Abstract
Titanium alloys are expected to be used as heat-resisting structural materials in the airplane and automotive industries. In this study, the creep properties of near-α Ti alloys consisting of a lamellar microstructure were studied. Ti–8.5wt%Al–8.0wt%Zr–2wt%Mo–1wt%Nb–0.15wt%Si alloy (alloy code, TKT34) and an alloy with 0.1 wt% of added boron (alloy code, TKT35) were used in this study. An ingot was hot forged at a temperature of 1,403 K and hot rolled (caliberrolling) at a temperature of 1,273 K to a reduction rate of approximately 90%. It then underwent solution treatment in a β single-phase region followed by air cooling. Finally, it was subjected to aging treatment for 28.3 ks at a temperature of 863 K and then air-cooled. Two solution treatment conditions were applied: a time of 1.8 ks at a temperature of 1,323 K (high temperature/short time (HS)) and a time of 3.6 ks at a temperature of 1,223 K (low temperature/long time (LL)). The average grain size of the prior β grains showed a tendency of the solution treatment temperature being low and the boron-added alloys tending to be small. The length and thickness of the lamellar of these alloys shortened or thinned owing to the addition of boron and at a low solution treatment temperature. The creep tests were carried out at an applied stress of 137 MPa and a temperature of 923 K in air. The creep rupture life of these alloys was excellent, in order of TKT35 (LL) < TKT34 (LL) < TKT35 (HS) ≦ TKT34 (HS). Therefore, the creep rupture life of these alloys was shown to be superior under the HS solution treatment condition as compared to the LL solution treatment condition. However, the minimum or steady-state strain rate of these alloys became slower in order of TKT 35 (LL)> TKT34 (LL)> TKT34 (HS) ≧ TKT35 (HS). The creep properties depended on the microstructure of the alloys.
