Structure and residual stress distribution in TiNi substrate after fabrication of surface alloy using electron-beam treatments

Abstract

In this work, an origin of the residual stresses formed in the sublayer of TiNi substrate adjacent to the [Ti-Ni-Ta-Si] surface alloy (SA) was proposed. The as-cast SA was fabricated through an additive thin-film electron-beam synthesis using a low-energy high current electron beam (LEHCEB) treatment of the [Ti60Ta30Si10 (at. %)/TiNi] system. An additional post-LEHCEB treatment was employed for the SA synthesis (denoted as EB-treated SA) in order to reduce the value of residual stresses. Based on the XRD, TEM/SEM/EDS data, it has been found that the transition zone between the outer SA zone and the TiNi substrate exhibits several sublayers: eutectic B2+Ti3Ni4, martensitic and heat-affected zone (HAZ). However, the martensitic interlayers formed beneath the as-cast and EB-treated SAs possess different types of martensitic structures. Indeed, in the as-cast SA the sublayer of TiNi substrate is a Ni-rich (Ti<50Ni>50) one, so the martensitic transformation В2→R takes place in this sublayer. In turn, in the EB-treated SA the TiNi sublayer shows the composition close to equiatomic (Ti∼50Ni∼50) one, therefore the В2→B19′ martensitic transformation has occurred. The formation of martensitic phases led to a partial decrease (by a factor of ∼1.5–2) of the residual stresses in the underlying HAZ of TiNi substrates. The mechanism of B2 phase stabilization in the HAZ is closely related with a substructural hardening accompanied by formation of dense dislocation structures and residual elastic stresses that could be relaxed through short-term annealings induced by electron beam treatments.