Thermal expansion and polymorphous transformations of sintered Ti and Fe alloys

The materials used were 180-300 and 300-500 μm size, PTES-grade electrolytic Ti powder; Fe powder produced by melt pulverization and annealing in a H2 atmosphere at 800-900°C, with up to 0.08% C content; and Fe powder plated with Ni followed by reduction with H2. Titanium powder with coarser particles has, at high temperatures, higher thermal expansion then that with fine particles. This correlates well with their different O2 contents. The thermal expansion coefficient variation with temperature for Fe is different from that of solid Fe, which might be related to purity of the materials. The starting temperature of the polymorphous transformation in Ti depends on residual porosity, which can be explained by temperature-induced stresses in the material's porous structure. Surface alloying of Fe powder with Ni has a strong effect on the α-γ transformation parameters.