In situ observation of nonmetallic inclusion formation in NiTi alloys

Abstract

It is well recognized that the fatigue characteristics of NiTi alloys are greatly influenced by the presence of nonmetallic inclusions. A high fatigue strength is required for the application of these alloys to medical devices, especially stents, to allow them to withstand the repeated processes of pulsation-induced expansion and contraction. In this study, the phases and morphology of the inclusions in commercially produced NiTi alloy were investigated. The inclusions in an as-cast ingot consisted of a single phase of Ti(C, O) and possessed a fiber shape with a diameter of about 0.5 μm, and were formed during the solidification of the NiTi alloy. In situ observation by confocal laser scanning microscopy revealed that the inclusions formed and grew in the region of residual melt (consisting of concentrated Ti, C, and O) during the solidification process.