Effect of Silicon Addition on Microstructures and Mechanical Properties of a Heat Resistant Titanium Alloy DAT54

摘要

Daido Steel had developed a heat resistant titanium alloy DAT54 (Ti-5.8Al-4.0Sn-3.5Zr-2.8Mo-0.7Nb-0.35Si-0.06C), which is modified IMI834 and superior to it in serviceable temperature limit. It contains 0.35 percent Si as one of crucial alloying elements to precipitation-strengthen the alloy. In this study the effect of Si up to 0.95 percent was reinvestigated on its micro-structure and mechanical properties. The silicon addition doesn't affect its liquidus, solidus nor beta-transus significantly, while it raises the solvus of titanium silicide, (Ti,Zr)_5Si_3, drastically over 1400K for 0.95 percent Si. The higher silicon increases the amount and the size of silicides retained at the solution treatment temperatures, beta-transus-35K. They tend to reduce the size of the pre-beta grains and the subsequent acicular colonies. The silicon improves tensile strength, creep-rupture life and fatigue limit, but the excessive silicon degrades tensile ductility and creep-rupture life. The Si content, 0.34 percent, is optimum to obtain the well-balanced properties. It is close to the solubility limit at the solution temperature and also to the nominal composition in DAT54.