| 1. |
- Zhang, Juqing, et al.
(author)
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Super-enhancers conserved within placental mammals maintain stem cell pluripotency
- 2022
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In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences (PNAS). - 0027-8424 .- 1091-6490. ; 119:40
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Journal article (peer-reviewed)abstract
- Despite pluripotent stem cells sharing key transcription factors, their maintenance involves distinct genetic inputs. Emerging evidence suggests that super-enhancers (SEs) can function as master regulatory hubs to control cell identity and pluripotency in humans and mice. However, whether pluripotency-associated SEs share an evolutionary origin in mammals remains elusive. Here, we performed comprehensive comparative epigenomic and transcription factor binding analyses among pigs, humans, and mice to identify pluripotency-associated SEs. Like typical enhancers, SEs displayed rapid evolu-tion in mammals. We showed that BRD4 is an essential and conserved activator for mammalian pluripotency-associated SEs. Comparative motif enrichment analysis revealed 30 shared transcription factor binding motifs among the three species. The majority of transcriptional factors that bind to identified motifs are known regulators associated with pluripotency. Further, we discovered three pluripotency-associated SEs (SE-SOX2, SE-PIM1, and SE-FGFR1) that displayed remarkable conservation in pla-cental mammals and were sufficient to drive reporter gene expression in a pluripotency-dependent manner. Disruption of these conserved SEs through the CRISPR-Cas9 approach severely impaired stem cell pluripotency. Our study provides insights into the understanding of conserved regulatory mechanisms underlying the maintenance of plu-ripotency as well as species-specific modulation of the pluripotency-associated regula-tory networks in mammals.
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| 2. |
- Li, Xiaojie, et al.
(author)
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Ab initio calculations of mechanical properties of bcc W-Re-Os random alloys : effects of transmutation of W
- 2016
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In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 28:29
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Journal article (peer-reviewed)abstract
- To examine the effect of neutron transmutation on tungsten as the first wall material of fusion reactors, the elastic properties of W1-x-yRexOsy (0 <= x, y <= 6%) random alloys in body centered cubic (bcc) structure are investigated systematically using the all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA). The calculated lattice constant and elastic properties of pure W are consistent with available experiments. Both Os and Re additions reduce the lattice constant and increase the bulk modulus of W, with Os having the stronger effect. The polycrystalline shear modulus, Young's modulus and the Debye temperature increase (decrease) with the addition of Re (Os). Except for C-11, the other elastic parameters including C-12, C-44, Cauchy pressure, Poisson ratio, B/G, increase as a function of Re and Os concentration. The variations of the latter three parameters and the trend in the ratio of cleavage energy to shear modulus for the most dominant slip system indicate that the ductility of the alloy enhances with increasing Re and Os content. The calculated elastic anisotropy of bcc W slightly increases with the concentration of both alloying elements. The estimated melting temperatures of the W-Re-Os alloy suggest that Re or Os addition will reduce the melting temperature of pure W solid. The classical Labusch-Nabarro model for solid-solution hardening predicts larger strengthening effects in W1-yOsy than in W1-xRex. A strong correlation between C' and the fcc-bcc structural energy difference for W1-x-yRexOsy is revealed demonstrating that canonical band structure dictates the alloying effect on C'. The structural energy difference is exploited to estimate the alloying effect on the ideal tensile strength in the [0 0 1] direction.
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| 3. |
- Li, Xiaojie, et al.
(author)
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First-principles calculations of the cleavage energy in random solid solutions : A case study for TiZrNbHf high-entropy alloy
- 2022
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In: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 212, s. 111575-
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Journal article (peer-reviewed)abstract
- The {100} and (110) cleavage energies of body-centered cubic TiZrNbHf high-entropy alloy are calculated using two alloy models: special quasi-random structures (SQSs) and the coherent potential approximation (CPA). The projector augmented wave method, as implemented in the Vienna ab initio simulation package (VASP), in combination with SQSs is adopted to evaluate the impact of local lattice distortions, whereas the exact muffin-tin orbitals (EMTO) method is used in combination with both SQSs and CPA to study the effect of chemical disorder using rigid underlying lattices. The variations of the cleavage energy as a function of surface chemistry and structure from the EMTO and VASP calculations are consistent with each other. Furthermore, the cleavage energies from CPA are in good agreement with those from SQSs, confirming that an averaged supercell approach reproduces well the mean-field CPA results. The alloy's cleavage energies estimated by the rule of mixtures compare well with those from the direct calculations, and the surface chemistry dependence of the cleavage energies is mainly controlled by the number of Nb atoms in the surface terminal layers owing to the large cleavage energy of Nb metal.
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| 4. |
- Li, Xiaojie, et al.
(author)
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Understanding the mechanical properties of reduced activation steels
- 2018
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In: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 146, s. 260-272
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Journal article (peer-reviewed)abstract
- Reduced activation ferritic/martensitic (RAFM) steels are structural materials with potential application in Generation-IV fission and fusion reactors. We use density-functional theory to scrutinize the micro-mechanical properties of the main alloy phases of three RAFM steels based on the body-centered cubic FeCrWVMn solid solution. We assess the lattice parameters and elastic properties of ferromagnetic alpha-Fe and Fe91Cr9, which are the main building blocks of the RAFM steels, and present a detailed analysis of the calculated alloying effects of V, Cr, Mn, and W on the mechanical properties of Fe91Cr9. The composition dependence of the elastic parameters is decomposed into electronic and volumetric contributions and studied for alloying levels that cover the typical intervals in RAFM steels. A linear superposition of the individual solute effects on the properties of Fe91Cr9 is shown to provide an excellent approximation for the ab initio values obtained for the RAFM steels. The intrinsic ductility is evaluated through Rice's phenomenological theory using the surface and unstable stacking fault energies, and the predictions are contrasted with those obtained by empirical criteria. Alloying with V or W is found to enhance the ductility, whereas additional Cr or Mn turns the RAFM base alloys more brittle.
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| 5. |
- Yang, Zhibiao, et al.
(author)
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Theoretically exploring covalent bonding effect on deformability of B2/beta Ti (AlxNb1-x) phase
- 2021
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In: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 194
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Journal article (peer-reviewed)abstract
- We used density-functional theory to assess the electronic structure, elastic properties and planar fault energies of the B2 Ti(AlxNb1-x) (0.2 <= x <= 0.8) phase in relation to the composition and chemical ordering. We found that the covalent bonding becomes stronger for B2 Ti(AlxNb1-x) with higher Al concentration and long range order (LRO) parameter. Based on a universal ductile-to-brittle criterion by integrating Pettifor's Cauchy pressure with Pugh's modulus ratio, the deformability becomes less for Ti(AlxNb1-x) with higher Al concentration and LRO parameter, which is well correlated with the bonding character. Rice's ratio has an anti-correlation with Pugh's modulus ratio for Ti(AlxNb1-x). According to Rice's criterion, Ti(AlxNb1-x) with various Al concentration and LRO parameter are brittle in pure mode I loading, however, Nb-enriched disordered and low-ordered Ti(AlxNb1-x) may satisfy Rice's criterion for nucleation of dislocation and thus, are ductile in mode II or III loading. The hardness increases but the fracture toughness decreases obviously with increasing the degree of covalent bonding in Ti(AlxNb1-x).
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| 6. |
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| 7. |
- Li, Ruihuan, et al.
(author)
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Effects of Cr and W additions on the stability and migration of He in bcc Fe : A first-principles study
- 2016
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In: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 123, s. 85-92
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Journal article (peer-reviewed)abstract
- Density functional theory calculations have been performed to study the effects of alloying Cr and W on the stability and diffusivity of interstitial He impurity in body-centered cubic (bcc) Fe host lattice. The interaction between two close Cr/W atoms is repulsive. The relative stable position for an interstitial He remains the tetrahedral interstitial site in the presence Cr. Energetically, He prefers to locate far away from W inside Fe host lattice due to the strong repulsive interaction between He and W. On the other hand, the He migration barrier becomes lower in the presence of Cr (0.026 eV) and W(0.049 eV), as compared to 0.059 eV for pure Fe. Addition of Cr is benefit for He self-trapping, while W is against. The effective diffusivity of He decreases with increasing Cr and W concentrations. Moreover, the additions of Cr and W slightly hinder He being trapped by monovacancy.
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| 8. |
- Li, Xiaojie, et al.
(author)
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First-principles study of crystal-face specificity in surface properties of Fe-rich Fe-Cr alloys
- 2019
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In: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 3:3
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Journal article (peer-reviewed)abstract
- A density-functional theory investigation of the (100) and (110) surfaces of the body-centered cubic (bcc) Fe1-xbCrxb binary alloys, x(b) <= 15 at.%, is reported. The energies and segregation energies of these surfaces were calculated for chemically homogeneous concentration profiles and for Cr surface contents deviating from the nominal one of the bulk. The implications of these results for the surface alloy phase diagram are discussed. The surface chemistry of Fe-Cr(100) is characterized by a transition from Cr depletion to Cr enrichment in a critical bulk Cr composition window of 6 < x(b) < 9 at.%. In contrast, such threshold behavior of the surface Cr content is absent for Fe-Cr(110) and a nearly homogeneous Cr concentration profile is energetically favorable. The strongly suppressed surface-layer relaxation at both surfaces is shown to be of magnetic origin. The compressive, magnetic contribution to the surface relaxation stress is found to correlate well with the surface magnetic moment squared at both surface terminations. The stability of the Cr surface magnetic moments against bulk Cr content is clarified based on the surface electronic structure.
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| 9. |
- Li, Xiaojie, et al.
(author)
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Generalized stacking faults energies of face-centered cubic high-entropy alloys : A first-principles study
- 2022
-
In: Intermetallics (Barking). - : Elsevier BV. - 0966-9795 .- 1879-0216. ; 145, s. 107556-
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Journal article (peer-reviewed)abstract
- Developing high-strength and ductile face-centered cubic (fcc) high-entropy alloys (HEAs) has attracted significant attention. The generalized stacking fault energy (GSFE) is a very useful concept to describe stable and unstable planar defects and their energies on a slip plane. It plays an essential role in designing high performance fcc HEAs and understanding the nanoscale plasticity phenomena. In this work, using first-principles simulations, we investigate the configuration-averaged GSFEs of 29 single-phase fcc HEAs and identify indicators that can be used to tune stacking fault energies. First we determine the equilibrium structural parameters for all considered alloys and compare them with available experimental data. With the obtained GSFEs, we analyze the relationship between the stacking fault energies and materials properties, and investigate scaling relations between planar fault energies and the tendencies to exhibit deformation twinning and transformation to hexagonal close-packed martensite. We find that unstable SFE and shear modulus correlates strongly. Moreover, we reveal that the ratio of intrinsic SFE to unstable SFE, gamma isf/gamma usf, is a characteristic materials measure, and the tendencies to twinning and martensitic transformation rank with it. Our results are expected to be useful for an efficient alloy design and selection of solutes in fcc HEAs.
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| 10. |
- Li, Xiaojie, et al.
(author)
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The influence of temperature on the elastic properties of body-centered cubic reduced activation steels
- 2021
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In: Materials & design. - : Elsevier BV. - 0264-1275 .- 1873-4197. ; 197
-
Journal article (peer-reviewed)abstract
- A first-principles based modeling approach to the effect of temperature on the isothermal single-crystal and polycrystalline elastic parameters of Fe-rich solid solutions is reported. The approach integrates alloy theory for chemical and magnetic disorders with accessible experimental data for the equilibrium volume and ferromagnetic phase transition, and is adopted to predict the temperature-dependent elastic parameters of the body-centered cubic phase of three reduced activation steels, CLAM/CLF-1, F82H, EUROFER97, considered as high-temperature material in power reactors. The predictions are assessed based on available experimental data for a reduced activation steel and both experimental and theoretical data for pure Fe. Alloying effects on the elastic constants relative to pure Fe are found to differ in the magnetically ordered and disordered phases. Contributions due to loss of long-range magnetic order, volume expansion, and entropy are important in determining the temperature dependence of the elastic parameters in all investigated materials. A previously reported, peculiar magneto-volume phenomenon on the equation of state in pure Fe is gradually removed by alloying and magnetic disordering, which requires particular attention when describing the thermo-chemical effects derived from the equation of state in Fe-rich solid solutions.
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