Unstable plastic flow in a zirconium alloy

Unstable plastic flow has been observed in the parabolic stage of work hardening of a zirconium alloy, which was manifested by periodic variations of a spatiotemporal pattern of the local strain distribution. A synergistic model explaining this behavior is proposed according to which the plastic flow evolution in the final stage is considered as an unstable limiting cycle of the dynamical system.     

Microstructural evolution and plastic stability during superplastic flow in a 7475 aluminium alloy

Abstract

Superplastic behaviour and microstructural evolution were examined at 788 K for strain rates in the range 2 × 10^-4–2 × 10^-3 s^-1 in a 7475 aluminium alloy of nominal composition Al–(1·2–1·9)Cu–(5·2–6·2)Zn–(1· 9–2·6)Mg (wt-%). In addition, the variation of the strain hardening and plastic stability parameters with strain was investigated based on experimental grain growth and cavitation data. The strain hardening parameter at 2 × 10^-4 s^-1 was high over a wide range of strain because of the high grain growth rate. Decrease in the strain hardening parameter due to cavitation was negligible. The highest plastic stability parameter was attained at 2 × 10^-4 s^-1, although the strain rate sensitivity was the lowest for the strain rate range investigated. This demonstrates the influence of grain growth on high plastic stability during superplastic deformation.     

Effect of heat treatments on mechanical properties and fracture behavior of a thixocast A356 aluminum alloy

The effect of different heat treatments (T5 and T6) on mechanical properties, fracture behavior and damage evolution of A356 Thixocast aluminum alloy have been examined in detail in the present work. Tensile tests of the material have been performed in the as cast and as treated conditions in order to observe the different fracture behavior in consequence of the heat treatments. Optical and scanning electron microscopy techniques have been used to characterize the microstructure and fracture surfaces of the specimens. Finally, the precipitation processes of the material have been analyzed by hardness and electrical conductivity measurements and EDS analysis has been used to characterize the different phases in the as-thixo and as-treated conditions.     

Structural-orientational instability of plastic flow in Zr-1% Nb alloy

It is established that the instability of plastic flow observed in a hardening-softening regime in Zr-1% Nb alloy at the parabolic stage of the curve of strain hardening (irrespective of the dislocation-related) is accompanied by the geometric hardening (softening) as a result of the reorientation of slip planes relative to the axis of loading. This reorientation accounts for a periodic change in the Schmid factors for prismatic and conjugate basal slip systems.     

The segregation of copper and silicon in Al-Si-Cu alloy during electromagnetic centrifugal solidification

The present paper investigates the segregation of copper and silicon in an Al–lwt%Cu–lwt%Si alloy solidified under the co-action of centrifugal and electromagnetic forces. The reasons for the solute segregation and the effect of electromagnetic force on segregation are discussed. Tubular samples cut from the solidified alloy are analyzed, the results showing that the segregation of copper and silicon occurs along the normal direction of the samples and that the electromagnetic field has a remarkable influence on the segregation of both copper and silicon. As the exciting current increases, the segregation of copper decreases, while the segregation of silicon first increases and then decreases. The migration of solute atoms in the melt depends not only on the density difference between the solute and aluminum atoms, but also on the strength of the electromagnetic force. The magnetic force changes the rotation velocity of the melt, reduces the migration velocity of copper and causes the reduction of copper segregation. Because of the difference of the electrical conductivity between the solute and the aluminum melt, the reductions of velocity are not equal.     

Direct observation of a plastic deformation autowave in a zirconium alloy

An analysis is made of the evolution of local deformations during the elongation of samples of fine-grained zirconium alloy. It is established that the deformation distribution patterns exhibit ordered behavior where the types of ordering are closely related to the stages of the deformation curve. The results are compared with similar data from studies of the deformation fields of single-crystal and polycrystalline materials with different types of crystal structures and deformation micromechanisms. It is noted that all these similar results, including those of the present study, may be interpreted using autowave representations. Pis’ma Zh. Tekh. Fiz. 24, 26–30 (January 12, 1998)     

锆基金属玻璃超声振动下的塑性流动行为

由于应力软化和表面快速扩散效应,超声振动可以用于金属玻璃微成型.然而,超声振动下的结构重排及其对金属玻璃力学响应机制的影响仍不清楚.本工作采用纳米压痕方法研究了超声振动能量为140 J的Zr(35)Ti(30)Cu(8.25)Be(26.75)金属玻璃的塑性流动行为.我们采用Kelvin和Maxwell-Voigt模型...     

Serrated plastic flow

This paper attempts an assessment of the current understanding of the phenomenon of “serrated plastic flow”, which manifests itself as serrations, load drops, jerkiness or other discontinuities in the stress-strain curves obtained in constant extension rate tensile tests, and as sudden bursts of strain in constant loading rate tests and in constant load (stress) creep tests (the so called staircase creep). Though one can identify at least seven physical processes that can cause serrations, the discussion here is restricted mainly to serrated yielding in tension tests originating from dynamic strain ageing (dsa). The characteristics of the five types of serrations that have been identified so far and the experimental conditions under which they occur are discussed. The various models of serrated flow that have been put forward are reviewed critically. Some recent results on 316 stainless steel are presented to illustrate the effects of grain size, temperature and strain rate on serrated flow. Manifestations ofdsa other than serrations such as a negative strain rate sensitivity, positive temperature dependence for flow stress and work hardening, and the ductility minimum are also discussed. Finally the various issues to be resolved are enumerated.     

Superplasticity in a 7055 aluminum alloy subjected to intense plastic deformation

Abstract

Superplasticity in a 7055 aluminum alloy subjected to intense plastic straining through equal channel angular extrusion (ECAE) was studied in tension over a range of strain rates from 1.4 × 10^-5 to 5.6 × 10^-2 s^-1 in the temperature interval 300 - 450 °C. The alloy had a grain size of ~ 1 μm. A maximum elongation to failure of ~750% occurred at a temperature of 425 °C and an initial strain rate of 5.6 × 10^-4 s^-1, with a strain rate sensitivity coefficient m of about 0.46. The highest m value was ~0.5 at a strain rate of 1.4 × 10^-3 s^-1 and T≥ 425 °C. Moderate superplastic properties with a total elongation of about 435% and m of ~0.4 were recorded in the temperature interval 350 - 400 °C; no cavitation was found. It was shown that the main feature of superplastic behaviour of the ECAE processed 7055 aluminum alloy is a low yield stress and strong strain hardening during the initial stages of superplastic deformation. Comparing the present results with the superplastic behaviour of the 7055 Al subjected to thermomechanical processing (TMP), the highest tensile elongation in the ECAE processed material occurred at lower temperatures because ECAE produces a finer grained structure.     

Decomposition process in a FeAuPd alloy nanostructured by severe plastic deformation

The decomposition process mechanisms have been investigated in a Fe50Au25Pd25 (at.%) alloy processed by severe plastic deformation. Phases were characterized by X-ray diffraction (XRD) and microstructures were observed using transmission electron microscopy (TEM). In the coarse grain alloy homogenized and aged at 450 °C, the bcc α-Fe and fcc AuPd phases nucleate in the fcc supersaturated solid solution and grow by a discontinuous precipitation process resulting in a typical lamellar structure. The grain size of the homogenized FeAuPd alloy was reduced in a range of 50–100 nm by high pressure torsion (HPT). Aging at 450 °C this nanostructure leads to the decomposition of the solid solution into an equi-axed microstructure. The grain growth is very limited during aging and the grain size remains under 100 nm. The combination of two phases with different crystallographic structures (bcc α-Fe and fcc AuPd) and of the nanoscaled grain size gives rise to a significant hardening of the alloy.     

Damage evolution under severe plastic deformation

The development and recovery of damage in continuously cast aluminium alloy 6061 due to plastic deformation is investigated for different stress histories. The processes of Equal Channel Angular Extrusion and Equal Channel Angular Drawing are used to introduce damage into the specimen for a specified stress history. The amount of plastic deformation is determined by the angle between the two intersecting channels, while the stress history is varied by applying different back-pressures. The damage is related to the density, measured using Archimedes' principle.The development of damage was observed to increase proportionally with the extent of accumulated plastic shear strain. The influence of stress history, characterised by a stress index, was found to be twofold. First, the stress index defines the intensity of the porosity development, which increases with the stress index as it changes from negative to positive values. Second, the stress index, when in the negative value region, governs the recovery process. A superimposition of high compressive stresses on the plastic shear deformation leads to a recovery of damage and an associated density increase. The kinematic equation for damage evolution is proposed and its coefficients are defined.     

Revealing the relationships between alloy structure,composition and plastic deformation in a ternary alloy system by a combinatorial approach

A high-throughput approach based on magnetron co-sputtering of alloy libraries is employed to inves-tigate mechanical properties of crystalline and amorphous alloys in a ternary palladium(Pd)-tungsten(W)-silicon(Si)system with the aim to reveal the difference in plastic deformation response and extract the relevant structure-property relationships of the alloys in the system.It was found that in contrast to crystalline alloys,the amorphous ones,i.e.,metallic glasses,exhibited a much smaller fluctuation range in the plasticity parameters(Er2/H and Wp/Wt),indicating a significant difference in the plastic deformation mechanism controlling the mechanical properties for the respective alloys.We propose that the inho-mogeneous deformation of amorphous alloys localized in thin shear bands is responsible for the weaker compositional dependence of both plasticity parameters,while dislocation gliding in crystalline materials is significantly more dependent on the exact structure,thus resulting in a larger scattering range.Based on the representative efficient cluster packing model,a set of composition-dependent atomic structural models is proposed to figure out the structure-property relationships of amorphous alloys in Pd-W-Si alloy system.     

新型铝锂合金板材塑性流动与各向异性性能试验研究

通过单向拉伸试验,研究了新型Al-Li-Cu-Mg合金板材的基本成形性能.针对材料显著的各向异性性能,选取屈服强度、抗拉强度、延伸率以及厚向异性指数等材料性能参数进行对比分析,绘制了7个不同取样方向的单向拉伸曲线,研究了材料各向异性的规律.基于对本构方程、各向异性屈服准则的研究及对比,建立了新型Al-Li-Cu-Mg合金的本构模型,并根据实验曲线计算得到Hill48、Barlat89屈服准则中的各向异性参数,结合各屈服准则绘制了新型Al-Li-Cu-Mg合金屈服轨迹.对比分析各试件的断口方向,并结合第一、第三强度理论,分析了材料的各向异性.利用SEM观察试样的断口形貌,分析对比试件断口的韧窝特征及带状特征.研究发现:试件的延伸率越大,其韧窝特征越明显;反之,其带状特征越明显.从微观角度印证了Al-Li-Cu-Mg合金板材存在的各向异性.     

The relationship between plastic flow macrolocalization and a dislocation structure

We have established a quantitative relationship between the spatial period of localization of plastic straining and the parameters of the dislocation structure of a zirconium alloy. It is found that the wavelength of localized straining is proportional to the average size of elements of a dislocation substructure formed in the material in various stages of plastic flow. A quantitative interpretation of this relationship is proposed.     

Strain hardening and softening in a nanocrystalline Ni-Fe alloy induced by severe plastic deformation

The strain response of an electrochemically deposited nanocrystalline Ni-20 wt.% Fe alloy processed by high-pressure torsion (HPT) was investigated by monitoring changes in hardness. Strain hardening was observed in the very early stage of HPT, followed by strain softening before the onset of a second strain hardening stage. Structural investigations revealed that the two hardening stages were associated with an increase in dislocation density, whereas the strain softening stage was accompanied by a reduction in the dislocation and twin densities, thereby demonstrating the main dependence of hardness on the dislocation density in this material. Grain growth occurred during HPT and its role in the hardness evolution is also discussed.     

The plastic flow and finite element simulation of plastic fine blanking

An idea of fine blanking is put forward where negative clearance affects plastic shearing and the plastic state of material in the shearing zone is examined under the condition of negative clearance. By analyzing of blanking force and stress, the paper brings forward a method of setting up the three-directions pressed stress. The method determines if the material of deformation field enters the plasticity regime. Some parameters that control when the material of blanked zone enters into the plastic state are given. The analysis indicates that the material stays in the plastic regime, when the specific energy of elastic deformation for common carbon steel Q235 reaches 8.34 MPa. The status of plastic flowing in the shearing zone is simulated.