A Review On Inclusions In NiTi Shape Memory Alloys

Since 1968, when melting and casting procedures for NiTi shape memory alloys (SMAs) were investigated by Drennen, Jackson, and Wagner, it is known that NiTi SMAs with commercial purity contain different types of inclusions that are stabilized by impurity elements. In the present paper, we provide a summary on what is known about these phases today. The most common impurity-related phases are TiC and oxygen-stabilized Ti₂Ni, which is often referred to as an oxide of type Ti₂Ni in shape memory-related literature. Both phases form during the solidification of molten NiTi within a quasi-binary eutectic reaction and a peritectic reaction, respectively. As specified in ASTM F2063-05, NiTi SMAs for medical applications can contain up to 0.05wt.% carbon and 0.05wt.% oxygen. The impurity levels of NiTi SMAs strongly depend on the quality of the raw materials (elemental Ni and Ti) and on the melting procedures. In the present paper, we give a detailed review on the formation mechanisms of these phases and on the evolution of phase morphologies during thermomechanical treatments. We provide thermodynamic information and show how volume fraction, size and density of these particles depend on impurity levels. We further discuss the effect of impurity-related secondary phases on phase transformation behavior, workability, corrosion and fatigue.