Toughness of Two-Phase 6Al-4V Titanium Microstructures

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Abstract
A duplex heat treatment which provides different shapes, sizes, and volume fractions of alpha platelets in 6Al-4V titanium is presented. It is shown that a 40 per cent increase in fracture toughness may be attained without any significant decrease in yield or ultimate strength. With respect to toughness, critical aspects of both the interstitial oxygen content and the shape, size, and spacing of the alpha phase are discussed. Metallographic and fractographic evidence supports the contention that the second-phase alpha in platelet form acts as a crack arrestor and causes a deviation in the crack path, resulting in additional energy absorption. A tentative hypothesis for fracture in two-phase microstructures is presented based upon the theoretical crack tip displacement. Preliminary evidence indicated that the critical toughness, size, and spacing of the second phase may all be related to the crack tip displacement concept.

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