CREEP-PROPERTIES OF ALPHA-2 TYPE TI-AL-SN-ZR-NB-SI ALLOYS DESIGNED WITH THE AID OF THERMODYNAMICS(ALPHA)

Effects of microstructural and compositional factors on creep and room temperature tensile properties of alpha + alpha2 titanium alloys were examined by using five Ti-Al-Sn-Zr-Nb-Si alloys designed by thermodyn amic calculation to have the volume fraction of alpha2 precipitate (Va lpha2) of 0.1 or 0.2. Increase in the creep strength due to additions of Nb and Si was confirmed in beta treated alloys. In the relationship between the creep strength and the amount of primary alpha phase (Val pha(p)), the peak was observed for the alloys GT-88, 89, and 90, while the creep strength of the GT-91 decreased rapidly with increasing Val pha(p), and that of the GT-79 was not changed so much with the change in Valpha(p). The observed dependence of Valpha(p) on the creep streng th seemed to result from variations of the Valpha2 and the grain size due to the heat treatment. In a room temperature tensile test, remarka ble improvements in elongation were obtained by the alpha + beta solut ion treatment in alloys having Valpha2 of about 0.1. From the present results, the Valpha2 of about 0.1 at 823 K combined with the alpha + b eta solution treatment is proposed as the best design condition for al pha + alpha2 type high temperature titanium alloys.