Basal-plane stacking-fault energies of Mg alloys: A first-principles study of metallic alloying effects

Generalized stacking-fault energies （GSFEs） of basal-plane stacking faults 11 and 12 in Mg alloys have been studied based on first-principles calculations, where 43 alloying elements were considered. It is found that the most contributing features of alloying elements to GSFEs are bulk modulus, equilibrium volume, binding energy, atomic radius and ionization energy. Both bulk modulus and ionization energy exhibit positive relationships with GSFEs, and the others show opposite relationships. Multiple regressions have been performed to offer a quantitative prediction for basal-plane GSFEs in Mg-X systems. GSFEs, alloying effects of elements and the prediction model established within this work may provide guidelines for new Mg alloys design with better ductility.

Synergistic Effect of Alloying Atoms on Intrinsic Stacking-Fault Energy in Austenitic Steels

Intrinsic stacking-fault energy is a critical parameter influencing the various mechanical performances of aus- tenitic steels with high Mn concentrations. However, quantitative calculations of the stacking-fault energy （SFE） of the face-centered cubic （fcc） Fe, including the changes in concentrations and geometrical distribution of alloying atoms, cannot be obtained by using previous computation models. On the basis of the interaction energy model, we evaluated the effects of a single alloying atom （i.e., Mn, A1, Si, C and N）, as well as its aggregates, including the Mn-X dimer and Mn2-X trimer （X = A1, Si, C and N） on the SFE of the fcc Fe via first-principle calculations. Given low concentrations （〈10 wt%） of alloying atoms, dimers and trimers, theoretical calculations reveal the following： （1） Alloying atom Mn causes a decrease in the SFE, whereas A1, Si, C and N significantly increase the SFE; （2） combination with other alloying atoms to form the Mn-X dimer （X = A1, Si, C and N） exerts an effect on SFE that, to a certain extent, is close to that of the corresponding single X atom; （3） the interaction between Mnz-X and the stacking fault is stronger than that of the corresponding single X atom, inducing a significant increase in the SFE of fcc Fe. The theoretical results we obtained demonstrate that the increase in SFE in high-Mn steel originates from the synergistic effect of Mn and other trace alloy atoms.

Influence of Neighboring Layers on Interfacial Energy of Adjacent Layers

The binding energy and generalized stacking-fault energy (GSFE) are two critical interface properties of two dimensional layered materials, and it is still unclear how neighboring layers affect the interface energy of adjacent layers. Here, we investigate the effect of neighboring layers by comparing the differences of binding energy and GSFE between trilayer heterostructures (graphene/graphene/graphene, graphene/graphene/boron nitride,boron nitride/graphene/boron nitride) and bilayer heterostructures (graphene/graphene,graphene/boron nitride) using density functional theory. The binding energy of the adjacent layers changes from -2.3% to 22.55% due to the effect of neighboring layer, with a very small change of the interlayer distance. Neighboring layers also make a change from -2% to 10% change the GSFE, depending on the property of the interface between adjacent layers. In addition, a new simple expression is proven to describe the GSFE landscape of graphene-like structure with high accuracy.

EFFECT OF STACKING－FAULT ENERGY ON RESIDUALSUBSTRUCTURE OF EXPLOSIVE SHOCK LOADED METALS

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Composition design study of strong and ductile Mo-alloyed CoCrNi medium-entropy alloys

The assistance of alloying elements provides enormous opportunities for the discovery of high-performance face-centered cubic(FCC)medium-entropy alloys(MEAs).In this work,the influence of al-loying element Mo on the phase stability,stacking fault energy(SFE),deformation mechanisms,lattice distortion,and mechanical properties of(CoCrNi)100-x Mox(0≤x≤10)MEAs was synthetically explored with the first-principles calculations.It indicates that the FCC phase remains metastable at 0 K,and its stability degenerates with increasing Mo content.The monotonous decrease of SFE is revealed with the rise of Mo content,which promotes the activation of stacking faults,deformation twinning,or martensitic transformation.Raising Mo content also causes the aggravation of lattice distortion and thus triggers in-tense solid solution strengthening.Significantly,the essential criterion for the composition design of FCC(CoCrNi)100-x Mo MEAs with superior strength-ductility combination was established based on the syner-gistic effects between multiple deformation mechanisms and solid solution strengthening.According to the criterion,the optimal composition is predetermined as(CoCrNi)93 Mo7 MEA.The criterion is proved to be effective,and it can provide valuable inspiration for the development of alloying-element reinforced FCC multi-principal element alloys.

Design and optimization of the composition and mechanical properties for non-equiatomic CoCrNi medium-entropy alloys

The development of multi-principal element alloys(MPEAs,also called as high-or medium-entropy al-loys,HEAs/MEAs)provides tremendous possibilities for materials innovation.However,designing MPEAs with desirable mechanical properties confronts great challenges due to their vast composition space.In this work,we provide an essential criterion to efficiently screen the CoCrNi MEAs with outstanding strength-ductility combinations.The negative Gibbs free energy difference△E_(FCC-BCC)between the face-centered cubic(FCC)and body-centered cubic(BCC)phases,the enhancement of shear modulus G and the decline of stacking fault energy(SFE)γ_(isf)are combined as three requisites to improve the FCC phase stability,yield strength,deformation mechanisms,work-hardening ability and ductility in the criterion.The effects of chemical composition on△E_(FCC-BCC),G andγisf were investigated with the first principles calculations for Co_(x)Cr_(33)Ni_(67-x),Co_(33)Cr_(y)Ni_(67-y)and Co_(z)Cr_(66-z)Ni_(34)(0≤x,y≤67 and 0≤z≤66)alloys.Based on the essential criterion and the calculation results,the composition space that displays the neg-ative Gibbs free energy difference△E_(FCC-BCC),highest shear modulus G and lowest SFEγ_(isf)was screened with the target on the combination of high strength and excellent ductility.In this context,the optimal composition space of Co-Cr-Ni alloys was predicted as 60 at.%-67 at.%Co,30 at.%-35 at.%Cr and 0 at.%-6 at.%Ni,which coincides well with the previous experimental evidence for Co_(55)Cr_(40)Ni_(5)alloys.The valid-ity of essential criterion is further proved after systematic comparison with numerous experimental data,which demonstrates that the essential criterion can provide significant guidance for the quick exploitation of strong and ductile MEAs and promote the development and application of MPEAs.