B. C. Maji, M. Krishnan, Bhabha Atomic Research Centre, Mumbai, India; V. Hiwarkar, I. Samajdar, Indian Institute of Technology, Mumbai, India; R. K. Ray, Tata Steel, Jamshedpur, India
The shape memory effect (SME) observed in Fe-based shape memory alloys is governed by the g to e martensitic transformation and strongly dependent on the crystallographic orientation. Therefore, introducing proper texture and microstructure by processing could be an effective approach for improving the imperfect SME shown by these alloys. With this objective, the development of texture and microstructure in cold rolling and annealing of Fe-14Mn-6Si-9Cr-5Ni shape memory alloy was studied. Vacuum arc melted ingots of the Fe-14Mn-6Si-9Cr-5Ni alloy were hot rolled into strips and solution annealed at 1050oC temperature for 1 hr. The strips were subjected to different extents of reduction (0-45%) by cold rolling. The effect of annealing was studied on specimens cold rolled to 45% reduction at different temperature between 700-1100oC for time ranging from 2 minutes to 8 hrs. The microstructural evolution was characterized by optical microscopy, transmission electron microscopy and electron back scatter diffraction (EBSD). The bulk texture measurement was carried out at the mid thickness of the specimens using X-ray of MoKa1 ( l = 0.7093 Ao) radiation. Pole figure and orientation distribution function (ODF) methods were used to study the texture development in the parent austenite phase. The solution treated material has a nearly random texture and cold deformation induces a strong alloy type texture with Brass {011}<211> and Goss {011}<100> components. It is observed that annealing of the cold deformed material in the single phase austenite region (1000-1100oC) produces a nearly random recrystallization texture. Annealing in the two-phase region (700-1000oC) also produces random recrystallization texture but is kinetically a slower process. This is perhaps related to the precipitation of the Fe5Ni3Si2 intermetallic phase.
Summary: In this study, the crystal structure, substructure, intervariant interfaces and self-accommodating microstructure of the 5M martensite in Ni50Mn28.9Ga21.1 alloy was investigated using transmission electron microscopy (TEM) and x-ray diffractometry for determining its. The 5M martensite was found to be a monoclinic structure with twin like internal defect substructure. The main features of the martensite are found to be consistent with the predictions of computations based on the phenomenological theory of martensite crystallography.
