Synthesis of NiTi intermetallics by self-propagating combustion

The production of NiTi intermetallic compounds from elemental powder compacts was conducted by self-propagating high-temperature synthesis (SHS) in this study. Effects of initial sample density, preheating temperature, and Ni particle size on the combustion characteristics, as well as on the final composition and morphology of products were studied. For the samples with densities between 50 and 60% theoretical maximum density (TMD) under preheating temperatures in the range of 100-300 degreesC, the synthesis process showed two consecutive combustion stages, including the propagation of combustion front and the subsequent bulk combustion. Under these conditions, high-density products containing NiTi as the major phase with small amounts of secondary phases of NiTi2 and Ni3Ti were obtained. However, porous combustion products with a significant amount of unreacted Ni were produced from the 45% TMD samples under preheating temperatures of 100 and 150 degreesC, due to the lack of the second combustion stage. The amount of unreacted Ni in the final composition was greatly reduced by increasing the initial sample density and preheating temperature, or by using smaller Ni particles in the samples. In addition, the use of smaller Ni particles also led to an increase in the product density up to about 95% relative to the density of NiTi intermetallic. (C) 2004 Elsevier B.V. All rights reserved.