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Neutron diffraction measurements, performed in the presence of an external magnetic field, have been used to show structural evidence for the kinetic arrest of the first order phase transition from (i) the high temperature austenite phase to the low temperature martensite phase in the magnetic shape memory alloy Ni37Co11Mn42.5Sn9.5, (ii) the higher temperature ferromagnetic phase to the lower temperature antiferromagnetic phase in the half-doped charge ordered compound La0.5Ca0.5MnO3 and (iii) the formation of glass-like arrested states in both compounds. The cooling and heating under unequal fields protocol has been used to establish phase coexistence of metastable and equilibrium states, and also to demonstrate the devitrification of the arrested metastable states in the neutron diffraction patterns. We also explore the field–temperature dependent kinetic arrest line TK(H), through the transformation of the arrested phase to the equilibrium phase. This transformation has been observed isothermally in reducing H, as also on warming in constant H. TK is seen to increase as H increases in both cases, consistent with the low-T equilibrium phase having lower magnetization.

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The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (c) up to 6 pct Si and, beyond that, becomes two-phase c + d ferrite. The Fe 5 Ni 3 Si 2 type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition does not affect the transformation temperature and mechanical properties of the c phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct Si show poor recovery due to the formation of d-ferrite. The shape memory effect (SME) in these alloys is essentially due to the c to stress-induced e martensite transformation, and the extent of recovery is proportional to the amount of stress-induced e martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced a¢ martensite through c-e-a¢ transformation and the large volume fraction of d-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and results in easy nucleation of stress-induced e martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal e martensite formed during cooling is also observed to decrease with the increase in Si.

The microstructure and phase stability of the Fe-15Mn-7Si-9Cr-5Ni stainless steel shape memory alloy in the temperature range of 600 °C to 1200 °C was investigated using optical and transmission electron microscopy, X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and chemical analysis techniques. The microstructural studies show that an austenite single-phase field exists in the temperature range of 1000 °C to 1100 °C, above 1100 °C, there exists a three-phase field consisting of austenite, d-ferrite, and the (Fe,Mn) 3 Si intermetallic phase; within the temperature range of 700 °C to 1000 °C, a two-phase field consisting of austenite and the Fe 5 Ni 3 Si 2 type intermetallic phase exists; and below 700 °C, there exists a single austenite phase field. Apart from these equilibrium phases, the austenite grains show the presence of athermal martensite. The athermal aЈ martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through the ␥--␣Ј transformation sequence.

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