Effects of volume fraction and grain size on creep characteristics of .ALPHA./.BETA. titanium alloys.

Abstract

Effects of volume fraction of α phase and grain size on the creep properties of a series of two-phase titanium alloys were examined. The chemical compositions of α and β phases of the alloys were designed to be constant.The alloys were quenched after solution-treatments for 1, 100, and 1 000 h at 1 173 K, followed by aging for 4 h at 773 K. The volume fraction of α phase (VFAP) was varied from 17 to 73%. Creep rupture test was carried out at 392 MPa and 773 K in air.The creep rupture life was the longest at 50 % of VFAP, and the shortest at the minimum VFAP when the specimens were solution-treated for 1 and 100 h. It increased with VFAP when solution-treated for 1 000 h. The steady state creep rates decreased with the increase in VFAP. This might be due to the higher creep resistance of the α phase. Irrespective of the wide range of the average grain sizes, the steady state creep rates become insensitive of grain size as VFAP decreased. This would be attributed to grain boundaries of fine α precipitates generated in the prior β phase. The nucleation of voids occurred in most cases at the α/β and α/α interfaces, and within α phase. The void growth rate decreased with the volume-fraction-ratio of β and α phases and increased with the average transverse grain size of α phase.