Martensitic Transformation Cycling and the Phenomenon of Two-Way Shape Memory Training
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MARTENSITIC TRANSFORMATION MEMORY TRAINING
CYCLING AND
THE PHENOMENON
J. PERKINS Materials Science Group, Naval Postgraduate School, Monterey, CA 93940, USA
OF
TWO-WAY SHAPE
Code 69,
ABSTRACT The character and mechanism of two-way shape memory (TWSM) "training" has been investigated in a Cu-Zn-Al alloy by means of detailed thermomechanical evaluation and TEM observations. The progressive effects of training on the TWSM behaviour, as well as an accompanying substructural evolution, have been established. The results indicate a relationship between the substructural effects of cyclic thermoelastic martensitic transformation and the ability to exhibit TWSM. Microscopic studies reveal that as the number of cycles of thermoelastic transformation under stress increases, specific physical features develop in the parent phase submicrostructure. These take the form of dislocation structures which evolve into "vestigial" martensite markinqs. These in turn assist in the nucleation and growth of a preferred martensite plate arrangement on cooling which is similar to that induced under stress during the training cycles.
INTRODUCTION Shape memory alloys can undergo surprisingly large amounts of strain, and then, upon temperature increase or unloading, revert to their original shape. In the ordinary shape memory effect (SME), a sample which has been deformed maintains that deformation when the applied stress is removed, but recovers its original shape when heated. In two-way shape memory (TWSM), the sample changes shape in both directions as a function of temperature (heating and cooling), in effect remembering both high and low temperature shapes. TWSM behaviour can be achieved only after suhjecting the sample to a "training" routine, such as a series of SME cycles (coolstrain-heat etc.), or a series of SIM (stress-induced martensite) cycles (strain above Md-unload etc.), or a series of combined SIM-SME-cycles (strain above Md-cool-unload-heat etc.) 11]. These routines, as well as the resultant TWSM behaviour, are represented in Figure 1. The purpose of this work was to investigate the mechanism of TWSM training.
EXPERIMENTAL The alloy studied was Cu-26.75 wt.% Zn-4.0 wt.% Al, with an M. of 2680K, provided by Delta Materials Research Limited, Ipswich, England. Training routines were carried out on a specially equipped and instrumented Instron Model 1102-TM-S-L testing machine at a constant strain rate of 0.02 cm/min, using electropolished cylindrical specimens 3 mm in diameter with a 25 mm gage length. The apparatus has the ability to strain the sample uniformly along the gage length, continuously change temperature below and above ambient, and sensitively monitor stress and axial strain in the sample by means of load cell and proximeter devices respectively.
Mat. Res.Soc.Symp. Proc. Vol. 21 (1984) oElsevier Science Publishing Co., Inc.
670
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