Fatigue improvement in low temperature plasma nitrided Ti–6Al–4V alloy

In this study a low temperature (600 °C) treatment was utilized to improve the fatigue performance of plasma nitrided Ti–6Al–4V alloy by optimization of microstructure. In order to study the fatigue properties, rotation bending tests were conducted, the S–N curves were constructed, and the results were compared with those obtained by an elevated temperature treatment (900 °C) as well as conventional gas/plasma nitriding treatments reported in literature. The plasma nitrided alloy at 600 °C showed an endurance limit of 552 MPa which was higher than those achieved by conventional nitriding treatments performed at 750–1100 °C. In contrast, plasma nitriding at 900 °C resulted in the reduction of fatigue life by at least two orders of magnitude compared to the 600 °C treatment, accompanied by a 13% reduction of tensile strength and a 78% reduction of ductility. The deterioration of mechanical properties after the elevated temperature treatment was attributed to the formation of a thick compound layer (~6 µm) on the surface followed by an α-Case (~20 µm) and phase transformation in the bulk microstructure from fully equiaxed to bimodal with coarse grains (~5 times higher average grain size value). The microstructure developed at 600 °C consisted of a thin compound layer (<2 µm) and a deep nitrogen diffusion zone (~45 µm) while the bulk microstructure was maintained with only 40% grain growth. The micromechanisms of fatigue failures were identified by examination of the fracture surfaces under a scanning electron microscope (SEM). It was found that fatigue failure in the plasma nitrided alloy initiated from the surface in the low cycle region (N≤105 cycles) and propagated in a ductile manner leading to the final rupture. No failures were observed in the high cycle region (N>105 cycles) and the nitrided alloy endured cyclic loading until the tests were stopped at 107 cycles. The thin morphology of the compound layer in this study restricted the extent of premature crack initiation from the surface. Moreover, a deep diffusion zone with a well-bonded interface decreased the likelihood of fatigue initiation at (or below) the compound layer interface. Another notable feature was that the fatigue strength of the nitrided alloy was correlated with the surface roughness and in fact when the nitrided surfaces were polished, a higher number of cycles were dedicated to the formation of fatigue cracks compared to the as-treated condition resulting in an improved fatigue life.