Mechanical Activation Of Powder Sintered Porous Ni-ti Alloys

The preparation of porous Ni-Ti alloys by mechanical milling and powder metallurgy has been studied in this paper in order to reveal the phase transformation and densifying behaviors of different degree milled Ti-50.5at%Ni powders during sintering. When the different degree mechanically activated powders are sintered at 1173 K for 0.5 hr, the short time milled powder is not completely alloyed and a metastable phase Ni4Ti3 as well as two pure elements Ni and Ti are resulted other than three intermetallics- NisTi, NiTi and Ti2Ni. When the compacts are sintered at elevated conditions (1173 K for 6 hr or 1123 K for 1 hr) and thus make the phase transformation complete, the resulted porous alloys consist of three Phase-Ni3Ti, NiTi and Ti2Ni and have a porosity between 31-37% with both open and close pores . It is indicated that mechanical milling dramatically enhances the activity of the powder and thus making the solid state diffusion between the two elements much easier. On the other hand, green samples made of deep milled powders are hard to sinter to their densification. This may be due to the larger and more homogeneous powder particles after milling which decrease the specific surface areas between particles, and the oxide coatings on the particles which impede substances to diffuse and transport to the sintering necks. For the powder milled to amorphous state, because of its very fine structure, it is easy to occur phase transformation and has good size stability when sintered.