Compression and Compressive Creep Behaviors in Titanium Aluminides Alloyed with Vanadium comprizing Gamma + Beta Dual Phase Microstructures

Abstract

AbstractFine grained gamma+beta dual phase microstructures were obtained in aluminum-titanium-vanadium ternary alloys containing 40 at.% aluminum and 60 at.% (titanium and vanadium). Average grain size was about 5, 3, and 2 micrometers in the recrystallized materials containing 20, 30, and 40 at.% vanadium, and the phase constitution was around 50vol.% gamma phase and 50vol.% beta phase. Compression behavior was investigated at temperatures ranging from the room temperature up to 1200K, and compressive creep tests were carried out at 1050-1200K in order to characterize the temperature and chemical composition dependences of strength and deformation. These gamma+beta microduplex materials showed very high strength at room temperature; 0.2% proof stress was around 1200MPa. The compression deformability decreased from about 0.2 to 0.05 true strain with increasing vanadium content. The grain size effect was not clarified yet, but it was rather disappointing that smaller-grained material with Al40Ti20V40 composition did not show effective improvement either in strength or in deformability. 0.2% proof stress showed a considerable weakening at temperatures higher than 900K; the onset temperature of softening became lower as the vanadium content increased. In Al40Ti40V20 material with about 5 micrometer grains, both gamma and beta grains were flattened up to 1000K, however, above 1100K the gamma grain showed no significant shape change even after a heavy deformation. This is probably because the gamma grains were relatively stronger than the beta grains. The gamma grains showed tendency toward agglomeration, which is similar to rafting of precipitate particles. Surface relief was observed after high temperature deformation suggesting activity of boundary sliding on grain boundaries and interfaces. Compressive creep behavior was investigated under a constant true stress in vacuum. Creep curves consisted of a small amount of normal primary transient, the minimum creep rate region, and a steady or slightly accelerating creep region. Stress exponent decreased to about 2 with decreasing vanadium content. It was rather unexpected that smaller grained Al40Ti20V40 material showed larger stress exponent around 3.