The role of β-phase on crack nucleation and propagation in dual phase zirconium polycrystals: a crystal plasticity finite element modeling

This paper aims to numerically investigate the nucleation and propagation of microcracks in dual phase Zirconium (Zr) containing both Hexagonal Close-Packed (HCP) α-Zr and Body Centered Cubic (BCC) β-Zr crystals. For this purpose, a subroutine that incorporates different damage criteria is coupled with a crystal plasticity finite element model to investigate the effects of crystals elastic and plastic anisotropy. Attention is given to the role of the BCC β-phase in the crack nucleation of notched zirconium polycrystals. First, the maximum shear strain accumulated on the predominant slip system is used as the crack initiation criterion. The modeling results reveal that for single phase HCP α-grains cracks lie on the prismatic planes, but for dual phase α/β cases, cracks may lie on either basal or prismatic planes depending on the α/β crystal orientations, and the adjacent β-phase features such as its thickness or distance from the notch. Moreover, numerical results indicate that the presence of thin layered β-phase hinders crack propagation, regardless of its geometrical or crystallographic features. The performance of other damage criteria is also discussed. Lastly, it is shown that in comparison to α-grains undergoing cyclic loads, the crack propagation rate is reduced in β-crystals.