Effect of hybrid surface treatment composed of plasma nitriding and fine particle bombarding on fatigue strength of Ti-6Al-4V alloy

Abstract

This study investigated the effect of hybrid surface treatment on the fatigue strength of Ti-6Al-4V alloy. The hybrid surface treatment was composed of plasma nitriding and fine particle bombarding (FPB). Plasma nitriding generated a hardened layer with no marked growth of the microstructure of the substrate, and subsequent FPB eliminated a compound layer formed outermost through plasma nitriding. Tensile strength was maintained at the initial level, although ductility was somewhat reduced by fracture of the hardened layer. Plasma nitriding decreased the fatigue strength because the brittle compound layer fractured at relatively low stress amplitude and hastened the propagation of cracks. On the other hand, the effect of the hardened layer appeared through the elimination of the compound layer, so that hybrid surface treatment greatly improved the fatigue strength. Correspondingly, a crack initiation site was found beneath the hardened layer and suggested that the formed layer strongly suppressed crack initiation from the surface. © 2012 The Japan Institute of Metals.