Grain Refinement in Pure Aluminum Severely Deformed by Equal Channel Angular Extrusion via Extended Processing Routes

AA1060 pure aluminum billets were processed by eight passes of equal channel angular extrusion (ECAE) using 90 and 120 deg dies via processing routes characterized by an inter-pass billet rotation angle (χ) varying from 0 to 180 deg. The grain refinement efficiencies achieved in the different processing conditions were investigated by comparing misorientation and grain size in the deformed samples measured by electron back-scatter diffraction. The results reveal an overall decrease of grain refinement efficiency with an increase of χ for both dies. This trend corroborates the general observations in various face-centered cubic metals processed using a 120 deg die and can be satisfactorily explained by correlating the relative grain refinement efficiency to the relative significance of newly activated slip systems at pass-to-pass transitions. For ECAE with the 90 deg die, the route-dependency of grain refinement found in the AA1060 samples contradicts some of the observations in the literature, and the main discrepancies are located for routes with χ = 0 to 90 deg. Comparison of the present results with those of pure copper processed under similar conditions further reveals that these discrepancies could be mainly ascribed to differences in the characteristics of the materials, and that it is irrational to simply claim the route with χ = 0 or 90 deg as the optimal route without necessary experimental validations for a specific material.     

The effect of temperature and equal-channel angular-pressing routes on the form of powders and structure in pressings

The results of investigations on varying the shape and fragmentation of aluminum powders of the PA-2 grade of 300–500 μm in size, briquette made of which was subjected to equichannel angular pressing (ECAP) at room temperature and 350°C. ECAP was performed by different routes up to five passages. The sample structure was studied by metallographic method in the longitudinal and transverse sections. It is established that ECAP at room temperature, including fivefold pressing (ɛ > 5.5), no deformation fragmentation occur. In the case of hot ECAP, density of samples after the fourth passage was close to the theoretical one. The structure of the material obtained by routes B _C and A consists of fine grains, while after the ECAP by route C it has the duplex nature and includes both fine and coarse grains.     

Effects of Route on Microstructural Evolution and Mechanical Properties of Cu-8 Wt Pct Ag Alloy Processed by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) is applied to investigate the microstructural evolution and mechanical properties of Cu-8 wt pct Ag alloy subjected to one to four passes via four different routes (A, B_A, B_C, and C). It is demonstrated that better mechanical properties, a higher fraction of high-angle boundaries, and a smaller grain size can be obtained most rapidly with route A, whereas the specimen processed by route B_C contains relatively inhomogeneous microstructure and has poor mechanical properties. The ultimate tensile stress (UTS) of the Cu-Ag alloy processed by route B_C saturates after four passes; in contrast, the UTS of the Cu-Ag alloy processed by route A increases continuously in relation to the number of ECAP passes. Based on the experimental results, the strengthening mechanisms of the Cu-8 wt pct Ag alloy processed by different routes as well as the efficiency of different routes in refining the binary alloy are discussed.     

Microstructural Evolution and Mechanical Response of Equal-Channel Angular Extrusion-Processed Al-40Zn-2Cu Alloy

Microstructural evolution, tensile properties, and impact toughness of an aluminum-zinc-copper (Al-40Zn-2Cu) alloy subjected to repetitive equal-channel angular extrusion (ECAE) up to four passes following either route A or route B_C were investigated. The experimental results reveal that the ECAE eliminated as-cast dendritic microstructure along with casting defects such as microporosities almost completely. The ECAE-processed samples consisted of mostly elongated microconstituents via route A and equiaxed microconstituents via route B_C. The high stresses imposed in ECAE lead to the fragmentation of the copper-rich θ phase into smaller particles with significant fragmentation occurring in the first pass and additional breaking in the subsequent passes in both routes. The ECAE processing simultaneously increased both the strength and ductility of the alloy as compared to the as-cast state, regardless of the processing route and number of passes. The deformation behavior of as-cast Al-40Zn-2Cu alloy has changed from brittle to ductile mode after ECAE due to the microstructural refinement, deformation-induced homogenization, and reduction of porosities. The limited impact toughness of as-cast alloy was significantly improved by multipass ECAE, especially in route A.     

On the Microstructural Stability of Ultrafine-Grained Interstitial-Free Steel under Cyclic Loading

The microstructural stability of ultrafine-grained (UFG) interstitial-free (IF) steel under cyclic loading was investigated. The samples were extracted from material processed along two different equal channel angular extrusion (ECAE) routes (4C and 4E) at room temperature. Low-cycle fatigue tests were carried out in addition to electron and optical microscopy in order to characterize the microstructural evolution induced by cyclic deformation. The results revealed substantial differences in microstructure resulting from different processing routes. Specifically, the volume fraction of high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs) varied significantly depending on the processing route. The different microstructural characteristics stemming from different ECAE routes expressively influence the fatigue response. Route-4C-processed material displays cyclic softening, while processing along route 4E leads to microstructural stability under cyclic loading. This highly route-dependent trend in the cyclic stress-strain response is attributed to the instability of the LAGBs and stability of HAGBs during cyclic deformation, which is further supported by electron backscattering diffraction results. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.     

Mechanical Properties and Microstructure of AZ31B Magnesium Alloy Processed by I-ECAP

Incremental equal channel angular pressing (I-ECAP) is a severe plastic deformation process used to refine grain size of metals, which allows processing very long billets. As described in the current article, an AZ31B magnesium alloy was processed for the first time by three different routes of I-ECAP, namely, A, B_C, and C, at 523 K (250 °C). The structure of the material was homogenized and refined to ~5 microns of the average grain size, irrespective of the route used. Mechanical properties of the I-ECAPed samples in tension and compression were investigated. Strong influence of the processing route on yield and fracture behavior of the material was established. It was found that texture controls the mechanical properties of AZ31B magnesium alloy subjected to I-ECAP. SEM and OM techniques were used to obtain microstructural images of the I-ECAPed samples subjected to tension and compression. Increased ductility after I-ECAP was attributed to twinning suppression and facilitation of slip on basal plane. Shear bands were revealed in the samples processed by I-ECAP and subjected to tension. Tension–compression yield stress asymmetry in the samples tested along extrusion direction was suppressed in the material processed by routes B_C and C. This effect was attributed to textural development and microstructural homogenization. Twinning activities in fine- and coarse-grained samples have also been studied.     

Microstructure and properties of copper and aluminum alloy 3003 heavily worked by equal channel angular extrusion

A technique invented in the former Soviet Union and recently introduced in the United States, called equal channel angular extrusion (ECAE), produces intense and uniform deformation by simple shear and is applied to 25 × 25 × 152-mm billets of Cu 101 and Al 3003. Microcrystalline structures with a grain size of 0.2 to 0.4 μm are created during room-temperature multipass ECAE deformation for true strains lying in the range ε=2.31 to 9.24. Evidence shows that intense simple shear deformation promotes dynamic or continuous recrystallization by subgrain rotation. The effects of the number of extrusion passes and deformation route for Cu 101, and the deformation route after four passes for Al 3003, are studied. Increasing the number of ECAE passes in Cu 101 causes strength to reach saturation and grain refinement stabilization after four passes (true strain of 4.68), and subgrain misorientation to increase as the number of passes increases. For multipass ECAE with billet orientation constant (route A) or rotated 90 deg between all passes (route B), two levels of structures are created inside the original grains: shear bands (first level) and very fine subgrains (second level) within the shear bands. For a billet rotation of 180 deg between passes (route C), an unusual event is observed. At each even numbered pass, shear bands nearly disappear and only subgrains are present inside the original grains. Route B gives the highest strength, whereas route C produces a more equiaxed and stable microstructure. Subsequent static recrystallization increases the average grain size to 5 to 10 μm.     

Routes Investigation in Equal Channel Multi-angular Pressing Process of UFG Al−3% Mg Alloy Strips

Equal channel angular pressing is one of the most efficient techniques among severe plastic deformation methods that enhances the mechanical properties of polycrystalline metals by refining subjected grains. In this article, equal channel multi-angular pressing was conducted on Al5754 strips. At the first step, finite element analysis was applied to evaluate the possible routes (A and C). The initial analysis showed that the route C was better in strain homogeneity compared with route A. Thus, the route C was considered for further investigations. Then, the effects of the die geometrical parameters on the created equivalent plastic strain (PEEQ) and process force were investigated by FEM. The results showed that the die channel angle (Φ₂) was the most effective parameter on both PEEQ and process force, while the die corner angle (ψ₁) had the least effect on both objectives.     

Strain-path effects on the evolution of microstructure and texture during the severe-plastic deformation of aluminum

Microstructure and texture evolution during the severe-plastic deformation (SPD) of unalloyed aluminum were investigated to establish the effect of processing route and purity level on grain refinement and subgrain formation. Two lots of aluminum with different purity levels (99.998 pct Al and 99 pct Al) were subjected to large plastic strains at room temperaturevia four different deformation processes: equal-channel angular extrusion (ECAE), sheet rolling, conventional conical-die extrusion, and uniaxial compression. Following deformation, microstructures and textures were determined using orientation-imaging microscopy. In commercial-purity aluminum, the various deformation routes yielded an ultrafine microstructure with a ∼1.5-μm grain size, deduced to have been formedvia a dynamic-recovery mechanism. For high-purity aluminum, on the other hand, the minimum grain size produced after the various routes was ∼20 μm; the high fraction of high-angle grain boundaries (HAGBs) and the absence of subgrains/deformation bands in the final microstructure suggested the occurrence of discontinuous static recrystallization following the large plastic deformation at room temperature. The microstructure differences were underscored by the mechanical properties following four ECAE passes. The yield strength of commercial-purity aluminum quadrupled, whereas the high-purity aluminum showed only a minor increase relative to the annealed condition.     

An investigation of microstructure and grain-boundary evolution during ECA pressing of pure aluminum

High-purity aluminum (99.99 pct) was processed by equal-channel angular pressing (ECAP) at room temperature through a die with a 90 deg angle between the die channels. Samples were examined by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM) methods after one, four, and 12 passes through the die. Repetitively pressed samples were rotated by 90 deg in the same sense between successive pressing operations (route B_C). After one pressing, TEM showed a subgrain structure which was elongated in the shearing direction. Corresponding OIM data illustrated an inhomogeneous microstructure in which bandlike features were also aligned with the shearing direction. The lattice orientation varied from location to location in the material. The boundary disorientation distribution determined from the OIM data exhibited a peak at 2 to 5 deg, in agreement with a predominance of subgrains in the microstructure. After four pressings, the microstructure data obtained by TEM and OIM were mutually consistent. The disorientation data revealed a decrease in the population of 2 to 5 deg boundaries accompanied by an overall upward shift in the distribution. Two orientations were generally apparent in the texture, although specific orientations varied with location. Often, a 〈111〉 orientation tended to align with the shear direction. Following 12 ECA passes, the grain size was reduced further to about 1.0 μm. The populations of high-angle boundaries (≥15 deg) increased in the disorientation distribution. A texture characteristic of shear deformation of fcc metals became apparent, although the orientations and particular components varied with location. Microstructural refinement during severe straining includes the development of large fractions of high-angle boundaries.     

Evolution of crystallographic texture during equal channel angular extrusion of copper: The role of material variables

The evolution of crystallographic texture during equal channel angular extrusion (ECAE) using route A has been investigated experimentally as well as by simulations for three types of materials: pure, commercially pure, and impure (cast) copper. The ECAE texture of copper can be compared with simple shear textures. However, there are deviations in terms of location of the respective components. These differences can be nearly reproduced using a recent flow line approach for ECAE deformation (L.S. Tóth, R. Arruffat-Massion, L. Germain, S.C. Baik, and S. Suwas: Acta Mater., 2004, vol. 52, pp. 1885–98) with the help of the viscoplastic self-consistent polycrystal model. The main texture components common to all three materials are A_1E and B_E/B_E; the latter ones are significantly stronger in the cast material. The effect of further deformation on texture modification depends on material variables, such as purity, initial microstructure, and texture.     

Interfacial Microstructure in a B4C/Al Composite Fabricated by Pressureless Infiltration

In this work, B₄C particulate-reinforced Al composite was fabricated by a pressureless infiltration technique, and its interfacial microstructure was studied in detail by X-ray diffraction as well as by scanning and transmission electron microscopy. The B₄C phase was unstable in Al melt during the infiltration process, forming AlB₁₀-type AlB₂₄C₄ or Al_2.1B₅₁C₈ as a major reactant phase. The Al matrix was large grains (over 10 μm), which had no definite orientation relationships (ORs) with the randomly orientated B₄C or its reactant particles, except for possible nucleation sites with { 011}_{\textB4 \textC} \{ 011\}_{{{\text{B}}_{4} {\text{C}}}} almost parallel to {111}_Al at a deviation angle of 1.5 deg. Both B₄C–Al and reactant–Al interfaces are semicoherent and free of other phases. A comparison was made with the SiC/Al composite fabricated similarly by the pressureless infiltration. It was suggested that the lack of ORs between the Al matrix and reinforced particles, except for possible nucleation sites, is the common feature of the composites prepared by the infiltration method.     

Relationship between texture and DC magnetic induction after various stages of processing in a 1 pct Si steel

Hot-rolling using a finishing temperature of 1450°F (series A) produces at the mid-thickness a strong partial fiber texture with the [011] axis parallel with the rolling direction and a sharp texture gradient towards the surface. A finishing temperature of 1600°F (series C) results in a near random mid-thickness texture and less texture heterogeneity. The dc magnetic induction at 100 oe,B(100), was measured at 0, 45, and 90 deg to the rolling direction. For series A,B ₄₅>B ₀>B ₉₀ while for series C,B(100) was more isotropic in accordance with the observed texture. After rolling 25 through 50 pct and annealing, series A was less highly textured than series C with the principal differences between the two materials being the intensities of the (200), (220) and (222) planes. The individual magnetic properties of both series showed changes with increasing deformation in whichB ₄₅ decreased whileB ₀ andB ₉₀ increased in A and changed little in C. The averaged magnetic properties of each series based onB ₀ andB ₉₀, , differed from those based uponB ₀,B ₄₅, andB ₉₀, , since in the hot-rolled condition while . After annealing, with increasing prior cold reduction, increased more rapidly than while remained constant and decreased slightly.     

Quantitative Microstructural Characterization of Thick Aluminum Plates Heavily Deformed Using Equal Channel Angular Extrusion

A detailed quantitative analysis of the microstructure has been performed in three orthogonal planes of 15-mm-thick aluminum plates heavily deformed via two equal channel angular extrusion (ECAE) routes. One route was a conventional route A with no rotation between passes. Another route involved sequential 90?deg rotations about the normal direction (ND) between passes. The microstructure in the center of these plates, and especially the extent of microstructural heterogeneity, has been characterized quantitatively and compared with that in bar samples extruded via either route A or route Bc with 90?deg rotations about the longitudinal axis. Statistically robust data were obtained in this work using gallium enhanced microscopy and EBSD mapping of large sample areas. For the plate processed using route A, the fraction of high-angle boundaries was found to strongly depend on the inspection plane, being smallest in the plane perpendicular to the ND (plane Z), where the largest subgrain size and most profound microstructural heterogeneities were also revealed. In comparison, the plate extruded with 90?deg rotations about the ND was less heterogeneous and contained smaller subgrains in plane Z. Comparing the plate and bar samples, the most refined and least heterogeneous microstructure was observed in the route Bc bar sample. The differences in the microstructure are reflected in the hardness data; the hardness is lowest after ECAE via route A and greatest in the bar sample processed using route Bc.     

Effect of Texture and Grain Size on Bio-Corrosion Response of Ultrafine-Grained Titanium

The bio-corrosion response of ultrafine-grained commercially pure titanium processed by different routes of equal-channel angular pressing has been studied in simulated body fluid. The results indicate that the samples processed through route B_c that involved rotation of the workpiece by 90 deg in the same sense between each pass exhibited higher corrosion resistance compared to the ones processed by other routes of equal-channel angular pressing, as well as the coarse-grained sample. For a similar grain size, the higher corrosion resistance of the samples exhibiting off-basal texture compared to shear texture indicates the major role of texture in corrosion behavior. It is postulated that an optimum combination of microstructure and crystallographic texture can lead to high strength and excellent corrosion resistance.     

Phase equilibria in the ternary system zirconium-molybdenum-carbon

Summary The system Zr-Mo-C was investigated by means of x-ray diffraction and microstructural analyses. Phase equilibria in the system at 1400 and 600°C were established (Fig. 1). The compound Mo₂C dissolves about 4 at.% Zr. The ZrC-base solid solution extends up to 90 mol.% Mo₃C₂. The hexagonal phase Mo₃C₂ may be stabilized at 1400°C and below by zirconium.     

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.     

Deformation of porous materials during hot forging combined with extrusion

Conclusions A study has been made of the character of the redistribution of volumes of a porous blank during hot forging with elements of extrusion; the feasibility is shown of controlling the process with the aim of ensuring that certain volumes of material occupy the required positions. It has been confirmed by experiment that the process of hot forging of porous materials with elements of extrusion involves more than one stage. The character of pressing pressure variation in various stages has been determined. In pressing in conical dies densification of central volumes of material lags slightly behind that of peripheral regions. For specimens of the geometric parameters investigated, the width of the peripheral blank zone, which in the steady-state extrusion stages moves parallel to the compressing part of the die, is (0.15–0.20)D_c. The degree of deformation and the redistribution of volumes of material in peripheral layers are affected mainly by the temperature conditions of the pressing process and to a much smaller extent by the starting blank porosity.Translated from Poroshkovaya Metallurgiya, No. 6(282), pp. 15–19, June, 1986.