KathoZieke Universiteit Leuven, Depmtement MetaaZkunde, BeZgiwn

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1 JOURNAL DE PHYSIQUE CoZZoque C4, suppt6ment au no 12, Tome 43, de'cembre 1982 page C4-673 ON SOME LESSER KNOWN PLANAR DEFECTS IN 6; Cu-Zn-A1 MARTENSITE M. Andrade*, L. Delaey and M. Chandrasekaran KathoZieke Universiteit Leuven, Depmtement MetaaZkunde, BeZgiwn (Accepted 9 August 1982) Abstract.- The crystallography of some lesser known stacking faults within a martensite variant in a Cu-Zn-A1 alloy has been analysed. The faults have been found to liz on the (T28) planesc of the R i martensite, that are different from the (128) twinning planes separating specific self-accommodating variants. The defects have also been found to appear in combination with faults on the basal plane, forming a domain-like configuration. The relevance of these (128) planes is discussed in relation to the correspondance between parent I? and martensite lattices. Introduction.- Planar defects on non-basal planes have been observed within the martensite variants of 183 Cu-A1 [I], Cu-Al-Ni [2] and Cu-Zn-Ga [3] alloys. These defects have been claimed by the authors to be stacking faults. In particular, Brooks [2] has suggested that such faults lie on (1%) planes. However, in the abscence of a reference to the parent I3 lattice or another martensite variant belonging to the same self accommodating group, it is not obvious which of the two ecpivalent 1i2~8) planes one is referring to. While reaortin~ the observation of sirnilar defects in the q-183 martensi'ce in a Cu-Zn-A1 alloy, Che present work is airned at resolving this doubt. The relevance of these defects in relation to t'ne crystallography of a self-accommodating group of I?; martensite is also briefly discussed. Zxperimenta1.- Induction melted Cu-18.5at%Zn-12.4at?LAl ing;ots (8 kg).,;ere hot rolled in the I? condition at 800OC into 1.2 mm thick sheets. Strips of the hot rolled,iiaterial (5 cm x 1 cm) were betatised at 75a C for 15/30 minutes, subsequently quenched into water at 75OC and finally cooled in air to room temperature. Three nm diameter discs of the as rolled or heat treated martensitic material were electropolished in a solution of 250 ml phosphoric acid ml water ml ethanol - 50 ml proaanol - 5 gr urea by a double jet method. The sarngles were examined In a jeol 200 CX transaission electron microscope operatin,?/c 200 kv. Results and discussion.- Typical contrast exhibited by the planar faults on nonbasal planes, as observed in this and earlier [1,2] investigatiors,is shown in fig. 1. The bright field micrograph was obtained under two beam condition with the 0018 as the operating reflection, which also provesthat the defects do not lie on the basal planes. Preliminary trace analysis indicated that the stacking faults lay on {i28} planes. However, as mentioned earlier in the introduction, two crystallographic equivalent planes, (i28) and (i.?8), exist in the 18R structure. Of these, the (I%) planes, as per the convention adopted here, act as the twinning as well as interface planes between specific martensite variants in a self-accommodating group [4]. Nuch importance, on the other hand, has not been attached, so far, to the (i28) planes. In order to find out to which of these two planes the stacking fault corresponded, tilting experiments were conducted inside the electron microscope. Fig. 2 shows the situation when the sample was tilted so that the (128) twinning planes were parallel to the electron beam. It demonstrates that the defects are still inclined Y also from ~unda~ao Centro ~ecnolg~ico de Minas Gerais - CETEC - Brazil +when not explicitly indicated, the indices refer to the 18R structures. Article published online by EDP Sciences and available at

2 C4-674 JOURNAL DE PHYSIQUE with respect to the electron beam and thus do not lie on (i28) planes. they lie on (158) planes. Instead, Fig. Fringe contrast exhibited by the stacking faults on non-basal planes. The bright field micrograph was taken under two beam condition with the 0018 as the operating reflection and electron beam nearly parallel to [ (a) ;b) Fig. 2 (a) Micrograph showing defects inclined to the electron beam and, thus, to the (158) twinning plane separating the martensite variants A and B. Electron beam //[2?0]. (b) Diffraction pattern of Fig. 2a. It 1s necessary to emphasize here that it would not have been possible, without a prior knowledge of the crystallography of the transformation, to differentiate, based only on diffraction patterns from a single variant, the (158) plane from the (i28) plane. The significance of tne present finding may be discussed with the help of the stereographic projection shown in fig. 3. The position of the six I110 >B planes before and after the transformation are given in this projection for a single martensite variant [5].

3 Fig. 3 Stereographic projection showing the position of the {lloib.planes before (e) and after (x) transformation. Tndices of the corres~ondrng martensite planes are also given. It may be concluded from the projection that, not only the (i28) twinning planes, but also the (128) planes, are derived from '&loib planes. The (128) is parallel to a {llo)b plane, whereas the (i28) is rotated about lo0 with respect to the corresponding {110>~ plane. The relevance of those latter planes becomes more evident when one considers the crystallography of a self-accommodating group of martensite variants, as shown in fig. 4 [ 5]. In forming such a self-accommodating group from a parent 8 crystal, four of the six {11018 planes are transformed into basal planes of the four variants, a fifth Ill0 lb plane is transformed into the (728) plane common to all four variants and the last {lloib plane gives rise to the (T28) planes, on which the defects discussed here are observed to lie. Accordingly, the fringe contrast exhibited by the defects in each variant are nearly parallel to each other, as can be verified from the two self-accommodating variants shown in fig. 5. Another interesting characteristic of the (T28) faults is shown in fig. 6. It may be seen from the figures presented that the (728) faults always terminate on other faults on the basal plane of the martensite. The microstructure then gives the appearance of a domain-like network. In conclusion, it would appear that the (328) defects are a common feature of 8'-18R martensite. The question whether they are to be treated as deformation or. g&wth faults, relevant or otherwise to the formation of martensite still remains to be solved.

4 C4-676 JOURNAL DE PHYSIQUE Fig. 4 Stereographic projection showing the position of the IllOIB and the (0018), (?28) and (i28) planes of the four martensite variants (a,b,c,d) in a selfaccommodating group. Fig. 5 (i28) faults in two self-accommodating variants twin related about the (i28) plane. Note that the fault fringes are nearly parallel to each other. Electron beam -[2i0].

5 Fig. 6 Easal plane and (i28) faults connected to each other forming a domain-like configuration. (a) beam axis -[210]. (b) beam axis -[010].

6 JOURNAL DE PHYSIQUE References.- 1. NISHIYAMA Z., KAJIWARA S., Trans. JIM 3 (1962) BROOKS J.W., Electron Microscopy 1 (1980) DELAEY L., WARLIMON'C H., 2. Metallkde 56 (1965) TAS H., DELAEY L. and DERUYTTERE A., Met. Trans. 2 (1973) DE VOS J., AERNOUDT E. and DELAEY L., 2. Metllkde 64 (1978) 438. Acknowledgements.- M. Andrade would like to thank Conselho Nacional de Desenvolvirnento ~ientfico e ~ecnold~ico, Brazil, for financial support and M. Chandrasekaran would like to thank Katholieke Universiteit Leuven for the award of a visiting fellowship during the course of this work.