Showing papers by "Terence G. Langdon published in 2003"

Equal-channel angular pressing using plate samples

Abstract: Equal-channel angular pressing (ECAP) was applied to plate samples of high-purity aluminum using processing route BCZ in which the plate is rotated by 90° in the same sense about the Z-axis between each pass. Samples were taken to a total of 4 passes, equivalent to an imposed strain of ~4, and then examined microscopically. In the as-pressed condition, the grain size was ~0.9 ?m but the microstructure exhibited some inhomogeneity with extensive areas (~70–80% of the total) of equiaxed grains and other areas containing elongated grains. This limited inhomogeneity was not apparent in the macroscopic properties since the Vickers microhardness, Hv, was essentially identical at all points on all planes of sectioning and the tensile behavior at room temperature was independent of the direction of orientation of the specimen gauge lengths. The tendency to develop an inhomogeneous microstructure is explained by reference to the predicted shearing patterns for processing using route BCZ with plate samples.

Achieving superplasticity in ultrafine-grained copper: influence of Zn and Zr additions

Abstract: Equal-channel angular pressing (ECAP) was applied to samples of pure Cu, a Cu–30%Zn alloy and alloys of Cu–0.18%Zr and Cu–30%Zn–0.13%Zr. All materials exhibited ultrafine grain sizes in the range of ∼0.1–0.4 μm after ECAP but the microstructures were inhomogeneous and the grains were elongated in the as-pressed condition. Tensile testing showed that superplasticity was not achieved in any of these materials at a temperature of 573 K but at 673 K the Cu–Zn–Zr alloy exhibited superplastic elongations at strain rates at and below ∼10 −3 s −1 . The results demonstrate that the presence of Zn and Zr are both beneficial in promoting the occurrence of superplastic ductilities. Zirconium is needed because it increases the recrystallization temperature, inhibits grain growth and, therefore, serves to retain a small grain size at elevated temperatures, and zinc is beneficial because it introduces solute atoms into the matrix so that dislocation creep is inhibited and superplastic flow can occur more easily.

Developing a superplastic forming capability in a commercial aluminum alloy without scandium or zirconium additions

Abstract: Tests were undertaken to determine the feasibility of producing a superplastic forming capability in a commercial Al-2024 alloy through processing by equal-channel angular pressing (ECAP), where this alloy was selected because it contains no scandium or zirconium additions that are generally beneficial in retaining an array of small grains. Processing by ECAP produced grain sizes in the range from ∼0.3 to ∼0.5 μm and static annealing showed these ultrafine grains were reasonably stable at temperatures up to ∼700 K. Superplastic elongations were achieved after ECAP with a maximum elongation of ∼500% at 673 K when using a strain rate of 1.0×10−2 s−1. The strain rate sensitivity was measured as ∼0.3 suggesting that dislocation glide is the rate-controlling mechanism. These results demonstrate the potential for achieving high tensile ductilities in conventional commercial aluminum alloys through processing by ECAP.

Factors influencing microstructural development in equal-channel angular pressing

Abstract: Processing through the imposition of severe plastic deformation (SPD) provides an opportunity for achieving very significant grain refinement in bulk materials. Although different SPD procedures are available, the process of equal-channel angular pressing (ECAP) is especially attractive because it can be scaled easily to produce relatively large samples. This paper describes the principles of ECAP processing and demonstrates the potential for achieving unusual mechanical properties in the samples subjected to ECAP. Special emphasis is placed on the possibility of attaining a high strain rate superplastic forming capability in the as-pressed materials: examples are presented for an Al−Mg−Sc alloy prepared in the laboratory by casting and for a commercial Al-2024 alloy.

Using atomic force microscopy to evaluate the development of mesoscopic shear planes in materials processed by severe plastic deformation

Abstract: Samples of pure Al and a Zn-22% Al alloy were processed by equal-channel angular pressing (ECAP) to give grain sizes of ∼1.3 μm in both materials. Following ECAP, tensile specimens were cut from the as-pressed billets and tested at room temperature using an initial strain rate of 1×10 −4 s −1 . The surface morphology of these two materials was examined using atomic force microscopy (AFM) and it was found that long mesoscopic shear planes are formed in pure Al over a large fraction of the surface area but no mesoscopic shear planes are formed in the Zn-22% Al alloy. The results are examined with reference to earlier reports of the occurrence of mesoscopic shear planes in pure Cu and pure Ni after processing by ECAP. It is concluded that mesoscopic shear planes do not form in two-phase alloys because of the difficulties of achieving local re-arrangements of the grain boundaries and in other materials the extent of the mesoscopic shear planes depends upon the homologous testing temperature and the grain size of the material.

Achieving a Superplastic Forming Capability through Severe Plastic Deformation

Abstract: Processing by severe plastic deformation (SPD) leads to very significant grain refinement in metallic alloys. Furthermore, if these ultrafine grains are reasonably stable at elevated temperatures, there is a potential for achieving high tensile ductilities, and superplastic elongations, in alloys that are generally not superplastic. In addition, the production of ultrafine grains leads to the occurrence of superplastic flow at strain rates that are significantly faster than in conventional alloys so that processing by SPD introduces the possibility of using these alloys for the rapid fabrication of complex parts through superplastic forming operations. This paper examines the development of superplasticity in various aluminum alloys processed by equal-channel angular pressing (ECAP).

A model investigation of the shearing characteristics in equal-channel angular pressing

Abstract: Model experiments were conducted to evaluate the shearing characteristics associated with processing through equal-channel angular pressing (ECAP). A Cu matrix was used with embedded inserts of either pure Al or a Cu–30% Zn alloy. When using aluminum, the experiments show good agreement with the theoretical predictions except that the embedded element, which has less strength than the Cu matrix, experiences greater shearing than anticipated. Conversely, the Cu–30% Zn alloy is stronger than the Cu matrix and, since the insert is not fully constrained, the shearing is then less than anticipated. Using an embedded sample of pure Al cut to the dimensions of the parallelepiped anticipated for a single pass of ECAP, it is shown that processing by route C, where the specimen is rotated by 180° between passes, effectively restores the cubic element in the second pass. The experiments confirm the pressing speed has no significant effect on the shearing characteristics, at least within the range from 0.2 to 18 mm s−1.

The creep behavior of discontinuously reinforced metal-matrix composites

Abstract: Discontinuously reinforced metal-matrix composites (MMCs) have extensive potential for use in structural applications at elevated temperatures. However, any use at high temperatures will require a detailed understanding of the creep characteristics of the MMCs and especially the dependence of the measured creep rates on the magnitudes of the applied stress and the testing temperatures. This report examines these characteristics with special reference to the well-documented creep behavior of conventional solid-solution alloys. It is shown that creep of these MMCs is controlled by deformation in the matrix alloys and this deformation may be interpreted using a similar approach to that already developed for the creep of solid solution alloys. However, additional parameters must be considered in analyzing the creep of MMCs, including the possible presence of a threshold stress and the potential for load transfer and additional substructural strengthening.

An Investigation of Deformation in Copper Single Crystals Using Equal-Channel Angular Pressing

Abstract: This paper describes experiments in which high purity copper single crystals of two different orientations were processed for one pass by equal-channel angular pressing (ECAP) and the deformed structures were examined using optical microscopy (OM), orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). The first single crystal (0° specimen) was oriented within the entrance channel of the die so that the {111} slip plane and the slip direction were parallel to the theoretical shear plane and shear direction, respectively. The second crystal (20° specimen) was oriented with the {111} slip plane and the slip direction rotated by 20° in a clockwise sense from the theoretical shear plane and shear direction, respectively. For the 0° specimen, after passing through the shear plane there were two crystallographic orientations representing the initial orientation and an orientation rotated by 60° in a counter-clockwise sense from the initial orientation. For the 20° specimen, there was an orientation rotated by 20° in a counter-clockwise sense from the initial orientation after passing through the shear plane.

Calorimetric and X-Ray Measurements in Ultrafine-Grained Nickel

Abstract: The procedures of X-ray diffractometry and peak profile analysis, together with differential scanning calorimetry (DSC), were used to measure the grain size, dislocation density and corresponding elastic energy stored in ultrafine-grained (UFG) nickel processed by different methods: equal channel angular pressing, high pressure torsion and their combinations. It has been shown that grain growth in these samples begins at temperatures in the range from 500 to 700 K and the measured activation energy for grain growth is close to the activation energy for grain boundary self-diffusion. A difference between the released enthalpy in DSC experiments and the elastic energy evaluated by x-ray diffractometry is attributed to the decrease in surface energy during grain growth in UFG nickel. The grain boundary (GB) surface energy of high angle boundaries was also evaluated.

Microstructural Control of a Precipitate-Hardenable Al-Ag Alloy Using Severe Plastic Deformation

Abstract: Departments of Aerospace & Mechanical Engineering and Materials ScienceUniversity of Southern California, Los Angeles, CA 90089-1453, USA, langdon@usc.eduKeywords: Al-Ag alloy, equal-channel angular pressing, precipitate particles, aging, metastablephaseAbstract. An Al-10.8wt%Ag alloy was subjected to aging treatment followed by Equal-ChannelAngular Pressing (ECAP) (designated process AE) or ECAP followed by aging treatment(designated process EA). Hardness measurements were undertaken with respect to the number ofECAP passes for process AE or with respect to aging time for process EA. Microstructures wereexamined by transmission electron microscopy (TEM) including X-ray mapping. It is shown thatage hardening is observed for the ECAP sample due to the precipitation of very fine particles withinthe small grains.IntroductionIt is well known that the process of equal-channel angular pressing (ECAP) is used to reduce thegrain size of metallic materials to the submicrometer range or the nanometer range [1-3]. However,it is not so well established that the ECAP process may also be used to control the morphology anddistribution of second phase particles in two-phase metallic materials. Thus, grain refinement andsecond-phase control are both feasible because severe strain is created through the process of ECAPbased on the principle that a sample is pressed through a channel bent into an L-shape within a dieand this pressing may be repeated without any change in the cross-section of the sample.Fragmentation not only of the grains but also of second-phase particles may occur due to theintroduction of severe strain in the material.Although many reports have now been published for grain refinement using the ECAP process[2,3], there are only a limited number of examples of the application of the ECAP process tosecond-phase control [4,5]. It was shown that the application of ECAP led to dissolution of 0'particles precipitated by aging of an Al-3.7wt%Cu alloy [4]. Supersaturation thus occurred in thealloy and subsequent aging gave rise to the formation of a stable 0-phase. The 0-phaseprecipitation was also recognized after aging of a severely deformed Al-3.7wt%Cu alloy usingECAP. An application of ECAP to an A1-0.9wt%Mg