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.     

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.     

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.     

Ti-Al系金属间化合物力学性能的电子理论

应用固体与分子经验电子理论，在价电子结构层次，系统地研究了Ti-Al系金属间化合物的力学性能.根据该理论的基本思想，比较了相关平行电子理论由电荷密度对力学性能的处理方法，模型化了价电子结构空间分布图象，揭示了Ti-Al系金属间化合物的脆性本质.据此提出了能反映金属间化合物均匀变形能力的价电子结构对称性形因子α，表明α与塑性有很好的对应关系，并给出了其脆性判据.初步建立了价电子结构与力学性能定量的经验关系. 关键词：     

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.     

Evolution of defects and deformation mechanisms in different tensile directions of solidified lamellar Ti-Al alloy

Two-phase γ-TiAl/α₂-Ti₃Al lamellar intermetallics have attracted considerable attention because of their excellent strength and plasticity. However, the exact deformation mechanisms remain to be investigated. In this paper, a solidified lamellar Ti-Al alloy with lamellar orientation at 0°, 17°, and 73° with respect to the loading direction was stretched by utilizing molecular dynamics (MD) simulations. The results show that the mechanical properties of the sample are considerably influenced by solidified defects and tensile directions. The structure deformation and fracture were primarily attributed to an intrinsic stacking fault (ISF) accompanied by the nucleated Shockley dislocation, and the adjacent extrinsic stacking fault (ESF) and ISF formed by solidification tend to form large HCP structures during the tensile process loading at 73°. Moreover, cleavage cracking easily occurs on the γ/α₂ interface under tensile deformation. The fracture loading mechanism at 17° is grain boundary slide whereas, at 73° and 0°, the dislocation piles up to form a dislocation junction.     

Grain boundary sliding and lattice dislocation emission in nanocrystalline materials under plastic deformation

A theoretical model is proposed to describe the physical mechanisms of hardening and softening of nanocrystalline materials during superplastic deformation. According to this model, triple interface junctions are obstacles to glide motion of grain boundary dislocations, which are carriers of grain boundary glide deformation. Transformations of an ensemble of grain boundary dislocations that occur at triple interface junctions bring about the formation of partial dislocations and the local migration of triple junctions. The energy characteristics of these transformations are considered. Pileups of partial dislocations at triple junctions cause hardening and initiate intragrain lattice sliding. When the Burgers vectors of partial dislocations reach a critical value, lattice dislocations are emitted and glide into adjacent grains, thereby smoothing the hardening effect. The local migration of triple interface junctions (caused by grain boundary sliding) and the emission of lattice dislocations bring about softening of a nanocrystalline material. The flow stress is found as a function of the total plastic strain, and the result agrees well with experimental data.     

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.     

Variation of apparent creep activation energy in plastic deformation of molybdenum in diffusion contact with nickel

The variation of the apparent creep activation energy as a function of the state of grain boundaries is investigated in the deformation of molybdenum in the presence of diffusion fluxes of nickel at the grain boundaries. It is shown that this energy varies in the same way as in the plastic deformation of classical superplastic materials.Physics Institute of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 110–113, May, 1993.     

Grain Growth During Superplastic Deformation

Significant grain growth occurring during superplastic deformation is related to the micro-mechanism of superplastic flow. Observations performed on the deformed surface of superplastically deformed tensile and shear Pb-62%Sn samples and bi-axially formed AA7475 samples directly indicate that cooperative grain boundary sliding, i.e. sliding of grain groups, is accompanied by cooperative grain boundary migration that can result in an enhanced grain growth. Such a long range correlation in migration of sliding grain boundaries is related to movement of grain boundary dislocations having a step associated with its core. Observed correlation between grain size and strain measured in different regions of a superplastically formed Ti-alloy part and alignment of grain boundaries along shear surfaces support coupling of grain boundary sliding and migration. A model of grain growth considering climb of cellular dislocations, topological defects in a grain array, has been expanded to incorporate gliding and mixed cellular dislocations.     

Generation of dislocation loops in deformed nanocrystalline materials

A theoretical model is suggested which describes the generation of lattice perfect, lattice partial and grain boundary dislocation loops (DLs) at pre-existent DLs in mechanically loaded nanocrystalline materials (NCMs). The energy characteristics of various modes of the DL generation are calculated and compared. With these calculations and comparison, the two basic ranges of the grain size in NCMs are revealed each is characterized by its specific set of effectively operating modes of the DL generation and plastic deformation. The role of the DL generation in plastic and superplastic deformation processes in NCMs is discussed.     

The Possibility of liquid-like and amorphous states existing in nano- and microregions of deformed metallic materials

The possibility of the periodic formation of a liquid-like state in nanoregions of highly nonequilibrium grain boundaries and other regions of defects in metallic materials upon superplastic deformation and intense plastic deformation is considered, based on the results from a thermodynamic analysis of a number of experimental data. Important differences between a liquid-like state and a glass-like (amorphous) state are also considered.     

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.     

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.     

Formation and study of electrospark coatings based on titanium aluminides

Coatings containing Ti-Al intermetallics are fabricated by the electrospark deposition of titanium on aluminum and aluminum on titanium. The microstructure and composition of the grown coatings is studied by scanning electron microscopy, X-ray diffraction, and S-ray microanalysis. It is found that the surface layer formed in argon mostly contains the α-TiAl₃ intermetallic independent of the duration and frequency of discharge pulses. The γ-TiAl and α₂-Ti₃Al phases can be obtained by aluminum deposition on titanium followed by the subsequent deposition of a second titanium layer. Aluminum oxide and titanium nitride are additionally formed during the deposition of electrospark coatings in air.     

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.     

铜射流侵彻穿孔处的温度及微观组织研究

 分析金属射流侵彻钢靶后钢靶孔壁上残留的射流材料的微观组织,将有助于了解射流的超塑性动态变形以及射流在侵彻和冷却过程中的状态变化。通过对铜射流侵彻钢靶时钢靶侵彻孔壁处的组织进行分析,利用数值模拟和理论计算,对铜射流的侵彻过程进行研究,得到了侵彻后孔壁上铜射流的温度及晶粒度的变化曲线,分析结果与扫描电镜的观察结果吻合。通过对孔壁处铜和钢的微观组织进行观察,判断出铜射流在侵彻时没有发生熔化,而是发生了动态再结晶,并且晶粒在随后的冷却过程中发生了明显的长大,此外在铜冷却过程中也产生了孪晶。     

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.     

纳米多晶铜的超塑性变形机理的分子动力学探讨

在聚能装药爆炸压缩形成射流的过程中, 伴随着金属药型罩的晶粒细化, 从原始晶粒30-80 μm细化到亚微米甚至纳米量级, 从微观层面研究其细化机理和动态超塑性变形机理具有很重要的科学意义. 采用分子动力学方法模拟了不同晶粒尺寸下纳米多晶铜的单轴拉伸变形行为, 得到了不同晶粒尺寸下的应力-应变曲线, 同时计算了各应力-应变曲线所对应的平均流变应力. 研究发现平均流变应力最大值出现在晶粒尺寸为14.85 nm时. 通过原子构型显示, 给出了典型的位错运动过程和晶界运动过程, 并分析了在不同晶粒尺寸下纳米多晶铜的塑性变形机理. 研究表明: 当晶粒尺寸大于14.85 nm时, 纳米多晶铜的变形机理以位错运动为主; 当晶粒尺寸小于14.85 nm时, 变形机理以晶界运动为主, 变形机理的改变是纳米多晶铜出现软化现象即反常Hall-Petch关系的根本原因. 通过计算结果分析, 建立了晶粒合并和晶界转动相结合的理想变形机理模型, 为研究射流大变形现象提供微观变形机理参考.     

The Effect of Severe Plastic Deformation on the Structure and Mechanical Properties of Al–Mg–Li Alloys

The structural-phase state and mechanical properties of commercial aluminum alloys produced by severe plastic deformation are studied and compared to the initial polycrystalline state. This kind of treatment is found to give rise to the formation of a homogeneous ultrafine-grained structure with second-phase particles occurring predominantly along grain boundaries. With this structure, the strain-temperature and strain-rate intervals wherein the superplastic properties of the alloys under study are observed are shifted to lower temperatures and higher rates.