First-principles study of the effects of selected interstitial atoms on the generalized stacking fault energies, strength, and ductility of Ni

We analyze the influences of interstitial atoms on the generalized stacking fault energy （GSFE）, strength, and ductility of Ni by first-principles calculations. Surface energies and GSFE curves are calculated for the （112） （111） and / 101） （ 1 1 1） systems. Because of the anisotropy of the single crystal, the addition of interstitials tends to promote the strength of Ni by slipping along the （10T） direction while facilitating plastic deformation by slipping along the （115） direction. There is a different impact on the mechanical behavior of Ni when the interstitials are located in the slip plane. The evaluation of the Rice criterion reveals that the addition of the interstitials H and O increases the brittleness in Ni and promotes the probability of cleavage fracture, while the addition of S and N tends to increase the ductility. Besides, P, H, and S have a negligible effect on the deformation tendency in Ni, while the tendency of partial dislocation is more prominent with the addition of N and O. The addition of interstitial atoms tends to increase the high-energy barrier γmax, thereby the second partial resulting from the dislocation tends to reside and move on to the next layer.

Impact of replacement of Re by W on dislocation slip mediated creeps of γ'-Ni_(3)Al phases

The anomalous flow behavior of γ'-Ni_(3)Al phases at high temperature is closely related to the cross-slip of 1/2<110>{111}super-partial dislocations.Generalized stacking fault energy curves(i.e.,Γ-surfaces)along the lowest energy path can provide a great deal of information on the nucleation and movement of dislocations.With the first-principles calculation,the interplay between Re and W,Mo,Ta,Ti doped at preferential sites and their synergetic influence on Γ-surfaces and ideal shear strength(τ_(max))in γ'-Ni_(3)Al phases are investigated.Similar to single Re-addition,the Suzuki segregation of W at stacking faults is demonstrated to enable to impede the movement of 1/6<112>{111} Shockley partial dislocations and promote the cross-slip of 1/2<110>{111}super-partial dislocations.With the replacement of a part of Re by W,a decreased γ_(APB)^(111)/γ_(APB)^(001) indicates that the anomalous flow behavior of γ'phases at high temperature is not as excellent as the double Re-addition,but an increasedτmax means that the creep rupture strength of Ni-based single crystal superalloys can be benefited from this replacement to some extent,especially in the co-segregation of Re and W at Al−Al sites.As the interaction between X1_(Al) and X2_(Al) point defects is characterized by an correlation energy function ΔE^(X1_(Al)+X2_(Al))(d),it is found that both strong attraction and strong repulsion are unfavarable for the improvement of yield strengths of γ'phase.

Theoretical study of the effects of alloying elements on Cu nanotwins

Nanotwins form in many metallic materials to improve their strength and toughness.In this study,we thoroughly studied the alloying effects of 10 common metal and nonmetal elements on Cu nanotwins by density functional theory(DFT).We calculated the segregation energies to determine if Cu nanotwins attract both the metal and nonmetal alloying elements;these segregation energies were then decomposed to mechanical and chemical components.The Cu-Sn bonds are different from other metal alloying elements,and the strong bond between Cu and the nonmetal element results in the negative values of the chemical contribution.Furthermore,the temperature and concentration have different effects on the nanotwin formation energy of the metal and nonmetal alloying elements.As determined by the Generalized Stacking Fault Energy,Al and nonmetals can inhibit the migration of Cu nanotwin boundary,and the effects of Li,Mg,and Sn are opposite.Our theoretical study serves as the foundation for the engineering nanotwin structures through alloying elements,the elements that may lead to new alloy compositions and thermomechanical processes,and are important complements to the experimental research.

Application of the Peierls–Nabarro Model to Symmetric Tilt Low-Angle Grain Boundary with Full Dislocation in Pure Magnesium

Three types of symmetric （1120） tilt low-angle grain boundaries （LAGBs） with array of basal, prismatic, and pyramidal edge full 〈a〉 dislocations in pure Mg have been studied by using the improved Peierls-Nabarro model in combination with the generalized stacking fault energy curve. The results show that with decreasing distance between the dislocations in all the three types of tilt LAGBs, the stress and strain fields are gradually suppressed. The reduction extent of the stress and strain fields decreases from the prismatic to basal to pyramidal dislocations. The variation of dislocation line energy （DLE） for all tilt LAGBs is divided into three stages： DLE changes slightly and linearly when the distance is larger than 300 A, - 10%; DLE declines exponentially and quickly when the distance goes from 300 to 100 A, ,- 70%; and finally, the descent speed lowers when the distance is smaller than 100 A and the dislocation core energy is nearly half of the DLE. The grain boundary energy （GBE） decreases when the tilt angle of LAGB increases from1 ° to 2° for all cases. The tilt LAGB consists of pyramidal dislocations always has the largest GBE, while that with array of prismatic dislo- cations has the smallest one in the whole range. The Peierls stress of dislocation in tilt LAGB is nearly unchanged, the same as that of single dislocation. This work is useful for further study of dissociated dislocation, solute segregation, precipitate nucleation in tilt LAGB and its interaction with single dislocations.

Dominant Deformation Mechanisms in Mg–Zn–Ca Alloy

The coaddition of Zn and Ca has great potential to improve the ductility of Mg alloys.Herein,the mechanical properties of an extruded Mg-Zn-Ca solid-solution alloy were studied by quasi-in situ electron backscatter diffraction(EBSD)-assisted slip trace analysis.The dominant deformation mechanisms of the Mg-Zn-Ca alloy were studied,and the origins of enhanced ductility were systematically revealed.The results indicate that most grains deformed by basal slip.In addition,multiple non-bas al slip traces were detected(particularly prismatic,pyramidal I,and pyramidal I<c+a>slip traces),and their activation frequency was promoted with increasing tensile strain.The enhanced participation of non-basal slip systems is believed to play a critical role in achieving homogeneous plastic deformation,thus effectively promoting the ductility of the Mg-Zn-Ca alloy.Furthermore,first-principle calculations revealed that the coaddition of Zn and Ca significantly reduces the unstable stacking fault energy for non-basal slip,which contributes to the activation of non-basal slip systems during plastic deformation.