Abstract
The instrumented Charpy impact test, static and dynamic fracture toughness tests were carried out on Ti-6A1-2Sn-4Zr-6Mo alloy in which the prior, β grain size was variously changed by heat treatments. Then, the effect of microstructure on the toughness was examined.
The elongation, crack initiation and propagation toughness increased with the slight decrease in strength in the specimens with the increased prior β grain size with prolonged solution treatment time in the β region. In particular, the crack propagation toughness increased remarkably. The colony size, width of grain boundary α, width and spacing of widmanstätten α also increased, but the subcolony spacing decreased with the increase in the prior β grain size.
The increase in the crack initiation toughness was mainly caused by the increase in the widmanstätten a lath or lath spaing. The increase in the crack propagation toughness was caused by the deflection of the crack pass, which was brought by the decrease in the intersubcolony spacing. The intersubcolony spacing decreased with the increase in the number of a nucleation during diffusion-controlled α→β transformation; such nucleation sites increased with the increase in the prior, β grain size. In such situation, α nucleated in the interior of the β grain and it increased its number by the introduction of the working strain.
