Defect-based fatigue crack nucleation and strength evaluation of additively manufactured TiC/Ti6Al4V titanium matrix composite at different temperatures

Abstract

In mechanical applications, defect-based fatigue cracking is a significant strength-limiting failure mode of additively manufactured composites that is not yet fully understood in the service environment. In this paper, axial loading tests were carried out at 25 ℃ and 450 ℃, and then the effects of temperature on the long-life fatigue properties of the TiC-reinforced Ti6Al4V composite fabricated by Laser Powder Bed Fusion (LPBF) were investigated by combining the techniques of electron backscatter diffraction, X-ray computed tomography, and 3D morphological reconstruction. With the increase in temperature, the fatigue strength of the LPBF TiC/Ti6Al4V composite decreases significantly. The main causes of the interior failure at both temperatures are the inhomogeneous hardening zone (IHZ) formed by TiC agglomeration and the pore defect generated during the manufacturing process. The presence of brittle IHZ and pores causes stress concentration, which leads to crack nucleation. When a crack reaches the specimen surface, the combined effects of temperature and oxygen accelerate the ensuing crack propagation at high temperature. Based on the temperature effect, material properties, and the statistical distribution of defects, a model has been developed to predict the P-S-N curves at both temperatures. This model performed well in estimating the fatigue strength at 109 cycles. These findings provide new insights into the defect-related fatigue failure mechanisms and strength evaluation of the LPBF TiC/Ti6Al4V composite at different temperatures.