Ti-3Al-8V-6Cr-4Mo-4Zr (β-C ) 합금의 기계적 성질과 피로한도에 미치는 용체화처리 온도와 미세조직의 영향

Effects of solution treatment(ST) and aging temperatures on the behavior of β decomposition and the mechanical properties of β-C alloy have been investigated mainly by means of TEM observations. In the initial stage of transformation at temperatures below 480℃, a modulated(or tweed) structure forms on a {110}_β and aligns _β. And then a metastable β′ nucleates along the modulated structure. At the later stage of transformation, the equilibrium a phase nucleates at the β/β′interface and grows preferentially to _β with Burgers relationship to β matrix. The β grain size and the fatigue limit are strongly dependent on ST temperature. The elongation, the yield and tensile strengths, however, are approximately constant independently of the ST temperature. Aging at 480℃ after ST at 800℃ gives the highest tensile strength but shows the lowest ductility in the present study. This result is considered to occurred by the large amounts of β′ and extremely fine needlelike α precipitates, which induce much elastic strains in the β matrix. Aging at 530℃ after ST at 800℃ gives improved tensile strength and ductility combination and represents the fatigue strength as high as 605㎫ with the high endurance ratio of 0.536. The mechanical properties of the specimens solution treated at 800℃ followed by aging treatment are better than those of the specimens solution treated at 900℃ followed by aging treatment. Therefore, it is indispensable for obtaining a better combination of strength, ductility and fatigue limit in the β-C alloy to select an adequate ST and aging temperatures.