Effects of conventional machining on high cycle fatigue behavior of the intermetallic alloy Ti47Al2Nb2Cr (at.%)

The TiAl based alloy, Ti47Al2Nb2Cr (at.%), was used as a model system to explore effects of machining-induced surface deformation on fatigue behavior of an intermetallic alloy with limited ductility. Conventional machining processes, such as grinding and turning, harden TiAl to depths ranging from 40 to 180 μm. Turning doubled the hardness to approximately 500 VHN in the outer 20 μm, and hardness increased at least 50 VHN to a depth of 180 μm. The deformed layer formed during machining recrystallized after 1 h at 760°C. Axial fatigue tests were performed by step loading every 106 cycles through a fixed set of stress levels until failure. At room temperature, mean fatigue strength was not affected by surface condition. The outer 20 μm of the electropolished surface hardened to the same level as the turned samples during the fatigue test. After hardening, these two surfaces would have similar crack initiation resistance and, therefore, similar fatigue strengths. At 760°C, turning improved the average fatigue strength by 5%, and the average life at the final stress level by about 1.5 orders of magnitude. Fatigue resistance of the turned samples was improved by formation of a continuous, crack initiation resistant, recrystallized layer in the outer 30–50 μm during the test.