Lattice image studies on the intervariant boundary structure and substructure of Cu-Zn-Al 18R martensite
- PDF / 2,164,623 Bytes
- 16 Pages / 612 x 792 pts (letter) Page_size
- 4 Downloads / 205 Views
I.
INTRODUCTION
R E C E N T studies of the thermoelastic martensitic transformation in/3-phase Hume-Rothery alloys have shown that the mobility of various intercrystalline boundaries in the martensitic microstructure plays a key role in regard to the two technologically interesting phenomena known as "the shape memory effect" and "pseudoelasticity". ~ 5 In the internally faulted martensites in Cu-Zn-A1 alloys forming from the B2 or DO3 ordered parent phase, the most significant boundaries which become mobile in response to external stress are the intervariant boundaries between the various orientations of martensite plates. ~5 Thus, an understanding of the nature and of the fine structure of these interfaces is vital to a further understanding of the shape memory behavior. The present work was initiated in order to study the intervariant boundary structure in a Cu-Zn-A1 shape memory alloy, using a lattice imaging technique. As pointed out by many workers, 2'4's the variant units of thermoelastic martensites are produced from the parent phase in such a fashion that they appear as a collection of the plate groups, each consisting of three basic combinations, the A:C, A:B, and A:D couplings,* where the letter desig*The A:B. A:C, and A:D type interfaces as denoted by Saburi and Wayman5 are referred to as the habit type interface, the twin type interface of the first kind, and the twin type interface of the second kind, respectively, according 1o Tas et a l . 2
nation given to each variant is according to the scheme presented by Saburi and Wayman. 5 The A:B and A:C type variant plate couplings appear because they are highly favorable in terms of the self accommodation of the martensite shear deformation, while the non-self-accommodating A:D pairs are thought to appear in order to adjust the local stress field, or to accommodate changes in growth directions. The observations reported in this paper are concerned only with the A:C and A:D type boundaries because, for the A:B type, the simultaneous lattice imaging of the basal planes [closeKENJI ADACHI, Adjunct Research Professor, and JEFF PERKINS, Professor of Materials Science, are with the Department of Mechanical Engineering, Naval Postgraduate School, Code 69Ps, Monterey, CA 93940. Manuscript submitted February 22, 1984. METALLURGICALTRANSACTIONS A
packed (0018hsR planes] in the A and B variants leads to a blurred interface which must be inclined considerably to the incident beam. 6 In addition to the basal plane fault-type structures, internal faults on nonbasal (]-28)18R planes have been reported previously_in Cu-A1-Ni, 7 Cu-A1, ~ and Cu-Zn-A19 alloys. Since the (128)~8R plane is crystallographically equivalent to the A:C type intervariant boundary, we have extended our experiments and analysis in this paper to a general consideration of basal and nonbasal faulting in the 18R structure.
II.
EXPERIMENTAL
The alloy studied in this work was provided by Delta Metals Research, Ltd., Ipswich, Suffolk, England, with a nominal composition of 69.3 at. pct Cu-14.6 at. p
Data Loading...
