Improved shape memory properties and internal structures in Fe-Mn-Si-based alloys containing Nb and C

It has recently been found that the shape memory properties of Fe-Mn-Si-based alloys are notably improved by addition of small amounts of Nb and C elements. In these newly modified alloys, the so-called training process is replaced by a simpler thermomechanical treatment. For example, in an Fe-28Mn-6Si-5Cr-0.5NbC alloy (mass %), a combination of pre-rolling at 870 K followed by 10 min ageing at 1070 K results in a shape recovery of 90% for 4% initial strain and a shape recovery stress of 295 MPa for 4.5% initial strain. In this work, we present the shape memory properties of the samples pre-rolled up to 20% in Fe-28Mn-6Si-5Cr-0.5NbC, and atomic force microscopy (AFM) and transmission electron microscopy (TEM) observations of the stress-induced martensite. Detailed discussion is made on the relations between the microstructures observed by AFM and TEM and the shape recovery and shape recovery stress, and the main factors responsible for drastic improvement of the shape memory properties in this alloy system are drawn.