Effect of microstructures on mechanical properties of heat resistant titanium alloys at elevated temperatures

The weight reduction of rotating and reciprocating automobile engine components such as intake and exhaust valves, valve springs and connecting rods has been vital to the improvement of engine performance. Titanium alloys are one of the candidate light weight materials for these. Among them, near alpha alloys, Ti-6242S (Ti-6Al-2Sn-4Zr-2Mo-0.1Si), which is applied to the compressor discs of aerospace jet engines and DAT54 (Ti-5.8Al-4.0Sn-3.5Zr-2.8Mo-0.7Nb-0.35Si-0.06C), which is a newly developed alloy in place of Ti-6242S, are promising heat resistant alloys for exhaust valves. As exhaust valves for an automobile locally reach up to 1000 K, it is necessary to understand mechanical properties at elevated temperatures. However, their maximum operating temperature for titanium alloys for aerospace components has been limited to 873 K, and few data exist on their mechanical properties over 873 K. To apply Ti-6242S and DAT54 to exhaust engine valves, the effect of microstructures on tensile, fatigue and creep properties were evaluated from room temperature to 1073 K. In each alloy, the acicular material exhibited an excellent combination of all properties. The equiaxed structure with the highest fatigue strength at room temperature showed the lowest mechanical strength at 1073 K. The bi-modal material possessed strength that was between that of the acicular and the equiaxed materials. Mechanical properties of DAT54 at various temperatures were superior to those of Ti-6242S. DAT54 with acicular microstructure showed the best combination of all properties and it is the most applicable to exhaust valves.