Microstructure evolution and comprehensive mechanical properties of β/B2 processed Ti-22Al-23Nb-2（Mo，Zr） alloy

Abstract

Quinary Ti-22Al-23Nb-2(Mo, Zr) alloy has a broad application prospect in the aeroengine field. The temperature ranges of phase regions, β/B2 processing, heat treatment and the corresponding mechanical properties were investigated by SEM, TEM and multi mechanical testing machines. The results show that the alloy goes through B2, B2+α2, B2+α2+O and B2+O four phase regions in the sequence from high temperature to low temperature, the corresponding critical temperatures are 1060 ℃, 950 ℃ and 930 ℃ respectively. The composition and morphology of the grain boundary phase are influenced by the collective effect of distortion degree of B2 grains and heat treatment. Coarse discontinuous α2 phase is easy to form at the boundaries of original B2 grains with high distortion degree, and is converted into α2+Orim structure if heat treated at B2+O phase region; The fine continuous O phase is inclined to form at the boundaries of dynamic recrystallized B2 grains with low distortion degree, which can be completely transformed into α2 if heat treated at B2+α2 phase region; no phase is formed at the boundaries of static recrystallized B2 grains. Precipitates within B2 grains are mainly influenced by heat treatment. α2 and O can directly and independently precipitate from B2, O can also attach α2 to form an α2/Orim core-shell structure. The α2/Orim core-shell structure indicates α2 has good stability at low temperature. After solid solution and aging treated, mixed microstructure with bi-size precipitates emerge within B2 grains, which can be coarse α2/Orim+ fine O or coarse OI+OII with the corresponding solution treatment at B2+α2 and B2+O respectively. The size distribution of B2 precipitated phase in grain is the main factor affecting the mechanical properties . Larger size is favorable for the ductility and toughness of alloy, but unfavorable for strength and creep resistance.

Keywords

• ti2alnb alloy
• β/b2 processing
• heat treatment
• microstructure evolution
• mechanical properties