Interfacial Stresses and Anomalous Shape of Pseudoelastic Deformation Curves in Ni₄₉Fe₁₈Ga₂₇Co₆ Alloy Crystals Compressed along the [011]_A Axis

Abstract

We have performed experimental and theoretical investigation of the anomalous form of the compression diagrams and shape memory restoration curves in Ni₄₉Fe₁₈Ga₂₇Co₆ alloy crystals deformed by uniaxial compression along the [011]_A crystallographic direction (A-austenite) in the temperature range of 200–350 K. It is found that in the investigated temperature range, all compression diagrams contain anomalous segments of smooth and sharp decrease in deforming stresses. It is shown that the segments of a smooth decrease in stress are associated with peculiarities in martensite reaction L1₂ → 14M, while segments of a sharp drop are due to instability of martensite reactions 14M → L1₀ and L1₂ → L1₀. A possible source of reaction instability is associated with interfacial stresses at the interfaces between the martensite and austenite phases (lamellas) due to different elastic moduli of contacting phases. The magnitude of these stresses is significant in the case of 14M → L1₀ and L1₂ → L1₀ transformations, which induces a sharp drop of the deforming stress, while the restoration of the shape memory effect is of a burst nature. It is established that the contribution of interfacial stresses to the free energy of martensite transformation is smaller than the dissipative (entropy) contribution to this energy; however, interfacial stresses higher than a certain threshold strongly affect transformation kinetics and, hence, determine the strongly anomalous shape of pseudoelastic deformation curves and burst restoration of the shape memory effect.