Pan, Z. (2012). COMPARATIVE STUDY OF THE EFFECT OF IMPURITIES ON THE DUCTILITY OF TANTALUM AND TUNGSTEN BASED ON ATOMISTIC AND FIRST PRINCIPLES CALCULATIONS. Unc Charlotte Electronic Theses And Dissertations.
Tungsten and tantalum are neighbors in the Periodic Table of the Elements and, as refractory metals, both have very high melting points (tungsten: 3422oC, tantalum: 2996oC). However, the ductility of the two metals is quite different especially at commercially available purity levels. Commercial purity polycrystalline tungsten shows brittle behavior in room temperature tensile tests, and its ductile-to-brittle transition temperature (DBTT) can be as high as 400oC. In contrast, commercial purity polycrystalline tantalum shows completely ductile behavior at room temperature, and its DBTT can be as low as -250oC. Based on published work, it has been well accepted that the brittleness of commercial purity tungsten is attributed to weakened grain boundaries (GBs) by segregated impurities. However, this consensus is far less sufficient to elucidate why there is a remarkable difference in ductility between the two metals.In this work, based on the understanding that ductility is the competition between grain boundary (GB) separation and dislocation activities, we used density functional theory and molecular dynamics to systematically calculate the pristine and contaminated GB separation energy, the GB and dislocation core segregation energy of various impurities, and the effect of impurities on generalized stacking fault energy and Peierls energy of screw dislocations for tungsten and tantalum. The results show that for each impurity species, the GB and core segregation energies in tungsten are always significantly higher than those in tantalum, indicating that impurities in tungsten are more likely to segregate to GB regions and the vicinity of dislocation core to influence them. The binding energy difference between GB and free surface site for each impurity species in tungsten is always higher than that in tantalum, indicating that the presence of impurities, if deemed undesirable, will cause a greater reduction in GB separation energy for tungsten. In addition, tungsten is more sensitive to sulfur impurity concentration level inside the GB than tantalum. Although both literature and our work have shown that the ductility of pure tungsten is already lower than that of pure tantalum, the remarkable difference of impurity effect on between the two metals makes the ductility of tungsten suffers more from the deleterious impurities than the ductility of tantalum. The analyses of the chemical and mechanical effects of impurities suggest that tungsten is more sensitive to impurities because of its low lattice constant and thus small interstitial sites despite other possible causes. The calculations of the effect of impurities on the dislocation related properties are not adequate to compare the impurity effect on between tungsten and tantalum due to the inappropriately chosen reaction paths.