Effect of electron-beam treatments on the level and evolution of residual stresses in the “surface Ti – Ni – Ta – Si alloy/TiNi-substrate” system

Abstract

TiNi shape memory alloys exhibiting martensitic transformations are attractive materials in the industry of micromechanical systems. The key issue responsible for degradation of functional properties of TiNi during operation is residual stresses arising from the sample processing and surface treatments. In this work, we investigated TiNi substrates modified after synthesis of Ti – Ni – Ta – Si surface alloys (SA) by additive thin film electron-beam (ATF-EB) method. The effect of electron-beam treatments of the system “[Ti – Ni – Ta – Si]SA/TiNi substrate” at the energy density (ES = 1.7 J/cm2) and pulse number n = 10 on the elastic-stress state of the B2 phase in TiNi substrate has been studied. Detailed analysis by X-ray diffraction (XRD) and transmission electron microscopy (TEM) has shown the formation of two isostructured B2 phases with different lattice parameters, chemical composition, and microstructure. It has been found that changes in the B2 lattice parameters are closely related to the variation of the chemical composition and residual elastic stresses of the first kind. The study of residual stresses in the samples reveals that compression stresses in the direction perpendicular to the irradiated surface reach a value of –350 MPa. After additional electron-beam treatment, the compression stress value decreases to –270 MPa.