it can be seen that an equivalent set of angles is obtained. Thus, solutions A and B are crystallographically equivalent variants. So far, a b.c.c. -t f.c.c. transition has been assumed. However, due to a CuAu type ordering, a small tetragonality may be expected after the Bain deformation (i.e. c/a e0.98). The influence of the tetragonality on the crystalIographic features has been computed (again using the (111) [Ii?] shear system) for c/a = 0.98. The lattice parameters were calculated assuming no change in volume; i.e. 7a 3 = a,” = a%. Again, only two of the four predidcted variants had to be considered. For the tetragonal calculation the values of %, m2 and 4 decreased (compared nitah corresponding values from the b.c.c. -+ f.o.c. calculation) and the predicted habit plane pole moves slightly to\%-ards (2, 111 121,. Small o~horhombi~ ‘~perturbations” on the basic f.c.c. or f.c.t. (c/a flrl 0.98) lattices (as noticed on the electron diffraction patterns) only slightly influence the predictions of the crystallographic theory; e.g. the habit plane varies between (2, 11, 12), and the pole obtained for the b.c.c. + f.c.c. transition. The above results obtained using the phenomenological crystallographic theory should hold equally well for the isothermal /3’ --f a1 transformation, which can be regarded as a martensitic transformation, as will be discussed in Part II. CONCLUSIONS

AND

SUMMARY

One of the most significant results of the present work is that the internal inhomogeneities in “bulk” martensite plates in Cu-38.4 wt. % Zn are twins: and not stacking faults as was previously beliered.(17) The twinning phenomenon is of considerable interest in explaining the shape memory (or marmen)(27) effect, not only in CuZn but in other alloys vrhose martensitic phase exhibits a memory such as TiNi, AuCd, Aged, InTI, CuANi and FeaPt. The essential and common characteristics of all these alloys appear to be:(2i) (1) thermoelastic martensitic transformation (2) an ordered arrangement of atoms, and (3) twinning as the inhomogeneous shear. The present results also indicate that the different kinds of martensite which can form in a given alloy should be carefully distinguished. It is indeed found that thermal-, stress inducedand thin film-martensites exhibit different morphological characteristics and structures. Therefore, a generalization of eharacteristics found for any one of these is misleading. Moreover other factors such as specimen size, grain

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fact,ors, especially with regard to the shape memory effect, is presently under investigation. Good agreement was obtained between the esperimentally determined and theoretically calculated lattice orientation relationship and habit plane. A maximum macroscopic shear angle of 11.5’ was found from the scratch displacement measurements. The theoretically determined shear angle is .~13” if a b.c.c. + f.c.c. transformation is assumed and slightly less (~11.4”) if a tetragonality of 0.9s is taken into account. Although it is difficult to determine if the largest measured angle corresponds with the maximum possible angle (i.e. when the direction of displacement of the invariant plane strain lies in the surface of the specimen), it may be concluded that close agreement is again obtained between theoretical and esperimental results. The agreement in lattice orientation relationship, habit plane and shear angle comparing theoretical and experimental values clearly indicate that the assumed Bain relationship betx-een the parent and product phases is ralid. It should be noted that t,he lattice orientation relationship, habit plane and magnitude of surface tilt observed for the martensite plates are identical to the respective characteristics found for x,-plates formed isothermally from the /?-phase. The only major differences between both transformation products is that the x,-plates are much smaller, show a different morphology and are untninned (3R structure), as n-ill be discussed in Part II. ACKNOWLEDGEMENTS

This work, based on the Ph.D. thesis of I. Cornelis, was supported by the U.S. Atomic Energy Commission through the Materials Research Laboratory at the University of Illinois (Contract AT-11-l-1198). REFERENCES

Fig. 11. FIGS. 10. 11. Stereographic representations of the lattice orientation relationship between the parent b.c.c. and product f.c.c. phases, obtained by solutions B and A respectively from the computer output data.

size, quenching rate (and amount of massive a), composition, and purity (i.e. influence of a ternary element) may vary the morphology (e.g. plate size) and structure. The influence of a number of these

1. A. L. TITCEESER and 11. B. BEYER, Tram. -U-M,!? 200, 303 (1954). 2. H. POPS and T. B. JL~ssa~ss~, Tran.s. AINE 230, 1662 (1964). 3. T. B. Yass~ss~ and C. S. B.URETT, Trans. AIME 209, 4.55 (1935). 4. A. B. GRESI~GER and V. G. MOOR.~LU, Trans. 8IME 128, 337 (1938). _ 0. E. HORSBOGES and G. WUSER~US, 2. Metallk. 47, d_“i (1966). 6. G. Bass1 and B. STRGX, 2. XetaZZk. 47, 16 (1936). G. K~SZE, 2. Xetallk. 53, 339, 396, 565 (1962). :: D. B. JLissos and R. K. Goma, 2. Xetallk. 54, 293 (1963). R. D. G=~~ooD and D. H-L, Acta Mel. 6, 98 (1958). 1:: 11. WLGESS and H. WARLNOST, dda _Uet. 11, 1099 t 1963). 11. ti. \V_&RLIxOSTand 31. IVILSESS, 2. >fet&k. 55, 362 (1964). 12. 2. NISBIT.UL&and S. K.%JIwv;lR+Jap. J. appl. Phys. 2, 458 (1963). 13. H. S~TO, R. S. TOTE and G. HOSJO, dcta Xet. 15, 1361 (196i).

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300

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14. J. BRETTSCHSEIDER and H. WARLIMOXT, 2. Net&k. 59, 740 (1968). 13. H. WARLIXOST and D. HARTER, Proc. 6th 1~. c’onf. Elect r. _1ficroac.. -Wv-uren, 433 (1966). 16. D. HL-LL, ELectron ~~imo.9copy and Strength of Cry.3taZ.s. Interscience (1963). 1;. S. SATO and K. TAKEZAWA, Trans. Jap. Inst. _lIetnb 9, 9% (196s). 1s. H. POPS and L. DELAEY. Trans. AINE 242, 1849 (196s). 20. 19. L. DELAEP DELAEY and and I. H. CORSELIS, WARLIMOST, dcta Uet. 2. _IfetaZZk. 18, 1061 58, (1970). 437 (1963).

T-OL.

22,

1974

-71. L. DELLEY and H. WARLIMOST, Z. Xetallk. 57, 793 (1966). 22. H. POPS, -Vet. Trmw 1, 251 (1970). 23. 11. HASSEX. C'o~~di~tcrtio~~ of Bimry dlloys, p. 650. McGranHill (IYS). 24. H. 11. LEDBETTER and C. 31. WAY~IAS, Not. Sci. Er~g. 7, 1.51 (1971). 23. J. S. BOWLES and J. Ii. MACGESZIE, Acta Net. 2, 129, 138, 2-4 (19%)). 194, lOi 26. L. H. BECK and C. S. SMITH. Tmr~. NME (192). ‘7. C. 31. ~~ATZC.~Sand K. 8~rzrrzr, _lfet. &i. .J. 6, 155 (1972).