Wear behavior of an aluminum alloy processed by equal-channel angular pressing

Wear tests were conducted on an aluminum Al-1050 alloy after processing by equal-channel angular pressing (ECAP). The results show that the coefficient of friction remains unchanged after processing by ECAP, but there is a decrease in the wear resistance and a mass loss that increases with increasing numbers of ECAP passes. The results are consistent with a wear mechanism map and confirm the occurrence of a severe wear mechanism. The decreasing wear resistance after ECAP is attributed to the significant grain refinement introduced by ECAP and the lack of a strain hardening capability.     

Characteristics of face-centered cubic metals processed by equal-channel angular pressing

This review surveys the characteristics of face-centered cubic (fcc) metals and alloys processed by equal-channel angular pressing (ECAP). The significance of the Hall–Petch relationship for ultra-fine grained structures is examined and the dependence of the saturated stress obtained in ECAP on the absolute melting temperature is described and discussed. In addition, the flow processes at low temperatures in ultrafine-grained materials and the microstructural evolution of the dislocation densities and precipitates in some alloys of practical importance are also considered briefly.     

Dry sliding wear of an AZ31 magnesium alloy processed by equal-channel angular pressing

A magnesium AZ31 alloy was processed by equal-channel angular pressing (ECAP) for up to 8 passes to reduce the grain size to ~1.0 μm. Following ECAP, microhardness measurements were taken to evaluate the mechanical properties of the material. Ball-on-disc dry sliding tests were conducted to compare the wear behaviour of the as-received alloy and the alloy processed by ECAP. The surface topography and volume loss were recorded for all samples. The results show that the fluctuations and average values of the coefficient of friction are improved after processing by ECAP. In addition, there is a decrease in the wear depth and volume loss with increasing numbers of ECAP passes. The ECAP-processed alloy has a higher wear resistance than the unprocessed alloy and it is a suitable candidate material for use in industrial applications.     

High temperature thermal stability of ultrafine-grained silver processed by equal-channel angular pressing

The high temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking-fault-energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures in two samples having purity levels of 99.995 and 99.99 at.% were achieved by four passes of equal-channel angular pressing at room temperature. The defect structure was studied by electron microscopy, X-ray line profile analysis, and positron annihilation spectroscopy before and after the exothermic DSC peak related to recovery and recrystallization. The heat released in the DSC peak was correlated to the change of defect structure during annealing. It was found for both compositions that a considerable fraction of stored energy (~15–20 %) was retained in the samples even after the DSC peak due to the remaining UFG regions and a large density of small dislocation loops in the recrystallized volumes. The larger impurity level in Ag yielded a higher temperature of recrystallization and a lower released heat. The latter observation is explained by the much lower vacancy concentration before the DSC peak which is attributed to the segregation of dopants at grain boundaries resulting in a smaller free volume in the interfaces.     

Processing by equal-channel angular pressing: Applications to grain boundary engineering

Equal-channel angular pressing (ECAP) is a processing technique in which a sample is pressed through a die constrained within a channel so that an intense strain is imposed without incurring any change in the cross-sectional dimensions of the sample. This procedure may be used to achieve considerable grain refinement in pure metals and metallic alloys with as-pressed grain sizes lying typically within the submicrometer range. Careful experiments reveal only a minor change in the grain size with increasing numbers of passes through an ECAP die but there is a significant change in the distribution of grain boundary misorientations as a function of the total imposed strain. In practice, the microstructure evolves with increasing strain from an array of grains where the boundaries are predominantly in low-angle misorientations to an array of grains where a high fraction (typically ≥60%) is in high-angle misorientations. This evolution has a significant effect on the characteristics of the as-pressed materials including the high temperature mechanical properties and the measured rates of diffusion. In addition, the evolution provides an opportunity to use Grain Boundary Engineering in order to optimize the behavior of the material.     

Microstructure and mechanical properties of ultrafine grain ZK60 alloy processed by equal channel angular pressing

The microstructure evolution and tensile properties of ZK60 magnesium alloy after equal channel angular pressing (ECAP) have been investigated. The results show that the two-step ECAP process is more effective in grain refinement than the single-step ECAP process due to the lower deformation temperature, a mean grain size of ~0.8 μm was obtained after two-step ECAP process at 513 K for four passes and 453 K for four passes. The EBSD examination reveals that ZK60 alloy after two-step ECAP process exhibits a more homogeneous grain size and misorientation distribution than single-step ECAP process. Both alloys after ECAP process present similar strong {0002} texture. The tensile strength of two-step ECAP alloy has also been improved compared with the single-step ECAP alloy. The strengthening effect was mainly ascribed to grain refinement.     

Principles of equal-channel angular pressing as a processing tool for grain refinement

During the last decade, equal-channel angular pressing (ECAP) has emerged as a widely-known procedure for the fabrication of ultrafine-grained metals and alloys. This review examines recent developments related to the use of ECAP for grain refinement including modifying conventional ECAP to increase the process efficiency and techniques for up-scaling the procedure and for the processing of hard-to-deform materials. Special attention is given to the basic principles of ECAP processing including the strain imposed in ECAP, the slip systems and shearing patterns associated with ECAP and the major experimental factors that influence ECAP including the die geometry and pressing regimes. It is demonstrated that all of these fundamental and experimental parameters play an essential role in microstructural refinement during the pressing operation. Attention is directed to the significant features of the microstructures produced by ECAP in single crystals, polycrystalline materials with both a single phase and multi-phases, and metal-matrix composites. It is shown that the formation of ultrafine grains in metals and alloys underlies a very significant enhancement in their mechanical and functional properties. Nevertheless, it is demonstrated also that, in order to achieve advanced properties after processing by ECAP, it is necessary to control a wide range of microstructural parameters including the grain boundary misorientations, the crystallographic texture and the distributions of any second phases. Significant progress has been made in the development of ECAP in recent years, thereby suggesting there are excellent prospects for the future successful incorporation of the ECAP process into commercial manufacturing operations.     

Cyclic softening of ultrafine-grained AZ31 magnesium alloy processed by equal-channel angular pressing

The low-cycle tension–compression fatigue tests were performed at ambient temperature on ultrafine grained AZ31 magnesium alloy processed by equal channel angular pressing. All samples exhibited cyclic softening, and the softening effect increased with increasing total strain amplitude. Observations by optical microscope revealed that pronounced recrystallization occurred, and the direction of larger axis of recrystallized grains was nearly 45° with respect to the loading axis. Local grain coarsening leads to the formation of dense shear localization, while line-like damage traces remain in fine grains. A model is proposed to account for the recrystallization, based on the characteristic distribution of defects introduced by equal channel angular pressing.     

Thermal stability of ultrafine grains size of pure copper obtained by equal-channel angular pressing

Ultrafine grain of pure copper 99.98% was produced by severe plastic deformation using the equal-channel angular pressing (ECAP) method. Copper samples were ECAPed from 1 to 8 passes following route B_C; fine grain sizes of 250 nm were developed after eight passes. Important enhancement in the mechanical strength properties was obtained. Subsequent heat treatments (HT) were carried out to evaluate the thermal stability of the grains of the ECAPed samples. Microstructure and mechanical properties were evaluated to determine the recovery and recrystallization temperatures. Differential scanning calorimetry (DSC) tests were conducted to all samples in order to determinate these temperatures. The activation energy of the recrystallization process was also determined by the DSC technique and good correlation was obtained with the microstructure and mechanical properties. An important decrease in the mechanical properties and an increasing heterogeneous grain size distribution were observed when heat treatments were performed.     

Microstructures and properties of ultrafine-grained pure titanium processed by equal-channel angular pressing and cold deformation

Equal-channel angular pressing (ECAP) has been used to refine the grain size of commercially pure (CP) titanium as well as other metals and alloys. CP-Ti is usually processed at about 400 degrees C because it lacks sufficient ductility at lower temperature. The warm processing temperature limits the ability of the ECAP technique to improve the strength of CP-Ti. We have employed cold deformation following warm ECAP to further improve the strength of CP-Ti. Ti billets were first processed for eight passes via ECAP route Bc, with a clockwise rotation of 90 degrees between adjacent passes. The grain size obtained by ECAP alone is about 260 nm. The billets were further processed by cold deformation (cold rolling) to increase the crystalline defects such as dislocations. The strength of pure Ti was improved from 380 to around 1000 MPa by the two-step process. This article reports the microstructures, microhardness, tensile properties, and thermal stability of these Ti billets processed by a combination of ECAP and cold deformation.     

Achieving superplastic properties in a Pb–Sn eutectic alloy processed by equal-channel angular pressing

Experiments were conducted on a Pb-62% Sn eutectic alloy containing 160 ppm of Sb. The alloy was processed by equal-channel angular pressing (ECAP) through 1 to 5 passes at room temperature and then tested in tension at a temperature of 423 K using initial strain rates from 1.0 × 10⁻⁴ to 1.0 × 10⁻¹ s⁻¹. Excellent superplastic elongations were achieved at intermediate strain rates with a maximum elongation to failure of 2,665%. It is shown that, for processing through similar numbers of ECAP passes, these elongations are higher than in an earlier investigation using a Pb-62% Sn alloy of higher purity. The results are presented pictorially in the form of a deformation mechanism map by plotting normalized grain size against normalized stress at a temperature of 423 K.     

Microstructure and tensile strength of grade 2 titanium processed by equal-channel angular pressing and by rolling

Commercially pure titanium strengthened by severe plastic deformation constitutes an alternative to the use of complex Ti alloys in many medical or industrial applications. In this research, rods of grade 2 Ti were processed by up to six passes using Equal-channel angular pressing (ECAP) at 573 K followed by cold rolling at room or subzero temperatures. After four passes of ECAP, the grain size was refined down to the submicrometer scale and subsequent rolling led to further refinement. The microstructure was characterized by taking Vickers microhardness measurements and tensile testing was performed both at room temperature and in the temperature range of 573–773 K. The results show that at all temperatures the tensile strength is significantly improved by means of these processing techniques. At room temperature, the ultimate tensile strength of pure Ti after ECAP plus subzero rolling is close to that of the traditional Ti-6Al-4V alloy while maintaining adequate levels of elongation to failure.     

Effects of grain size on compressive behaviour in ultrafine grained pure Mg processed by equal channel angular pressing at room temperature

Ultrafine-grained pure magnesium with an average grain size of 0.8 μm was produced by refining coarse-grained (980 μm) ingot by multi-pass equal channel angular pressing (ECAP) at room temperature with the application of a back pressure. The compressive deformation behaviour at room temperature depended on grain size, with deformation twinning and associated work hardening observed in coarse-grained Mg, but absent in the ultrafine grained material as decreasing grain size raised the stress for twinning above that for dislocation slip. The ultrafine grained Mg showed good plasticity with prolonged constant stress after some initial strain hardening.     

Review: Processing of metals by equal-channel angular pressing

Equal-channel angular pressing (ECAP) is a processing method in which a metal is subjected to an intense plastic straining through simple shear without any corresponding change in the cross-sectional dimensions of the sample. This procedure may be used to introduce an ultrafine grain size into polycrystalline materials. The principles of the ECAP process are examined with reference to the distortions introduced into a sample as it passes through an ECAP die and especially the effect of rotating the sample between consecutive presses. Examples are presented showing the microstructure introduced by ECAP and the consequent superplastic ductilities that may be attained at very rapid strain rates.     

Visco-plastic self-consistent modelling of a grain boundary misorientation distribution after equal-channel angular pressing in an AZ31 magnesium alloy

This study applies a visco-plastic self-consistent (VPSC) model to an AZ31 alloy processed by equal-channel angular pressing. The study focuses on the possibility of reproducing a grain misorientation distribution and the distribution of coincident site lattice boundaries in the model framework. Co-rotation and a magnesium misorientation scheme are employed together with the conventional VPSC model to improve its predictions. The results of the model are then compared with experimental data.