Shape Memory Steels

Shape memory alloys are characterized by reversible diffusionless/martensitic transformation, i.e., stress-induced martensitic transformation upon loading and its reverse transformation upon unloading or heating. In deformed shape memory alloys, a certain portion of the non-linear strain can be recovered upon unloading and further recovery can be attained by heating above a critical temperature. The former phenomenon is referred to as superelasticity and the latter shape memory effect (SME). Both phenomena arise from the reverse martensitic transformation. In the past half century, Ti-Ni-based alloys have attracted the most extensive research efforts among all shape memory alloys owing to their excellent superelasticity and SME (recovery strain up to 8%) as well as the associated complicated martensitic transformations. Numerous applications such as pipe couplings, coronary stents and micro-actuators have been found for Ti-Ni-based shape memory alloys. Nevertheless, these alloys suffer from high alloying costs and low workability, limiting their wider application. In 1982, Sato observed a large recovery strain (up to 9%) comparable to that of TieNi-based shape memory alloys in single-crystalline Fee30Mne1Si alloy, opening up the possibility of developing commercially more attractive ferrous shape memory alloys with lower alloying costs and better workability. However, subsequent studies revealed that the as-solution treated polycrystalline Fe-Mn-Si-based alloys suffered from a low recovery strain. Even the three alloys, Fe-28Mn-6Si-5Cr, Fe-20Mn-5Si-8Cr-5Ni and Fe-16Mn-5Si-12Cr-5Ni, which were claimed to have a combination of good corrosion resistance and good SME, exhibited a recovery strain <2.5%. In the past decades, considerable effort has been devoted to improving the SME of these alloys and their derivatives.