| 1. |
- Li, Wei, et al.
(författare)
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First-principles prediction of the deformation modes in austenitic Fe-Cr-Ni alloys
- 2016
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 108:8
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Tidskriftsartikel (refereegranskat)abstract
- First-principles alloy theory is used to establish the gamma-surface of Fe-Cr-Ni alloys as function of chemical composition and temperature. The theoretical stacking fault energy (SFE) versus chemistry and temperature trends agree well with experiments. Combining our results with the recent plasticity theory based on the gamma-surface, the stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective stacking fault energy below similar to 18 mJ m(-2). Alloys with SFE above this critical value show both twinning and full slip at room temperature. Interestingly, twinning remains a possible deformation mode in addition to full slip even at elevated temperatures, in line with observations.
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| 2. |
- Makela, Jaakko, et al.
(författare)
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Comparison of chemical, electronic, and optical properties of Mg-doped AlGaN
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:50, s. 28591-28597
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogen, carbon, and oxygen are common unintentional impurities of Al(x)Ga(1−x)N crystals. This impurity structure and its interplay with Mg impurities in Al(x)Ga(1−x)N semiconductors are relevant to develop the p-type nitride crystals for various devices (e.g, LEDs, transistors, gas sensors) but are still unclear. Here we have investigated Mg-doped Al0.5Ga0.5N before and after postgrowth annealing with valence-band and core-level photoelectron spectroscopy, photoluminescence, and resistivity measurements. First, it is found that a surface part of the Al0.5Ga0.5N crystal is surprisingly inert with air and stable against air exposure-induced changes. Thus, the relatively surface-sensitive photoelectron spectroscopy measurements reflect in this case also the bulk crystal characteristics. The measurements reveal the presence of deep states up to 1 eV above valence-band maximum before and after the annealing and that oxygen and carbon occupy N lattice sites (i.e., ON and CN). The model where CN-induced acceptor states in the band gap participate in the blue emission (photoluminescence) is supported. Furthermore, the presented Mg 2p core-level spectra demonstrate that part of Mg atoms forms direct bond(s) with oxygen in the bulklike structure of Al0.5Ga0.5N and that the chemical environment of Mg atoms is much richer than was expected previously.
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| 3. |
- Tian, Li-Yun, et al.
(författare)
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Exchange-Correlation Catastrophe in Cu-Au : A Challenge for Semilocal Density Functional Approximations
- 2016
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 117:6
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Tidskriftsartikel (refereegranskat)abstract
- Semilocal density functional approximations occupy the second rung of the Jacob's ladder model and are thus expected to have certain limits to their applicability. A recent study [Y. Zhang, G. Kresse, and C. Wolverton, Phys. Rev. Lett. 112, 075502 (2014)] hypothesizes that the formation energy, being one of the key quantities in alloy theory, would be beyond the grasp of semilocal density functional theory (DFT). Here, we explore the physics of semilocal DFT formation energies and shed light on the connection between the accuracy of the formation energy and the ability of a semilocal approximation to produce accurate lattice constants. We demonstrate that semilocal functionals designed to perform well for alloy constituents can concomitantly solve the problem of alloy formation energies.
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