MICROSCOPIC STUDIES ON STRESS-INDUCED MARTENSITE-TRANSFORMATION AND ITS REVERSION IN AN FE-MN-SI-CR-NI SHAPE-MEMORY ALLOY

Martensitic transformation induced by extension at room temperature and its reverse transformation on heating in an Fe-14Mn-6Si-9Cr-SNi (mass%) shape memory alloy were studied in detail by optical microscopy, electron microscopy and electron diffraction. The stress-induced transformation is initiated by random formation of extremely thin martensite plates with 1-2 nm width and then these plates are clustered and some of them coalesce to form thicker martensite plates with increasing deformation. The clustered regions are 400-600 mn wide and considered to correspond to the thinnest martensite plates observable with optical microscope. It is presumed that these clustered regions may also correspond to the lamella structures of a mixture of f.c.c. and h.c.p. phases reported in a previous work. In optical microscopic scale, the thin martensite plates with the smallest width are formed rather uniformly in an austenite grain, and with further increasing deformation, they are clustered and coalesce into thicker plates with 3-8 mu m width. On heating, the reverse transformation is accomplished exactly by the reverse process of the martensite formation mentioned above. Stacking disorders in martensite revealed by electron diffraction analysis are such that the stacking sequence at the fault represented by parameter alpha is equal to that of the parent phase, in other words, extremely thin layers of f.c.c. phase (parent phase) with the minimum width of 3 layers are contained in the martensite with a high density.