Effect of precipitates on plastic anisotropy for polycrystalline aluminum alloys

The effects of crystallographic texture and precipitate distribution on macroscopic anisotropy in aluminum alloys were investigated. In order to simultaneously consider the effects of crystallographic texture and precipitate distribution on macroscopic anisotropy, predictions of plastic properties were carried out using an anisotropic yield function based on the material texture and a combined isotropic-kinematic hardening rule. The input to the model was a single stress-strain curve, the crystallographic texture, and the precipitate volume fraction, shape, and habit planes. It was shown that the kinematic hardening rule, which expresses a translation of the yield surface in stress space, was a function of all the parameters describing the precipitate distribution. The model was applied to the case of an extruded and recrystallized binary Al-3 wt pct Cu alloy deformed in uniaxial compression in different directions. Excellent agreement was observed between the experimental and predicted yield stress anisotropy and the specimen cross section shape anisotropy. Gaussian distributions of grain orientations around ideal texture components typical of aluminum alloys were generated using computer simulations. These textures were combined with the isotropic-kinematic hardening rule determined for the Al-3 wt pct Cu binary alloy to theoretically assess the influence of precipitates on the r-value (the width-to-thickness plastic strain ratio in uniaxial tension) and yield stress anisotropy for aluminum sheets. It was shown that, for these textures, the precipitate distribution had the effect of reducing plastic anisotropy, in agreement with the trends generally observed in practice.     

Effect of Process Variables on Transformation-Texture Development in Ti-6Al-4V Sheet Following Beta Heat Treatment

The effect of preheat time, prestrain, cooling rate, and concurrent deformation during cooling on the preferential selection of hcp alpha variants during the decomposition of the high-temperature, bcc beta phase in two-phase titanium alloys was established using Ti-6Al-4V sheet material. For this purpose, sheet tension samples were pre-soaked in the beta phase field for 0 or 10 minutes (to vary the beta grain size), subjected to a prestrain of 0 or 0.1, and cooled at a rate of 11 or 155 K/min (11 or 155 °C/min) under conditions comprising free ends, fixed ends, or concurrent deformation at a strain rate between ~10^?5 and 3 × 10^?4 s^?1. Electron-backscatter diffraction was used to determine the orientations of the alpha variants so formed, from which the underlying high-temperature, beta-grain microstructure and orientations were reconstructed. These measurements revealed that the parent beta texture changed due to grain growth during preheating. A comparison of the alpha- and beta-phase textures indicated that preferential variant selection was most noticeable under conditions involving a slow cooling rate especially when prestrain or concurrent straining was imposed.     

Non-schmid effects on the behavior of polycrystals—with applications to ni3al

The elastoviscoplastic single crystal constitutive model incorporatingnon-Schmid effects developed by Dao and Asaro (Mater. Sci. Eng. A, 1993, vol. 170, pp. 143-60) is introduced into Asaro and Needleman’s (Acta Metall., 1985, vol. 33, pp. 923-53) Taylor-like polycrystal model as well as Harren and Asaro’s (J. Mech. Phys. Solids, 1989, vol. 37, pp. 191-232) finite element polycrystal model. The single crystal non-Schmid effects, strain hardening, latent hardening, and rate sensitivity, are all described on the individual slip system level, while polycrystal mechanical properties on macroscale are predicted. In general, it is found that non-Schmid effects can have important influences on the “constant offset plastic strain yield surfaces,” stress-strain behavior, texture development, and shear band formation. Finite element calculations show that with moderate non-Schmid effects, localized deformation within a polycrystal aggregate tends to initiate earlier and form sharper and more intense shear bands. Heavy shear banding is found to produce less pronounced textures, which is consistent with existing experimental evidence on Ni₃Al. Examples with Ni₃Al demonstrate that the kind of non-Schmid effects existing in Ni₃Al can increase the generalized Taylor factor to values much higher than 3.06, raise the polycrystal strain hardening rate much higher than that which would be obtained using Schmid’s rule, and influence the deformation texture.     

Deformation textures of AA8011 aluminum alloy sheets severely deformed by accumulative roll bonding

A fully annealed AA8011 aluminum alloy sheet containing a number of large particles (∼5 μm) was severely deformed up to an equivalent strain of 12 by an accumulative roll-bonding (ARB) process. The texture evolution during the ARB process was clarified, along with the microstructure. The ARB-processed aluminum alloy sheets had a different texture distribution through the sheet thickness, due to the high friction between the roll and the material during the ARB process. The shear textures composed of {001} 〈110〉 and {111} 〈110〉 orientations developed at the sheet surface, while the rolling textures, including Cu {112} 〈111〉 and Dillamore {4,4,11} 〈11,11,8〉 orientations, developed at the sheet center. The textural change from a shear texture to a rolling texture at the sheet center during the ARB process contributed to an increase in the fraction of high-angle boundaries. Also, a large number of second-phase particles in the AA8011 alloy sheets weakened the texture. Up to the medium strain range (below ɛ=6.4), relatively weak textures developed, due to the inhomogeneous deformation around the second-phase particles; after the strain of 6.4, strong rolling-texture components, such as the Dillamore and Cu orientations, developed. This remarkable textural change can be explained by the reprecipitation of fine particles in grain interiors.     

Effect of intermediate annealing on texture evolution and plastic anisotropy in an Al-Mg autobody alloy

The effect of intermediate annealing (IA) on texture evolution and plastic anisotropy of an Al-Mg autobody sheet was investigated. The formation of annealing textures after two kinds of final heat treatments was discussed with the aid of microstructural characterization. The results showed that the IA treatment had obvious influences on the final annealing texture as well as the resulting plastic anisotropy. It led to relatively lower recrystallization extent and stronger retained rolling orientations in the sheets after final heat treatment. The final annealing textures were very weak due to the dominant influence of particle stimulated nucleation. The other orientations occurring can either be attributed to genuine recrystallization or to recrystallization in situ. As a result of good suppression of the cube and rotated cube orientations, a good combination of normal anisotropy and planar anisotropy was achieved in the sample with IA and after O treatment.     

Modeling the mechanical behavior of tantalum

A crystal plasticity model is proposed to simulate the large plastic deformation and texture evolution in tantalum over a wide range of strain rates. In the model, a modification of the viscoplastic power law for slip and a Taylor interaction law for polycrystals are employed, which account for the effects of strain hardening, strain-rate hardening, and thermal softening. A series of uniaxial compression tests in tantalum at strain rates ranging from 10⁻³ to 10⁴ s⁻¹ were conducted and used to verify the model’s simulated stress-strain response. Initial and evolved deformation textures were also measured for comparison with predicted textures from the model. Applications of this crystal plasticity model are made to examine the effect of different initial crystallographic textures in tantalum subjected to uniaxial compression deformation or biaxial tensile deformation.     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190 °C and 260 °C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases.     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190°C and 260°C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases. T. FOGLESONG formerly with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801     

Mechanical anisotropy and grain interaction in recrystallized aluminum

Axial compression tests up to 50 pct deformation were performed on rolled and fully recrystallized aluminum sheet stock (AA5754, AA5182, and AA6016) in the direction perpendicular to the sheet. Textures were measured using both X-rays and orientation imaging microscopy (OIM). In all three cases, a systematic in-plane anisotropy was observed, with more strain taking place in the transverse than in the rolling direction. Previous attempts to simulate this in-plane anisotropy for AA5754, starting from the X-ray initial textures and using a one-site polycrystal model, resulted in predictions of more deformation along the rolling than along the transverse direction. An analysis of the OIM textures indicates that there is a nonrandom spatial correlation of the recrystallization and retained rolling components. As a consequence, we implemented grain interaction and co-rotation in a viscoplastic self-consistent (VPSC) polycrystal model, in order to be able to account for orientation correlations. Such an approach allows us to describe the large- and small-angle misorientation distributions, as a function of deformation and to compare them with the available experimental evidence. Concerning the in-plane anisotropy, we conclude that it is very sensitive to details in the texture representation, rather than on grain interactions. Grain-interaction and co-rotation effects, however, have the effect of inducing less severe deformation textures, which is in better agreement with the experimental evidence.     

Effect of Strain and Strain Path on Texture and Twin Development in Austenitic Steel with Twinning-Induced Plasticity

High-manganese (15 to 30 wt pct) austenitic steels exhibit extreme strain hardening because of twinning with increased strain. Twinning in these low stacking fault materials promotes retention of the austenitic microstructure and impedes dislocation motion. A dearth of information is available concerning the extent to which strain path influences twinning in so-called twinning-induced plasticity (TWIP) steels. The present study focuses on the influence of strain level and strain path on texture and twinning in a high-Mn content TWIP steel (Fe17.2Mn0.6C). Electron back-scatter diffraction was employed to measure the twin fraction, twin deviation, twin boundary length, grain misorientation, and volume fraction of different texture components as a function of both uniaxial and biaxial deformation. This information, which is part of the necessary first step toward linking crystallographic texture and twinning to mechanical properties, was used to quantitatively assess the extent to which these critical metallurgical features depend on the amount of straining and the strain path.     

The influence of strain path on subsequent mechanical properties—Orthogonal tensile paths

Several aluminum alloys have been subjected to two stage tensile straining, an initial prestrain followed by a subsequent tensile strain at 90 deg to the initial direction. In AA1100-0 and AA3003-0 the prestrain produces dislocation tangling and diffuse cell walls resulting in an enhanced flow stress and decrease in ductility when the material, is subsequently strained in the orthogonal direction. In a fine grained experimental Al−Fe−Ni alloy the prestrain is accompanied by a very low accumulation of dislocations and in this case the flow stress is reduced and ductility enhanced in subsequent orthogonal straining. The commercial alloys AA2036-T4 and AA5182-0 are unaffected by the two stage tensile strain path. The results are considered in terms of the forming limit curve and it is also shown that the behavior is consistent with the concept of an “alien” dislocation distribution being generated during the prestrain.     

Effects of grain interactions on deformation and local texture in polycrystals

Detailed simulations of grain deformation and crystal orientation evolution in a small region at the interior of polycrystal have been performed. The finite element model accounts for deformation by crystallographic slip and for crystal lattice rotation with deformation. The initial shapes and orientations of the grains and the crystal hardening relations were determined from polycrystalline aluminum samples. The results clearly demonstrate the effects of grain interaction on local deformation and texture evolution. A comparison of the predicted lattice orientations with results from plane strain compression experiments shows good agreement for some of the grains and little agreement for others. Part of the discrepancy results from kinematic restrictions which were necessary to model the 3D microstructure with 2D models. The model shows very nonuniform strain fields and provides detailed information on grain interactions.     

Effects of strain path changes on the formability of sheet metals

The effects of a change in strain path on the deformation characteristics of aluminum-killed steel and 2036-T4 aluminum sheets have been studied. These sheets were pre-strained various amounts in balanced biaxial tension and the resulting uniaxial proper-ties and forming limits for other loading paths were determined. In comparison to uni-axial prestrain the steel was found to suffer a more rapid loss in uniform strain upon the strain path change from biaxial to uniaxial. In contrast, the uniform strain in aluminum does not drop as rapidly after the same change. In keeping with this behavior, the form-ing limit diagram of steel is found to decrease with prestrain at a much faster rate than that of aluminum. Such effects can be explained in terms of the transition flow behavior of the metals occurring upon the path change. Thus, the path change produces strain soften-ing and premature failure in steel, while causing additional strain hardening and consequent flow stabilization in aluminum. AMIT K. GHOSH, formerly with General Motors Research Laboratories     

降低管线钢拉伸强度各向异性的热轧工艺

首先利用Taylor模型分析了某种工业用热轧钢板轧制中出现的几种主要织构与材料拉伸性能各向异性的关系,发现了{112}110织构是引起板材性能各向异性的主要原因。为了减少{112}110织构组分,降低板材的各向异性,有必要研究影响织构演化的热轧工艺。通过设定不同的热轧工艺,得出几种主要织构组分的变化趋势。通过对热轧工艺与织构演化趋势及相应机理的分析,发现在保持良好组织性能的基础上,适当升高未再结晶区开轧温度、提高未再结晶区终轧温度,可以减少热轧中产生的{112}110织构,从而有利于减少板材拉伸性能的各向异性。     

Effect of prestrain on the ductile fracture behavior of an interstitial-free steel

The effect of tensile prestrain on the ductile fracture behavior of an interstitial-free (IF) steel has been studied using primarily (1) the analysis of void density by optical microscopy, (2) characterization of the size of dimples by scanning electron microscope (SEM) and image analyzer, and (3) estimation of strain hardening behavior of a series of prestrained tensile specimens, loaded until fracture. The variation of void density with local plastic strain around the necked region of the specimens indicated the existence of two types of void nucleation pertaining to inclusions and precipitate particles. The critical strain for void nucleation (ε{inn}) for the precipitate particles initially increases and then decreases with the increase in percentage prestrain. This phenomenon has been explained using the strain hardening exponent and nature of dislocation-particle interaction. The nature of variation of the average size of the dimples and that of ε{inn} with prestrain are found to be similar. The dimple size thus bears a proportional relationship with the void, nucleation strain ε{inn} and hence the former can be used to predict (ε{inn}) for IF steel.     

Comparison of strain hardening in polycrystals and <111> single crystals: The example of copper

Tests on copper single crystals oriented about 2 deg from <111> were in agreement with earlier work on similar crystals. Comparison to polycrystals using the Taylor factor showed agreement only for large-grained material. Literature data for single crystals in plane strain compression also agreed only with large-grained copper strain hardening curves. These results suggest that grain size strengthening occurs which is not accounted for in the Taylor analysis. It is therefore concluded that polycrystal strain hardening in copper is not simply a Taylor-averaged single crystal process.     

Approach to saturation in textured soft magnetic materials

A model has been developed for calculating the anisotropic magnetic properties of soft magnetic materials with the objective of accounting for variations in permeability in textured materials. The model in its current form takes account of the rotation of the magnetization direction in each domain of a textured polycrystal and, thus, is applicable to large applied fields. The magnetization direction is determined by minimizing the sum of the magnetocrystalline anisotropy energy and the energy of interaction between the applied field and local magnetization. Examples are given of the application to idealized textures, such as fiber textures, in which all grains share a common axis parallel to the sheet normal (ND). The cube fiber (〈100〉‖ND) has the highest permeability at any applied field, followed by a randomly oriented polycrystal, with the gamma fiber (〈111〉‖ND) having the lowest permeability. Two further examples are given of textured steel sheets, often referred to as “nonoriented electrical steels,” intended for use as laminations in rotating electrical machinery. In one case, the two samples show that a random texture is preferable to one in which the rolling texture is retained. The second example demonstrates the importance of a particular texture component, the Goss or 〈001〉{110}, for producing an anisotropic permeability.     

Influence of strain path on the forming-limit curve in aluminum

The influence of a two-stage strain path on the forming-limit curve (FLC) in aluminum has been investigated. Uniaxial tensile straining followed by orthogonal uniaxial tensile straining decreases the forming-limit strains. Equibiaxial tensile straining followed by uniaxial tensile straining increases the forming-limit strains in the region of plane-strain deformation. Uniaxial tensile straining followed by biaxial tensile straining increases the forming-limit strains in the region of biaxial tensile deformation in some cases. The texture evolution related to the deformation along the two-stage strain path was observed.     

Microstructure Refinement and Mechanical Properties of 304 Stainless Steel by Repetitive Thermomechanical Processing

Repetitive thermomechanical processing (TMP) was applied for evaluating the effect of strain-induced α′-martensite transformation and reversion annealing on microstructure refinement and mechanical properties of 304 austenitic stainless steel. The first TMP scheme consisted of four cycles of tensile deformation to strain of 0.4, while the second TMP scheme applied two cycles of tensile straining to 0.6. For both schemes, tensile tests were conducted at 173 K (? 100 °C) followed by 5-minute annealing at 1073 K (800 °C). The volume fraction of α′-martensite in deformed samples increased with increasing cycles, reaching a maximum of 98 vol pct. Examination of annealed microstructure by electron backscattered diffraction indicated that increasing strain and/or number of cycles resulted in stronger reversion to austenite with finer grain size of 1 μm. Yet, increasing strain reduced the formation of Σ3 boundaries. The annealing textures generally show reversion of α′-martensite texture components to the austenite texture of brass and copper orientations. The increase in strain and/or number of cycles resulted in stronger intensity of copper orientation, accompanied by the formation of recrystallization texture components of Goss, cube, and rotated cube. The reduction in grain size with increasing cycles caused an increase in yield strength. It also resulted in an increase in strain hardening rate during deformation due to the increase in the formation of α′-martensite. The increase in strain hardening rate occurred in two consecutive stages, marked as stages II and III. The strain hardening in stage II is due to the formation of α′-martensite from either austenite or ε-martensite, while the stage-III strain hardening is attributed to the necessity to break the α′-martensite-banded structure for forming block-type martensite at high strains.