A COMPARATIVE STUDY ON MICROSTRUCTURE AND COMPRESSIVE PROPERTIES OF NITI ALLOYS SINTERED FROM Ni/Ti AND Ni/TiH2 MIXTURES

摘要

This work reports a comparative study using nickel powder and two different titanium powder sources to synthesize porous NiTi alloys via a conventional press-and-sinter powder metallurgy process. Two batches of powder mixture (i.e., Ni/Ti and Ni/TiH2) were prepared in a ball mill, pressed under five different compaction pressures, and subsequently sintered in vacuum at 1,100 degrees C for 2 h. The results of microstructural observations reveal porosity ranging from 5.1 +/- 0.6% to 31.2 +/- 0.3%, and density varying from 4.38 +/- 0.02 g/cm(3) to 5.74 +/- 0.04 g/cm(3) in the sintered samples. The predominant phase identified in all the NiTi alloys produced is B2-NiTi, with the presence of minor Ni3Ti and NiTi2 phases. The sinter shrinkage is not uniform, with axial shrinkage (A) being larger than radial shrinkage (R). The A:R ratio increases with increasing compaction pressure. The cyclic compressive behaviors of the porous NiTi alloys made from Ni/Ti and Ni/TiH2 mixtures were also investigated. In comparison with the sintered Ni/Ti samples, the sintered Ni/TiH2 samples exhibit more porosity, a larger pore size, lower fracture strength, and lower secant modulus.