Strengthening of a FeMnSi-based shape memory alloy by dispersion of χ-phase domains

Abstract

The FeMnSi-based shape memory alloy (SMA), governed by the γ ⇔ ϵ phase transformation, is shown to be strengthened remarkably by the introduction of a non-deformable phase via beating and heating treatments, improving the shape memory effect (SME) against an opposing stress as high as 450 MPa. Dispersion of the non-deformable phase is also found to increase pseudo-elasticity. The non-deformable phase contributing to the strengthening is identified as having a bcc-based structure of I 3m, i.e. α-Mn-type χ-phase, by analyzing the zero- (ZOLZ) and first-order Laue zones (FOLZ) in microdiffraction patterns. This phase is found to be non-deformable, at least up to 1.2 GPa, by semi-in situ compression tests made on thin foil specimens, which were prepared for observation with a high-voltage electron microscope (HVEM). The origin of the strengthening and pseudo-elasticity of the FeMnSi-based SMA is discussed, with reference to dispersion–hardening, under the chemical driving force acting on the Shockley partial dislocations, which convey the γ ⇔ ϵ phase transformation.