Ti-6Al-4V合金のRカーブに及ぼす微視組織及び不純物の影響

Abstract

Effect of microstructure, impurity content and testing temperature on crack initiation (δi) and crack extension (slope of R curve) properties of Ti-6Al-4V alloys was examined. δi values and R curves were obtained by accurate measurement of microcrack length at a notch tip of unloaded specimens. At 0°C, microcracks of 10 -1 800μm in length preceded unstable fracture at a notch tip irrespective of microstructures examined. Specimens with a variety of microstructures had almost the same δi values, but showed a remarkable defference in the slope of R curves. A specimen with acicular microstructure, which have a higher fracture toughness (δc) at a given strength, showed a greater crack extension resistance than one with equiaxed microstructure. At -196°C, little difference in the crack initiation was obtained among the specimens with different kinds of microstructures and differnce in the crack extension resistance was also greatly reduced. But specimens with lower impurity contents, ELI grade, which had a higher δc, showed a greater crack extension resistance than specimens with standard impurity contents. It is concluded that the difference in fracture toughness at a given strength for Ti-6Al-4V alloy was largely dependent on crack extension resistance of microcracks preceding unstable fracture rather than crack initiation property.