Design of Ultra-Strong As-Cast Titanium Alloy at 600 ℃ by Using Cluster Formula

Abstract

A Ti–5.4Al–6.4Zr–6.2Sn–0.4Mo–1.6W–0.4Nb–3.2Ta–0.5Si alloy is designed following cluster formula approach that achieves a strength level of 1 GPa at 600 ℃ in the as-cast state, superior to any existing high-temperature Ti alloys. Its composition is formulated by 17 basic units, α-{[Al-Ti₁₂](AlTi₂)}₁₂ + β-{[Al-Ti₁₂Zr₂](Mo_0.125Nb_0.125Ta_0.5W_0.25Sn_1.5Si_0.5)}₅, each unit covering a nearest-neighbor cluster plus a few next-neighbor glue atoms. This design is on the basis of the composition formula of Ti65 alloy, with an enhanced β stability via more Zr, Mo, Nb, Ta, W, Sn, and Si co-alloying. Upon copper-mold pour casting, this alloy shows a good microstructure stability. In tensile testing below at 650 ℃, its α plates thickness is nearly at the same level of 0.2 μm, which is much smaller than 0.7–0.8 μm of Ti65 at the same condition. The changes in volume fraction of β phase are increased by 86%, much less than by 105% in Ti65. Its room-temperature strength reaches the ultra-high-strength level, with an ultimate tensile strength of 1328 MPa and a yield strength of 1117 MPa, with a moderate elongation of 3.8%. At 600 ℃, its ultimate tensile strength of 1017 MPa and yield strength of 936 MPa are superior to those of any existing high-temperature Ti alloys, with an elongation of 7.2%. At 650 ℃, its ultimate tensile strength of 848 MPa still maintains a high level.