F.C.C. reversible H.C.P. Transformations in Fe-Mn-Si-Cr-Ni Shape Memory Alloys

摘要

The thermal-induced and stress-induced fcc-hcp martnsitic transformation and its reversion charteristics were stuided utilizing of transmission electron microscopy (TEM) and hig resolution electron microscopy (HREM) in Fe-16.86Mn-4.50Si-10.30Cr-5.29Ni and Fe-25.89Mn-4.20Si-2.33Ni-1.29Cr shape memory alloys (SMAs). Some interesting results are obtaiend as follows: 1) Under normal condition, the wide hcp martensite bands can be observed always which consist of laminar hcp plates segregated by certain laminate of parent phase. However, in the early stage the small hcp plate as a subunit, forms first; subsequently certian small plates grow along their length direction to conjoin each other and beocme a single hcp thin plate with large lenglth to width ratio. 2) The hcp matensite nucleates at the boudnary of the overlapping region for the stacking faults anways, since where there are more Shockley partial dislocations. The streakds of the stacking faults bands can be bent at the rgion around the martensite nucleus, so, the formation of the hcp martensite exhibits shear character. 3) The process of the reverse transforamtion may describe as that the original ratained fcc phase grows into the hcp martensite.