Microstructure and shape memory characteristics of gas-atomized TiNi powders

For the fabrication of bulk near-net-shape shape memory alloys and porous metallic biomaterials, consolidation of TiNi alloy powders is more useful than that of elemental powders of Ti and Ni. In the present study, TiNi shape memory alloy powders were prepared by inert gas atomization, and martensitic transformation temperatures and microstructures of those powders were investigated as a function of powder size. The size distribution of the powders was measured by conventional sieving, and sieved powders with the specific size range of 0–200 μm were chosen for this examination. XRD analysis showed that the B2–B19' martensitic transformation occurred in powders smaller than 200 μm. In DSC curves of the as-atomized Ti50Ni50 powders as a function of powder size, only one clear peak was found on each cooling and heating curve. The martensitic transformation start temperature (Ms) of the 0–20 μm powders was 21.9 °C. The Ms increased with increasing powder size, and the difference in Ms between 0 and 20 μm powders and 150–200 μm powders is only 1 °C. The typical microstructure of the rapidly solidified TiNi powders showed cellular/dendrite morphology and exhibited a small volume fraction of Ti2Ni phase, which is located in interdendritic/intercellular regions.