Fabrication of bulk ultrafine-grained materials through intense plastic straining

Ultrafine grain sizes were introduced into samples of an Al-3 pct Mg solid solution alloy and a cast Al-Mg-Li-Zr alloy using the process of equal-channel angular (ECA) pressing. The Al-3 pct Mg alloy exhibited a grain size of ∼0.23 μm after pressing at room temperature to a strain of ∼4, but there was significant grain growth when the pressed material was heated to temperatures above ∼450 K. The Al-Mg-Li-Zr alloy exhibited a grain size of ∼1.2 μm, and the microstructure was heterogeneous after pressing to a strain of ∼4 at 673 K and homogeneous after pressing to a strain of ∼8 at 673 K with an additional strain of ∼4 at 473 K. The heterogeneous material exhibited superplastic-like flow, but the homogeneous material exhibited high-strain-rate superplasticity with an elongation of >1000 pct at 623 K at a strain rate of 10⁻² s⁻¹. It is concluded that a homogeneous microstructure is required, and therefore a high pressing strain, in order to attain high-strain-rate superplasticity (HSR SP) in ultrafine-grained materials. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Precipitate-induced plastic anisotropy: Explicit solutions of the plastic anisotropy due to plate-shaped precipitates

In some aluminum alloys, the observed plastic anisotropy cannot be explained solely by the measured Taylor factor variation. Qualitatively, it has been suggested that this difference results from a secondary effect due to plate-shaped precipitates. Models addressing the effect of plastically-deforming and elastically-deforming precipitates have been previously proposed. In the present article, explicit solutions of the anisotropic strengthening increment are presented for the case of plate-shaped precipitates. These solutions allow a quantitative consideration of the effect of precipitates on different habit planes and of the effect due to stress aging. Generally, in fcc materials, precipitates on {100} habit planes are predicted to minimize the anisotropy due to texture; precipitates on {111} habit planes are predicted to accentuate the anisotropy due to texture; and precipitates on other habit planes are predicted to produce a minor effect resulting from an averaging over a greater number of crystallographcally equivalent habit planes. Stress aging to alter the relative orientation distribution of a single precipitate type is predicted to produce only slight changes in the plastic anisotropy. Larger effects on the yield variation will be observed when stress aging alters the relative volume fractions of two precipitate types on different habit planes.     

Thermal effects during uniaxial straining of steels

When metals are deformed, most of the strain energy absorbed is converted to heat resulting in a temperature increase. Such temperature increases could affect mechanical properties during forming operations and were studied during rapid uniaxial tensile straining at strain rates of 3 x 10³ and 10–² _s–¹ in a dual phase steel, a high strength low alloy (HSLA) steel, and a plain carbon steel, using an infrared thermometer. The maximum temperatures observed were directly related to the strain energy absorbed, as measured by the area under the stress-strain curve. The dual phase steel absorbed the largest amount of strain energy and therefore registered the largest temperature increase. However, the observed temperature increases were lower than those predicted by calculations assuming adiabatic heating, indicating that such heating did not occur at the strain rates studied.     

The role of plastic anisotropy in the fatigue behavior of zircaloy

The changes in the plastic properties and the mode of fracture were examined with highly textured Zircaloy under strain-controlled push-pull cyclic loading condition. Since the loading direction was nearly normal to the (0002) poles of hcp structure, deformation occurred predominantly by prism slip. Different twinning systems were also activated when the sign of shear stress changed. The magnitude of plastic anisotropy also changed differently for warm cross-rolled and recrystallized materials. In spite of these structural anomalies, the Coffin-Manson relationship was obeyed, independent of the particular method used for control of diametral strain limits. Depending on the particular orientation of specimen surface, the process of crack initiation could be closely related to the detailed slip morphology. The crack propagation, however, occurred in the direction normal to the surface where the corresponding plastic strain range was the largest. Twinning was also shown to contribute importantly to the process of fracture in the cyclic loading condition.     

The plastic anisotropy of an Al-Li-Cu-Zr alloy extrusion in unidirectional deformation

The plastic anisotropy resulting from the initial deformation microstructure and various aging treatments applied to several regions of an AA2090 near-net-shape extrusion has been investigated. Yield behavior was measured by uniaxial compression in multiple orientations of each region. Two models of the plastic anisotropy were generated: the Taylor/Bishop-Hill model, based on crystallographic texture, and the plastic inclusion model, developed by Hosford and Zeisloft,^[5] which incorporates anisotropic-precipitate effects. In overaged conditions, the Taylor/Bishop-Hill model adequately describes the observed plastic anisotropy. As the strengthening increment due to second-phase particles increases, there is a concurrent increase in the magnitude of the precipitate contribution to anisotropy. This anisotropy can not be accurately predicted solely by crystallographic texture. By incorporation of terms describing the precipitate anisotropy, the plastic inclusion model correctly predicts the yield strength variation in all regions tested. Examination of the fundamental interaction between matrix and precipitation strengthening reveals that there is a stronger basis for taking the critical resolved shear stress (CRSS) of the precipitates as a constant, rather than their effective yield strength. This consideration provides a more consistent and accurate form of the plastic inclusion model.     

Modeling of the plastic anisotropy of textured sheet

An alternative is proposed to the classical crystallographic and continuum techniques for representation of polycrystal anisotropy. It involves the use of continuum yield criteria to reproduce the yielding behavior of a collection of disoriented grains displaying typical experimental spreads. It is shown that the anisotropic properties pertaining to single ideal orientations are readily assessed. Yield surfaces as well as strain rateR(θ) and yield stressσ(θ)/σ(0) ratios are calculated for polycrystalline materials displaying several texture components. The Taylor, Sachs, and Kochendörfer grain interaction models are used for this purpose, the last of which leads to the fastest computations because it permits the texture/plastic properties relationship to be described analytically. Such methods are particularly well suited to FEM and CAD-CAM calculations. The predictions obtained from the present analysis are compared to experimental observations reported in the literature.     

Malleability and plastic anisotropy of iridium and copper

Room-temperature plane-strain compression tests were performed on iridium and copper single crystals with (110) [110] and (110) [001] orientations that represent contrasting flow strengths. Both copper and iridium show plastic anisotropy in agreement with flow theories. At a given shear strain, the shear strengths of copper and iridium are proportional to the elastic shear modulusG calculated for the fcc slip system. The ability of fcc metals to be cleaved appears to be associated with small values of the quotientK/G, whereK is the elastic bulk modulus. It is concluded that the brittleness of iridium is probably inherent. Formerly with Department of Physical Metallurgy Science of Materials of the University of Birmingham, Birmingham, England     

The distribution of plastic deformation in a metal matrix composite caused by straining transverse to the fibre direction

Distributions of equivalent plastic strains in an A16061/SiC fibre composite measured using the electron back scatter pattern (EBSP) technique were compared to plastic strain and stress distributions calculated using a continuum mechanics model solved by finite element analysis (FEA). Close to the interface EBSP measurements indicated higher dislocation densities than expected for strains calculated in such a region using the FEA model, the excess dislocations presumably being necessary to preserve the continuity of the interface. EBSP measurements also indicated considerable dislocation density in matrix regions where the FEA model calculated small plastic strains due to the production of a nearly hydrostatic tensile stress state. Inhomogeneities in the microstructure of the real matrix material can generate local shear stresses and so lead to production of dislocations even though the far field stress state has no shear component. Thus the dislocation density was controlled by the magnitude of the hydrostatic tension rather than the deviatoric stress components.     

The effect of processing on plastic anisotropy of Ti-6Al-4V

The plastic anisotropy of Ti-6Al-4 V was examined after various thermomechanical treatments, including heat treating, rolling, and forging. Processing temperatures were varied from room temperature to 1340 K. The anisotropy, in terms of the strain ratioR, was measured by post-yield strain gages in the three principal directions, and results were correlated with the (0002) pole figures for each thermomechanical treatment. The plastic strain anisotropy, which was consistent with the basal pole orientation, was found to depend upon both the method and the temperature of mechanical working. The greatestR values occurred for the cold-rolled material where the basal poles rotate to within 15 deg from the sheet normal. In addition,R is not constant under uniaxial tension, but generally increases with the amount of plastic strain. The variation ofR with uniaxial strain depends upon the forming temperature, and the largest changes occur in samples that were rolled at room temperatures.     

Microstructural stability of ultrafine grained low-carbon steel containing vanadium fabricated by intense plastic straining

Two grades of low-carbon steel, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing (ECAP) at 623 K up to an effective strain of ∼4. After equal channel angular pressing, a static annealing treatment for 1 hour was undertaken on both pressed steels in the temperature range of 693 to 873 K. By comparing the microstructural evolution during annealing and the tensile properties of the two steels, the effect of the addition of vanadium on the thermal stability of ultrafine-grained (UFG) low-carbon steel fabricated by intense plastic straining was examined. For the steel without vanadium, coarse recrystallized ferrite grains appeared at annealing temperatures above 753 K, and a resultant degradation of the strength was observed. For the steel containing vanadium, submicrometer-order ferrite grain size and ultrahigh strength were preserved up to 813 K. The enhanced thermal and mechanical stabilities of the steel containing vanadium were attributed to its peculiar microstructure, which consisted of ill-defined pearlite colonies and ultrafine ferrite grains with uniformly distributed nanometer-sized cementite particles. This microstructure resulted from the combined effects of (a) the preservation of high dislocation density providing an effective diffusion path, due to the effect of vanadium on increasing the recrystallization temperature of the steel; and (b) precipitation of fine cementite particles at ferrite grain boundaries through the enhanced diffusion of carbon atoms (which were dissolved from pearlitic cementite by severe plastic straining) along ferrite grain boundaries and dislocation cores.     

Measurement and prediction of plastic anisotropy in deep-drawing steels

R-values and yield stresses were measured as a function of inclination with respect to the rolling direction on 15 steels selected from four basic types [high-strength low-alloy (HSLA), Al-killed drawing quality (AKDQ), interstitial-free (IF), and rimmed]. Orientation distribution functions (ODF’s) were also determined for these steels, using both X-ray and neutron diffraction techniques. The series expansion method was employed for predicting the plastic anisotropy of the rolled sheets. Comparison with the experimental measurements indicates that the “pancake” relaxed constraint model is a more accurate predictor of behavior than the Taylor, Sachs-Kochendörfer, or two other relaxed constraint models. The best quantitative agreement is obtained when the critical resolved shear stress (CRSS) ratio for glide on the {112} (111) and {110} (111) slip systems (t₁₁₂/t₁₁₀) is 0.95. A “lath” relaxed constraint model (with ?₂₃ relaxed), associated with the same CRSS ratio, leads to good results for steels with elongated microstructures.     

Elastic and plastic anisotropy in sheets of cubic metals

Methods are described for the calculation of the elastic and plastic properties of textured polycrystalline metals. The calculations involve the crystallite orientation distribution function derived from X-ray texture data together with data which describe the single crystal behavior. Subsequently, it is shown how a knowledge of the elastic properties can be used to make an analytical prediction of the plastic properties. In making this prediction use is made of the fact that for cubic metals both the elastic and plastic properties are influenced predominantly through the zeroth and fourth order coefficients of the crystallite distribution function. The methods are illustrated by application to the analysis of the data of Stickels and Mould. The statistical correlation they observed is shown to have a justifiable analytical basis.     

The relation between preferred orientation and the Lankford parameter r of plastic anisotropy

Calculation of the r-value of plastic anisotropy from experimentally determined texture data on the basis of the Taylor theory with different assumptions about the glide systems. Experimental determination of the r-value at different angles to the rolling direction in a steel RSt 14. Comparison of the experimental results with the theory on the basis of texture and cell formation.     

The influence of texture inhomogeneities on the plastic anisotropy of hot-rolled low-carbon sheet steel

Measurement of the r-value in low-carbon steel sheet with and without surface removal. Comparison with calculated values on the basis of the Taylor theory with pencil glide. Good agreement can be obtained if texture inhomogeneities are taken into account.     

Bauschinger effect during cyclic straining of two ductile phase alloys

The Bauschinger effect has been examined for monotonie and cyclic straining conditions in a series of binary α, α + β and β Ti-Mn alloys, heat treated to produce constant composition of phases with different morphologies and sizes. For monotonic straining the average Bauschinger strain (ABS) was found to increase with strain over the range of plastic strains used, up to 2%, to be maximum at about 26 vol.% β and to be higher for equiaxed E(α) than for Widmanstätten plus grain boundary (W + GB)α structures. Over the range of particle sizes (either α or β) studied (1–12 μm), the ABS decreased with increasing particle size. After a number of constant strain cycles, the ABS increased and the reverse stress, at which flow began, decreased in the β alloy, when the cyclic flow stress was the same as the monotonic flow stress. In a number of instances permanent softening, as illustrated by parallelism of forward and reverse curves, was not observed. Plastic flow commences in the softer phase, α. The maximum ABS in 26 vol.% β was attributed to interface effects, which restrict flow in α, and to strain concentration in α, which occurs because the β generally begins to deform plastically at much higher stresses. The lower ABS in W + GBα structure was attributed to the greater ease of slip transfer from α to β because of the Burgers orientation relationship between the two phases [29]. The greater ABS in β and the decreased reverse flow stress under cyclic straining conditions is ascribed to an increase in mobile dislocation density or to decreased flow resistance because of possible metallurgical changes and to an increased back stress. The form of the reverse stress-strain curve and conditions for the absence of permanent softening are considered.     

High temperature plastic anisotropy in MoSi2 single crystals

The temperature dependence of the 0.2% yield stress in MoSi₂ single crystals has been determined from 900 to 1600°C for three specimen orientations, [001], [021] and [771]. Four different slip systems have been identified: 013〈100〉, 011〈100〉, {1¯10}1/2<111> and {1¯10}1/2<331>; the critical resolved shear stresses (CRSS) of these systems differ significantly from one another. MoSi₂ is especially strong in the [001] orientation at temperatures above I100°C, because decomposition of l/2<33l> dislocations forces slip on a fifth system, either {123¯}1/2<111> or {11¯1}1/2<111>. The CRSS of each slip system is discussed in terms of the influence of the dislocation core structure on the Peierls stress.     

Texture and anisotropy of zirconium in relation to plastic deformation

Abstract

Possible modes of plastic deformation of tubular materials are analyzed generally. Based on symmetry considerations applied to known textures of zirconium wire and strip, textures of zirconium tubes are predicted as resulting from a wide range of deformation modes. This is made possible by focusing on the main feature of texture, Le., the orientation distribution of basal poles. A relation between texture and the mechanical anisotropy of zirconium is developed, and rules are suggested for softening and hardening caused by texture change. Experimentally, Zircaloy tubes were deformed by various techniques. The resulting textures were determined by X-ray diffraction and the mechanical anisotropy by Knoop-hardness measurement. Each determination is condensed into two representative figures, one describing the type of texture or anisotropy and the other describing the sharpness of texture or the degree of deviation from isotropy.

The experiments verify the predicted relation between type of texture and mechanical anisotropy, and also indicate how degree of anisotropy depends on sharpness of texture. Changes in texture are investigated in relation to plastic deformation, and further refined rules are derived for the direction and rate of these changes. The initially predicted textures are concluded to be stable, i.e., they are reached only after a certain degree of deformation. Zircaloy tubes of three different textures were subjected to tensile testing, closed-end burst testing and open-end burst testing. The observed texture-softening phenomenon corroborates the rules suggested and is considered to commonly occur, as opposed to texture hardening, which requires special combinations of texture and testing method.

Résumé

Les différents modes de déformation plastique possibles dans les tubes ont été analysés d'une façon générale. En se basant sur des considérations de symétrie appliquées aux textures connues pour le zirconium en fil et en bandes, la texture des tubes de zirconium resultant de divers modes de déformation a été prédite. Ceci est possible si l'on tient surtout compte des principes de la texture, c'est à dire, la distribution des orientations des poles de base. Une relation entre la texture et l'anisotropie du zirconium est développée, et des règles sont suggérées pour l'adoucissement et la consolidation dûs à un changement de texture. Dans les expériences, les tubes de zirconium ont été déformés par plusieurs modes. Les textures résultantes ont été déterminées par la diffraction des rayons-X. et l'anisotropie par des mesures de dureté Knoop. Chacune de ces déterminations est représentée dans deux figures, l'une décrivant le genre de texture ou anisotropie, l'autre décrivant la netteté de la texture ou l'écart du caractère isotrope. Les expériences vérifient la relation prévue entre la texture et l'anisotropie. Les changements de texture sont examinés en fonction de la déformation plastique, et d'autres règles plus rigoureuses sont dérivées pour la direction et la vitesse de ces changements. On conc1ut que les textures prévues initialement sont stables, c'est à dire qu'elles ont lieu apres une certaine déformation. Des tubes de Zircalloy de trois textures différentes ont été soumis à des essais detraction, des essais d'éc1atement à extrémités ouvertes ou fermées.

Le phénomène d'adoucissement observé, causé par la texture, est en accord avec les règles suggérées et est généralement présent, ce qui n'est pas le cas du durcissement causé par la texture, durcissement nécessitant des combinaisons spéciales de texture et de méthodes d'essai.     

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.