Development of Fe-Mn-Si based shape memory alloys with no necessity of "training"

Abstract

In ar. ttempt to modify the current Fe-Mn-Si based shape memory alloys so that so-called the "training" treatment may be no more necessary, a small amount of Nb and C were added to Fe-Mn-Si based alloys to form NbC precipitates by aging. The existence of small coherent NbC precipitates in austenite greatly increases the shape recovery and the recovery force without the "training". For Fe-28Mn-6Si-5Cr-0.5NbC (mass %), 100-80 % shape recovery are obtained for tensile strains of 1-4 % and, for Fe-l4Mn-6Si-9Cr-5Ni-0.5NbC, 100-80 % shape recovery are attained for tensile strains of 1-3 %. The maximum recovery force is 155 MPa for the former alloy and 130 MPa for the latter. The increase in recovery force of the former alloy compared with the corresponding NbC free alloy is 45 MPa. Our preliminary experiments with transmission electron microscope and atomic force microscope indicate that the stress-induced martensite structures formed in the NbC containing alloys are nearly the same as those in the "trained" sample of the NbC free alloys in accordance with the prediction recently made by Kajiwara.