Origin of Microstructural Irreversibility in Ni-Ti Based Shape Memory Alloys during Thermal Cycling

Abstract

Different microstructures of Ni-Ti-and Ni-Ti-Fe-based shape memory alloys were subjected to thermal cycling: dipping in liquid nitrogen, for approximately 5 minutes, and then bringing it back to room temperature or austenite (cubic: B2) M martensite (monoclinic: B19¢) reversible solid-state phase transformation. Direct electron backscattered diffraction (EBSD) observations could bring out aspects of microstructural irreversibilities: namely, changes in grain size, misorientation buildup, and presence of retained martensite. The average changes in grain size (Dd) differed by almost 2 to 4 times between different microstructures. The highest Dd was typically observed in structures having maximum clustering of fine (d < 5 lm) grains. The sample with highest Dd was also subjected to multiple thermal cycling. Although Dd scaled linearly with d after the first thermal cycle, the scatter increased during subsequent thermal cycles. Grain or orientations deviating from the linear behavior were clearly anisotropic crystallographically. With repeated thermal cycling, the patterns of changes in Dd, austenite misorientation, and retained martensite content were similar. A phenomenological model or hypothesis, based on 40 deg 001 h i orientation relationship between austenite and martensite phases, was proposed to address the observed patterns of microstructural irreversibility.

Lattice instability during the martensitic transformation in the high temperature shape memory alloy Zr(Cu0.5Co0.25Ni0.25)

“…3(a) and (d). For long term cyclic stability it is desirable for an SMA to undergo complete transformation from one phase to another but in reality a small amount of low temperature martensite remains untransformed in the microstructure on heating above the austenite finish temperature (A f ) [26] and vice-versa on cooling below the martensite finish temperature (M f ) [16,2]. This presence of retained austenite in between growing martensite variant groups is expected to be due to the stress stabilisation of the austenite by the transformation strains [2].…”

Microstructural Irreversibilities under Thermal Cycling in Ni-Ti-Fe Shape Memory Alloys

“…The subsequent thermal cycling deals with two different microstructures: both were hot rolled (~64% deformation at 930°C), marformed ((~8% deformation in the martensite phase: under liquid nitrogen) and annealed. Annealing temperature of 750°C and respective times of 1 h and 1.5 h created two different microstructures (A and B) [8] These samples were electropolished: 20 volts dc, 0°C and an electrolyte of methanol with 10% perchloric acid. A suitable area on the sample surfaces were marked (using conducting permanent 'ink') for thermal cycling and further analysis.…”

Microstructural investigation on marforming and conventional cold deformation in Ni–Ti–Fe-based shape memory alloys

“…These dislocations stabi-lize the remaining martensite variants. These dislocations form an array to establish aphase boundary with the austenite phase upon reversal that has the minimum energy and continuity of the slip system.Ithas been pointed out before that an orientation relationship (OR) of 408 <001> between martenite and austenite is needed for reverse austenite transformation [28]. Any deviation from the exact relationship will lead to the presence of remnantm artensite.…”