Creep and stress relaxation of a FeMnSi-based shape memory alloy at low temperatures

The creep and stress relaxation behavior of a Fe-17Mn-5Si-10Cr-4Ni-1(V,C) (wt%) shape memory alloy at low homologous temperatures (-45 degrees C < T < 50 degrees C) was systematically studied in stress and strain controlled tensile tests. At constant stresses at -45 degrees C, the alloy exhibits pronounced creep up to 0.6% at 600 MPa after only 30 min holding time. If the strain is kept constant, a pronounced stress relaxation of up to 10% of the initial stress was observed. The final creep strains, the creep rates at a constant stress as well as the stress relaxation at a constant strain increase with decreasing temperature. In addition, the change of the recovery stress as a function of time in a restrained sample after 4% elongation and heating to different constant temperatures was monitored. It was observed that the increase of the final recovery stress is more pronounced when the holding temperature is increased. This behavior was explained with the time and temperature dependent formation of stress induced epsilon-martensite from the parent gamma-austenite phase during mechanical loading according to the model of Kajiwara et al. as well as with the increased number of stacking faults at lower temperatures, which serve as nucleation sites for the epsilon-martensite formation. (C) 2016 Elsevier B.V. All rights reserved.