Microstructure, Phase Transformations and Properties of Hot Extruded Ni-Rich NiTi Shape Memory Alloy

The processing technology of NiTi shape memory alloys strongly influence their microstructure, phase transformations and mechanical and shape memory properties. In these studies the Ni_50.8Ti_49.2shape memory alloy obtained by conventional VIM melting technique is presented. The alloy has promising further application in construction of medical devices.

The studied alloy was prepared from high purity nickel and titanium rods using vacuum induction melting furnace VSG-10. For reducing carbon contamination of the ingot the high quality, low porosity and high density graphite crucible and graphite mould were used. During the hot forging or hot rolling intense oxidation of the surface takes place. In order to reduce oxidation the material was processed by hot-pack working. After homogenization the ingot was cut into two parts. One of them was packed in steel pipe and subsequently hot forged and the second one was hot extruded. The forged rods and extruded blanks were subsequently worked by hot rotary forging and hot drawing to wires with different diameters.

     The characteristic temperatures of phase transformations, transformations hysteresis and one-way shape memory effect in the samples after hot forging and hot extrusion were studied by: differential scanning calorimetry (DSC) method, three-point bending and free recovery ASTM F2082-06 test, low-temperature XRD technique. The X-ray powder diffraction measurements were performed on a PANalytical Empyrean diffractometer equipped with Anton Paar TTK450 Low-Temperature Chamber. The performed X-ray phase analysis showed that after casting and after hot extrusion the material at the room temperature has B2 parent phase structure which was also confirmed by electron microscopy studies. DSC measurements performed on Perkin-Elmer DSC-7 calorimeter proved that during cooling and heating the phase transformations occur with the sequence B2↔R↔B19’.