Effect of a submicron Ti-Ni-Ta surface alloy on the mechanical behavior of a TiNi thin wire in torsion tests
摘要
The mechanical and inelastic properties of a TiNi thin wire with a diameter of ~950 µm are studied after the synthesis of a surface alloy (SA) based on the Ti-Ni-Ta system with an amorphous structure and a thickness of ≤ 1 µm. The SA is synthesized by a 2-fold alternation of deposition of the alloying film (Ti60Ta40 (at.%), with a thickness of ~100 nm) and liquid-phase mixing of the [film/substrate] system using a low-energy high-current electron-beam. The deformation behavior of the samples is examined in torsion tests before and after thermal cycling (TC) of the [SA/TiNi-substrate] system within the temperature range of a B2 ⇄ B19′ martensitic transformation. The cyclic tests reveal that the presence of SA does not lead to an increase in martensitic shear stress τM and stress hysteresis loop ∆τ, but it affects the material ability to accumulate and recover superelastic strain γSE. It is found out that the TC of the initial and modified samples does not significantly affect the deformation behavior of the wire. It is noted that the amorphous SA does not crack after TC and does not peel off from the surface after torsion tests. As a result of testing to failure at an accumulation shear strain of γ ≥ 25%, the presence of an SA leads to a decrease in the shear fracture strength τmax by ~200 MPa and in the fracture strain γmax by ~15% compared to the values for the initial TiNi samples.