Microstructure and martensitic transformation characteristics of gas-atomized Ti-Ni-Cu powders

Three batches of Ti-Ni-Cu powders (Ti50Ni45Cu5, Ti50Ni40Cu10 and Ti50Ni30Cu20) were prepared by gas atomization and porous specimens were fabricated by spark plasma sintering (SPS). The microstructure of as-solidified powders exhibited a cellular structure. XRD analysis showed that one-step martensitic transformation of B2-B19 occurred in all alloy powders and SPS specimens. DSC measurements of as-atomized powder, sintered bulk specimens and as-cast ingots were performed in order to study the effect of rapid solidification on martensitic transformation behaviors. The dependence of powder size on martensitic transformation temperature is very small in the rapidly solidified powders. However, the transformation temperatures were strongly dependent on the Cu-content. The martensitic transformation starting temperatures (Ms) of Ti50Ni45Cu5, Ti50Ni40Cu10 and Ti50Ni30Cu20 powders ranging between 25 and 150 μm are −4.9, −17.6 and 32.1 °C and the austenite transformation finishing temperatures (Af) of Ti50Ni45Cu5, Ti50Ni40Cu10 and Ti50Ni30Cu20 powders were 15.6, −4.4 and 38.3 °C, respectively. The Ms and Af of the as-atomized powders are much smaller than those of SPS specimens and as-cast ingots for all Ti50Ni45Cu5, Ti50Ni40Cu10 and Ti50Ni30Cu20 alloy systems. The temperature hysteresis (Af − Ms) of Ti50Ni45Cu5 powders was 20 °C and continued to decrease with increasing Cu-content. The temperature hysteresis of Ti50Ni30Cu20 powders was only 7 °C.