Structure control and densification of sintered titanium alloys

以市售钛铬粉为原料，通过粉末混合、成型、真空烧结等常规粉末冶金工艺，可方便地制备出钛铬合金。由于该合金在900℃以上具有良好的元素弥散性的体心立方结构(β相)，1250℃-1小时的烧结处理使化学成分均匀，2)烧结材料的密度提高到相对密度的92%。该合金在烧结态下具有良好的力学性能，但由于存在残留的气孔，在断裂韧性和疲劳强度方面也存在一些不足。为了克服这些缺点，烧结材料的密度应提高到相对密度的99%或以上。β相在高温下足够软的性质使烧结材料容易形成和完全致密成为可能。烧结材料中残留的气孔为闭合气孔，气孔内表面清洁，变形时气孔完全消失，不会发生气孔内氧化。此外，还发现该合金在700℃左右发生超塑性变形，预先对烧结材料进行适当的处理：将烧结材料从β相区一次水冷得到马氏体组织，然后在700℃充分变形，使马氏体组织转变为细小的等轴晶。超塑性变形有助于提高该合金的成形性。

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.