The effect of aluminum alloying on strength properties and deformation mechanisms of the 〈123〉 Hadfield steel single crystals

The role of aluminum alloying on strength properties and deformation mechanisms (slip, twinning) of 〈123〉 single crystals of Hadfield steel under tensile loading at T = 300 K is demonstrated. It is found out that aluminum alloying suppresses twinning deformation in the 〈123〉 single crystals and, during slip, results in a dislocation structure change from a uniform dislocation distribution to a planar dislocation structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 3–7, October, 2007.     

Localization of twinning plastic strain in doped γ-Fe single crystals

The patterns of plastic flow localization in high-manganese γ-Fe fcc single crystals oriented for twinning upon stretching are obtained. Basic space-time features of strain localization at the stages of yield plateau, easy glide, and linear hardening are established. The velocity of strain localization sites during stretching is determined. Conditions under which plasticity autowaves appear in the strained medium are discussed. It is demonstrated that the local strain distributions in the case of twinning are similar to those due to dislocation glide.     

Effect of cyclic stresses on the microstructures of hexagonal close packed metals

Slip band extrusions are formed in cadmium, magnesium and titanium, but not in zinc. The extrusions form preferentially in untwinned crystals. Filamentary growths occur at {10¯12} and {11¯21} twin interfaces during cyclic twinning.Possible dislocation interactions at these twin interfaces are described. The dislocation debris produced during cyclic strain in the slip bands and by cyclic twinning is shown to be similar and composed of a high density of dipole loops. It is therefore concluded that the occurrence and distribution of slip band extrusions in metals and the formation of twin boundary filamentary growths can be accounted for by a model based upon the glide of interstitial type dipole loops. Vacancy type loops will then cause crack nucleation in slip bands and deformation twin boundary regions.Twin boundary debris can also cause the observed fragmentation of twins by acting as a barrier to twin boundary movement.The author is grateful to Dr. A. G. Crocker, University of Surrey, for many discussions on the twinning mode in h.c.p. metals and to P. J. E. Forsyth for his interest and encouragement. The paper is published by permission of the Controller, H. M. Stationery Office. Crown copyright is reserved.     

Effect of coherency strain on the deformation of In x Ga1− x As superlattices under nanoindentation and bending

It has been shown elsewhere that the room temperature yield pressure of In _x Ga_{1? x} As superlattices measured by nanoindentation, decreases from a high value as the volume averaged strain modulation is increased, while at 500°C under uniaxial compression or tension the yield stress increases from a low value with increasing strain modulation. We have used cross-sectional transmission electron microscopy to examine the deformation mechanisms in these two loading regimes. At room temperature both twinning and dislocation flow was found with the proportion of twinning decreasing with increasing strain modulation. The coherency strain of the superlattice is retained in a twin but partially relaxed by dislocation flow. The strain energy released by the loss of coherency assists dislocation flow and weakens the superlattice. Twins are only nucleated when a critical elastic shear of about 7° is achieved at the surface. The plastic zone dimensions under the indent are finite at the yield point, with a width and depth of approximately 1.3?µm and 1.1?µm respectively. Under uniaxial compression and tension at 500°C the superlattices deform by dislocation flow along {111} planes. The most highly strained samples also partially relax through the formation of misfit dislocations.     

Effect of a dislocation forest on the bauschinger effect during the twinning of zinc single crystals

A study was made of the effect of a dislocation in the {{11¯22}} 11¯23 system (pyramidal slip) on the characteristics of the Bauschinger effect during the twinning of single crystals of zinc and of the alloy Zn + 0.6% Cu. The dislocation forest retards the backward motion of the twinning dislocations, leading to the effect.Translated from Izvestiya VUZ. Fizika, No. 5, pp. 101–104, May, 1971.In conclusion the authors thank candidates of physicomathematical sciences F. F. Lavrent'yev for useful discussion.     

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.     

Evidence for a transition in deformation mechanism in nanocrystalline pure titanium processed by high-pressure torsion

Nanocrystalline titanium with an average grain size of about 60–70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening–softening–hardening–softening behaviour occurs. For coarse-grained titanium, 〈a〉-type dislocation multiplication, twinning and a high pressure-induced α-to-ω phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for 〈a〉-type dislocations, leading to a slight strain softening. The 〈c〉-component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and 〈c〉-component dislocation multiplication, grain refinement and the α-to-ω phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery.     

Surface nanocrystallization mechanism of a rare earth magnesium alloy induced by HVOF supersonic microparticles bombarding

A nanostructured surface layer with a thickness up to 60 μm was produced on a rare earth Mg-Gd-Y magnesium alloy using a new process named HVOF-SMB (high velocity oxygen-fuel flame supersonic microparticles bombarding). The microstructural features of the treated surface at various depth of the deformed layer were characterized by optical microscopy (OM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with an aim to reveal the formation mechanism. Results showed that three steps during grain refinement process were found, i.e., twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets at the initial stage, stacking faults are generated and a number of dislocation slip systems are activated leading to the cross slips with increasing strain and strain rate, eventually high-density dislocation networks, dislocation cells and dislocation arrays are formed, which further subdivides the twin platelets and residual microbands into sub-microstructures. As a result, homogeneous nanostructure with a grain size of about 10-20 nm is formed through dynamic recrystallization in the topmost surface layer. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation with higher strain rate during the HVOF-SMB treatment in the rare earth Mg-Gd-Y alloy was proposed.     

Grain size effects on microstructural stability and creep behaviour of nanotwinned Ni free-standing foils at room temperature

Creep tests were performed on the high stacking fault energy (SFE) nanotwinned (NT) Ni free-standing foils with nearly the same twin thickness at room temperature (RT) to investigate the effects of grain size and loading rate on their microstructural stability and creep behaviour. The grain growth mediated by the twinning/detwinning mechanism at low applied stresses (<800 MPa) and grain refinement via the detwinning mechanism at high applied stresses (>800 MPa) were uncovered in the present NT-Ni foils during RT creep, both of which are attributed to the interactions between dislocations and boundaries. It appears that a higher initial dislocation density leads to a faster primary creep strain rate and a slower steady-state creep strain rate. Unlike the non-twinned metals in which grain growth often enhances the creep strain rate, the twinning/detwinning-mediated grain growth process unexpectedly lowers the steady-state creep strain rate, whereas the detwinning-mediated grain refinement process accelerates the creep strain rate in the studied NT-Ni foils. A modified phase-mixture model combined with Arrhenius laws is put forward to predict the scaling behaviour between the creep strain rate and the applied stress, which also predicts the transition from grain growth-reduced to grain refinement-enhanced steady-state creep strain rate at a critical applied stress. Our findings not only provide deeper insights into the grain size effect on the mechanical behaviour of nanostructured metals with high SFE, but also benefit the microstructure sensitive design of NT metallic materials.     

High resolution transmission electron microscope observation of zero-strain deformation twinning mechanisms in Ag

We have observed a new deformation-twinning mechanism using the high resolution transmission electron microscope in polycrystalline Ag films, zero-strain twinning via nucleation, and the migration of a Σ3{112} incoherent twin boundary (ITB). This twinning mechanism produces a near zero macroscopic strain because the net Burgers vectors either equal zero or are equivalent to a Shockley partial dislocation. This observation provides new insight into the understanding of deformation twinning and confirms a previous hypothesis: detwinning could be accomplished via the nucleation and migration of Σ3{112} ITBs. The zero-strain twinning mechanism may be unique to low staking fault energy metals with implications for their deformation behavior.     

Effects of torsional deformation on the microstructures and mechanical properties of a CoCrFeNiMo0.15 high-entropy alloy

Abstract

The effects of torsional deformation on the microstructures and mechanical properties of a CoCrFeNiMo_0.15 high-entropy alloy have been investigated. The torsional deformation generates a gradient microstructure distribution due to the gradient torsional strain. Both dislocation activity and deformation twinning dominated the torsional deformation process. With increasing the torsional equivalent strain, the microstructural evolution can be described as follows: (1) formation of pile-up dislocations parallel to the trace of {1 1 1}-type slip planes; (2) formation of Taylor lattices; (3) formation of highly dense dislocation walls; (3) formation of microbands and deformation twins. The extremely high deformation strain (strained to fracture) results in the activation of wavy slip. The tensile strength is very sensitive to the torsional deformation, and increases significantly with increasing the torsional angle.     

层厚度和应变率对铜-金复合纳米线力学性能影响的模拟研究

采用分子动力学模拟方法, 研究了层厚度和应变率对铜-金多层复合纳米线在均匀拉伸载荷下力学性能的影响, 并分析了铜-金位错成核机理. 研究结果表明, 随着铜-金层厚度的增加, 复合材料的屈服强度也随之增大; 高应变率时复合材料的力学性能比低应变率时要强, 低应变率的塑性形变主要是位错运动和孪晶形变, 而高应变率主要以单原子运动为主, 表现出了非晶化. 该研究对制备高性能的多层复合材料提供了一定的理论依据.     

In-situ strain localization analysis in low density transformation-twinning induced plasticity steel using digital image correlation

The effect of deformation temperature on the strain localization has been evaluated by an adapted digital image correlation (DIC) technique during tensile deformation. The progress of strain localization was traced by the corresponding strain maps. The electron backscatter diffraction analysis and tint etching technique were utilized to determine the impact of martensitic transformation and deformation twinning on the strain localization in both elastic and plastic regimes. In elastic regime the narrow strain bands which are aligned perpendicular to the tension direction were observed in temperature range of 25 to 180 °C due to the stress-assisted epsilon martensite. The strain bands were disappeared by increasing the temperature to 300 °C and reappeared at 400 °C due to the stress-assisted deformation twinning. In plastic regime strain localization continued at 25 °C and 180 °C due to the strain-induced alfa-martensite and deformation twinning, respectively. The intensity of plastic strain localization was increased by increasing the strain due to the enhancement of martensite and twin volume fraction. The plastic strain showed more homogeneity at 300 °C due to the lack of both strain-induced martensite and deformation twinning.     

Fabrication of well-defined individual dislocations in SiGe as a novel one-dimensional system

We show a new way to fabricate well-defined individual dislocations in SiGe. We started with a fully pseudomorphic but metastable SiGe layer grown on Si(0 0 1) by molecular beam epitaxy. Next, elongated (1 mm) mesa stripes with various widths (0.5–3 μm) were fabricated by a combination of isotropic and anisotropic etching. For smaller stripes, elastic relaxation of the strained SiGe layer can occur, transforming the originally biaxial strained layer into uniaxial strained subsystems. Subsequent strain relaxation caused by high temperature treatments leads to the formation of individual dislocation along the mesa stripes. The number of parallel dislocation can be adjusted by the original strain (Si:Ge ratio and layer thickness) and the mesa widths. We were able to fabricate structures with exactly one dislocation. Finally, contact pads were added to the stripes enabling the electrical characterization of individual dislocation.     

Effect of solid-solution concentration on the type and parameters of the dislocation structure formed in the process of deformation of the alloys Cu-Mn and Cu-Al

A comparative study of the evolution of dislocation structure accompanying deformation in the alloys Cu-Mn and Cu-Al for concentrations 0–19 at.% Mn and 0.5–14 at.% Al was performed by the method of transmission diffraction electron microscopy. It was determined that the concentration of the solid solution affects the character of the evolution and the parameters of the substructure. It was found that the type of observed substructures are virtually identical for the alloys of both systems, but in contrast to the alloys Cu-Al twinning does not occur in Cu-Mn. In the solid solution Cu-Mn the substructures are observed to change at higher concentrations than in the alloys Cu-Al. It was established that a decrease in the packing-defect energy affects the change in the substructure in the same direction as does an increase in solid solution hardening.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 60–66, October, 1991.     

Investigation of the electromechanical effect in twinning of bismuth crystals in the temperature range 77–530°K

A study was made of the influence of high-density current pulses on twinning of bismuth single crystals by a concentrated load at temperatures of 77–530° K. Twinning was stimulated by current pulses throughout the investigated temperature range and at high temperatures these pulses widened the interval of twinning temperatures. The electroplastic effect in twinning appeared most clearly at low temperatures. The temperature dependences of the quantitative characteristics of twinning had three regions in which there were considerable differences in the qualitative pattern of the distribution of twins originating from indentations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 29–33, July, 1980.     

Change in the Bauschinger effect during the relaxation of twinned crystals

Changes taking place in the quantitative characteristics of the Bauschinger effect in wedge-shaped twins are studied in relation to the period for which these are held before untwinning. All the measurements are made in the cleavage plane of single-crystal zinc and Zn-Cu alloys. The Bauschinger displacement of the twin boundaries first increases slightly and then starts diminishing during the delay period. In the course of untwinning, the region of easy untwinning on the (b) relationship diminishes with time in the softened zone; the extent of the softened zone remains constant during the relaxation of the crystals. The suppression of the Bauschinger effect takes place more intensively if the crystal is held under load before untwinning. The change in the quantitative characteristics of the Bauschinger effect may be explained by the diffusion of impurities to the twinning dislocations and dislocation sources and the consequent blocking of these, and also by the relaxation of stresses in the twin boundaries as a result of plastic shears and dislocation reactions between complete and twinning dislocations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 113–117, April, 1971.