The effect of short-time duplex heat treatment on the microstructure and mechanical properties of Ti-6Al-4V alloy was investigated. This heat treatment consisted of solution treatment at 1203 K for 60 s and subsequent water-quenching plus aging at 753, 853 and 953 K for 40 s. The yield strength and tensile strength of the alloy were significantly increased by the above heat treatment and their maximum improvement rates reached about 25%. It was thought that this strengthening was caused by the formation of 0 martensite phase with quenching after the short-time solution treatment and the precipitation of fine phase in the retained phase during the short-time aging. In spite of the remarkable improvement in the strength, the ductility of the heat-treated materials remained above the level of the non-treated material, most likely because strain-induced martensite transformation occurred in phase which was retained even after the short-time aging.

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This analysis was conducted to investigate the influence that dispersion in crystal grain sizes has upon the yield behavior of nanocrystalline materials. In the model proposed for this purpose, the distribution of crystal grains was expressed in a log-normal manner. It was assumed that the yield stress of each crystal grain was determined by the relationship between the grain size and the stress required for the generation of dislocations from a grain boundary. Furthermore, using the micromechanics of inclusions, we carefully considered the internal stresses in the crystal grains yielded at a different remote applied stress and in the matrix which is still elastic. The result of the analysis showed that the increase of compliance with the yielding of crystal grains from large to small can cause the macroscopic yielding of nanocrystalline materials. It was also inferred that the distribution of crystal grains was one of the important factors which affect the shape of the stress-strain curve and the macroscopic yield stress.

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