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Grain boundary engineering is an effective and feasible metal strengthening strategy to enhance the properties of nanopolycrystalline alloys by changing the number, configuration, and connectivity of different types of grain boundaries, especially for the twin boundaries. In the present contribution, the effect of twin spacing and loading mode on the deformation behavior and mechanism of NiCoAl columnar polycrystalline alloy is investigated. The results show that the nanotwins can not only increase the bearing capacity of dislocations but also emit many dislocations, resulting in the coupling effect of dislocation strengthening and twin strengthening. When the twin spacing is large, intrinsic stacking faults occur and gradually transform into deformation twins. In this stage, Shockley partial dislocation controls plastic deformation. When the twin spacing is small, the high‐density twin layers and stacking faults are more likely to interweave, showing a combination action of Shockley partial dislocation and stair‐rod dislocation. With the loading changing to Z axis, the yield strength decreases due to reduced resistance to the dislocation and a weakened number of Shockley partial dislocations of the emission, leading to less strengthening of the twins. The insights provide a solid theoretical foundation for the further application of NiCoAl in industrial production. [ABSTRACT FROM AUTHOR]

In this contribution, molecular dynamics method is used to accurately add quantitative Co atoms in the grain boundary (GB) region of nano-polycrystalline Ni to study the effect of solute atoms on the GB and the deformation mechanism of the material. It was found that adding an appropriate amount of Co atoms at the GBs can improve the tensile strength of the material to a certain extent through solid solution strengthening. However, excessive introduction of solute atom will increase the activity of GB atoms, destroy the stability of GBs, and accelerate the appearance of material yielding. At the same time, the Co atoms at the GB gradually detached from the GB region and diffused into the grains. In addition, the dislocations are distributed in a grid at the GBs, and with the increase in strain, the number and length of dislocations gradually increase, forming a large number of entanglements. Moreover, the Shockley dislocation interacts with other dislocations to form a more stable dislocation structure by hindering the generation and movement of other dislocations, which greatly enhances the strength of the system. [ABSTRACT FROM AUTHOR]

Herein, the effects of different Co content and temperature factors on the shear mechanical properties of nano‐polycrystalline nickel–cobalt alloy are analyzed by molecular dynamics (MD) simulations. It is found in the work that the highest shear modulus of 63.03 GPa is obtained when the Co content is 10%, attributed to the lattice distortion caused by the difference in the lattice parameters between two elements. During the shearing process, the multilevel twins are present with intensification of the deformation process, resulting in excellent dynamic shear performance. Shockley dislocations will drive the stacking faults and other partial dislocations to constitute a tetrahedral edge, and finally forms a tetrahedral structure. In addition, as the temperature of the shear environment increases, the highest strain and stress decrease. The grain boundary atoms increase significantly, and the atoms inside the grains decrease accordingly. As the degree of atomic disorder increases, the grain boundaries will melt gradually. The decrease of partial dislocation density can weaken the entanglement effect caused by dislocations. The previous conclusions promote the product development and application of nano‐polycrystalline alloys in the future, and provide important guidance significant for the engineering application of nickel–cobalt alloys. [ABSTRACT FROM AUTHOR]

The stabilization of grain boundaries (GBs) is beneficial for improving the stability and mechanical properties of nanocrystalline (NC) metals. Molecular dynamics (MD) calculations were performed to investigate the shear response of Ni Σ 17 [110](223) symmetrical tilt GB. It was found that under the action of shear, the nucleation and evolution of the GB source Shockley partial dislocations ultimately result in the low-energy-state transformation of the GB structure units (SUs). However, the Ag atom contained in the GB increases the shear stress and strain required for the GB relaxation, and the strain range for the GB relaxation is expanded, indicating the inhibitory effect of the Ag atom on the structural relaxation of Ni Σ 17 [110](223) GB. As the temperature increases from 10 K to 250 K, the structural relaxation of Ni Σ 17 [110](223) GB becomes easier to proceed. In addition to segregation-induced GB stabilization, strain-induced GB relaxation and the roles of foreign atom and temperature clarified in this work could provide several new entry points for stabilizing high-energy GBs. [ABSTRACT FROM AUTHOR]

• The deformation mechanism of nanotwins under Hall-Petch enhancement effect was studied via MD simulation. • Excessive volume fraction of nano twins causes Hall-Petch effect breakdown. • Critical volume fraction nanotwins effectively inhibit grain rotation and stabilize grain boundaries. • Nanotwin layer inhibits dislocation nucleation and leads to deformation mechanism transformation. Grain boundary plasticity dominates the mechanical behavior of materials, and the introduction of intragranular nanotwins can achieve coordination between strength and ductility in metals and alloys. In this effort, nanotwins with different volume contents are introduced in NiCoAl alloys to explore the strengthening mechanism under the Hall-Petch effect from perspectives such as grain boundary energy, grain rotation angle, and grain coarsening. The results indicate that the introduction of nanotwins with critical volume fraction effectively inhibits grain boundary migration and grain combination, and improves the strength of the alloy by reducing grain boundary energy and hindering grain rotation. Meanwhile, it leads to a change in the strengthening mechanism from the combined effects of dislocation movement, grain boundary migration and deformation twinning to the dual strengthening of local movement of dislocations within the grain and Lomer-Cottrell locks. The latter hinders the slip of local dislocations within the grain and improves the mechanical property. In addition, based on the average grain size and atomic mean square displacement after high-temperature annealing, the stability mechanism of nanotwins in the alloy can obtain an insight from the aspect of atomic motion. This article explains the Hall-Petch strengthening mechanism from a more detailed and interesting point and provides insights into the strengthening mechanism of nanotwins under the Hall-Petch effect. [Display omitted] [ABSTRACT FROM AUTHOR]

Precisely controlling grain boundary (GB) thermodynamic stability and strength is important for processing nanograined alloys. The segregation-induced stabilization and strengthening GB mechanisms of 20 transition metal elements in Ni Σ11 [110](113) STGB with low interfacial excess energy were clarified from first-principles total-energy calculations in this work. The results indicate that the segregation energy and GB energy are inversely related to the radius difference between TM and matrix Ni atoms. The segregation of 11 elements were identified to enable the Ni Σ11 [110](113) STGB to be stable according to Schuh criterion for GB stability. The electronic mechanism of GB stabilization through TM segregation is mainly due to the less anti-bonding states of Ni -X pairs than those of Ni–Ni pairs in unsegregated GB, from the analysis of crystal orbital Hamiltonian populations. The GB embrittlement resulted from TM segregation is mainly from the mechanical contribution caused by atomic size mismatch between segregation atom and matrix atom. The segregation caused GB strengthening comes from the chemical contribution, which is due to the fact that the segregation of X leads to the formation of stronger Ni -X covalent-like bonds instead of Ni–Ni metallic bonds. This work could provide better understanding on designing high-stable Ni nanograined alloys. • The elements that stabilize grain boundaries are screened by Schuh criterion. • The grain boundary's cohesion was characterized by first-principles tensile tests. • Stabilizing and strengthening mechanisms were uncovered at electronic level. [ABSTRACT FROM AUTHOR]

The thermodynamic stability and mechanical strength of grain boundaries (GBs) govern many key properties of nanocrystalline or polycrystalline metals. In order to design novel Cu-based alloys with extraordinary properties and broaden the application scopes of Cu alloys, it is essential to clarify the effects of GB structure and composition on the stability and cohesion of Cu GBs. Firstly, the GB energies and works of separation of 12 Cu symmetrical tilt GBs with various GB orientations were figured out by using first-principles calculations, to link the GB properties with structures. Our results shown that the GB energies and works of separation of 12 Cu GBs are notably anisotropic. The GB energies for these GBs have an inverted and linear relationship with their works of separation. As to GB composition, the segregation tendencies of 8 alloying elements (Mg, Ca, Cr, Ni, Zn, Zr, Ag and Sn) in the Cu Σ5 [001](210) symmetrical tilt GB and their cohesive and embrittling effects on the GB were studied. It was found that all considered solutes excluding the element Ni can segregate to the Cu GB and thus play stabilizing roles into the Cu GB. Among 8 solutes, only the Zr and Cr segregations can improve the thermodynamic stability and fracture strength of Cu GB simultaneously. The GB strengthening effects of Zr and Cr segregations was attributed to their significant chemical contributions to the embrittling potencies, i.e., the presence of covalent-like bonding features in newly-formed Cu–Zr/Cr bonds. The reduction/increase of anti-bonding/bonding states below the Fermi level in Cu–Zr/Cr atomic pairs should be responsible for the GB stabilizing effects of Zr and Cr segregations. [Display omitted] • Zr and Cr segregation can strengthen Cu Σ5 [001](210) symmetrical tilt grain boundary. • Zr and Cr segregation can stabilize Cu Σ5 [001](210) symmetrical tilt grain boundary. • Grain boundary energy present inverted linear relationship with work of separation. [ABSTRACT FROM AUTHOR]

To reduce or even eliminate the negative effects of grain boundaries (GBs) in nickel-based superalloys, the GB-segregation behaviors of 14 main alloying elements X of superalloys and their effects on the stability and fracture strength of Ni Σ5 [001](210) GB were systematically investigated based on the concept of GB segregation engineering (GBSE), using first-principles calculations. We found that 10 alloying elements can segregate towards the GB. Their GB-segregation tendencies increase in the order of W < Mo < Al < Mn < Cu < Ti < Ru < Ta < Hf < Zr, due to the increase in atomic radius and electronegativity differences between X and host Ni atoms. The GB energies of X -segregated GBs depend linearly on the GB segregation energies of solutes X , the stronger segregation tendency of solute X could lower the GB energy and make the GB more stable. Except Cu, the segregations of the other 9 solutes increase the GB fracture strength. The segregation-induced GB stabilizing and strengthening mechanisms were further revealed on the electronic level. This work could be beneficial for manipulating the composition, structure and properties of Ni GBs and designing Ni-based superalloys with desired performances based on the GBSE. • The grain boundary segregation of alloying elements- X is affected by atomic radius and electronegativity differenc. • The distribution map of grain boundary energy and work of separation of X -segregated grain boundary is obtained. • The mechanism of grain boundary X -segregation induced stabilization is the reduction of anti-bonding states. • The strengthening mechanism of the grain boundary segregation of X is the increase in bond strength of Ni- X. [ABSTRACT FROM AUTHOR]

Hot tearing susceptibility (HTS) refers to the casting defect that existed owing to the appreciable shrinkage at the final stage of solidification. Present research studied the effect of Y on the HTS of 3Y2O3/Al5Cu composite. After 0.3%, 1% and 2% Y (in wt.%) added, the HTS value of 3Y2O3/Al5Cu composite decreased by 2.1%, 39.5% and 61.9%, respectively. Mechanism analysis indicated that the refined grain size and reduced solidification temperature range contributed to the HTS improvement. The grain size of 3Y2O3/Al5Cu was 36.7 μm, while the grain size refined to 32.99 μm, 24.35 μm and 20.9 μm for the 0.3Y-3Y2O3/Al5Cu, 1Y-3Y2O3/Al5Cu and 2Y-3Y2O3/Al5Cu. The result indicated that the increase of grain growth restriction factor, which was caused by the Y element, improved the grain refinement efficiency. Furthermore, adding Y resulted in the novel Al8Cu4Y phase formation and solidification temperature range reduction. In addition, the Al8Cu4Y and Al2Cu phases promoted grain boundary bridges formation and prevented the cracks initiation and growth during solidification process. • The severe hot tearing susceptibility was significantly reduced by adding Y. • Y increased the grain growth restriction factor and refined the grain size. • The solidification temperature range reduced due to the Al-Cu-Y phase formation. • Hot tearing susceptibility and grain refinement mechanisms were carefully discussed. [ABSTRACT FROM AUTHOR]