The microstructure of an Fe-Mn-Si-Cr-Ni stainless steel shape memory alloy

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, delta-ferrite, and the (Fe,Mn)3Si intermetallic phase; within the temperature range of 700°C to 1000°C, a two-phase field consisting of austenite and the Fe5Ni3Si2 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 martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through the gamma-epsilon-alpha transformation sequence.