Formation Of Twin Structure Research Articles

Impact of alloying elements on generalized stacking fault energy and twinning of Ag-based alloys

The creep strength of Ag-based alloys can be remarkably enhanced through the formation of twin structures. In this study, the effects of alloying elements, including Sn, In, Zn, Cu, Zr, Ni, Ir, Cr, and W, on the formation enthalpy of point defects and the generalized stacking fault energy of Ag-based alloys were investigated using first-principles calculations. The formation enthalpy of point defects serves as a measure of the difficulty of alloying element doping in Ag-based alloys. Calculation results shows that the ability of alloying element doping to form point defects follows the order of Sn > In > Zn > Cu > Zr > Ni > Ir > Cr > W. The generalized stacking fault energy along the [112‾](111) slip direction in Ag-based alloys characterizes the competition between dislocation slip and twinning. As the formation enthalpy of point defects increases, the intrinsic stacking fault energy increases, leading to a decrease in twinning tendency. Once the formation enthalpy of point defects corresponding to the alloying element exceeds 1 eV, the intrinsic stacking fault energy and twinning tendency remain relatively unchanged. Comprehensive analysis shows that alloying elements Sn and In effectively reduce the intrinsic stacking fault energy and promote twinning, which aligns with the behavior observed in commonly used Ag–Sn–In electrical contact materials.

Temperature-Driven Twin Structure Formation and Electronic Structure of Epitaxially Grown Mg3Sb2 Films on Mismatched Substrates.

Mg3Sb2-based compounds are one type of important room-temperature thermoelectric materials and the appropriate candidate of type-II nodal line semimetals. In Mg3Sb2-based films, compelling research topics such as dimensionality reduction and topological states rely on the controllable preparation of films with high crystallinity, which remains a big challenge. In this work, high quality Mg3Sb2 films are successfully grown on mismatched substrates of sapphire (000l), while the temperature-driven twin structure evolution and characteristics of the electronic structure are revealed in the as-grown Mg3Sb2 films by in situ and ex situ measurements. The transition of layer-to-island growth of Mg3Sb2 films is kinetically controlled by increasing the substrate temperature (Tsub), which is accompanied with the rational manipulation of twin structure and epitaxial strains. Twin-free structure could be acquired in the Mg3Sb2 film grown at a low Tsub of 573 K, while the formation of twin structure is significantly promoted by elevating the Tsub and annealing, in close relation to the processes of strain relaxation and enhanced mass transfer. Measurements of scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) elucidate the intrinsic p-type conduction of Mg3Sb2 films and a bulk band gap of ~0.89 eV, and a prominent Fermi level downshift of ~0.2 eV could be achieved by controlling the film growth parameters. As elucidated in this work, the effective manipulation of the epitaxial strains, twin structure and Fermi level is instructive and beneficial for the further exploration and optimization of thermoelectric and topological properties of Mg3Sb2-based films.

Effect of Solution Treatment on Microstructure and Properties of SLM 316L Stainless Steel for Nuclear Power Field

In order to characterize the effect of solution treatment on the properties of Selective laser melting (SLM) 316L material for nuclear power field, the effects of different solution treatment conditions on the microstructure evolution and mechanical properties of 316L material were studied. The results show that 316L material is arc-shaped fish scale structure for as-deposited condition. After solution treatment at 1050 °C and 1150 °C respectively, the material microstructure recrystallizes and the arc boundary between layers disappears. The fracture characteristics is typical ductile fracture characteristic. Compared with temperature of 1050 °C, the material is fully recrystallized at 1150 °C and the microstructure uniformity is improved together with the anisotropy is obviously reduced, meanwhile, the formation of twin structure will be beneficial of improving the mechanical properties which has been demonstrated through metallographic analysis combined with characterization of mechanical performance.

Microstructure and anisotropic mechanical properties of B6.5C-TiB2-SiC-BN composites fabricated by reactive hot pressing

B6.5C-TiB2-SiC-BN composite ceramics were prepared by a novel solid-state reaction using TiCN, B, and Si as raw materials. The final products obtained by hot pressing at 1950 °C possessed a fine microstructure, homogeneous distribution, and excellent mechanical properties. The obtained bulk B6.5C-TiB2-SiC-BN composite ceramic shows a high relative density (98.8 %). The mechanical properties of the composites are anisotropic because of the orientation growth and structural characteristics of TiB2 and h-BN grains. The values of hardness, bending strength, and fracture toughness measured along the hot-pressing direction were 19.6 GPa, 801 MPa, and 4.30 MPa m1/2, respectively, which were higher than those measured perpendicular to the hot-pressing direction. The formation of twin structures in B6.5C and SiC grains and the crack deflections induced by h-BN and TiB2 grains are beneficial for improving the mechanical properties of these composites.

Effects of Strain Rate, Temperature and Grain Size on the Mechanical Properties and Microstructure Evolutions of Polycrystalline Nickel Nanowires: A Molecular Dynamics Simulation

Through molecular dynamics (MD) simulation, the dependencies of temperature, grain size and strain rate on the mechanical properties were studied. The simulation results demonstrated that the strain rate from 0.05 to 2 ns–1 affected the Young’s modulus of nickel nanowires slightly, whereas the yield stress increased. The Young’s modulus decreased approximately linearly; however, the yield stress firstly increased and subsequently dropped as the temperature increased. The Young’s modulus and yield stress increased as the mean grain size increased from 2.66 to 6.72 nm. Moreover, certain efforts have been made in the microstructure evolution with mechanical properties association under uniaxial tension. Certain phenomena such as the formation of twin structures, which were found in nanowires with larger grain size at higher strain rate and lower temperature, as well as the movement of grain boundaries and dislocation, were detected and discussed in detail. The results demonstrated that the plastic deformation was mainly accommodated by the motion of grain boundaries for smaller grain size. However, for larger grain size, the formations of stacking faults and twins were the main mechanisms of plastic deformation in the polycrystalline nickel nanowire.

EBSD characterization of boundary types after discontinuous precipitation in a grit-blasted nickel-based superalloy

In the present work, direct and detailed insights with respect to the statistical information on boundary type of discontinuous precipitates and their advancing reaction fronts in a Re-containing single crystal superalloy DD6 have been obtained by EBSD. With the annealing time prolonging, the reaction fronts proceeded into the original matrix for gradual release of grit-blast induced stress. The average recrystallized grain diameter and the average depth of recrystallization were respective 0.83μm and 3.98μm for the specimen annealed at 1050°C for 10min, which, after 16h, had grown to the average values of 3.22μm and 31.17μm. The reaction fronts of discontinuous precipitation have been confirmed to be high-angle grain boundaries, but do not always correspond to CSL type. Besides the role of strain energy dissipation for recrystallization, the formation of twin structure has no direct relationship with the proceeding of reaction fronts. The information of boundary type of discontinuous precipitation and its advancing reaction fronts would provide new insight into the cellular recrystallization behavior.

Research on Mechanical Properties and Wear Behavior at Extremely Low Temperature of a 16Cr-6Ni Austenitic Steel

The tensile strength and plastic behavior of the Cr16Ni6 steel were investigated at 298K (298K) and 76K. The yield strength and tensile strength of the alloy increased significantly with lower temperatures, from 990 MPa to 1350 MPa and from 1313 MPa to over 1800 MPa, respectively. Young’s modulus and impact behavior were found when testing temperature decreased. It was revealed from the microstructure observation conducted by scanning electron microscopy (SEM) and x-ray diffraction technique (XRD) analysis that the face-centered-cubic (FCC) structure of the matrix remained stable with a very small amount of tangent phase transformation to martensitie during the tensile tests at 76 K. The matrix grains deformed at 298K were cut apart into many small sub-grains with similar crystallographic orientation in the size of only 60-200 nm wide. Very small amount of dislocation cells or bands observed in the specimens deformed at 76 K, although many sub-grains and areas containing dislocation network were found. The formation of twin-structures along the 1/3(420) planes was found in the deformation areas at 76 K. The improvement of strength was mainly attributed to the fact that it was more difficult for the dislocation slipping along lattice planes at 76 K than at 298K, due to the reduction of the movement and diffusion abilities of atoms. The friction coefficient was found to be a little higher at 93 K than at 298 K due to the bad lubricity of the graphite material at cryogenic temperature.

Sub-solvus Cellular Recrystallization and P Phase Formation in a Single-Crystal Superalloy Containing Re

Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated cellular recrystallization accompanied with formation of twin structure and TCP phase of P during annealing at sub-solvus temperature of 1,100 °C. The precipitation of P phase is considered to be attributed to the coarsening of γ′ phase in the recrystallized aggregates which lower the activation energy for atomic migration.

The formation of twin structures in an industrial Zr-2.5% Nb alloy

Very regular twin structures can be formed in textured industrial Zr-2.5% Nb alloy by compression along a direction which does not — or only to a very limited extent — lie in basai planes of the crystallites. Single or co-operative twins arise in rolled rods or tubes through plastic deformation if this exceeds 2%. Those twins have the appearance of regular strips at an angle about 45° to the rolling direction. Their length is determined by grain size or by dimensions of areas with stable crystallographic grain orientation. For sharper tube texture, the twin length exceeds that for rods. The thickness of the twins increases with the degree of deformation. The maximum twin thickness is limited by grain size, which is 4–7 μm for the material investigated. With increase in deformation, the form of the twins changes from strips to a lenticular shape and with further increase eventually to the shape of normal grains. This is the phenomenon of the transformation of twins into grains. The twin structure is stable and preserved after standard annealing. The above considerations for Zr-2.5% Nb may indeed be applicable to other α-Zr alloys.