Investigation of α phase evolution and tensile fracture behavior of Ti5Al2Sn2Zr4Mo4Cr alloy fabricated by directed energy deposition

Abstract

α phase evolution and tensile fracture behavior of Ti5Al2Sn2Zr4Mo4Cr alloy fabricated by directed energy deposition (DED) were studied. Samples with a different number of layers were fabricated by the same processing parameters. Microstructure evolution was analyzed based on the thermal history calculated through finite element analysis and isothermal transformation kinetics. The results suggested that the samples exhibit various α phase characteristics. Two kinds of starting microstructures at different layers, single β phase and mixed α/β phases can be obtained depending on the deposited layer’s temperature and cooling rate. Further evolution of α phase is closely related to the consecutive thermal cycles with different durations and different temperature amplitudes. The in-situ scanning electron microscopy tensile tests were performed in the horizontal (perpendicular to the build direction) and vertical (parallel to the build direction) directions under the as-deposited and heat treatment conditions. The results showed that the cracks and micro-voids usually initiate on the grain boundaries or α/β interface. α phase characteristic plays an important role in deformation behavior. The sample containing fine α laths usually fractured quickly after the first surface crack was observed. The coarse α laths resulted in a larger crack tip plastic zone and delayed the crack propagation.