Dynamic ageing and the mechanical response of Al–Mg–Si alloy through equal channel angular pressing

In this paper, dynamic ageing characteristics associated with the application of equal channel angular pressing (ECAP) to Al6061 alloy at elevated temperatures was investigated. Followed by ECAP, Vickers microhardness measurement on the cross-sectional planes and microstructural observations were undertaken using transmission electron microscopy. The combination of the ECAP process with dynamic ageing at both 100 °C and 150 °C resulted in a significant increase in hardness. The grain size was measured as ∼160 nm after four passes. A comparison with the published data on the same alloy processed by ECAP at room temperature and statically aged, suggests several advantages in incorporating dynamic ageing with ECAP. These advantages consist of the ability to attain better grain refinement, increased hardness and the potential for saving time and energy.     

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.     

Grain refinement in as cast Al–Mg–Mn alloy during high temperature equal channel angular pressing

Abstract

An as cast Al–Mg–Mn alloy with coarse equiaxed grain structure was processed by equal channel angular pressing (ECAP) at 350°C up to eight passes. Systematic studies were made on the microstructural evolution during ECAP by optical microscopy, electron backscattered diffraction and TEM. Equal channel angular pressing led to a considerable grain refinement, resulting in an average cell size of about 1 μm and a fraction of high angle boundaries of 75% after eight pressing passes. Deformation bands were not developed during the ECAP process, and a reasonably equiaxed substructure was obtained even after one pass. The main mechanism of grain refinement was attributed to the continuous dynamic recrystallisation based on the motion of deformation induced dislocations. Discontinuous recrystallisation at grain boundaries and triple junctions also contributed to the refinement, which played an important role especially at high strain of eight passes.     

Superplastic flow in a nanostructured aluminum alloy produced using high-pressure torsion

Samples of a spray-cast Al-7034 alloy were processed by high-pressure torsion (HPT) at temperatures of 293 or 473 K using an imposed pressure of 4 GPa and torsional straining through five revolutions. Processing by HPT produced significant grain refinement with grain sizes of 60 and 85 nm at the edges of the disks for the two processing temperatures. In tensile testing at room temperature, the alloy processed by HPT exhibited higher strength and lower ductility than the unprocessed material. Good superplastic properties were achieved in tensile testing at elevated temperatures with a maximum elongation of 750% for the sample processed at 473 K and tested in tension at 703 K under an initial strain rate of 1.0 × 10⁻² s⁻¹. The measured superplastic elongations are lower than in samples prepared by equal-channel angular pressing because of the use of very thin disks in the HPT processing.     

Microstructural evolution during extrusion and ECAP of a spray-deposited Al–Zn–Mg–Cu–Sc–Zr alloy

The microstructural evolution of an Al–Zn–Mg–Cu–Sc–Zr alloy prepared by spray deposition via extrusion and equal-channel angular pressing (ECAP) was investigated in this study. Deformation route A for Al–11.5 wt% Zn–2 wt% Mg–1.5 wt% Cu–0.2 wt% Sc–0.15% Zr super-strength alloy was carried out at 573 K by ECAP. The microstructures of extruded and ECAP samples were investigated by means of Electron Backscatter Diffraction (EBSD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). A large amount of dislocation tangles were formed inside grains during ECAP, which further evolved into sub-boundaries and high angle grain boundaries. Microstructure analyses showed that the grain size was refined to 800 nm after 8 passes ECAP from earlier 3.5 μm of sprayed and extruded alloy. A few finer MgZn₂ and Al₃(Sc,Zr) were dispersed uniformly after ECAP. The textures of 8 passes ECAPed sample were dominated by the strong Cu orientation and relatively weak S orientation.     

Thermal stability study on two aluminum alloys processed with equal channel angular pressing

Equal channel angular pressing was used to produce sub-micrometer size grain structures in two aluminum alloys (commercially pure 1200 and Al-Mn-Si 3103). ECAP was conducted at room temperature following the process via route C, which involves a 180° angular rotation between passes and strongly affects material microstructure by reversing the shear strain every second pass. This unique characteristic of route C induced limited build-up of new high-angle boundaries, at least for the first three passes. The equal channel angular pressing was extended to six passes for both alloys; three passes by route C throughout the die were sufficient to produce a very fine-structured material for both alloys. High-resolution electron back-scattered diffraction pattern analysis was carried out to measure boundary misorientation within the deformed structures. Measurements of subgrain and grain spacing revealed a more effective microstructure refining effect in the 3103 than the 1200 alloy. Thermal stability of the severely deformed materials was studied at temperatures of 130, 240, 330°C, corresponding to 0.2, 0.35, 0.5 of the melting temperature. The results showed considerable grain growth in both materials solely at temperatures from and above half of the melting one.     

Microstructural evolution in an aluminum solid solution alloy processed by ECAP

Samples of an Al-1% Mg solid solution alloy were processed by equal-channel angular pressing (ECAP) at room temperature for totals of 1-12 passes and the microstructures of the processed samples were examined using orientation imaging microscopy. The results demonstrate the alloy achieves a reasonably stable microstructure after 6 passes through the ECAP die with an ultimate equilibrium grain size of ∼700 nm. Measurements show both the fraction of high-angle boundaries and the average boundary misorientation increase with increasing numbers of passes up to 6 passes but thereafter there is only a minor additional increase up to 12 passes.     

Effect of equal channel angular pressing on grain refinement and texture evolution in a biomedical alloy Ti13Nb13Zr

Evolution of texture and concomitant grain refinement during Equal Channel Angular Pressing (ECAP) of Ti13Nb13Zr alloy has been presented. Sub-micron sized equiaxed grains with narrow grain size distribution could be achieved after eight pass at 873 K. A characteristic ECAP texture evolved in α phase till four passes while the evolution of characteristic ECAP texture in the β phase could be observed only beyond the fourth pass. On increasing the deformation up to eight passes, the texture in α phase weakens while the β phase shows an ideal ECAP texture. A weaker texture, low dislocation density and high crystallite size values in α phase suggest the occurrence of dynamic recrystallization. The absence of texture evolution in β phase till four passes can be attributed to local lattice rotations. The characteristic ECAP texture in the eight pass deformed sample is attributed to delayed dynamic recrystallization in the β phase.     

Texture and microstructure evolution in ultrafine-grained AZ31 processed by EX-ECAP

Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180 °C following route B_c, i.e. rotating the sample 90° between individual passes. Microstructure evolution was investigated using EBSD and TEM, as a function of strain imposed by ECAP. The first ECAP pass resulted in the formation of a new texture component which relates to the bimodal grain structure observed in this specimen. The grains larger than 10 μm show the orientation changes corresponding to the ECAP shear, which is characterised by the rotation of the basal poles by approximately 40° from the initial orientation. The fine grains with the average size of 1 μm maintain the initial orientation. The character of the bimodal grain structure and the distinct texture components between large and small grains remained unchanged up to 4 ECAP passes. Further ECAP pressing to 8 and 12 passes leads to a grain refinement through the whole sample volume and the orientation changes of all grains corresponding to the ECAP shear.     

Enhanced superplasticity of magnesium alloy AZ31 obtained through equal-channel angular pressing with back-pressure

Excellent superplastic elongations (in excess of 1,200%) were achieved in a commercial cast AZ31 alloy processed by low temperature equal-channel angular pressing (ECAP) with a back-pressure to produce a bimodal grain structure. In contrast, AZ31 alloy processed by ECAP at temperatures higher than 200 °C showed a reasonably uniform grain structure and relatively low ductility. It is suggested that a bimodal grain structure is advantageous because the larger grains contribute to strain hardening thus delaying the onset of necking, while grain boundary sliding associated with small grains provides a stabilizing effect due to enhanced strain rate sensitivity.     

Ultrafine-grained Ti–Nb–Ta–Zr alloy produced by ECAP at room temperature

To ascertain the influence of severe plastic deformation (SPD) on a Ti–Nb–Ta–Zr (TNTZ) alloy, we studied the room temperature mechanical behavior and microstructural evolution of an ultrafine-grained (UFG) Ti–36Nb–2Ta–3Zr (wt%) alloy prepared via equal-channel angular pressing (ECAP) of the as-hot-extruded alloy. The tensile behavior, phase composition, grain size, preferred orientation, and dislocation density of the UFG alloy, processed under different conditions, were analyzed and discussed. Compared to the as-hot-extruded alloy, the ECAP-processed TNTZ alloy (3 passes) exhibited approximately 40 and 88 % increase in average ultimate strength and yield strength, respectively. Moreover, as the number of ECAP passes increased from 3 to 6, the TNTZ alloy exhibited not only the expected increase in ultimate and yield strength values, but also a slight increase in elongation. Our results suggest that the deformation mechanisms that govern the behavior of the as-hot-extruded coarse grained (CG) TNTZ alloy during ECAP involve a combination of stress-induced martensitic transformation and dislocation activity. In the case of the ECAP-processed UFG TNTZ alloy, the deformation mechanism is proposed to involve two components: first, dislocation activity induced by the strain field imposed during ECAP; and second, the formation of α″ martensite phase during the early stages of ECAP which eventually transforms into β phase during continued deformation. We propose that the deformation mechanism governing the room temperature behavior of the TNTZ alloy strongly depends on the grain size of the β phase.     

T-shaped equi-channel angular pressing of Pb–Sn eutectic and its tensile properties

Equi-channel angular pressing (ECAP) of a Pb–Sn eutectic alloy up to six passes in a T-shaped die, rather than a conventional L-shaped die, was studied for grain refinement. The effect of ECAP on the hardness and tensile properties was studied. Microstructure predominately changed in the early part of the ECAP process and became equiaxed and uniformly distributed in both the longitudinal and the transverse sections after four passes. There occurred substantial softening over the first two passes—hardness of 10 Hv, yield strength of 14.2 MPa and tensile strength of 16.3 MPa in the as-cast condition decreased upon two passes to 6 Hv, 9.7 MPa and 13.0 MPa, respectively. The ductility (% elongation) increased drastically from <50% in the as-cast condition to 150% upon two passes, and further increased to 230% after four passes. Various tensile properties and concurrent microstructural evolution were used to develop a mutual relationship among them.     

Equal channel angular pressing of a TWIP steel: microstructure and mechanical response

A Fe–20.1Mn–1.23Si–1.72Al–0.5C TWIP steel with ultrafine grain structure was successfully processed through equal channel angular pressing (ECAP) at warm temperature up to four passes following the B _C route. The microstructure evolution was characterized by electron backscattered diffraction to obtain the grain maps, which revealed an obvious reduction in grain size, as well as a decrease in the twin fraction, with increasing number of ECAP passes. The texture evolution during ECAP was analyzed by orientation distribution function. The results show that the annealed material presents brass (B) as dominant component. After ECAP, the one pass sample presents A ₁* and A ₂* as the strongest components, while the two passes and four passes samples change gradually toward \( B/\bar{B} \) components. TEM analysis shows that all samples present twins. The twin thickness is reduced with increasing the number of ECAP passes. Nano-twins, as a result of secondary twinning, are also observed in the one and two passes samples. In the four passes sample, the microstructure is extensively refined by the joint action of ultrafine subgrains, grains and twins. The mechanical behavior was studied by tensile samples, and it was found that the yield strength and the ultimate tensile strength are significantly enhanced at increasing number of ECAP passes. Although the ductility and strain hardening capability are reduced with ECAP process, the present TWIP steel shows significant uniform deformation periods with positive work hardening rates.     

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 mechanical properties of nanocrystalline high strength Al–Mg–Si (AA6061) alloy by high energy ball milling and spark plasma sintering

In the present paper, the microstructure and mechanical properties of nanostructured Al–Mg–Si based AA6061 alloy obtained by high energy ball milling and spark plasma sintering were reported. Gas atomized microcrystalline powder of AA6061 alloy was ball milled under wet condition at room temperature to obtain nanocrystalline powder with grain size of 30 nm. The nanocrystalline powder was consolidated to fully dense compacts by spark plasma sintering (SPS) at 500 °C. The grain size after SPS consolidation was found to be 85 nm. The resultant SPS compacts exhibited microhardness of 190–200 HV_100 g, compressive strength of 800 MPa and strain to fracture of 15%.     

Exceptional superplasticity in an AZ61 magnesium alloy processed by extrusion and ECAP

Experiments were conducted on a commercial AZ61 alloy to evaluate the potential for achieving an ultrafine grain size and superplastic ductilities through the use of the EX-ECAP two-step processing procedure of extrusion plus equal-channel angular pressing. The results show that EX-ECAP gives excellent grain refinement with grain sizes of 0.6 and 1.3 μm after pressing at 473 and 523 K, respectively. The alloy processed by EX-ECAP exhibits exceptional superplastic properties including a maximum elongation of 1320% after pressing through four passes when testing at 473 K with an initial strain rate of 3.3 × 10⁻⁴ s⁻¹. This result compares with an elongation of 70% achieved in the extruded condition without ECAP under similar testing conditions.     

Microstructure, mechanical properties and electrical conductivity of industrial Cu-0.5%Cr alloy processed by severe plastic deformation

Microstructure, mechanical properties and electrical conductivity of industrial Cu-0.5% alloy subjected to equal channel angular pressing (ECAP) by route A and cold rolling with and without aging treatment were investigated. The lamellar grains in thickness of 100 nm were obtained after eight ECAP passes. They were not further pancake shaped, but fragmentary and obtained less sharp boundaries with more dislocations in addition to cold rolling. After aging at 450 °C for 1 h, high density of dislocations and some coarse grains were observable after ECAP and the additional cold rolling, respectively. The tensile tests show that tensile strength arrived at 460 MPa and 484 MPa after four and eight passes of ECAP, respectively, the corresponding tensile strength increased to 570 MPa and 579 MPa after the additional cold rolling. However, the electrical conductivity was not more than 35% IACS. It was proved that four passes of ECAP followed by 90% cold rolling and aging at 450 °C for 1 h offered a short process for Cu-0.5%Cr alloy to balance the paradox of high strength and electrical conductivity, under which the tensile strength 554 MPa, elongation to failure 22% and electrical conductivity 84% of IACS could be obtained. The high strength was explained by precipitation strengthening and fine grain strengthening.     

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.     

Effect of equal channel angular pressing on fracture toughness of Al-7075

In this paper, influence of equal channel angular pressing (ECAP) on the fracture behavior of Al-7075 alloy is experimentally investigated. The specimens are successfully processed by ECAP methodology up to four passes using different routes. Transmission electron microscope (TEM) images showed that after four passes of ECAP, the average grain size is refined from 40 μm to less than about 500 nm. The percentage increase in yield strength, ultimate strength and microhardness of the specimens after four ECAP passes was 230, 90 and 110 respectively. Standard tests on the disk-shaped compact DC(T) specimens showed that fracture toughness is decreased up to 8% at the first ECAP pass while after four passes, this parameter roused to 17% higher than that of annealed condition. Furthermore, scanning electron microscope (SEM) micrographs demonstrated that ductile fracture mechanism with large dimples occurred in the annealed samples, changed to limited ductile fracture with fine dimples after ECAP process. This research provides new insights into the effect of ECAP and grain refinement on the fracture behavior of materials.     

An investigation on the effects of equal channel angular pressing on the mixed‐mode fracture toughness and mechanical properties of 6063 aluminium alloy

In this study, effects of equal channel angular pressing (ECAP) on the mixed‐mode fracture toughness of Al‐6063 were investigated. The ECAP process continued up to 5 passes without failure. Grain refinement was obvious after 5 passes of the ECAP process. The average grain size reduced from 45μm to less than 1μm, and textural studies shows aligning the grains in known directions. After 4 passes, yield and ultimate strengths increase respectively from 100 and 209 MPa to 300 and 375 MPa and reduction in elongation was also observed. The microhardness improved after the process. The fracture toughness for different orientations was measured. For pure mode I (opening mode), its value decreased after the first pass from 18.4 to 15.71  ; however, it increased to about 18.8  after the fifth pass. For mixed‐mode loading condition, different orientations were investigated. The results revealed different fracture toughness reductions after the first passes of the process for specimens with different orientations. The fracture surfaces were studied by using scanning electron microscope, and refined equiaxed dimples were observed after the ECAP process.