素粉末混合Ti-6Al-4V合金の疲労特性に及ぼす微視組織の影響

抄録

Several different microstructural conditions were generated through combinations of processing and heat treatment to find out the optimum microstructure and also to investigate the effect of the alpha phase mor phology on the high cycle fatigue strength. The best combination of high cycle fatigue strength and ductility was obtained by the new Blended Elemental (BE) method, in which sintered material was waterquenched from the beta phase region prior to HlP'ing. The highest fatigue strength at 10⁷ cycles, 72 kgf/mm², was obtained by the STA treatment. Examination of the fatigue crack initiation facets and the underlying microstructures by the presicion sectioning method revealed that the fatigue failure initiated from the shear-across-colony facet for conventional BE material and from the shear-across-primary alpha facet for acicular microstructure. These shear facets were inclined at 45° to the tensile axis, which coincide with the maximum shear direction. In the case of the acicular microstructure, the slip direction was oriented almost parallel to the short axis of the elongated primary alpha grain and hence it seemed that there is no dependence of the high cycle fatigue strength on the aspect ratio of the alpha grain, but rather on the width of alpha grain. Based on these initiation analysis, it is concluded that, irrespective of microstructural categories, the high cycle fatigue strength can be described definitively as a function of the length of slip path in the alpha grain, with a colony considered as a single grain.