Equal Channel Angular Pressing Strain Research Articles

Grain Boundary Engineering of ECAPed OFHC Copper

Grain boundary character distributions (GBCD) of OFHC copper equal-channel angular pressing (ECAP) deformed and then annealed were analyzed by electron back scatter diffraction (EBSD). The experimental results showed that a combination of ECAP deformation and annealing treatments could significantly increase the fraction of low-Σ coincidence site lattice (CSL) boundaries (Σ≤29) and effectively interrupt the connectivity of random boundaries network in OFHC copper. An increase of low-Σ CSL boundaries from 45.27 to 71.06% was observed in as-received material after one pass ECAP strain followed by annealing at 350 °C for 48 h. The connectivity of random boundaries network was interrupted by high fraction of low-Σ CSL boundaries.

High-strain induced reverse martensitic transformation in an ultrafine-grained Ti-Nb-Ta-Zr alloy

We report on a novel phenomenon, that is a high-strain-induced reverse martensitic transformation in an ultrafine-grained Ti–36Nb–2Ta–3Zr (wt.%) alloy processed by equal channel angular pressing (ECAP) at room temperature. Our results show that a martensitic transformation from body-centred cubic β matrix to orthorhombic α″ martensite occurs under low-strain ECAP conditions and that a large portion (~34%) of martensite transforms into a matrix phase (i.e. reverse martensitic transformation) with increasing ECAP strain to a high value of 4 (i.e. 6 passes) with a corresponding reduction in the α″-lath thickness and a refinement of grain size in the matrix phase.

Microstructural and texture changes during equal channel angular pressing of an Al–Mg–Sc alloy

Evolution of the microstructure and microtexture of an Al–Mg–Sc alloy during equal channel angular pressing (ECAP) at 200 °C was studied. Increasing the ECAP strain of 1 < ε < 4 was accompanied by a rapid hardening followed by a saturation of microhardness at the higher strains. Straining to ε ≤ 4 did not result in any significant structural changes on a mesoscopic level: e.g. the original grains were mainly rotated toward the shearing direction in accordance with the simple shear deformation mode. At that, the texture changes were characterized by creation of a relatively strong duplex texture, typical for conventional high-strain axi-symmetric deformation of fcc metals, i.e. the directions of <100> and <111> were oriented parallel to the pressing direction (PD). At ECAP strains larger than 4, in contrast, significant changes took place in both the microstructure and the texture evolved. Those were mainly related to the frequent formation of microshear bands developed in various directions, that resulted in the local lattice rotations toward <110> and frequent nucleation of new ultrafine grains at high strains. Ultrafine crystallite structures were formed in the alloy characterized by the fraction of high-angle boundaries of about 40–45% and a texture with the near <110> maximum oriented in PD at the highest strain of ε = 12.

Ultrafine Grained Structure Formation in Low Carbon Steel Processed by SPD

The present work deals with grain refinement in low carbon steel (AISI 1010) by severe plastic deformation (SPD). The effect of structure modification was evaluated with respect to thermomechanical (TM) treatment of steel prior to SPD. The grain refinement was accomplished during warm angular channel pressing (ECAP) at 300°C. The evolution of microstructure during equal channel angular pressing (ECAP) was studied using SEM and TEM of thin foils. Ultrafine-grained structure development is described in relation to strainintroduced. At lower strain applied, the subgrain and/or polygonized structure was frequently found. Due to increased deformation temperature, the dynamic recovery contributed to structure refinement in both structural states. The amount of high angle boundaries increased with higher ECAP strain and was higher in TM-processed steel. There was only an indistinctive difference in structure refinement, considering different initial structures of the steel.The deformation behaviour of UFG steel in dependence on processing conditions was evaluated by a tensile test and correlated with structural characteristics.

Shear Induced Low- and High-Angle Boundary Characterization Using Kikuchi Bands in Transmission Electron Microscopy

Microstructure evolution with equal channel angular pressing (ECAP) using route Bc, that is a 90° axial rotation of the billet between passes, up to 8 passes, was investigated by transmission electron microscopy. The study has been focused on the induced development of boundary misorientation and spacing toward microstructure refinement. Cell (low-angle) and grain (high-angle) misorientation and spacing were determined from about 250 boundaries per pass of ECAP, systematically using whether Kikuchi patterns or Moiré fringes, these latter where possible. The average cell size and misorientation saturate within the first two passes. Misorientation and spacing of high-angle boundaries decrease with the number of passes. After 8 passes, mean cell size is ≈ 1.3 µm and the fraction of high-angle boundaries is ≈ 0.7. Differences in rate of grain structure evolution per pass are linked to differences in ability of dislocations introduced in new passes to recombine with the existing ones. As ECAP strain rises, the misorientation distribution develops strong deviations from the MacKenzie distribution for statistical grain orientation. This is interpreted as a result of the tendency to form equiaxed grains in a textured grain structure.

Continuous Recrystallization Phenomenon in High Purity Copper during Equal Channel Angular Pressing up to High Strain at Room Temperature

Equal channel angular pressing (ECAP) was conducted at room temperature to a high strain level of ~24 in high purity copper. Tensile testing, Transition Electron Microscopy (TEM) and Electron backscatter diffraction (EBSD) were used to characterize the microstructure and property evolution with the increase of ECAP strain. It was found that tensile yield strength and the stored energy increases upon ECAP processing until a peak reached at 8~12 passes of ECAP, and their saturation was observed at higher ECAP passes. Continuous recrystallization phenomenon in microstructure was observed, where dislocation free crystallites with large misorientation to their surrounding matrix and smaller than the nuclei for discontinuous recrystallization were observed embodied in the matrix of deformed structure with high dislocation density. A two-step process was observed for the formation of these small crystallites, first the condensing of dislocation tangles into a narrow boundary, mostly low angle boundary; And second local migration (in sub-micrometer range) of short grain boundaries, in strong contrast to the dramatic migration of long large angle grain boundaries during discontinuous recrystallization to swallow the deformed matrix, was observed leading to vanish of small subgrains.

Annealing of Al–Si alloys after equal-channel angular pressing

The annealing response after equal-channel angular pressing (ECAP) of three different Al–Si alloys has been investigated. The alloys were deformed at room temperature by ECAP up to four passes and the microstructural changes after deformation and annealing were studied by light and electron microscopy. The influence of ECAP strain, particle fractions, and annealing temperatures on microstructure and hardness evolutions of the ultra-fine grained materials was examined. High particle contents seemed to enhance recrystallization kinetics. Also, a change from continuous grain coarsening to discontinuous recrystallization occurred at about 200–250 °C.

Transmission Electron Microscopy Study of Strain-Induced Low- and High-Angle Boundary Development in Equal-Channel Angular-Pressed Commercially Pure Aluminum

The evolution of the microstructure in a commercially pure aluminum during equal channel angular pressing (ECAP) using route BC was investigated by transmission electron microscopy. Subgrains, or cells, form, which have both high (ϕ > 15 deg) and low (ϕ < 15 deg) misorientation. Misorientations and spacings of cell boundaries were determined from about 250 boundaries per pass of ECAP cell boundaries on the basis of Kikuchi patterns and Moire fringes. The average cell size and misorientation saturate within the first two passes. Misorientations and spacings of high-angle boundaries decrease with the number of passes. After eight passes, the cell size is ≈1.3 μm and the fraction of high-angle boundaries is ≈0.7. The marked differences in the rate of grain structure evolution per pass are linked to differences in the ability of dislocations introduced in new passes to recombine with the existing ones. With increasing ECAP strain, the distribution of misorientations develops strong deviations from the MacKenzie distribution for statistical grain orientation. This is interpreted as a result of the tendency to form equiaxed grains in a textured grain structure.

Superplastic Behavior of Ultrafine Grained Al Alloys Fabricated by Severe Plastic Deformation

Ultrafine grained (UFG) 5083 Al and 5154 Al alloys were prepared by equal channel angular pressing (ECAP) with an effective strain of ~ 4 or ~ 8. This investigation was aimed at examining the effect of the ECAP strain and post-rolling inducing different microstructure in these alloys on the deformation mechanisms at low temperature superplastic (LTS) and high strain superplastic (HSRS) regimes. The sample after 4 passes (a strain of ∼ 4) did not exhibit LTS, but superplastic elongations were obtained in the sample after 8 passes (a strain of ∼ 8). An analysis of the mechanical data in light of the standard deformation mechanisms revealed that deformation of the sample after 4 passes was governed by dislocation climb while grain boundary sliding attributed to LTS of the sample after 8 passes. In addition, the 5154 Al alloy processed by ECAP and postrolling was capable of enhancing HSRS elongation significantly. An analysis revealed that the deformation mode was changed from dislocation viscous glide to grain boundary sliding by additional ECAP strain and post-rolling.

Creep and Mechanical Properties of a Commercial Aluminum Alloy Processed by ECAP

A commercial spray-cast aluminum alloy, having a composition of Al-11.5% Zn-2.5% Mg-0.9% Cu-0.2% Zr, was processed by equal-channel angular pressing (ECAP) to give an ultrafine-grained microstructure with a grain size of ~0.3 μm and a fracturing of the rod-shaped MgZn2 precipitates. The mechanical properties of the alloy were investigated using microhardness testing at ambient temperature and tensile testing at a temperature of 673 K. In addition, creep tests were conducted using a double-shear testing facility at temperatures from 473 to 673 K. The results show the microhardness decreases with increasing numbers of passes in ECAP and high strain rate superplasticity is achieved after processing by ECAP at 473 K for 6 or 8 passes. A maximum elongation of &gt;1000% was recorded at a strain rate of 10-2 s-1 when testing at 673 K. The creep tests show the creep rates are faster in the as-pressed material than in the as-received material when testing at the same applied stress and testing temperature.

Comparison of the microstructure and thermal stability of an AZ31 alloy processed by ECAP and large strain hot rolling

The aim of this work is to compare the microstructure, the texture, as well as the thermal stability of an AZ31 Mg alloy processed via two different severe plastic deformation processing techniques, namely large strain hot rolling (LSHR) and equal channel angular pressing (ECAP). The microstructure was characterized by optical microscopy and the texture was measured both by X-ray diffraction and electron backscatter diffraction (EBSD). The microstructure obtained via LSHR has average grain sizes around 3 μm, but it is quite heterogeneous. Additionally, a well-defined basal texture develops. ECAP gives rise to a more homogeneous and slightly coarser microstructure, with an average grain size of 7 μm and a shear type texture. The higher resistance of the extruded sample to secondary recrystallization after severe post-deformation annealing is attributed to a texture effect.

Equal channel angular pressing of magnesium alloy AZ31

Equal channel angular pressing (ECAP) was applied to a wrought magnesium alloy AZ31 for up to 8 passes at temperatures as low as 100 °C. The application of a back pressure was critical in deforming Mg alloys at lower temperatures. Room temperature mechanical properties were obtained by tensile and hardness tests. With increasing ECAP strain, the initial coarse grained structure was transformed into a submicrometer-grained microstructure. In general, hardness increased with decreasing grain size although the changes in tensile strength and ductility were more complicated.

Enhancement of high strain rate superplastic elongation of a modified 5154 Al by subsequent rolling after equal channel angular pressing

Subsequent rolling after equal channel angular pressing (ECAP) resulted in a considerable enhancement of the high strain rate superplastic elongation of a modified 5154 Al alloy, compared to that of the alloy subjected to the identical ECAP strain without rolling. The mechanical data revealed that the deformation of the former was governed by grain boundary sliding but that of the latter was dominated by viscous glide.

Mechanical Properties and Microstructures of an AZ31 Mg Alloy through Severe Plastic Deformation/ Mechanische Eigenschaften und Gefüge einer AZ31-Mg-Legierung nach starker plastischer Verformung

Equal channel angular pressing (ECAP) technique was adopted to a dilute Mg-Al alloy (AZ31) for its enhancement in formability and strength through grain refinement. The microstructure is reasonably homogeneous and equiaxed after 4 passings. The grains of the ECAPed AZ31 alloy (2.5 μm after 4 passings) are not so fine as those of the ECAPed Al alloys (0.5-1 mm) with the same ECAP strain due to the difference in ECAP processing temperature. In the activation energy for grain growth, there is clear evidence for a transition having an activation of 0.18 Q 1 (activation for lattice diffusion) from an activation energy of 0.46 Q 1 at temperatures of 473 - 523 K to an activation energy of 0.86 Q, at temperatures of 673 - 773 K. The yield stresses of the ECAPed alloy are lower than the unECAPed material whereas the hardness of the former is higher than that of the latter. For YS, the softening effect due to texture anisotropy overwhelms the strengthening due to grain refinement.

Effect of ECAP Strain on Deformation Behavior at Low Temperature Superplastic Regime of Ultrafine Grained 5083 Al Alloy Fabricated by ECAP

A series of tensile testing was carried out on the ultrafine grained 5083 Al alloy, which was fabricated by equal channel angular pressing (ECAP) with different ECAP strains, at low temperature superplastic (LTS) temperature of 548 K. This investigation was aimed at examining the effect of the ECAP strain inducing different microstructure in the alloy on the deformation mechanisms at LTS regime. The most distinguishable microstructural evolution by increasing the ECAP strain from ∼4 to ∼8 was an increment of a portion of high angle boundaries while the (sub)grain size remained almost unchanged. The sample after 4 passes (a strain of ∼4) did not exhibit LTS, but superplastic elongations were obtained in the sample after 8 passes (a strain of ∼8). An analysis of the mechanical data in light of the standard deformation mechanisms revealed that deformation of the sample after 4 passes was governed by dislocation climb while grain boundary sliding attributed to LTS of the sample after 8 passes. The difference of the deformation mechanisms in the present case was discussed in terms of the microstructures developed under different ECAP strains.

Annealing behavior of submicrometer grained ferrite in a low carbon steel fabricated by severe plastic deformation

The annealing behavior of submicrometer grained ferrite in a low carbon steel processed by equal channel angular pressing (ECAP) was investigated. In particular, the effect of the ECAP strain on the annealing behavior of submicrometer grained ferrite was studied by examining the microstructural changes of the samples processed by two different ECAP strains, 4 and 8, during static annealing. Contrary to the general belief that more severely worked materials exhibit faster kinetics of microstructural evolution related to annealing, i.e. recovery, recrystallization and grain growth, the steel subjected to higher ECAP strain exhibited more sluggish recovery and recrystallization kinetics. The more sluggish recovery and recrystallization kinetics with increasing the ECAP strain is associated with the carbon dissolution from pearlitic cementite during ECAP. The grain growth kinetics of submicrometer grained ferrite was analyzed by the non-ideal grain growth law. The grain growth exponent was about 0.2 and the apparent activation energy for grain growth was 170–180 kJ mol −1. Neither value was influenced by the ECAP strain.

Ultrafine Grained Low Carbon Steels Fabricated by Equal Channel Angular Pressing: Microstructures and Tensile Properties.

Equal channel angular pressing (ECAP) was conducted on the two grades of low carbon steel, with or without vanadium, in order to produce an ultarfine grained structure. As a result, the ferrite grains were re-fined from 30 µm to 0.2–0.3 µm. The strength of the ECAPed steels increased remarkably, over twice of the strength of the steels before ECAP. A series of static annealing experiments showed that the increment of ECAP strain and the dilute addition of microalloying element such as vanadium were very effective on enhancing thermal stability of the ultrafine gained low carbon steels produced by ECAP in terms of microstructure and tensile properties. This enhanced thermal stability resulted from; (a) presence of excessive carbon content in the ferrite matrix by carbon dissolution from pearlitic cementite during ECAP; (b) preservation of high dislocation density due to addition of vanadium, providing the effective diffusion path for dissolved carbon atoms; (c) precipitation of excessive carbon as the form of nano-sized cementite particles during subsequent annealing and its effect on suppressing grain growth.