Ti-6Al-4V合金の疲労強度に及ぼす短時間2段階熱処理の影響

抄録

Influence of short-time duplex heat treatment on the fatigue strength of α + β titanium alloy Ti-6Al-4V was investigated. When the alloy was solution-treated for 60s at 1203K and water-quenched, prior β phase was hardened since a part of this phase was transformed to acicular α' martensite phase. With subsequent aging for 40s at 753∼903K, fine α phase was precipitated in retained β phase so that hardness of prior β phase further increased. Such hardening of prior β phase remarkably improved the fatigue strength of the alloy as well as its yield stress and tensile strength. That was because slip in primary α phase, where would be fatigue crack initiation sites, was restricted by hardened prior β phase so that fatigue crack initiation was suppressed to high stress amplitude level. In the examined treatment conditions, the combination of the above short-time solution treatment and aging for 40s at 803K was suitable to improve the strengths. In this case, the tensile strength and fatigue strength were increased by 29% and 22%, respectively. At the same time, ductility was also increased by 10% due to stress-induced martensitic transformation of retained β phase.