The microstructure and precipitated phase dependence of plastic deformation and fracture mechanism in repaired IN718 alloy by directed energy deposition

Abstract

This work studied microstructural evolution, precipitate behaviors, and micro-hardness distribution along the built direction of directed energy deposited IN718 alloy. In combination with digital image correlation and crystallographic orientation analysis, the plastic deformation and fracture mechanisms of coarse columnar grains were explored in detail. The columnar grains in the deposition zone (DZ) grow along the <001> direction in the crystallography and gradually widen with increasing built height. The misorientation angle per unit length within individual columnar grains first decreases and then slows down to ~ 0.07 °/μm when the deposition height is ~ 1.8mm. The micro-hardness in the DZ is gradually decreased along the built direction, which may be due to the increasing average columnar grain width and primary dendrite arm spacing, as well as reducing the volume fraction of the γ'' + γ' phases. During the uniaxial tensile loading, most plastic deformation always occurs in the DZ irrespective of the lowest hardness in the heat-affected zone (HAZ). The nucleation of microvoids is ascribed to the granular Laves phase debonding from the γ matrix. The number of microvoids gradually decreases with increasing distance from the fracture surface, which may be attributed to the strain-controlled nucleation of microvoids. EBSD results substantiated that the columnar grains near the fracture surface occur to severe lattice rotation, leading to the Kernel misorientation angle distribution in each grain interior increasing to about 1° ~ 4°.