時效處理對Ti-15V-3Cr-3Al-3Sn鈦合金機械性質之影響研究

Abstract

This study investigated one-step (317~648℃/8 h) and two-step (317~648℃/8 h + 426℃/24 h) aging treatments to the Ti-15V-3Cr-3Al-3Sn alloy (Ti-15-3 alloy). The effects of aging treatment on the microstructure and mechanical properties were also evaluated. The mechanical tests involved the tensile, notch tensile, J-integral and fatigue crack growth tests. For the Ti-15-3 alloy, the α phase precipitated from the β matrix and resulted in hardening during the aging treatment. Experimental results indicated that the hardening effect was insignificant for the 317℃/8 h (A317 specimen) treatment. Precipitation hardening could be observed for the one-step aging treatment at 371℃ (A371 specimen) or higher than that. In one-step aged specimens, the highest hardness could be obtained for the A426 specimen with a value of approximately Hv 428. Over-aging occurred when the temperature was higher than 482℃ for one-step aged specimens, and the hardness gradually decreased as the aging temperature increased. The second aging treatment (426℃/24 h) increased the hardness and the size of α precipitates for the specimens which were previously aged at temperatures equal to or below 426℃ in the one-step aging treatment. For higher one-step aging temperatures, e.g., 648℃, the second aging treatment caused more precipitation of fine α, leading to further hardening of the D648 specimen (648℃/8 h + 426℃/24 h) relative to the A648 specimen. On the other hand, the D538 and D593 (two-step aged) specimens exhibited similar hardness and coarser α precipitates compared to the A538 and A593 (one-step aged) specimens, respectively. In general, the specimen with a pronounced effect of age hardening had high tensile strength and low notched tensile strength, regardless of the aging conditions. Furthermore, the ductility was also relatively poor, and brittle fractures were observed for such specimens. The specimens in the over-aged conditions showed high elongation and notch strength ratio, except the D648 specimen. Dimple and grain boundary sliding were often observed on the fracture surfaces of the over-aged specimens. Compared to the one-step aged specimens, the two-step aged specimens had lower notched tensile strength and elongation. Other than the D648 specimen, the J-integral value increased as the first aging temperature increased of the two-step aged specimen. The ductility was also found to be the main factor to affect the J-integral value, i.e., the higher elongation, the higher J-integral value. The formation of fine α precipitates during the second stage aging of the D648 specimen led to increased brittleness and decreased fracture toughness. On the other hand, the results of fatigue crack growth tests also revealed that those specimens with higher fracture toughness exhibited slower crack growth rates.