Structural transformations during heating of a Zr-2.5% Nb alloy subjected to equal-channel angular pressing

The structure of a Zr-2.5% Nb alloy after equal-channel angular pressing (ECAP) at 690–700 K and annealing in the temperature range 670–1070 K is investigated. The structure of the Zr-2.5% Nb alloy deformed by ECAP is an irregular grain-subgrain oriented structure with an enhanced dislocation density, a cross-section of 30–150 nm of oriented structural elements, and an equiaxed-grain (subgrain) size of 50–200 nm. Heating after ECAP in the temperature range 720–770 K for 3–5 h is proposed for the formation of an ultrafine-grained equilibrium structure in the ECAP deformed Zr-2.5% Nb alloy. Heating of the Zr-2.5% Nb alloy after ECAP at 723 K for 5 h leads to the formation of a predominantly equiaxial submicrocrystalline structure with a grain size of 150–500 nm. Equal-channel angular pressing of the Zr-2.5% Nb alloy increases the yield strength to 622 MPa, which is higher than that in the as-delivered undeformed state by a factor of 1.6. In this case, the relative elongation decreases. Heating of the ECAP deformed Zr-2.5% Nb alloy at 723 K for 5 h decreases the yield strength to 504 MPa, but the relative elongation increases to 14%.     

Texture of the Mg-0.49% Al-0.47% Ca alloy after equal-channel angular pressing

Equal-channel angular pressing (ECAP) is used to refine grains and to change the texture of the initial pressed Mg-0.49% Al-0.47% Ca alloy rod in order to study the possibility of increasing the low-temperature ductility of the alloy. ECAP is performed at 300°C in six passes at a total true logarithmic strain ε = 6.8 according to route B _C . As a result, an ultrafine-grained structure with a grain size of 2–5 μm forms. The initial texture of the pressed rod is characterized by the [12 11] axial orientation parallel to the pressing direction. After ECAP, the texture changes its type and is characterized by a set of preferred orientations that represent basal planes located at an angle of 40°–50° with respect to the pressing direction. An analysis of the generalized Schmid factors, which were calculated for the main operating deformation systems with allowance for the critical shear stresses in them and the volume fractions of the preferred orientations, indicates that the texture caused by ECAP affects the decrease in the strength properties of the alloy measured at room temperature and the increase in the low-temperature ductility of the alloy.     

Grain refinement and superplasticity in a magnesium alloy processed by equal-channel angular pressing

The extrusion/equal channel angular pressing (EX-ECAP) processing procedure, in which magnesium-based alloys are subjected to extrusion followed by ECAP, was applied to a Mg-7.5 pct Al-0.2 pct Zr alloy prepared by casting. Microstructural inspection showed the EX-ECAP process was effective in reducing the grain size from ∼21 μm after extrusion to an as-pressed grain size of ∼0.8 μm. It is shown through static annealing that these ultrafine grains are reasonably stable up to 473 K, but grain growth occurs at higher temperatures. Tensile specimens were cut from the billets prepared by EX-ECAP and testing showed these specimens exhibited superplasticity at relatively low temperatures with maximum elongations up to >700 pct. By processing through EX-ECAP to a higher imposed strain and thereby increasing the area fraction of high-angle boundaries, it is demonstrated that there is a potential for achieving high-strain-rate superplasticity. This article is based on a presentation made at the Symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–18, 2004, in Charlotte, NC, under the auspices of the ASM-MSCTS Phase Transformations Committee.     

Fatigue strength of a magnesium MA2-1 alloy after equal-channel angular pressing

The fatigue strength of a magnesium MA2-1 alloy is studied after annealing and equal-channel angular pressing (ECAP). The ultrafine-grained structure formed upon ECAP is shown to increase the plasticity of the material during static tension, to decrease the cyclic life to failure, and not to decrease the fatigue limit. The mechanisms of crack nucleation and growth during cyclic deformation are investigated.     

Structural evolution of a strip-cast al alloy sheet processed by continuous equal-channel angular pressing

Experiments were conducted on strip-cast 1050 Al alloy sheets using an equal-channel angular pressing (ECAP) process to investigate the feasibility of the technique for producing metal strips. The developed process is capable of introducing shear deformation into metal strips in a continuous mode at a relatively fast forming speed of 10 to 50 m/min. The actual shear-flow patterns as a result of the continuous ECAP were demonstrated and compared with those obtained from numerical calculations. The effects of die geometry on the mechanical properties of the strips were investigated. Observations of the microstructural evolution in the equal-channel angular pressed (ECAPed) samples were conducted using transmission electron microscopy (TEM) as a function of oblique angles. The texture evolution was investigated using orientation distribution function (ODF) analysis. A possible application of this process for producing an Al alloy sheet with high formability and low earing was discussed by calculating the Lankford parameter and the planar anisotropy.     

Effect of equal-channel angular pressing on the fatigue strength of titanium and a zirconium alloy

The static and fatigue strength of commercial-purity VT1-00 titanium and a Zr-2.5% Nb alloy subjected to equal-channel angular pressing (ECAP) are studied. The formation of a submicrocrystalline structure after ECAP is shown to result in significant hardening, an increase in the fatigue life at high stress amplitudes, and an increase in the fatigue limit as compared to the annealed state. The mechanisms of fatigue fracture of the materials in various structural states are investigated.     

Dynamic deformation and fracture behavior of ultrafine-grained aluminum alloy fabricates by equal-channel angular pressing

In the present study, ultrafine-grained microstructures of a conventional 5083 aluminum alloy were fabricated by equal-channel angular pressing, and their dynamic deformation and fracture behavior were investigated. Dynamic torsional tests were conducted on four aluminum alloy specimens using a torsional Kolsky bar, and then the test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-banding behavior. The equal-channel angular-pressed (ECAP) specimens consisted of ultrafine grains and contained a considerable amount of second-phase particles, which were refined and distributed homogeneously in the matrix as the equal-channel angular pressing pass number increased. The dynamic torsional test results indicated that the maximum shear stress increased, while the fracture shear strain remained constant, with increasing equal-channel angular pressing pass number. Observation of the deformed area beneath the dynamically fractured surface showed that a number of voids initiated mainly at second-phase particle/matrix interfaces and that the number of voids increased with increasing pass number. Adiabatic shear bands of 200 to <300 μm in width were formed in the as-extruded and 1-pass ECAP specimens having coarser particles, whereas they were hardly formed in the four-pass and eight-pass ECAP specimens having finer particles. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation.     

Shear bands and anisotropy of the mechanical properties of an MA2-1pch magnesium alloy after equal-channel angular pressing

Effect of structure and texture on the anisotropy of the mechanical properties of the MA2-1pch magnesium alloy subjected to equal-channel angular pressing and subsequent annealing has been studied in two mutually perpendicular planes Y and X (along and across the pressing direction). The anisotropy of the mechanical properties is shown to be due to various orientations of shear bands and various types of texture inside the bands and outside them in planes X and Y.     

Microstructure transformation from lamellar to equiaxed microduplex through equal-channel angular pressing in an Al-33 pct Cu eutectic alloy

Strain-driven transformation of a lamellar structure into a homogeneous equiaxed microduplex structure was investigated in an eutectic Al-33 pct Cu alloy deformed by equal-channel angular pressing at 400 °C via route Bc. In route Bc, the rotation of sample is always 90 deg in the same sense, i.e., clockwise or counterclockwise. The transformation follows the subdivision of the colonies into smaller lamellar blocks, the separation of these lamellar blocks into isolated islands, and, finally, the shrinkage of these islands to disappear, through the breaking down of the lamellae near the boundaries of the lamellar block-and-equiaxed region. The kinetics of the strain-driven microstructure transformation process, described by the dependence of the relative lamellar area fraction on equivalent true strain accumulated by equal-channel angular pressing (ECAP), physiognomically resembles that of the thermally activated transformation process described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model for the recrystallization process.     

Structure,properties, and deformation behavior of corrosion-resistant steel 12Kh18N10T subjected to equal-channel angular pressing

The results of investigation into the evolution of structure, properties, and deformation behavior of corrosion-resistant steel 12Kh18N10T in the course of severe plastic deformation by equal-channel angular pressing are reported. The formation of a fibrous ultrafine-grained structure of the steel, its twofold hardening accompanied by a slight decrease in the plasticity, and changes in the cold-deformation mechanism are shown.     

Structure and strain hardening of a Ti-40 at % Nb alloy produced by equal-channel multiangular pressing and hydroextrusion

The effect of a combined treatment, which includes nonmonotic deformation by equal-channel multiangular pressing and monotonic forming by hydroextrusion, and subsequent heat treatment on the structure, phase composition, and strength of an Nb-60 at % Ti alloy has been studied.     

Shear, principal, and equivalent strains in equal-channel angular deformation

The shear and principal strains involved in equal channel angular deformation (ECAD) were analyzed using a variety of methods. A general expression for the total shear strain calculated by integrating infinitesimal strain increments gave the same result as that from simple geometric considerations. The magnitude and direction of the accumulated principal strains were calculated based on a geometric and a matrix algebra method, respectively. For an intersecting angle of π/2, the maximum normal strain is 0.881 in the direction at π/8 (22.5 deg) from the longitudinal direction of the material in the exit channel. The direction of the maximum principal strain should be used as the direction of grain elongation. Since the principal direction of strain rotates during ECAD, the total shear strain and principal strains so calculated do not have the same meaning as those in a strain tensor. Consequently, the “equivalent” strain based on the second invariant of a strain tensor is no longer an invariant. Indeed, the equivalent strains calculated using the total shear strain and that using the total principal strains differed as the intensity of deformation increased. The method based on matrix algebra is potentially useful in mathematical analysis and computer calculation of ECAD. An erratum to this article is available at .     

Structure formation in grade 20 steel during equal-channel angular pressing and subsequent heating

The structure formation and the mechanical properties of quenched and tempered grade 20 steel after equal-channel angular pressing (ECAP) at various true strains and 400°C are studied. Electron microscopy analysis after ECAP shows a partially submicrocrystalline and partially subgrain structure with a structural element size of 340–375 nm. The structural element size depends on the region in which the elements are formed (polyhedral ferrite, needle-shaped ferrite, tempered martensite, and pearlite). Heating of the steel after ECAP at 400 and 450°C increases the fraction of high-angle boundaries and the structural ferrite element size to 360–450 nm. The fragmentation and spheroidization of cementite lamellae of pearlite and subgrain coalescence in the regions of needle-shaped ferrite and tempered martensite take place at a high ECAP true strain and heating temperature. Structural refinement ensures considerable strengthening, namely, UTS 742–871 MPa at EL 11–15.3%. The strength slightly increases, whereas the plasticity slightly decreases when the true strain increases during ECAP. After ECAP and heating, the strength and plastic properties of the grade 20 steel remain almost the same.     

Superplasticity and internal friction of a superplastic Zn-22% Al eutectoid alloy

Temperature spectrums of internal friction, in other words, specific points have been investigated and discussed in the wide temperature range from room temperature to equilibrium eutectoid isotherm for Zn-22% Al eutectoid alloy in order to measure grain boundary peaks. Three large grain boundary peaks of Pα, Pαβ and Pβ which are associated with superplasticity have been observed over the range of temperatures from 447K (174°C) to 525K (252°C). The Activation energies of Pα, Pαβ and Pβ have been calculated to be 109, 93 and 62 kJ·mol⁻¹ respectively in internal friction measurements. These peaks seem due to the grain boundary sliding which can be accommodated by the diffusive flux on a boundary between like phases, α/α, for Pα and β/β for Pβ, and on an interphase boundary, α/β for Pαβ. Furthermore, it has been indicated that a utilization of Pα as the damping alloys would be possible.     

Formation of a submicrocrystalline structure in St10 and 08R steels during equal-channel angular pressing

The effect of cold equal-channel angular pressing (ECAP) on the structure and properties of low-carbon St10 and 08R steels in the initially ferritic-pearlitic and bainitic states is studied. Directly after ECAP, only a partially submicrocrystalline structure with a grain size of 150–300 nm (depending on alloying and the initial state) can be obtained. Along with a granular structure, a subgrain and/or cellular structure, including an oriented structure, are observed. The finest structure forms in a boron-containing 08R steel with a structural-element size of 190 nm. The strength of the 08R steel subjected to cold ECAP (σ_u = 805–1235 MPa) corresponds to a hardware strength class of 8.8–12.9. The strength of the deformed St10 steel is close to a strength class of 8.8.     

Factors influencing the equilibrium grain size in equal-channel angular pressing: Role of Mg additions to aluminum

Experiments were undertaken to compare the equal-channel angular (ECA) pressing of Al-1 pct Mg and Al-3 pct Mg solid-solution alloys with pure Al. The results reveal both similarities and differences between these three materials. Bands of subgrains are formed in all three materials in a single passage through the die, and these subgrains subsequently evolve, on further pressings through the die, into an array of grains with high-angle boundaries. However, the addition of magnesium to an aluminum matrix decreases the rate of recovery and this leads, with an increasing Mg content, both to an increase in the number of pressings required to establish a homogeneous microstructure and to a decrease in the ultimate equiaxed equilibrium grain size. It is concluded that alloys exhibiting low rates of recovery should be especially attractive candidate materials for establishing ultrafine structures through grain refinement using the ECA pressing technique.     

Discontinous precipitation in a Cu-12 at.% in alloy

The feasibility of discontinuous precipitation from phase boundaries vis-a-vis grain boundaries has been investigated for the first time. Suitable heat treatments with a Cu-12 at.% In alloy produce measurable amounts of peritectic and eutectoid areas in the microstructure prior to discontinous precipitation. It has been demonstrated that the boundary between a supersaturated matrix and a eutectoid colony, under favourable conditions, can initiate discontinuous precipitation in the former. A suitable initiation mechanism from such phase boundaries has been proposed. Detailed kinetic analysis of the precipitation reaction and exhausted comparison with the published data reveal that the matrix/precipitate type phase boundaries are equally capable of undertaking discontinuous precipitation as the grain boundaries.     

Formation of a nanostructure and properties of titanium rods during equal-channel angular pressing CONFORM followed by drawing

Severe plastic deformation by equal-channel angular pressing according to the Conform schedule (ECAP-Conform) is used for the first time to produce long titanium rods in a nanostructured state. As a result of ECAP-Conform followed by drawing, the ultimate strength of Grade 4 titanium increases to 1350 MPa, and its yield strength increases to 1300 MPa at a high retained plasticity. The high efficiency and output of the ECAP-Conform method allow the development (on its basis) of a commercial technology for the production of high-strength nanostructured titanium, which is a promising material for medical implants.