Gradient structure of ternary Ti-Ni-Ta surface alloy synthesized on TiNi substrate by electron-beam method

Abstract

Ti-Ni-Ta surface alloy was fabricated on a TiNi substrate through a pulsed melting of the “Ti60Ta40 (at. %) film/TiNi substrate” system by a low-energy high-current electron beam. The influence of the electron-beam treatment on the microstructure and stressed state of the B2 phase preserved in the surface modified area was studied. An analysis, performed by both X-ray diffraction and transmission electron microscopy, has shown a formation of the gradient structure in the outer surface layer of the surface alloy of the sample. The surface alloy consists of two layers: a sharply textured outer layer of ~150 nm thickness based on α-Ta phase and an amorphous sublayer having ~500 nm thickness. A heat-affected zone is formed beneath at a depth above ~750 nm and includes sublayers possessing B2 and B19' structures. After electron beam treatments, the B2 lattice parameter decreases from 3.0137 Å to 2.9950 Å. It has been found that the variation of the B2 lattice parameters in the sublayers of the TiNi substrate are partly due to the residual compressive stresses distributed in the direction perpendicular to the irradiated surface. X-ray diffraction analysis has shown that the maximum residual stresses reach values of ~ –290 MPa after the surface alloy synthesis. The residual stresses are distributed in the heat-affected zone adjacent to the synthesized surface alloy.