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A first-principles investigation of refractory alloy systems united by a common computational framework

Description

Abstract:
The cluster expansion formalism for alloys is used to investigate phenomena in several refractory alloys: Co3W (superalloy), NbTiVZr, HfNbTaTiZr, and AlHfNbTaTiZr (high entropy alloys). Density functional theory (DFT) is used to calculate the energies of many small structures which are used to construct a cluster expansion model for each alloy system. These models are then used as Hamiltonians for Monte Carlo simulations in order to explore the dependence of the antiphase boundary (APB) energy on temperature and Al and Hf impurity concentration in metastable FCC Co3W as well as phase segregation behavior in NbTiVZr, HfNbTaTiZr, and AlHfNbTaTiZr. First, the effects of Al and Hf impurities on the (111) antiphase boundary (APB) energy of metastable FCC Co3W are investigated via ab initio calculations. It is found that Hf increases the APB energy far more than Al, particularly at higher concentrations of the impurity, and both systems exhibit little variation with respect to temperature. It is further shown that at higher concentrations of Hf, and most noticeably for Co3(W0.5Hf0.5), Hf and W tend to segregate into alternating planes, unlike the corresponding Co3(W0.5Al0.5), which explains the different impacts of the two impurities on the APB energy. Finally, the ratio of (111) to (100) APB energies is studied for sacrificial W compositions to understand cross slip behavior in both ternary systems. Second, the phase segregation behavior of three key refractory high entropy alloys (NbTiVZr, HfNbTaTiZr, and AlHfNbTaTiZr) is studied using first-principles calculations. Phase segregation and intermetallic phases documented in the experimental literature are reproduced in all three high entropy alloys and thermodynamic integration is used to calculate the configurational entropy of these alloys as a function of temperature. Our results show that at low temperatures, the configurational entropy of these materials is largely independent of N, which suggests that alloy design guidelines based on the ideal entropy of mixing require further examination.
Notes:
Thesis (Ph. D.)--Brown University, 2021

Citation

Nataraj, Chiraag M., "A first-principles investigation of refractory alloy systems united by a common computational framework" (2021). Mechanics of Solids Theses and Dissertations. Brown Digital Repository. Brown University Library. https://repository.library.brown.edu/studio/item/bdr:ay8pgsa7/

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