Microstructural characteristics of ultrafine-grained aluminum produced using equal-channel angular pressing

The shearing associated with equal-channel angular (ECA) pressing was examined using optical microscopy. Samples of pure A1 with a large grain size were subjected to ECA pressing to different strains and then examined on three orthogonal planes. Samples were pressed without any rotation or with rotations of either 90 or 180 deg between each consecutive pressing. The experimental observations are compared with models which predict the shearing characteristics associated with ECA pressing under different conditions. It is demonstrated that there is good agreement, in terms of both the grain elongation and the shearing within individual grains, between the experimental results and the predictions of the models. 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.     

Influence of pressing speed on microstructural development in equal-channel angular pressing

The influence of pressing speed in equal-channel angular (ECA) pressing was investigated using samples of pure Al and an Al-1 pct Mg alloy and a range of pressing speeds from ∼10⁻² to ∼10 mm s⁻¹. The results show that the speed of pressing has no significant influence on the equilibrium grain size, at least over the range used in these experiments. Thus, the equilibrium grain sizes were ∼1.2 μm for pure Al and ∼0.5 μm for the Al-1 pct Mg alloy for all pressing conditions. However, it is shown that the nature of the microstructure is dependent on the pressing speed, because recovery occurs more easily at the slower speeds, so that the microstructure is then more equilibrated. There is also indirect evidence for the advent of frictional effects when the cross-sectional dimensions of the samples are at or below ∼5 mm.     

Dynamic deformation and fracture behavior of ultrafine-grained aluminum alloy fabricates by equal-channel angular pressing

In the present study, ultrafine-grained microstructures of a conventional 5083 aluminum alloy were fabricated by equal-channel angular pressing, and their dynamic deformation and fracture behavior were investigated. Dynamic torsional tests were conducted on four aluminum alloy specimens using a torsional Kolsky bar, and then the test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-banding behavior. The equal-channel angular-pressed (ECAP) specimens consisted of ultrafine grains and contained a considerable amount of second-phase particles, which were refined and distributed homogeneously in the matrix as the equal-channel angular pressing pass number increased. The dynamic torsional test results indicated that the maximum shear stress increased, while the fracture shear strain remained constant, with increasing equal-channel angular pressing pass number. Observation of the deformed area beneath the dynamically fractured surface showed that a number of voids initiated mainly at second-phase particle/matrix interfaces and that the number of voids increased with increasing pass number. Adiabatic shear bands of 200 to <300 μm in width were formed in the as-extruded and 1-pass ECAP specimens having coarser particles, whereas they were hardly formed in the four-pass and eight-pass ECAP specimens having finer particles. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation.     

Effect of die shape on the deformation behavior in equal-channel angular pressing

Experimental studies and finite element analysis of equal-channel angular pressing (ECAP) were carried out to clarify the deformation behavior in a sharp corner die and a round corner die under the condition without a frictional effect. It was found in both the experiment and the finite element simulation that the geometry of the die itself has a great influence on the homogeneity in deformation, resulting in more uniform shear deformation in the sharp corner die than in the round corner die under the condition without friction. The shear deformation was concentrated homogeneously on the diagonal plane of the sharp corner, which was in good agreement with the postulation of the conventional theory. In the case of the round corner die, however, plastic deformation was spread over a wide sector of the corner where shear deformation was confined to the inner part, and nonshear deformation was found in the outer part. The inhomogeneous deformation of the round corner die due to geometrical effects exhibited forward-curved flow in the outer part of the corner differently from backward-curved flow caused by frictional effects. The numerical analysis showed that more inhomogeneous distribution of stress was generated on the inlet cross section of the round corner, resulting in a variation of the normal stress from a compressive stress in the inner part to a tensile stress in the outer part. Tension followed by compression was a dominant deformation mode of the material during passing through the outer corner, and a gradual bending of the material occurred instead of shear deformation.     

Structural transformations during heating of a Zr-2.5% Nb alloy subjected to equal-channel angular pressing

The structure of a Zr-2.5% Nb alloy after equal-channel angular pressing (ECAP) at 690–700 K and annealing in the temperature range 670–1070 K is investigated. The structure of the Zr-2.5% Nb alloy deformed by ECAP is an irregular grain-subgrain oriented structure with an enhanced dislocation density, a cross-section of 30–150 nm of oriented structural elements, and an equiaxed-grain (subgrain) size of 50–200 nm. Heating after ECAP in the temperature range 720–770 K for 3–5 h is proposed for the formation of an ultrafine-grained equilibrium structure in the ECAP deformed Zr-2.5% Nb alloy. Heating of the Zr-2.5% Nb alloy after ECAP at 723 K for 5 h leads to the formation of a predominantly equiaxial submicrocrystalline structure with a grain size of 150–500 nm. Equal-channel angular pressing of the Zr-2.5% Nb alloy increases the yield strength to 622 MPa, which is higher than that in the as-delivered undeformed state by a factor of 1.6. In this case, the relative elongation decreases. Heating of the ECAP deformed Zr-2.5% Nb alloy at 723 K for 5 h decreases the yield strength to 504 MPa, but the relative elongation increases to 14%.     

Effect of equal-channel angular pressing on the fatigue strength of titanium and a zirconium alloy

The static and fatigue strength of commercial-purity VT1-00 titanium and a Zr-2.5% Nb alloy subjected to equal-channel angular pressing (ECAP) are studied. The formation of a submicrocrystalline structure after ECAP is shown to result in significant hardening, an increase in the fatigue life at high stress amplitudes, and an increase in the fatigue limit as compared to the annealed state. The mechanisms of fatigue fracture of the materials in various structural states are investigated.     

Texture of the Mg-0.49% Al-0.47% Ca alloy after equal-channel angular pressing

Equal-channel angular pressing (ECAP) is used to refine grains and to change the texture of the initial pressed Mg-0.49% Al-0.47% Ca alloy rod in order to study the possibility of increasing the low-temperature ductility of the alloy. ECAP is performed at 300°C in six passes at a total true logarithmic strain ε = 6.8 according to route B _C . As a result, an ultrafine-grained structure with a grain size of 2–5 μm forms. The initial texture of the pressed rod is characterized by the [12 11] axial orientation parallel to the pressing direction. After ECAP, the texture changes its type and is characterized by a set of preferred orientations that represent basal planes located at an angle of 40°–50° with respect to the pressing direction. An analysis of the generalized Schmid factors, which were calculated for the main operating deformation systems with allowance for the critical shear stresses in them and the volume fractions of the preferred orientations, indicates that the texture caused by ECAP affects the decrease in the strength properties of the alloy measured at room temperature and the increase in the low-temperature ductility of the alloy.     

Formation of a submicrocrystalline structure in St10 and 08R steels during equal-channel angular pressing

The effect of cold equal-channel angular pressing (ECAP) on the structure and properties of low-carbon St10 and 08R steels in the initially ferritic-pearlitic and bainitic states is studied. Directly after ECAP, only a partially submicrocrystalline structure with a grain size of 150–300 nm (depending on alloying and the initial state) can be obtained. Along with a granular structure, a subgrain and/or cellular structure, including an oriented structure, are observed. The finest structure forms in a boron-containing 08R steel with a structural-element size of 190 nm. The strength of the 08R steel subjected to cold ECAP (σ_u = 805–1235 MPa) corresponds to a hardware strength class of 8.8–12.9. The strength of the deformed St10 steel is close to a strength class of 8.8.     

Microstructure transformation from lamellar to equiaxed microduplex through equal-channel angular pressing in an Al-33 pct Cu eutectic alloy

Strain-driven transformation of a lamellar structure into a homogeneous equiaxed microduplex structure was investigated in an eutectic Al-33 pct Cu alloy deformed by equal-channel angular pressing at 400 °C via route Bc. In route Bc, the rotation of sample is always 90 deg in the same sense, i.e., clockwise or counterclockwise. The transformation follows the subdivision of the colonies into smaller lamellar blocks, the separation of these lamellar blocks into isolated islands, and, finally, the shrinkage of these islands to disappear, through the breaking down of the lamellae near the boundaries of the lamellar block-and-equiaxed region. The kinetics of the strain-driven microstructure transformation process, described by the dependence of the relative lamellar area fraction on equivalent true strain accumulated by equal-channel angular pressing (ECAP), physiognomically resembles that of the thermally activated transformation process described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model for the recrystallization process.     

Shear modulus and internal friction in a Cu-0.17% Zr alloy and pure copper subjected to equal-channel angular pressing

The effect of temperature on the shear modulus and low-frequency internal friction of pure copper and a Cu-0.17 wt % Zr alloy subjected to equal channel angular pressing (ECAP) is studied in the temperature range 100–550 K. In both materials, ECAP significantly decreases the shear modulus. It is found that the temperature dependences of the shear modulus of the alloy and pure copper are qualitatively similar and those of the internal friction are markedly different. Possible mechanisms responsible for the anomalous behavior of the elastic moduli and internal friction in the materials subjected to ECAP are discussed.     

Structure,properties, and deformation behavior of corrosion-resistant steel 12Kh18N10T subjected to equal-channel angular pressing

The results of investigation into the evolution of structure, properties, and deformation behavior of corrosion-resistant steel 12Kh18N10T in the course of severe plastic deformation by equal-channel angular pressing are reported. The formation of a fibrous ultrafine-grained structure of the steel, its twofold hardening accompanied by a slight decrease in the plasticity, and changes in the cold-deformation mechanism are shown.     

Shear bands and anisotropy of the mechanical properties of an MA2-1pch magnesium alloy after equal-channel angular pressing

Effect of structure and texture on the anisotropy of the mechanical properties of the MA2-1pch magnesium alloy subjected to equal-channel angular pressing and subsequent annealing has been studied in two mutually perpendicular planes Y and X (along and across the pressing direction). The anisotropy of the mechanical properties is shown to be due to various orientations of shear bands and various types of texture inside the bands and outside them in planes X and Y.     

Equal-channel angular pressing of commercial aluminum alloys: Grain refinement, thermal stability and tensile properties

Using equal-channel angular (ECA) pressing at room temperature, the grain sizes of six different commercial aluminum-based alloys (1100, 2024, 3004, 5083, 6061, and 7075) were reduced to within the submicrometer range. These grains were reasonably stable up to annealing temperatures of ∼200 °C and the submicrometer grains were retained in the 2024 and 7075 alloys to annealing temperatures of 300 °C. Tensile testing after ECA pressing through a single pass, equivalent to the introduction of a strain of ∼1, showed there is a significant increase in the values of the 0.2 pct proof stress and the ultimate tensile stress (UTS) for each alloy with a corresponding reduction in the elongations to failure. It is demonstrated that the magnitudes of these stresses scale with the square root of the Mg content in each alloy. Similar values for the proof stresses and the UTS were attained at the same equivalent strains in samples subjected to cold rolling, but the elongations to failure were higher after ECA pressing to equivalent strains >1 because of the introduction of a very small grain size. Detailed results for the 1100 and 3004 alloys show good agreement with the standard Hall-Petch relationship.     

Formation of a submicrocrystalline structure in austenitic 08Kh18N10T steel during equal-channel angular pressing followed by heating

The structure and mechanical properties of austenitic 08KhN10T steel subjected to equal-channel angular pressing (ECAP) at room temperature (? = 3.2) and subsequent heating are studied. In the course of ECAP, the steel undergoes a martensitic transformation; the martensite content reaches 45%. Upon heating, martensite (ferrite) transforms into austenite. The partly submicrocrystalline oriented structure of the 08Kh18N10T steel in the austenitic (55%)-martensitic (45%) state (formed upon ECAP) provides its high strain hardening (σ_0.2 = 1315 N/mm²), as compared to the initial state (σ_0.2 = 250 N/mm²), and high plasticity δ = 11%. After heating to 550°C, the steel has a predominantly submicrocrystalline austenitic (80%)-ferritic (20%) structure, σ_0.2 = 1090 N/mm², and δ = 11%.     

Effect of grain size on the thermal diffusion of copper in aluminum

The effect of grain size upon the thermal diffusion of copper in aluminum has been investigated. It is found that the heat of transport of copper in fine-grained aluminum is 10.8 kcal/mole, and -7.1 kcal/mole is found in large-grained samples. This grainsize dependence is opposite to that found in self-thermal diffusion of aluminum by Matlock and Stark¹ where the large-grained samples had the larger heat of transport. This copper result might be expected if vacancies are tightly bound to copper atoms in aluminum since the copper flux is antiparallel to that of aluminum and parallel to the vacancy flux. The difference in the heat of transport of copper in aluminum for small and large-grained samples is apparently equal to the heat of vacancy formation in aluminum. The observed difference of 17.9 kcal/mole is in numerical agreement with the value of 17.5 kcal/mole obtained by Simmons and Balluffi.2 R. E. DOERR, Formerly Graduate Student, Department of Mechanical Engineering, University of Texas at Austin, Austin, Tex.     

Effect of equal-channel angular pressing and annealing conditions on the texture,microstructure, and deformability of an MA2-1 alloy

Equal-channel angular pressing (ECAP) of am MA2-1 alloy according to routes A and Bc is used to study the possibility of increasing the low-temperature deformability of the alloy due to grain refinement and a change in its texture. To separate the grain refinement effect from the effect of texture on the deformability of the alloy, samples after ECAP are subjected to recrystallization annealing that provides grain growth to the grain size characteristic of the initial state (IS) of the alloy. Upon ECAP, the average grain size is found to decrease to 2–2.4 μm and the initial sharp axial texture changes substantially (it decomposes into several scattered orientations). The type of orientations and the degree of their scattering depend on the type of ECAP routes. The detected change in the texture is accompanied by an increase in the deformability parameters (normal plastic anisotropy coefficient R, strain-hardening exponent n, relative uniform elongation δ_u) determined upon tensile tests at 20°C for the states of the alloy formed in the IS-4A-4Bc and IS-4Ao-4B_cO sequences. The experimental values of R agree with the values calculated in terms of the Taylor model of plastic deformation in the Bishop-Hill approximation using quantitative texture data in the form of orientation distribution function coefficients with allowance for the activation of prismatic slip, especially for ECAP routes 4B_c and 4B_cO. When the simulation results, the Hall-Petch relation, and the generalized Schmid factors are taken into account, a correlation is detected between the deformability parameter, the Hall-Petch coefficient, and the ratio of the critical shear stresses on prismatic and basal planes.     

Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys

A highly sensitive thermal analysis technique has been used to study the mechanisms of grain refinement in high-purity aluminum. Additions of Al-Ti-B master alloys were made both below and above the peritectic concentration in reference to the Al-rich corner of the binary Al-Ti phase diagram (0.15 pct Ti in solution). The experiments were conducted at various times after the addition of grain refiner. From the results, except for formation of TiB₂, no effect of boron on the Al-rich portion of the binary Al-Ti phase diagram can be observed. With hypoperitectic additions of Al-Ti-B master alloys, TiB₂ particles are the most frequent nucleant for aluminum grains. Also, when Al-5Ti-lB additions are made, nucleation frequently occurs above the equilibrium liquidus temperature. From a thermodynamic point of view, this phenomenon can occur only if regions of the melt (which contain bondes and nucleate new grains) have a higher Ti concentration than is present in the bulk of the liquid. A mechanism has been proposed to account for this observation. When hyperperitectic additions of grain refiner were made, a metastable formation of Al solid was often observed to occur at 2 to 5 deg above the equilibrium peritectic temperature. Other researchers have made this observation and proposed that a metastable aluminide phase was formed, even though no X-ray evidence of this phase was found. The experiments reported here show that the metastable nucleation occurs on boride particles when cooling from high temperature, which allow high (metastable) quantities of dissolved Ti to be retained in portions of the melt.     

Factors affecting the final grain size of decarburized lamination steels

A study was made of the effects of carbon content, strain, initial grain size, carbide morphology, annealing time, and temperature on the final grain size produced after the decarburization anneal. The results show that columnar grains form only in higher carbon (0.05/0.06 Pct C) steels with low initial grain sizes (<20 μm) when decarburized at 1450 °F (788 °C). Equiaxed grains, however, are obtained in low carbon (0.02/0.03 Pct C) lamination steels for all processing conditions. It was found that the formation of both equiaxed and columnar final grain size (GS _f ) was dependent upon percent strain (ε), initial grain size (GS _i ), and carbide distribution (D _c ) according to the following regression equations (all grain size measurements in μm):G5 _f (columnar grain) = 204 + 0.9GS_i - 12.3ε + 37.8D _C logGS _f (equiaxed grain) = 2.151 + 0.022GS_i - 0.038ε - 0.0005GS _i · ε- 0.0002(GS _i )² + 0.079D _c A.R. Marder, formerly with the Research Department, Bethlehem Steel Corporation, Bethlehem, PA 18016 This paper is based on a presentation made at the symposium “Physical Metallurgy of Electrical Steels” held at the 1985 annual AIME meeting in New York on February 24–28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee.