HREM STUDY OF STRESS-INDUCED TRANSFORMATION STRUCTURES IN AN FE-MN-SI-CR-NI SHAPE-MEMORY ALLOY

Transformation structures produced by extension of an Fe-14Mn-6Si-9Cr-5Ni (mass%) shape memory alloy have been studied by high resolution electron microscope, in order to know the origin of the improved shape recovery of a thermomechanically treated specimen. Nanometric substructures in the ''deformation band'' which is produced by stress-induced transformation are clarified by taking (00.1) and {10.0} lattice images of the h.c.p. structure of martensite and {111} lattice images of the f.c.c. structure of parent phase. It is found that a distinct difference exists between the deformation bands of the thermomechanically treated specimen and the non-thermomechanically treated specimen, namely, the minimum unit of deformation band with about 0.2 mu m thickness consists of lamella structures having a mixture of f.c.c. phase and h.c.p. phase with 1-10 nm widths for the former case, but, for the latter case, it mostly consists of h.c.p. phase only. This difference is discussed in relation to the difference in shape memory effect. Stacking faults in martensite are also studied by electron diffraction.