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.     

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.     

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.     

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.     

Superplasticity of Mg-Zn-Y alloy containing quasicrystal phase processed by equal channel angular pressing

Equal channel angular pressing (ECAP) has been conducted on as-cast Mg-4.3 wt.%Zn-0.7 wt.%Y Mg alloy containing quasicrystal phase at a temperature of 623 K. After 8 ECAP passes, the grain size of the as-cast alloy is decreased from ∼ 120 to ∼ 3.5 μm, and the coarse eutectic quasicrystal phases are broken and dispersed in the alloy. Tensile testing has been performed on the ECAPed Mg-Zn-Y alloy at temperatures of 523 K and 623 K with initial strain rates from 1.5 × 10^− 3 to 1.5 × 10^− 4 s^− 1. The ECAPed alloy exhibits a maximum elongation of about 600% when testing at 623 K using an initial strain rate of 1.5 × 10^− 4 s^− 1. Grain boundary sliding is considered to be the dominant deformation mechanism of the Mg-Zn-Y alloy in the temperature and strain-rate range investigated.     

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.     

Microstructural characteristics of an ultrafine grain metal processed with equal-channel angular pressing

Equal-channel angular pressing is a procedure for producing a fully dense material with an ultrafine grain size, typically in the submicrometer or nanometer range, by subjecting the material to a very high plastic strain. This paper describes the principle of equal-channel angular pressing and illustrates the capability of the technique by reference to a series of detailed experiments conducted on an Al-3%Mg solid solution alloy in which the grain size was successfully reduced by equal-channel angular pressing from an initial size of ˜500 μm in the hot-rolled condition to a final size of ˜0.2 μm.     

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.     

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.     

Finite element analysis of rotary-die equal channel angular pressing

In this paper, the finite element method (FEM) was applied to analyze the plastic flow and strain hardening behavior of pure copper, subjected to rotary-die equal channel angular pressing (RD-ECAP) up to four passes. The die was rotated 90° counter clockwise between the passes in the simulation. The effective strain distribution and load–stroke curves were investigated. The load was increased with the number of rotary-die equal channel angular pressing passes. The results show that, plastic deformation becomes inhomogeneous with the number of passes due to an end effect, which was not found seriously in conventional equal channel angular pressing (ECAP). Especially, decreasing corner gap with increasing the number of passes was observed and explained by the strain hardening effect.     

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.     

Multicomponent materials from machining chips compacted by equal-channel angular pressing

Al and Mg machining chip blends were compacted by equal-channel angular pressing with back pressure. By varying the weight fraction of the constituent materials, temperature and processing route, as well as employing subsequent heat treatment, the microstructure and the mechanical properties of the compact were varied. The width of the interdiffusion zone and the formation of intermetallic phases near the interfaces between the two metals were studied by energy-dispersive X-ray spectroscopy and nanoindentation. It was shown that substantial improvement of mechanical properties, such as an increase of strength, strain-hardening capability and ductility, can be obtained. This is achieved by changing the processing parameters of equal-channel angular pressing and the annealing temperature, as well as by optimising the weight fraction of the constituent metals.     

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.     

Crack growth in ultrafine-grained AA6063 produced by equal-channel angular pressing

Crack growth behaviour of ultrafine-grained AA6063, processed by equal-channel angular pressing (ECAP) via route E at room temperature, was evaluated with special emphasis on the effect of grain size distribution and work hardening. A bimodal, two times ECAPed condition and a monomodal ultrafine-grained condition after eight ECAP passes are compared with the coarse grained peak aged material. Depending on their microstructure, the ECAPed materials show significantly lower fatigue threshold values (ΔK _th) and higher crack growth rates (da/dN) than their coarse grained counterparts. Micrographs of the crack propagation surfaces reveal the reduced grain size as major key to increased crack growth rates of the ECAPed material, as it influences roughness-induced crack closure and crack deflections. Furthermore, the effects of other features, such as ductility, work hardening capability and grain boundary characteristics, are discussed.     

Static and cyclic creep behaviour of SiC whisker reinforced aluminium composite

Abstract

Static and cyclic creep tests were carried out in tension at 573–673 K on a 20 vol.-%SiC whisker reinforced aluminium (Al/SiC_w ) composite. The Al/SiC_w composite exhibited an apparent stress exponent of 18·1–19·0 at 573–673 K and an apparent activation energy of 325 kJ mol^-1 for static creep, whereas an apparent stress exponent of 19·6 at 623 K and an apparent activation energy of 376 kJ mol^-1 were observed for cyclic creep. A cyclic creep retardation (CCR) behaviour was observed for the Al/SiC_w composite. The steady state creep rate for cyclic creep was three orders of magnitude lower than that for static creep. Furthermore, the steady state creep rates of the composite tended to decrease continuously with increasing percentage unloading amount. The static creep data of the Al/SiC_w composite were rationalised by the substructure invariant model with a true stress exponent of 8 together with a threshold stress. The CCR behaviour can be explained by the storage of anelastic strain delaying non-recoverable creep during the onload cycles.