Understanding of grain boundary embrittlement and its correlation with polycrystalline tungsten fracture
Abstract
Grain boundary (GB) embrittlement has been repeatedly reported as one of the important factors affecting fracture of refractory metals. It has been suggested that the GB embrittlement is due to ductility loss by material addition and its segregation. While there are a few hypotheses available, one of the guiding explanations is that mechanism of GB embrittlement by segregation is from creation of a barrier to dislocation propagation by formation of a hardened separation region near the boundary. In the case of tungsten (W) - nickel (Ni) alloy, ductility loss by Ni addition is known to be not related to the change of grain size. However, geometry of GBs is found to have high dependence on the GB embrittlement. GBs in W-Ni alloy are found to have thickness as a function of the level of saturation of W atoms with respect to Ni atoms in the GBs. The present work focuses on both atomic scale and continuum scale analyses with focus on understanding one question: How chemical additional in GBs affects microstructure dependent fracture? The study at atomic scale examines (110)-(210) W GB mechanical strength as a function of thickness using an ab initio calculation framework based on Car-Parrinello molecular dynamics (CPMD) simulations. The atomic fraction of Ni atoms is varied to understand the influence of Ni addition and its correlation with thickness variation on the GB mechanical strength. Based on the analyses performed, an analytical relation to predict GB peak tensile strength as a function of the GB cohesive energy, GB thickness (level of saturation), and the Ni atomic fraction is proposed. Thereafter extended finite element method (XFEM) simulations are performed at continuum scale, with an account of obtained GB peak tensile strength at atomic scale, to understand crack propagation through GBs at continuum length scale. These analyses are used to predict GB embrittlement through a quantitative expression based revised brittleness index. In order to predict influence of length scale change, the definition of brittleness index is revised. Based on the understanding of GB embrittlement, microstructure dependent fracture in a range of W microstructure is examined. For this purpose, crack propagation and fracture toughness of a variety of curved interface is studied in order to analyze influence of GB angle inside microstructure on crack propagation. For this purpose a combined XFEM- cohesive finite element method (CFEM) based strategy is pursued. By utilizing combined XFEM-CFEM simulation based approach, crack propagation path is correlated with GB inclination angle as well as GBs' mechanical properties. Based on the findings a criterion which determines failure type between inter-granular and trans-granular fracture in W microstructures is proposed. Furthermore, a method of estimating GB fracture strength based on W microstructure images is introduced.
Degree
Ph.D.
Advisors
Tomar, Purdue University.
Subject Area
Aerospace engineering|Mechanical engineering|Materials science
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