Effect of heat treatment at the temperature above β‐transus on the microstructures and fatigue properties of pure Ti

Abstract

Commercially pure Ti (CP-Ti) was heat treated at 1273 K (1000 C), which was higher than β-transus temperature, followed by water quenching. The dwell time at 1000 C was changed from 1 to 240 h. The heat treatment at 1000 C resulted in α grain coarsening, whereas martensitic transformation occurred due to the rapid cooling from β phase. The hardness increased by the martensitic transformation, where higher hardness was achieved by longer dwell time at 1000 C. However, the fatigue strengths of the heat-treated CP-Ti were lower than that of the as-received one. The fatigue strength of the specimen with the shortest dwell time of 1 h was the lowest among the heat-treated samples with different dwell times. Untransformed coarse α grains were seen in the heattreated specimens, which resulted in the lower fatigue crack initiation resistance and fatigue strength than the as-received specimen.

Effect of a Heterogeneous Nitrogen Diffusion Phase on Four-Point Bending Fatigue Properties in Commercially Pure Titanium

“…The present authors have previously reported 27,34,3740) that a relatively coarse microstructure with low strength was the predominant crack initiation site in CP titanium and a Ti6Al4V alloy with a bimodal grain size distribution during fatigue tests. Uematsu et al 41) reported that a fatigue crack was initiated at the untransformed coarse grains in the CP titanium heat-treated at a temperature above the ¢ transus. These studies indicate that the weak region, which is the un-nitrided region with low hardness and low nitrogen content in the present study, acts as a crack initiation site in CP titanium having a heterogeneous microstructure.…”