Computer Simulation of Hydride Precipitation in Bi-crystalline Zirconium
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Computer Simulation of Hydride Precipitation in Bi-crystalline Zirconium X.Q.Ma , S.Q.Shi@ , C.H. Woo and L.Q.Chen† Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong † Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA ABSTRACT γ-hydride precipitation and growth in a zirconium bi-crystal were simulated using a phase field kinetic model. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg-Landau equations for the structural variables and the Cahn-Hilliard diffusion equation for the concentration variable. The morphology evolution of γ-hydride with and without external load was simulated. It is demonstrated that nucleation density of the hydride at the grain boundary increases as compared to the bulk and favorable hydride precipitation at the grain boundary become weaker when an external load is applied. INTRODUCTION A special case of phase transformations, hydride precipitation in zirconium is chosen for the current study. Zirconium is a primary structural material in nuclear power industry owing to its combination of good mechanical properties, excellent corrosion resistance and low neutron absorption cross-section. However, zirconium gradually pickup hydrogen from environment during service. At a certain level of hydrogen concentration, hydride will form. Since the brittleness of the hydride, the mechanical properties of the material will degrade, and fracture initiation at the hydride may occur[1]. It can be very expensive to study experimentally the morphological evolution of hydride precipitates in irradiated materials because of the cost involved for irradiation protection. Unfortunately, such expensive test is still the only way to study the evolution of hydride morphology in irradiated materials. Computational materials engineering methods have advanced significantly in recent years [2]. Among them, the phase-field kinetic model is an effective model in describing the morphological evolution during phase transformation. It has been successfully applied to study the morphology of the second phase precipitation in many materials [3-7]. It is now possible to apply computer simulation to acquire characteristics of hydride pattern formation, or to have better understanding of the mechanism and to predict the properties and morphology of new phases. Zirconium has a hexagonal close-pDFNHG VWUXFWXUH 7KH K\GULGH LV IRUPHG DV D UHVXOW RI either high rates of cooling or long-hold at low temperature, and has a face-centered tetragonal structure. They appear needle-like with axis along three < 1120 > directions [8]. The γ-hydride precipitation under an applied load in a zirconium single crystal has been investigated [9] using the phase-field model based on the elasticity theory of Khachaturyan [10]. The morphological evolution process of this system is in well accordance with the experimental observations [8]. @
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