Alloying effect on grain-size dependent deformation twinning in nanocrystalline Cu–Zn alloys

Grain-size dependency of deformation twinning has been previously reported in nanocrystalline face-centred-cubic metals, which results in an optimum grain-size range for twin formation. Here, we report, for the first time in experiments, the observed optimum grain sizes for deformation twins in nanocrystalline Cu–Zn alloys which slightly increase with increasing Zn content. This result agrees with the reported trend but is much weaker than predicted by stacking-fault-energy based models. Our results indicate that alloying changes the relationship between the stacking-fault and twin-fault energy and therefore affects the optimum grain size for deformation twinning. These observations should be also applicable to other alloy systems.     

γ-TiAl金属间化合物面缺陷能的分子动力学研究

运用分子动力学方法，对γ-TiAl金属间化合物的面缺陷能(层错能和孪晶能)进行了研究. 计算得到γ-TiAl不同滑移系(或孪生系)的整体堆垛层错能曲线，结果表明，γ-TiAl较一般fcc晶体结构的金属可动滑移系(孪生系)的数量减少，在外界条件下呈脆性. 研究孪生系(1/6)〈112〉{111}的弛豫的整体堆垛层错(GSF)能和整体孪晶(GTF)能曲线，对不稳定层错能γ_usf、稳定层错能γ_sf和不稳定孪晶能γ_usf值进行分析，可以预知， γ-TiAl的主要变形机理为孪生系(1/6)〈112〉{111}的孪生和普通滑移系(1/6)〈110〉{111}的滑移，以及超滑移系(1/2)〈011〉{111}的滑移. 关键词： γ-TiAl')" href="#">γ-TiAl 堆垛层错能 孪晶能 分子动力学     

Influence of Ni and N on generalized stacking-fault energies in Fe-Cr-Ni alloy: A first principle study

The alloying effects of Ni and N were examined in terms of the generalized stacking-fault (SF) energy (GSFE). GSFE is associated with the basal plane of Fe-Cr-Ni austenitic stainless steel. The GSFE profiles were obtained using first-principle calculations. The results show that Ni increases the intrinsic SF energy γ_isf and the unstable SF energy γ_us. N does the opposite. However, a γ_isf/γ_us ratio close to zero accompanies the addition of Ni or N. This ratio implies that deformation by partial dislocation is preferred. Overall, the effect of N on Fe-Cr-Ni alloys is more evident than that of Ni.     

The peierls energy and kink energy in fcc metals

In fcc crystals, dislocations are dissociated on the {111} glide plane into pairs of partial dislocations. Since each partial interacts individually with the Peierls potential and is coupled to its neighbour by a stacking fault, periodic variations in the separation distance d of the partials occur when dislocations running along closed packed lattice directions are displaced. This can drastically reduce the effective Peierls stress. By using the Peierls model the structure of 0°, 30°, 60° and 90° dislocations in a typical fcc metal with the elastic properties of Cu and a stacking-fault energy γ₀ in the interval 0.04?≤?γ₀?≤?0.05?J/m² was studied, and the magnitude of the Peierls energy ΔE _P and the resulting kink energies E _K were determined. Since the energies involved are of the order of 10^?3?eV/b or less, their magnitude cannot be asserted with high confidence, considering the simplifying assumptions in the model. The difference in the changes of the core configuration during displacement of dislocations of different orientations should, however, be of physical significance. It is found that a dissociated 60° dislocation generally has a higher effective Peierls energy than a screw dislocation, but the reverse is true for the kink energy, at least in Cu.     

Analysis of structural factors that control necking during tension of FCC metals and alloys

The relation between the strength and ductility of structural materials is theoretically analyzed using stress-strain curves for a number of fcc metals and alloys. The theoretical analysis is based on the criterion of necking in a tensile specimen and on a stress-strain curve, which reflects the evolution of the dislocation density in a material with increasing strain and the effect of structural factors on this evolution. Theoretical relationships are obtained for the uniform strain and the ultimate tensile strength. The effects of the stacking-fault energy, the solid-solution hardening, and the grain size on these quantities are considered.     

Nanostructures and deformation mechanisms in a cryogenically ball-milled Al-Mg alloy

An Al-7.6 at.% Mg alloy was ball milled in liquid N₂ for 8 h and its microstructures were investigated using transmission electron microscopy. Electron diffraction confirmed that the resulting powder is a supersaturated Al-Mg solid solution with an fcc structure. Three typical nanostructures with different grain-size ranges and shapes were observed and the deformation mechanisms in these structures were found to be different. High densities of dislocations were found in large crystallites, implying that dislocation slip is the dominant deformation mechanism. The dislocations rearranged to form small-angle subboundaries upon further deformation, resulting in the formation of medium-sized crystallites with diameters of 10-30 nm. In very small crystallites with dimensions less than 10 nm, twinning becomes an important deformation mechanism. The reasons for the different deformation mechanisms were discussed. Some defects, such as twin boundaries, and small- and large-angle grain boundaries were investigated in detail.     

Electronic structure and plasticity anomalies of the intermetallic compound Co3Ti

In order to explain the physical causes of the mechanical anomalies of the intermetallic compound Co₃Ti we theoretically determine the dependence of the stacking-fault energy on the composition. The problem of evaluating that energy for Co₃Ti is reduced to a problem of examining the difference of the specific energies of close-packed disordered phases of Co−Ti alloys in the ground state. The energies are calculated within the framework of a model based on the locator method of the electron theory of alloys. The stacking-fault energy is found to become zero in the direct vicinity of the Co₃Ti composition. The possibility that alloying may have an effective influence on that quantity is demonstrated. The density ofd states and the enthalpy of formation of Co−Ti alloys are calculated. V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 14–21, June, 1996.     

Polygonization and restoration of long-range order on heating a cold-worked iron −6.5% silicon alloy

Rearrangements taking place in the dislocation structure of single crystals of cold-worked (rolled) Fe-6.5% Si alloy (initially ordered in accordance with the DO₃ system) on subsequent annealing, accompanied by the restoration of the equilibrium degree of long-range order, are considered on the basis of an electron-microscope examination. The microscopic polygonization processes detected are analogous to those which occur in alloys with low stacking-fault energies. The processes underlying the perfection of the defect structure and the restoration of the long-range order disrupted by plastic deformation have a marked influence on one another.     

Effect of alloying elements on room temperature tensile ductility in magnesium alloys

The impact of alloying elements on the room temperature tensile behaviour was investigated for a wide range of strain rates using eight types of extruded Mg-0.3 at.% X (X = Ag, Al, Li, Mn, Pb, Sn, Y and Zn) binary alloys with an average grain size of 2–3 μm. The solid solution alloying element affected not only tensile plasticity but also rate-controlling mechanism for these fine-grained magnesium alloys. Most of the alloys exhibited an elongation-to-failure of 20–50% , while the alloys with a high m-value exhibited large tensile plasticity, such as an elongation-to-failure of 140% in a strain rate of 1 × 10^?5 s^?1 for the Mg–Mn alloy. This elongation-to-failure is more than two times larger than that for pure magnesium. This is due to the major contribution of grain boundary sliding (GBS) on the deformation. Microstructural observations reveal that grain boundary segregation, which is likely to affect gain boundary energy, plays a role in the prevention or enhancement of GBS. The present results are clearly expected to open doors to the development of magnesium alloys with good secondary formability at room temperature through the control of alloying elements.     

Crystallographic Analysis of FCC ? BCT Martensitic Transformation in the Alloy Fe-22% Ni-0.8% C

The infinitesimal deformation (ID) approach is applied to analyse the crystallography involved in the fcc to bct martensitic transformation for the case of (101)_γ[<formula><overline>1</overline>01</formula>]_γ twinning shear as LIS (lattice invariant shear) system in the alloy Fe-22% Ni-0.8% C. Analytical solutions are derived for habit plane orientation, orientation relationships between austenite and martensite phases, and the magnitude of the total shape deformation, etc. In order to compare numerical solutions with the ID approach and phenomenological crystallographic theory, the corresponding crystallographic parameters are calculated by using the Ledbetter and Dunn (L-D) theory. The numeric values obtained are compared with the predictions of the phenomenological crystallographic theories, and with experimental results.     

The anomalous exhibition of GSF energy at surface of fcc metals

The generalized stacking fault (GSF) energy curves for (1 1 1) surface of fcc metals are calculated by the second nearest-neighbor modified embedded atom method (2NN-MEAM), in order to investigate the deformation mechanism of (1 1 1) surface. Except the energy reduce for all these metals, strange energy curves are found for Au, Pd and Pt, especially for Au. Combining the surface GSF energy data and the experimental results, we find that the deformation mechanism should be explained by not only the values of the stable stacking fault energy γ_sf and unstable stacking fault energy γ_usf, but the whole shape of a metal’s energy curve.     

Strain Hardening and Fracture of Austenitic Steel Single Crystals with High Concentration of Interstitial Atoms

Stages in the flow curves, mechanisms of deformation (slip or twinning), evolution of the dislocation structure and fracture are studied in austenitic stainless steel single crystals alloyed with nitrogen (C _N = 0–0.7 wt. %) and Hadfield steel in relation to the orientation of the crystal axis of tension, test temperature, and atomic concentrations of nitrogen and carbon. The dislocation-structure pattern (cellular or planar) and deformation mechanisms (slip or twinning) are shown to depend on the matrix stacking-fault energy _sf, friction forces due to solid-solution hardening by interstitial atoms, and crystal orientation. An interrelation between the stages in the flow curves and the type of dislocation structure is found. The contribution of mechanical twinning to the plastic flow of steel crystals is shown to increase with increase in nitrogen and carbon concentrations. The mechanical twinning develops in the early stages of deformation and determines the strain-hardening coefficient and fracture of crystals in high-strength states for interstitial atomic concentration C 0.5–0.7 wt. %. High deforming stresses due to solid-solution strain hardening by interstitial atoms of nitrogen and carbon in combination with low _sf are found to result in twinning in the <001> orientations. The values of _sf in Hadfield steel single crystals and in austenitic stainless steel single crystals are found experimentally depending on the concentration of nitrogen atoms and test temperature.     

New deformation twinning mechanism generates zero macroscopic strain in nanocrystalline metals

Macroscopic strain was hitherto considered a necessary corollary of deformation twinning in coarse-grained metals. Recently, twinning has been found to be a preeminent deformation mechanism in nanocrystalline face-centered-cubic (fcc) metals with medium-to-high stacking fault energies. Here we report a surprising discovery that the vast majority of deformation twins in nanocrystalline Al, Ni, and Cu, contrary to popular belief, yield zero net macroscopic strain. We propose a new twinning mechanism, random activation of partials, to explain this unusual phenomenon. The random activation of partials mechanism appears to be the most plausible mechanism and may be unique to nanocrystalline fcc metals with implications for their deformation behavior and mechanical properties.     

Crack-stimulated generation of deformation twins in nanocrystalline metals and ceramics

A theoretical model is proposed that describes the generation of deformation twins near brittle cracks of mixed I and II modes in nanocrystalline metals and ceramics. In the framework of the model, a deformation twin nucleates through stress-driven emission of twinning dislocations from a grain boundary distant from the crack tip. The emission is driven by both the external stress concentrated by the pre-existent crack and the stress field of a neighbouring extrinsic grain boundary dislocation. The ranges of the key parameters, the external shear stress, τ, and the crack length, L, are calculated within which the deformation-twin formation near pre-existent cracks is energetically favourable in a typical nanocrystalline metal (Al) and ceramic (3C-SiC). The results of the proposed model account for experimental data on observation of deformation twins in nanocrystalline materials reported in the literature. The deformation-twin formation is treated as a toughening mechanism effectively operating in nanocrystalline metals and ceramics.     

Influence of the Order-Disorder Phase Transition on the Grain Structure of Iron–Palladium Alloy

Transmission diffraction electron microscopy and optical metallography are used to investigate the grain structure and the crystallographic parameters of grain boundaries in Pd₃Fe alloy with short- and long-range atomic orders having the superstructure L1₂. It has been found that ordering annealing of the Pd₃Fe alloy with the ordering temperature T _K lower than the recrystallization temperature is accompanied by changes in the grain structure. The average distance between the nearest boundaries and the average grain size decrease. The fraction of twinning boundaries for which the inverse density of coinciding lattice sites () is equal to 3 increases. This results in a decrease of the twinning boundary energy. A comparative analysis of changes in the grain structure of Ni₃Fe and Pd₃Fe alloys at the A1 L1₂ phase transition is performed. The mechanisms of changes of the grain structure during ordering annealing are discussed.     

Crystal structure and compressibility of FePt nanoparticles under high pressures and high temperatures

FePt nanoparticles with an average grain size of 4 nm and equiatomic composition of Fe and Pt was studied under high pressures in a diamond anvil cell to investigate its structural stability and compressibility under high compression. The ambient pressure disordered face-centered-cubic (fcc) phase was found to be stable to the highest pressure of 61 GPa (compression of 15%) at room temperature. The compression of Fe₅₀Pt₅₀ nanoparticles is closer to the compression curve for pure Pt and shows lower compressibility than what would be expected for a bulk Fe₅₀Pt₅₀ alloy. The nanoparticle character of Fe₅₀Pt₅₀ sample is maintained to the highest pressure without any observable grain coarsening effects at ambient temperature. Laser heating of disordered fcc phase at 32 GPa to a temperature of 2000 K resulted in a phase transformation to a microcrystalline phase with the distorted fcc structure.     

First-principles study of the structural and elastic properties of AuxV1–x and AuxNb1–x alloys

Ab initio total energy calculations, based on the Exact Muffin-Tin Orbitals (EMTO) method in combination with the coherent potential approximation (CPA), are used to calculate the total energy of Au_xV_1–x and Au_xNb_1–x random alloys along the Bain path that connects the body-centred cubic (bcc) and face-centred cubic (fcc) structures as a function of composition x (0 ≤ x ≤ 1). The equilibrium Wigner–Seitz radius and the elastic properties of both systems are determined as a function of composition. Our theoretical prediction in case of pure elements (x = 0 or x = 1) are in good agreement with the available experimental data. For the Au–V system, the equilibrium Wigner–Seitz radius increase as x increases, while for the Au–Nb system, the equilibrium Wigner–Seitz radius is almost constant. The bulk modulus B and C₄₄ for both alloys exhibit nearly parabolic trend. On the other hand, the tetragonal shear elastic constant C′ decreases as x increases and correlates reasonably well with the structural energy difference between fcc and bcc structures. Our results offer a consistent starting point for further theoretical and experimental studies of the elastic and micromechanical properties of Au–V and Au–Nb systems.     

Structure and stability of Σ3 grain boundaries in face centered cubic metals

Σ3 grain boundaries form as a result of either growth twinning or deformation twinning in face centered cubic (fcc) metals and play a crucial role in determining the mechanical and electrical properties and microstructural stability. We studied the structure and stability of Σ3 grain boundaries (GBs) in fcc metals by using topological analysis and atomistic simulations. Atomistic simulations were performed for Cu and Al with empirical interatomic potentials to reveal the influence of stacking fault energy on the morphology of the twinned grains. Three sets of tilt Σ3 GBs were studied with respect to the tilt axis parallel to ?111?, ?112?, and ?110?, respectively. We showed that Σ3{111} and Σ3{112} GBs are thermodynamically stable and the others will dissociate into terraced interfaces regardless of the stacking fault energy. The morphology of the nano-twinned grains in Cu is predicted from the above analysis and found to match with experiments.     

多晶Mn1-xCux(0.1≤x≤0.3)合金的磁诱发应变

采用X射线衍射分析、显微形貌观察、差示扫描量热法、标准电阻应变计法等实验方法,研究了室温下多晶Mn_1-xCu_x(0.1≤x≤0.3,原子分数)合金在低磁场中的磁诱发应变性能.结果表明,Mn_1-xCu_x合金经过长时间的固溶处理,在冷却过程中会出现fcc(γ)→fct(γ’)马氏体相变,形成(γ+γ 关键词： 磁诱发应变 MnCu合金 马氏体相变