Abstract
The fracture path follows grain boundaries (GB) in most metallic system under tensile test. In general, impurities, even in ppm concentration, that segregate to these boundaries can remarkably change materials mechanical properties. Predicting impurities segregation effects in Nickel super-alloys might not be seen as intuitive and perhaps more fundamental understanding is needed. We performed a density functional theory calculation to elucidate the effect of eight light elements (B, C, N, O, Al, Si, P and S) and twelve transition metal elements (Tc, Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta, W, Re) on Nickel ∑5(210) grain boundary formation and its Ni free surface. The effect of impurities was carefully examined by calculating different properties such as segregation, binding and cohesive energies, strengthening/embrittling potency and the theoretical tensile strength. Additionally, we employed the electron density differences and magnetic effects to explain why and how impurities such as B, S, V, Nb, Mn and W affect Nickel ∑5 GB. We used the generated data calculated on equal footing, to develop a fundamental understanding on impurity effect. A clear and strong correlation is found between difference in magnetic moment change between isolated and imbedded impurity atom on one hand and the tensile strength on the other hand. The higher the loss of the magnetic moment, the more the impurity consolidates the GB.
Highlights
Grain boundaries (GBs) are common type of defects in crystalline solids that strongly affect the physical and mechanical properties of materials
For the present of GB unit cells, we considered k-points sampling on different models as follow: 4 × 4 × 1 k-points for the 80-atom model and 3 × 5 × 1 for the 44-atom model
The comparison of our results conducted on the effect on Ni GB of eight light elements to those in the literature have shown a good agreement
Summary
Grain boundaries (GBs) are common type of defects in crystalline solids that strongly affect the physical and mechanical properties of materials. Since we are trying to understand the mechanisms and effects of different impurity atoms which involve very small energy differences, it is important to conduct calculations on equal discrepancies for all impurities i.e. using the same model and approximations for each type of targeted physical property calculation This should allow us to make sensible comparisons and draw meaningful conclusions. We have to mention that in order to get relevant and comparable total energies, and for each above calculated property for all considered impurities, we used the same calculation parameters, the same GB unit cell size and the same number of atoms per model This allowed us to calculate the energy difference when the impurity segregates to the surface, bulk and GB for each property. All calculations were carried out with P1 symmetry by forcing symmetry breaking of models, in order to get relevant and comparable total energies
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