The influence of the microtexture and orientation of columnar grains on the fatigue crack growth of directed energy deposited Ti-6.5Al-2Zr-Mo-V alloys

Abstract

This work aims to investigate the influence of the orientation and microtexture of columnar grains on the fatigue crack growth of a Ti-6.5Al-2Zr-Mo-V titanium alloy fabricated by directed energy deposition. In this paper, the fatigue crack growth rate test in three sampling directions in a directed energy deposited Ti-6.5Al-2Zr-Mo-V titanium alloy using compact specimens was carried out. The crack length was measured visually, and the fatigue crack growth rate of the stable crack growth stage was obtained. During the test, the influence of the microstructure on the crack growth was directly observed. In addition, the complete crack front shape was indicated on the fracture surface by the marker load technique, and the crack growth behavior was obtained. An optical microscopy, a scanning electron microscopy and a laser confocal microscopy were used to observe and clarify the influence of the columnar grain boundary on the crack growth behavior and the interaction between the crack front and microstructure. The results show that the fatigue crack growth rate in the three sampling directions is different in the low ΔK region; the columnar grain boundary has no significant effect on the fatigue crack growth behavior, but the columnar grain itself has an effect on the fatigue crack growth behavior, which is indicated by the irregularity of the crack front shape in different columnar grains. Microhardness testing and electron backscattered diffraction were used to explain the above phenomena based on static and orientation characteristics. It was found that the microtexture and orientation of the columnar grains are responsible for differences in the crack growth rates, and the orientation of the columnar grains also determines the extent of the difference.