Anomalys in the Tensile Properties at Elevated Temperatures of Metastable Beta Titanium Alloys

Two anomalys in the tensile properties at elevated temperatures were observed in the Ti-Mo base metastable beta titanium alloys annealed in the beta field. The one is the existence of a peak on the flow stress-temperature curve in the vicinity of 400°C and the other is the existence of a serration on load-elongation curve in the range of 200° to 400°C. With the objective of procuring more detailed information for these two anomalys, the tensile tests at elevated temperatures were carried out on metastable beta titanium alloys of different stability such as Ti-12Mo-6Sn, Ti-11.5Mo-6Zr-4.5Sn, Ti-15Mo-0.2Pd, Ti-15Mo-5Zr, Ti-15Mo-5Zr-3Al, Ti-15Mo-5Zr-3Al-2V, and Ti-13V-11Cr-3Al. At the same time the effect of strain rates and annealing conditions on these two anomalys were examined. From the results it becomes evident that these two anomalys are not observed in a rather stable beta alloy, Ti-13V-11Cr-3Al, and Ti-15Mo-5Zr, prestabilized by annealing at 400°C or 500°C for 8 hr before tensile testing. The new type of serration observed in this study is different from the one due to the Portevin-Le Chatelier effect which has been suggested for mild steel and aluminium alloys. It is concluded from this investigation that both the peak of flow stress at 400°C and the serration on the load-elongation curve in metastable beta titanium alloys are due to the transformation of the metastable beta to the omega phase which took place during deformation.