The experimental determination of the stress responses of an Al/Mg alloy to a polygonal strain path after three levels of prestraining

Summary A servohydraulic, computer-controlled MTS axial-torsion testing machine with a biaxial clip-on extensometer is employed to test thin-walled tubes of an Al/Mg alloy under nonproportional straining. Digital data acquisition is used to record axial and shear stress as well as axial and shear strain. The influence of three levels of prestrain (0%, 1% and 12%) on the subsequent stress response to a regular, 16-sided, polygonal strain path is investigated. At each corner of the polygon, yield surface probings were performed before straining continued to the next corner. Straining and yield surface probing were done under computer control with no human interaction. The alloy was donated by ALCOA and has a tensile ductility of about 16%.With no prestrain the stress response to the polygonal strain path is a spiral in the axial-stress — shear stress coordinates. The growth of the radius of the spiral is initially rapid but diminshes as straining continues. If the strain path length is long enough the growth of the spiral subsides at a radius equivalent to the ultimate tensile strength of the material. The effect of prestraining is primarily an increase of the initial radius of the spiral. Ultimately the growth levels off at the same radius as for the specimen without prestrain. It seems that an ultimate surface with a radius equal to the ultimate strength can be postulated. For the present tests at least, this surface is unaffected by prior deformation.When the effective stresses pertaining to the corners of the polygon are plotted vs. accumulated inelastic strain, an effective stress-strain curve is obtained within a reasonable scatter. A total of eight specimens comprise this plot which includes a tensile and a torsion test. However, not every part of the stress response falls on this curve. The results indicate that a universal response can only be obtained after the passage of a transient. It usually lasts about one percent effective strain. This observation is augmented with discussions pertaining to the direction of the initial and the final stress response for a piecewise linear straining.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 2—Microstructural study

Abstract

The morphologies and characteristics of microstructure, including dislocations, mechanical twins and α' martensite, in 304L stainless steel deformed under various strain, strain rate range from 10² to 5 × 10³ s^-1 for different prestrain levels at room temperature were examined by a split Hopkinson bar and TEM. The evolution of microstructure correlated with dynamic mechanical behaviour are presented and discussed in terms of prestrain and applied strain rate. The results show that characteristics of dislocations, mechanical twins and α' martensite varied with prestrains, strains and strain rates. They dominate the strengthening effects on the 304L stainless steel. Dislocation cell structures can be observed in all tested specimens. At larger prestrain under dynamic loading, the formation of elongated dislocation cells becomes evident. The presence of elongated dislocation cells leads to different work hardening behaviour. Twinning occurred at all testing conditions except for the 0·15 prestrain specimen deformed at 0·1 strain and 8 × 10² s^-1 strain rate. The formations of α' martensites were found to be confined to the microshear bands and were barriers of dislocation movement. As the heavy loading is imposed, irregular and blocky α' martensites were observed. Quantitative measurement revealed that dislocation and twin density, as well as the volume fraction of α' martensite increase with the prestrain, strain and applied strain rate, but a decay of twin density occurred as the prestrain of 0·5 is applied. These microstructrual changes can be related to the different work hardening stress (σσ_y and strengthening nature. The observed strengthening effect resulted from the dislocation multiplication, twin formation and α' martensite seems to reflect an enhancement of hardness. However, the increased hardness is less sensitive to the twin formation.     

Effects of changes in strain path on the anisotropy of yield stresses of low-carbon steel and 70-30 brass sheets

Uniaxial tensile tests in various directions following uniaxial extension, equibiaxial stretching or plane strain rolling have been performed to study the effects of changes in strain path on the anisotropy of yield stresses of aluminium-killed low-carbon steel and 70-30 brass sheets. The anisotropy could be predicted from the specimen textures, if dislocation structure were equiaxed, as in the case of equibiaxial stretching. However, elongated dislocation cell structures, developed in the steel specimens prestrained in uniaxial tension or plane strain rolling, gave rise to the second-stage yield stresses higher than predicted from textures in the directions different from the maximum prestrain direction. Planar dislocation structures in the brass specimens prestrained in uniaxial tension or plane strain rolling gave the second-stage yield stresses lower than predicted from the textures in the directions different from the maximum prestrain direction. The phenomena are discussed based on textures and dislocation structures.     

The influence of dynamic strain aging on resistance to strain reversal as assessed through the Bauschinger effect

The Bauschinger effect of three commercially produced medium carbon bar steels representing different microstructural classes with similar tensile strengths and substantially different yielding and work-hardening behaviors at low-strain was evaluated at room temperature and in situ at temperatures up to 361 °C. The influence of deformation at dynamic strain aging temperatures as a means to produce a more stable dislocation structure was evaluated by measuring the resistance to strain reversal during in situ Bauschinger effect tests. It was shown that the three medium carbon steels exhibited substantial increases in strength at dynamic strain aging temperatures with the peak in flow stress occurring at a test temperature of 260 °C for an engineering strain rate of 10⁻⁴ s⁻¹. Compressive flow stress data following tensile plastic prestrain levels of 0.01, 0.02 and 0.03 increased with an increase in temperature to a range between 260 °C and 309 °C, the temperature range where dynamic strain aging was shown to be most effective. The increased resistance to flow on strain reversal at elevated temperature was attributed to the generation of more stable dislocation structures during prestrain. It is suggested that Bauschinger effect measurements can be used to assess the potential performance of materials in fatigue loading conditions and to identify temperature ranges for processing in applications that utilize non-uniform plastic deformation (e.g. shot peening, deep rolling, etc.) to induce controlled residual stress fields stabilized by the processing at temperatures where dynamic strain aging is active.     

Effect of prestrain with a path change on the strain rate sensitivity of AA5754 sheet

The effect of prestrain with a path change on the strain rate sensitivity of AA5754 sheet was investigated. Prestrain magnitudes between 0% and 12% were applied in plane strain in either the transverse or longitudinal (rolling) material direction. Samples were then loaded in uniaxial tension in the longitudinal direction at strain rates of 0.001/s and 0.1/s. Results show that when a path change is involved between prestrain and subsequent uniaxial loading, the strain rate sensitivity of the hardening rate at 0.1/s compared to 0.001/s is reduced. The rate sensitivity of the yield stress remains constant with increasing magnitudes of prestrain, while the rate sensitivity of the elongation to failure decreases with increasing prestrain. A permanent softening of the flow stress is also observed, which is greater when the path change is combined with a change in orientation.     

The influence of temperature and grain size on substructure evolution in stainless steel 316L

The flow stress of polycrystals is controlled by the processes occurring in the grain interior as well as in the mantle, i.e. at the grain boundary and its immediate vicinity. The early stages of evolution of dislocation substructure in these two regions with strain in 316L stainless steel polycrystals have been studied at 293 K, 673 K and 1123 K representing the low temperature thermal, the intermediate temperature athermal and the high temperature thermal regimes respectively. Specimens with grain sizes of 4 and 12 m were employed to determine the effect of grain size.Transmission electron microscopy studies on deformed specimens show the different roles of grain boundary and grain interior in different temperature regimes. In the low temperature regime grain boundaries act as obstacles to moving dislocations and as such high density of dislocation is found in the grain boundary vicinity. In the intermediate temperature regime the dislocations which are easily spread into the grain interior rearrange to form cell walls. In the high temperature regime grain boundaries transform to the equilibrium state and do not contain any grain boundary dislocations, and the distribution of dislocations within grains is homogeneous at all strains. Significantly higher values of dislocation densities in the vicinity as well as in the grain interior were found in the finer grain size material in the whole strain region employed.     

Factors Affecting Transformation Temperatures in Fe-Mn-Si-Cr-Ni Shape Memory Alloy

1. IntroductionFe-Mn-St-Cr-Ni shape memory alloys exhibit notonly a good shape memory effect (SME), but also agood corrosion resistanced'2] ) compared with Fe-MuSt alloys. Fulthermore, it can be used in manufacturing pipe couplings because of its high phase transformation temperatures and great thermal hysteresis.But the recovery strain in these alloys is still low (lessthan 2%), which licits their application in pipe couplings. Therefore, to increase the recovery strain iskey to their pra…     

Analysis of isothermal and isochronal annealing in deformed Zn and Zn-Ag

Zn and Zn-Ag polycrystals have been deformed by rolling at 293 K to true strains =0.05–3.8. After deformation, samples were subjected to isothermal and isochronal anneals, and thereby investigated by intermittent measurements of strength, electrical resistivity, and TEM. Along the isotherms at 293 K, quite unusual hardening effects were observed, which turned out to be strongly affected by the applied prestrain and alloy content. The experimental results can be consistently ascribed to loop formation and loop coarsening from deformation-induced vacancies whereas other explanations, such as loop formation by oxidation and/or phase transformations, can be largely ruled out. Saada's model accounts satisfactorily for the vacancy concentrations measured. In the framework of a loop-hardening theory by Kirchner, the experimentally found values of vacancy concentration and loop density/size yield the right order of magnitude for the strength effects observed. With the isochronal anneals, three stages could be found which are related to loop annealing, dislocation rearrangement, and dislocation annealing.     

Characterization of deformation processes in a Zn-22% Al alloy using atomic force microscopy

The Zn-22% Al eutectoid alloy was subjected to equal-channel angular pressing at a temperature of 473 K to give an as-pressed grain size of 1.3 m. Subsequent tensile testing of the as-pressed alloy at room temperature revealed a transition from deformation by a dislocation mechanism at the higher strain rates to superplastic flow at strain rates below 5 × 10^–3 s^–1: this corresponds to the transition from region III to region II in conventional superplasticity. Samples were pulled to relatively low total strains, of the order of 0.2–0.5, and the surface topography was then examined using an atomic force microscope (AFM). The AFM observations confirm the transition in deformation mechanisms with decreasing strain rate and they provide direct evidence for the occurrence of grain boundary sliding within the superplastic regime.     

High-temperature creep of yttrium-aluminium garnet single crystals

High-temperature creep in single crystals of Y₃Al₅O₁₂ (YAG) was studied by constant strainrate compression tests. The creep resistance of YAG is very high: a stress of ~ 300 MPa is needed to deform at a strain rate of 10^–6 (s^–1) at a temperature as high as 1900 K (~0.84 T _m, (melting temperature)). YAG deforms using the 111 {1¯10} slip systems following a power law with stress exponent n ~ 3 and activation energy E* ~ 720 kJ mol^–1. However, a small dependence of n on temperature and of E* on stress was observed. This stress-dependence of activation energy combined with the observed dislocation microstructures suggests that the high creep resistance of YAG is due to the difficulty of dislocation glide as opposed to the difficulty of climb. Present dislocation creep data are compared with diffusion creep data and a deformation mechanism map is constructed. Large transition stresses (2–3 GPa for 10 m grain size) are predicted, implying that deformation of most fine-grained YAG will occur by diffusion creep.     

Role of recovery in high temperature constant strain rate deformation

A model based on the three-dimensional distribution of dislocations is used to delineate the role of recovery during high temperature constant strain rate deformation. The model provides a good semi-quantitative explanation for classical work-hardening as well as for high temperature work-softening resulting from rapid recovery. It predicts linear work-hardening, whereby the ratio of the work-hardening rate,H, to the shear modulus,G, is constant when a crystal is tested in the absence of recovery. The slope of the stress-strain curve, θ, for high temperature deformation is related to the low temperature work-hardening rateH; the dislocation annihilation rate , the flow stress a, the free dislocation density ρ, the strain rate , and a parameter which is sensitive to the dislocation distribution. A modified version of the Bailey-Orowan equation for simultaneous work-hardening and recovery during constant strain rate deformation which is derived from the model takes the form

镁合金塑性变形中孪生的研究

介绍了镁合金变形过程中孪生的晶体学、位错机理以及几何位向学;探讨了孪晶的形核、长大与演变机制;分析了孪生过程对塑性变形的作用;论述了影响孪生的几种基本因素,包括晶粒取向、变形温度、变形速度、晶粒尺寸、预变形.研究结果表明,镁合金塑性变形过程中孪生变形的作用在于,通过孪生过程改变晶粒取向或通过孪晶间或孪晶与滑移之间的相互作用,诱发新的滑移和孪生;孪晶也可抑制裂纹的产生和扩展,从而提高变形镁合金的室温塑性.     

Effect of orientation on work softening in aluminum single crystals

A single crystal of aluminum, prestrained at low temperature, exhibits a yield drop when it is deformed at a sufficiently higher temperature and this phenomenon has been termed the work softening. In this study, three types of aluminum single crystals with various tensile orientations were stretched at 77 K and 293 K, to clarify the effect of orientation on this work softening phenomenon. The single crystal orientated for single slip shows work softening when deformed at 293 K after prestraining satisfactorily at 77 K. In the deformation of this crystal at 293 K, a coarse slip accompanied by an intimate cross slip, was observed. The single crystal orientated along 1 0 0 also shows work softening at 293 K after giving moderate elongation at 77 K and a clustered slip accompanied by a prominent cross slip was observed in the deformation at 293 K. However, in the deformation of the single crystal orientated along 1 1 1 at 293 K, only a fine triple slip was observed and work softening was not observed even when prestrained to a large strain at 77 K. It is thought that the work softening found at 293 K after prestrain at 77 K is associated with the occurrence and propagation of coarse slip accompanied by cross slip.     

Grazing incidence reciprocal space mapping of partially relaxed SiGe films

Epitaxially grown lattice mismatched semiconductor structures are increasingly important for microelectronic and optoelectronic applications. Recently, a great deal of research has been carried out on strain relaxation mechanisms in lattice mismatched epitaxial films. Here, we describe triple-axis x-ray diffraction measurements that were performed to study strain relaxation mechanisms and dislocation formation in Si_1–x Ge _x alloys grown on (0 0 1) Si substrates. At low growth temperature (T_g 600°C) and small lattice mismatch (>2%), two different mechanisms of strain relaxation are observed, depending on the growth temperature and the magnitude of the strain. At Higher growth temperatures or larger lattice mismatch, strain relaxation occurs initially by surface roughening. Subsequently, 60° misfit dislocations nucleate in regions of high strain. At smaller lattice mismatch or lower growth temperature, the surface of the film does not roughen and the 60° misfit dislocations are formed primarily by Frank–Read multiplication. Triple-axis x-ray diffraction reciprocal space maps taken at grazing incidence on very thin epitaxial films can easily distinguish between these two mechanisms. Here, the lattice planes perpendicular to the interface are measured, whereas conventional diffractometry looks either at the planes parallel to the wafer surface or at planes having components both parallel and perpendicular to the surface. In the grazing incidence geometry, thickness broadening of the x-ray peak is eliminated, since the film is essentially infinitely thick parallel to the surface.     

Flow-stress recovery of nickel-aluminium alloys

Flow-stress recovery measurements along with structural observations using electron microscopy have been carried out on room-temperature prestrained alloys of Ni-11.9, 13.1 and 13.5 at.% Al aged at 735° C. These alloys contained respectively 0, 5, and 8 vol% Ni₃Al) in a fine dispersion. Samples were recovered at and below the ageing temperature for times ranging from 0.1 to 100 h. The influence of volume fraction of , distribution, amount of prestrain and recovery temperature on recovery kinetics was investigated.Results for samples recovered at 735° C showed a large fractional recovery (about 30%) following the first 0.1 h anneal for all samples. About 50 to 70% of the flow stress is recovered at the end of 100 h recovery anneals. The changes in dislocation structure agreed quantitatively with the changes in flow stress.Interpretation of the data in terms of a network growth model of recovery show the solid solution alloy to agree with theory for long recovery times (t>10 h) whereas the -strengthened alloys deviate considerably from the simple model.     

Mechanical damping and dynamic modulus measurements in alumina and tungsten fibre-reinforced aluminium composites

Simultaneous measurements of mechanical damping, or internal friction (Q ^–1 ), and dynamic Young's modulus (E) were made near 80 kHz and at strain amplitudes () in the range 10^–8 to 10^–4 on small specimens of continuous or chopped fibre-reinforced metal matrix composites (MMCs): 6061 aluminium reinforced with alumina (Al/Al₂O₃) and 6061 aluminium reinforced with tungsten (Al/W). Baseline experiments were also done on 99.999% aluminium (pure Al). The strain amplitude dependence of damping and the temperature dependence of dynamic modulus were of particular interest in this study. The temperature (T) dependence of the modulus from room temperature up to 475° C was determined for the Al/Al₂O₃ and pure Al specimens and a highly linear decrease in modulus with increasing temperature was observed. The rate of modulus loss (dE/dT –80 M Pa° C^–1 ) was the same for both materials and the reduction in modulus of the Al/Al₂O₃ was attributed to the reduction in modulus of the alu minium matrix, not the alumina fibres. The size, type, and amount of fibre reinforcement were found to have a significant effect on the strain amplitude dependence of the damping in both MMCs. Unreinforced aluminium exhibited classical dislocation damping trends with a region of strain amplitude independent damping at low strains (less than 10^–5) followed by a non linear, strain amplitude dependent region at higher strains. The addition of alumina fibres (chopped or continuous), while increasing stiffness, resulted in a significant reduction in damping capacity for the MMC relative to that for aluminium and near complete suppression of the amplitude dependent response. The damping levels increased as the volume fraction of fibre, and therefore, the amount of fibre/matrix (FM) interface decreased, indicating that the matrix, not factors such as increased dislocation densities at the FM interface, was the dominant influence on the damping. Analysis of the Al/Al₂O₃ results by Granato-Lücke (GL) theory indicated that dislocation densities were increased relative to those in aluminium, but the dis locations were well pinned and unable to increase damping levels effectively. Analysis of the Al/W results by GL theory also revealed high dislocation densities, but, unlike the Al/Al₂O₃ specimens, the Al/W specimens (continuous fibres) exhibited strong amplitude dependent damping (starting near strain levels of 2 × 10^–6) with damping levels approximately twice those of pure aluminium. Trends showed increased damping with increased fibre diameter, not with increased FM interface area. There was some evidence that it was the tungsten fibre itself that dominated the damping behaviour in Al/W composites, not the aluminium matrix or the FM interface.     

Effects of prestrain on the fracture properties of pressure vessel steel

In this paper, various methods have been used to measure the effects of prestrain on the fracture toughness of 20 g steel experimentally. Based on elastic-plastic FEM analysis, the relationships between crack tip opening displacement _T and the maximum strain _maxc have been obtained and the effects of prestrain _p on the critical fracture strain _maxc have been examined. At the same time, based on the Rice-Tracey's void growth model [4], the cause of the void formation ahead of the crack tip, the condition of the principal crack propagation and the effects of prestrain _p on the fracture properties of 20 g steel were investigated. For ductile material there was a maximum value of V _G ahead of the crack tip that was the cause of the void formation in that place. It is believed that when the maximum value of V _G ahead of the crack tip reaches a critical value, it may be the principal crack propagating condition for high ductile material.     

Dynamic mechanical analysis of prestrained Al2O3/Al metal-matrix composite

The effect of prestraining on the elastic modulus,E, and damping capacity, tan, of 10 and 20 vol% Al₂O₃ particle-reinforced composites has been investigated as function of temperature using dynamic mechanical analysis. Both elastic modulus and damping capacity were found to increase with volume fraction. At 10 vol% the modulus and damping were relatively insensitive to prestrain. However, at 20 vol% it was observed that the modulus decreased with increasing prestrain while damping increased significantly. These results are discussed in terms of fraction of broken particles, particle size, and differential in thermal expansion between the matrix and Al₂O₃ particulate.     

A case for Taylor hardening during primary and steady-state creep in aluminium and type 304 stainless steel

Previous elevated-temperature experiments on 304 stainless steel clearly show that the density of dislocations within the subgrain interior influences the flow stress at a given strain rate and temperature. A re-evaluation shows that the hardening is consistent with the Taylor relation if a linear superposition of solute hardening ( ₀, or the stress necessary to cause dislocation motion in the absence of a dislocation substructure) and dislocation (Gb ^1/2) hardening is assumed. The same Taylor relation is applicable to steady-state structures of aluminium if the yield stress of annealed aluminium is assumed equal to ₀. New tests on aluminium deforming under constant-strain-rate creep conditions show a monotonic increase in the dislocation density with strain. This and the constant-stress creep trends are shown to be possibly consistent with Taylor hardening.