Evaluating the effects of PIRAC nitrogen-diffusion treatments on the mechanical performance of Ti–6Al–4V alloy

Powder Immersion Reaction Assisted Coating (PIRAC) is a relatively simple nitrogen diffusion based process which has been proposed as a technique capable of considerable improvements in the tribological performance of ceramics and metals alike; however, the necessary exposure of the substrate material to high temperatures for several hours may have an adverse effect on the bulk properties of materials such as titanium alloys. The effect of PIRAC treatments on the bulk metallography and mechanical properties of Ti–6Al–4V has been studied. Following PIRAC nitrogen-diffusion treatment, studies using X-ray diffraction and cross-sectional microscopy have shown evidence of the formation of a thin (~1.4µm) TiN/Ti2N layer, together with the presence of some Ti3Al intermetallic phase. Semi-logarithmic S–N plots show a deleterious effect after PIRAC treatment in terms of material cyclic fatigue strength, particularly at higher treatment temperatures. Samples processed at 800°C for 4h however exhibit better fatigue performance than others treated at lower temperatures for longer nitriding times. Fractographic inspection has shown that fatigue cracks originate at (or near) the surface for the untreated Ti-alloy and from the subsurface regions following diffusion treatment, owing to the build-up of compressive stresses in the latter, which hinder crack propagation.