Slip-accommodated superplastic flow in Zn-22 wt%Al

Experiments were conducted on the Zn-22 wt% Al eutectoid that contained nanometre-scale dispersion particles. These particles were introduced in the matrix of the alloy via powder metallurgy followed by cryomilling. Transmission electron microscopy observations made on specimens crept at a strain rate near the centre of the superplastic region (the intermediate-stress region or region II in the sigmoidal relationship between stress and strain rate) reveal clear evidence for lattice dislocation activities during superplastic flow. Such evidence is demonstrated in part by the presence of attractive particle-dislocation interactions that are only noted in some of the grains. It is suggested that each one of these grains serves as an obstacle for a group of grains sliding together as a unit. In addition, the configurations of the lattice dislocations in the interiors of the blocking grains are suggestive of viscous glide and single slip in the blocking grain. Combining the present findings with earlier observations reported for superplastic deformation leads to the conclusion that the generation and movement of lattice dislocations provide an accommodation process for grain-boundary sliding.     

Strain-induced structural and phase transitions in superplastic eutectic

The features of diffuse X-ray scattering from samples of the superplastic Pb-62% Sn alloy, (i) subjected to preliminary compression by ～75% and (ii) deformed at room temperature under the conditions optimal for superplastic flow, have been studied. The diffuse peaks revealed are due to the occurrence of short-range stratification in the alloy crystal structure during superplastic deformation. It is found that the superplastic deformation is accompanied by the outflow of Pb atoms from the surface layers, enriched in lead under preliminary compression, to the sample bulk. The formation of viscous amorphous interlayers at grain boundaries under compression facilitates superplastic flow.     

Strain hardening under structural superplasticity conditions

A model is proposed for describing the hardening of fine-grained materials deformed under structural superplasticity conditions. Under these conditions, the strain dependence of the flow stress is shown to be caused by the internal stress fields induced by the defects introduced into grain boundaries during intragranular slip. Expressions describing the dependences of the flow stress on the rate and temperature of superplastic deformation and the structural parameters of the material are obtained.     

The influence of Cd on the superplasticity of Zn-Al alloys

In the paper the results of the superplastic deformation study in the fine grained Zn-1·1 wt.% Al and Zn-0·35 wt.% Al — 0·25 wt.% Cd alloy are presented. The influence of the long-termed ageing at room temperature on the deformation characteristics is investigated and their changes are explained on the basis of the grain growth. The presence of Cd is found to increase the stability of the fine grained structure. The influence of strain rate is studied at 293 and 373 K. Both alloys exhibit superplastic properties with maximum ductilitiesA600% and maximum values of the parameterm0·5. The region of the best superplastic properties is shifted to slower strain rates as a consequence of the Cd atoms presence. The flow stress corresponding to a given strain rate is found to be much higher in the Zn-Al-Cd alloy. The grain boundary segregation of Cd atoms is suggested as a possible reason for better stability of the fine grained structure in the Zn-Al-Cd alloy as well as for the differences observed in the deformation behaviour of both alleys studied.     

Accommodation mechanisms for grain boundary sliding as inferred from texture evolution during superplastic deformation

To gain insight into accommodation mechanisms for local stress concentrations produced by grain boundary sliding (GBS), we systematically examined texture evolution within a superplastic magnesium alloy undergoing deformation at a relatively low deformation temperature (at which basal slip is known to be the preferred slip system in magnesium). Although we did observe an overall weakening of the initial basal texture during superplastic deformation, we also observed within the interior of the specimen a convergent evolution that depends on loading direction. We attribute this texture evolution within the bulk to the competing effects of (a) orientation divergence due to grain rotation accompanied by GBS and (b) convergent evolution due to slip, which acts primarily as an accommodation mechanism for GBS. In contrast, at the near-surface, we found the initial orientation to be preserved, indicating that slip accommodation is less important near the surface than within the bulk.     

Superplasticity in A Zn-1·1 wt.% Al alloy III. The influence of grain size

The paper represents the third instalment of the series dealing with the superplastic deformation in a Zn-1·1 wt. % Al alloy and is devoted to the influence of grain size on the deformation behaviour of this alloy. Deformation characteristics were measured at two temperatures — 293 and 500 K. The grain size dependence of the flow stress observed was found opposite to that predicted by the Hall-Petch relation. Such a behaviour was explained under the assumption that grain boundaries might act as sites of rapid recovery of lattice dislocations. The results obtained at temperature 293 K proved that the transition between the regions of abnormal behaviour (with the flow stress increasing with increasing grain size) and normal behaviour (with the flow stress decreasing with increasing grain size) was not directly connected with the transition from the superplastic to the nonsuperplastic state. The results obtained at 500 K revealed dynamic recrystallization and a grain refinement in samples with initially coarser structures. Such a change in grain structure was accompanied with a development of superplastic characteristics.     

Changes in the diffusion properties of nonequilibrium grain boundaries upon recrystallization and superplastic deformation of submicrocrystalline metals and alloys

The effect of an increase in the coefficient of the grain-boundary diffusion upon recrystallization and superplastic deformation of submicrocrystalline (SMC) materials prepared by severe plastic deformation has been studied. It is shown that the coefficient of the grain-boundary diffusion of the SMC materials is dependent on the intensity of the lattice dislocation flow whose value is proportional to the rate of the grain boundary migration upon annealing of SMC metals or the rate of the intragrain deformation under conditions of superplastic deformation of SMC alloys. It is found that, at a high rate of grain boundary migrations and high rates of superplastic deformation, the intensity of the lattice dislocation flow bombarding grain boundaries of SMC materials is higher than the intensity of their diffusion accommodation, which leads to an increase in the coefficient of the grain-boundary diffusion and a decrease in the activation energy. The results of the numerical calculations agree well with the experimental data.     

Recrystallization of the superplastic Zn-Cd alloy

The initial fine-grained structure of superplastic alloys is attained by rather complicated thermomechanical treatment. Various processes can take place during annealing of such materials due to the release of deformation energy stored in specimens in the course of their preparation. This paper deals with the measurements of the electrical resistivity annealing curves of the superplastic Zn-0·25 wt. % Cd alloy. The pronounced electrical resistivity drop due to the recrystallization was observed between 320 and 360 K. The value of kinetic exponentn from Avrami's equation was found to be 1<n<2;n increases with increasing temperature of annealing. The influence of superplastic deformation on the electrical resistivity annealing curves was investigated. Due to the softening during superplastic deformation the recrystallization becomes less pronounced and shifts to higher temperatures. The kinetic exponentn decreases to unity and its temperature dependence disappears. The attempt was made to explain these results on the ground of literature findings on the kinetics of recrystallization after preceding hot deformation.     

Collective effects in an ensemble of dislocations and vacancies with the formation of the localized deformation zone

Abstract

Paperi have reported about the formation of the superplastic zone as the result of defect production in high strength alloys with the fine precipitates of a non-metalic phase under the intensive stress of about (10^-2 ÷ 10^-1)μ, where μ is a shear modulus. It turned out, that the stopping of slipping dislocations near the precipitates leaded to vacancy influx, which promoted dislocations climbing, on the one hand, and increased its concentration on the other. The higher vacancy concentration, the higher dislocation density increases     

Lüders deformation and superplastic flow of metals extruded by KOBO method

The work brings the results of the study on mechanical properties of some metallic materials subjected to very large plastic deformation by KOBO extrusion. The unexpected features of the KOBO products like Lüders deformation in pure metals and superplastic flow in coarse grain materials are discussed in terms of micro- and nano-scale elements of their structure. The choice to the experiment materials having different crystallographic and phase structure (commercial purity aluminium, multiphase aluminium 7075 alloy, pure zinc and multiphase magnesium AZ91 alloy) and different history (extrusion, casting) allowed to identify the common nano-size elements of the structure generated during the KOBO deformation which seems to be responsible for the mechanical behaviour of these materials. In particular, clusters of point defects (self-interstitials) formed under the KOBO extrusion conditions (cyclic change in the deformation path, high hydrostatic pressure) were found in these materials regardless of grain size and material early history. They correlate with appearance of unstable Lüders-like or even Portevin–LeChatelier deformation at ambient and superplastic flow at elevated temperatures.     

Current problems of superplasticity

The possibility of conversion of intermetallic materials (Ti-Al, Fe-Cr-Co and Mn-Al-C) into the superplastic state is shown. The microstructural changes occurring during the superplastic flow in the Ti-Al intermetallics are analysed. A very important role of grain boundary structure in the superplastic deformation of intermetallics is shown. The experimental results obtained are interpreted on the basis of a model taking into account interactions between lattice dislocations and grain boundaries during the superplastic deformation.     

In-situ small-angle X-ray scattering study of dynamic precipitation in an Al-Zn-Mg-Cu alloy

Dynamic precipitation during room-temperature deformation of an Al-Zn-Mg-Cu alloy in solid solution has been investigated using in-situ small-angle X-ray scattering measurements during tensile tests, performed at the European Synchrotron Radiation Facility. Guinier-Preston (GP) zones are observed to form continuously during the deformation process, and the quantitative measurement of their size and volume fraction shows that their precipitation kinetics are much faster than those of static precipitation. A strong negative strain-rate sensitivity of dynamic precipitation has been observed. A model for the hardening effect of the GP zones is deduced from the evolution of yield stress during static ageing at room temperature. This model is applied to the dynamic precipitation kinetics in order to describe the anomalously high strain-hardening rate observed in these deformation conditions. The kinetics of dynamic precipitation are discussed in terms of semiphenomenological models based on the dynamic strain-ageing theory.     

Thermal activation analysis of superplastic deformation of Bi2O3 ceramics

Activation energies for the conventional hot and superplastic deformations of Bi₂O₃ ceramics have been determined. It is shown that different deformation mechanisms are responsible for significant changes in the activation energy of plastic flow.     

Superplasticity in A Zn-1·1 wt. % Al alloy

In the paper the results of the deformation behaviour study in a fine-grained Zn-1·1 wt. % Al alloy are presented. The influence of strain rate ranging from 4·2×10^–5 to 4·2×10^–2 s^–1 on the true stress — true strain curves, ductility and strain rate sensitivity was investigated at temperatures 295 K and 375 K. At both temperatures the superplastic behaviour was observed. The increase in temperature improved the superplastic properties and shifted the region of the superplastic behaviour to higher strain rates. The maximum values of ductilityA=700% and strain rate sensitivity parameter m=0·48 were established at 375 K at strain rates . The true stress — true strain curves were found to be influenced by a grain growth taking place during the deformation. The grain growth was also found to be responsible for significant differences in the parameterm values obtained from the log plots and from the strain rat-changes.     

Deformation characteristics of nanocrystalline copper and nickel at low temperatures

Measurements were made of the deformation and fracture characteristics of nanocrystalline copper and nickel at temperatures between 4.2 and 300 K. It was observed that the flow stresses are sensitive to the sign of the load while deformation instability was observed at temperatures close to liquid-helium temperature. The temperature dependence of the yield stress was obtained. It was found that there is a range of a thermal deformation at low temperatures which extends to 60 K for nickel and 200 K for copper. Possible reasons for these characteristics in the deformation behavior of nanocrystalline materials are discussed, especially the role of quantum effects in the low-temperature deformation. Fiz. Tverd. Tela (St. Petersburg) 40, 1264–1267 (July 1998)     

Superplasticity in a Zn-1·1 wt.% Al alloy

In the paper the results of the deformation behaviour study in a Zn-1·1 wt. % Al alloy are presented. The influence of temperature ranging from 223 K (0·34T_m) to 645 K (0·98 T_m) on the flow stress, strain rate sensitivity parameterm and activation energy was investigated at various strain rates. The superplastic behaviour was observed in a very broad temperature interval the limits of which were dependent on the strain rate. The best superplastic properties—ductility A>1000 % and parameterm0·7—were established at strain rates at temperatureT=520 K. Superplastic characteristics were found in coarser-grained samples with grain sizes up to 50 m. An intensive grain growth occurring at temperaturesT370 K was found to be probably responsible for the origin of anomalies in temperature dependences of the flow stress and parameterm. The activation energy established in the region of superplastic behaviour was much lower than the activation energy measured in the non-superplastic region. The valueQ90 kJ/mol obtained in the non-superplastic region at very high temperatures is comparable with the activation energy of lattice selfdiffusion in pure Zn or heterodiffusion of Al in Zn along the hexagonalc-axis. This result suggests that the deformation mechanism in the non-superplastic region has a nature similar to the high-temperature dislocation creep. The activation energyQ 20;40 kJ/mol obtained in the superplastic region cannot be compared with activation energies of any known diffusion types in Zn. The explanation of this discrepancy could be find in the presence of other temperature dependent terms the contributions of which to the temperature dependence of the flow stress were neglected. The structure study confirmed the decrease in the contribution of grain boundary sliding when passing from the superplastic to the non-superplastic region.     

Transition of deformation mechanisms in nanotwinned single crystalline SiC

ABSTRACT

The ability to experimentally synthesise ceramic materials to incorporate nanotwinned microstructures can drastically affect the underlying deformation mechanisms and mechanics through the complex interaction between stress state, crystallographic orientation, and twin orientation. In this study, molecular dynamics simulations are used to examine the transition in deformation mechanisms and mechanical responses of nanotwinned zinc-blende SiC ceramics subjected to different stress states (uniaxial compressive, uniaxial tensile, and shear deformation) by employing various twin spacings and loading/crystallographic orientations in nanotwinned structures, as compared to their single crystal counterparts. The simulation results show that different combinations of stress states and crystal/twin orientation, and twin spacing trigger different deformation mechanisms: (i) shear localised deformation and shear-induced fracture, preceded by point defect formation and dislocation slip, in the vicinity of the twin lamellae, shear band formation, and dislocation (emission) avalanche; (ii) cleavage and fracture without dislocation plasticity, weakening the nanotwinned ceramics compared to their twin-free counterpart; (iii) severe localised deformation, generating a unique zigzag microstructure between twins without any structural phase transformations or amorphisation, and (iv) atomic disordering localised in the vicinity of coherent twin boundaries, triggering dislocation nucleation and low shearability compared to twin-free systems.     

A study of the hot and cold deformation of twin-roll cast magnesium alloy AZ31

Recent advances in twin-roll casting (TRC) technology of magnesium have demonstrated the feasibility of producing magnesium sheets in the range of widths needed for automotive applications. However, challenges in the areas of manufacturing, material processing and modelling need to be resolved in order to fully utilize magnesium alloys. Despite the limited formability of magnesium alloys at room temperature due to their hexagonal close-packed crystalline structure, studies have shown that the formability of magnesium alloys can be significantly improved by processing the material at elevated temperatures and by modifying their microstructure to increase ductility. Such improvements can potentially be achieved by processes such as superplastic forming along with manufacturing techniques such as TRC. In this work, we investigate the superplastic behaviour of twin-roll cast AZ31 through mechanical testing, microstructure characterization and computational modelling. Validated by the experimental results, a novel continuum dislocation dynamics-based constitutive model is developed and coupled with viscoplastic self-consistent model to simulate the deformation behaviour. The model integrates the main microstructural features such as dislocation densities, grain shape and grain orientations within a self-consistent viscoplasticity theory with internal variables. Simulations of the deformation process at room temperature show large activity of the basal and prismatic systems at the early stages of deformation and increasing activity of pyramidal systems due to twinning at the later stages. The predicted texture at room temperature is consistent with the experimental results. Using appropriate model parameters at high temperatures, the stress–strain relationship can be described accurately over the range of low strain rates.     

Strong magnetization by antiphase-boundary tubes in a cyclically deformed Fe-35 at.% Al alloy

Strong magnetization was observed in a cyclically deformed Fe-35 at.% Al alloy by introduction of a high density of antiphase-boundary (APB) tubes. In order to introduce a large plastic deformation without changing the specimen dimension, compressive deformation was given repeatedly along mutually orthogonal stress axes under a constraint imposed across the stress axes. Clear evidence was obtained in attributing the major origin of the large magnetization to the production of a high density of APB tubes. The contribution of the APB tubes to the strong magnetization and to the work hardening is discussed on the basis of quantitative analyses of the microstructures introduced by cyclic deformation.