Tini Shape Memory Alloy Fracture Mechanism

Shape-memory effect (SME) is a phenomenon where deformation suffered below a critical temperature can be recovered on heating. The micromechanism of this kind of recovery is the thermal-mechanical martensitic transformations. To date, there are nearly twenty intermetallic alloys which possess shape memory properties. However, due to several variables such as cost, strength, processability, and recoverable strain levels, only two shape memory alloy systems have been extensively used in engineering applications. One is copper based shape memory alloys, the other is TiNi. Over the last two decades, many research efforts have focused on characterizing martensitic transformations in TiNi.This paper sets about studying room-temperature fracture mechanisms of TiNi shape memory alloys through the measurements of mechanical properties, in-situ observations of fracture surfaces and calculations, the measurement of macro-parameters, the observations of fracture surface and calculations on notch specimens (4PB). Fracture mechanisms of titanium nickel shape memory alloys specimens are observed and analyzed carefully through above experiments. The investigation available recently in connection with the fracture of titanium nickel shape memory alloys is analyzed and summarized. The work about phase compositions, process of phase transformation and stress-induced martensite transformation in TiNi shape memory alloys conducted by the predecessors is analyzed and compared. Based on the results of experiments, calculation, the following are found: fracture of TiNi shape memory alloys is different from brittle cleavage fracture of steels. The fracture of TiNi shape memory alloys is controlled by cleavage coupled with void nucleation, growth, and coalescence from the second phase particles. The two mechanisms of material failure act in conjunction with one another and the dominance of one over the other depends on many factors. The reversible stress induced martensitic transformation has a very important effect on the fracture of TiNi shape memory alloys.