Abstract
The Ti–Al–Ta intermetallic alloys are potential candidates for high temperature structural applications. This study concentrates on the microstructure refinement and high temperature mechanical properties of a Ti–46Al–8Ta alloy by N addition up to 2 (at. %). The N-bearing alloys were fabricated by vacuum are remelting, followed by hot isostatic pressing, homogenization, and solution treatments. Microstructures were characterized by XRD, OM, SEM, and TEM techniques; while mechanical properties were evaluated by hardness measurement and small punch testing. Phase transformations were determined by DTA measurements and Thermo-Calc computations. The results showed that increasing N influenced considerably morphology of the as-cast alloys, changed solidification path, and resulted in a significant microstructure refinement. The maximum small punch load at 850 °C of the fully lamellar structures enhanced at the expense of displacement at onset of fracture with increasing N content up to 1.05 at. %, beyond which the maximum load was dropped due to Ti2AlN precipitation. The hardness increased monotonically with increasing N content. The fracture surface of the low-N alloys revealed crack-tip plastic deformation, crack-bridging ligaments and plasticity around crack tips, while that of the high-N alloys showed cleavage cracks which were responsible for dramatic drop in strength.
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