Effect of Heat Treatment on Microstructures and Properties of Ti90 Alloy

The effects of annealing temperature, cooling rate and multiple annealing processes on microstructures, tensile properties and corrosion behavior of a new near-alpha titanium alloy-Ti90 were comparatively investigated. The results show that when specimens are annealed in the alpha+beta phase region, as the annealing temperature increases, the deformed structure gradually spheroidizes with a decreasing fraction of primary alpha phase (alpha(P)) and an increasing fraction of beta-transformation structure (beta(t)), and the secondary alpha phase (alpha(s)) precipitates and coarsens gradually. These changes result in the reduced strength and improved plasticity. When specimens are annealed, followed by air cooling, in the single beta phase zone, a fine lamellar microstructure with fairly coarse original beta grains is obtained, which results in a sharp decrease in the plasticity of the alloy. After water-cooling, the acicular alpha' martensite phase precipitates inside beta grains, which significantly improves the strength while maintaining good plasticity. With multiple annealing, the size of alpha(P) and beta(t) increases, and alpha(s) coarsens, leading to a simultaneous decrease in the both strength and plasticity. The polarization curve test results show that samples with four different microstructures all exhibit passivation behavior in 3.5% NaCl (mass fraction) solution with low passivation current densities, implicating a good corrosion resistance of Ti90 alloy. The corrosion resistance of different microstructures is in the order of bimodal microstructure>equiaxed microstructure>lamellar microstructure.