Structure control and densification of sintered titanium alloys

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Ti-4mass%Cr alloy, which was developed in this study, can easily be produced from commercial powders of titanium and chromium through the usual powder metallurgy process ; mixing of powders, molding of the mixed powder and then sintering in vacuum. Since this alloy has BCC strructure (beta-phase) at elevated temperature above 900C which is excellent in difusivity of elements, the sintering treatment of 1250C-1hr results in 1) homogenization of chemical composition and 2) increasing of density of sintered materials to 92% in relative density. The alloy has good mechanical properties under the as-sintered condition, but due to the retained pores, it is also cleared that there are some disadvantages in fracture toughness and fatigue strength. To get over these disadvantages, the density of sintered materials should be increased to 99% or over in relative density. The nature that beta-phase is enough soft at elevated temperature makes easy forming and complete densification of sintered materials possible. Pores retained in sintered materials are closed ones and the innersurface of pores is clean, hence they can fully disappear during deformation for forming without inner-oxidation of pores. In addition, it was also found that this alloy undergoes superplastic deformation at around 700C,when sintered materials were subjected to an appropriate treatment in advance : Sintered materials was once water-quenched from beta-phase region to obtain martensitic structue and then sufficiently deformed at 700C to change the martensitic structure to fine-equiaxd grains. Superplastic deformation is very helpful for developing the rnrformance in formability of this alloy.