Effect of Nitrogen Addition on TiNi Shape Memory Alloys

In present paper we will show how nitrogen effects microstructures, transformation temperatures, and mechanical properties of equiatomic Ti 50 –Ni 50 and Ti-rich Ti 51 –Ni 49 binary shape memory alloys. 0.5 at.% of nitrogen was added to prepare Ti 50 –Ni 49.5 –N 0.5 , and Ti 51 –Ni 48.5 –N 0.5 (at.%) alloys by arc-melting. Microstructures were investigated by scanning electron microscope (SEM), phase constitutions were investigated by X-ray diffraction (XRD), transformation temperatures were investigated by differential scanning calorimeter (DSC) and mechanical properties were tested by tensile tests. Solutions treated Ti–Ni–N shape memory alloys contain TiNi matrix without nitrogen, Ti 2 Ni type phase containing a small amount of nitrogen and a new Ti 2 N type phase containing a small amount of nickel. Compared with Ti 50 –Ni 50 and Ti 51 –Ni 49 binary alloys, the martensitic transformation starts temperatures (Ms) of Ti 50 –Ni 49.5 –N 0.5 and Ti 51 –Ni 48.5 –N 0.5 ternary alloys decreased from 63.4 °C to 41.6 °C and from 85.3 °C to 79.4 °C, respectively. By adding N, fracture strain decreased and incomplete superelasticity was observed.