FATIGUE-CRACK INITIATION AT ND-RICH PARTICLES IN AN ND CONTAINING HIGH-TEMPERATURE TITANIUM-ALLOY

Scanning electron microscopy (SEM) and surface replica method have bee n employed to study the micromechanism of fatigue crack initiation at Nd-rich phase particles in a high temperature titanium (Ti-55) alloy. It was found that the microcrack initiates near the equator of Nd-rich particles in the matrix. The microcrack grows first at an angle of ab out 45 degrees with respect to the tensile axis, and then its growth d irection becomes approximately normal to the tensile axis. The experim ental results are analyzed in terms of the elastic stress distribution around soft particles imbedded in the matrix to account for the exper imental findings of particle cracking and the associated surface micro crack initiation near the particle ''equator.'' A model of fatigue cra ck initiation at a soft surface particle is proposed.