Papers by Author: Terence G. Langdon

Abstract: High purity aluminum was processed by equal-channel angular pressing (ECAP) to reduce the grain size to ~1.3 m. Tensile specimens were cut from the as-pressed billets and these specimens were tested under conditions of high temperature creep. The results show excellent creep properties with a well-defined region of steady-state flow. The flow behavior is analyzed by comparing the creep data with the predicted behavior for different fundamental creep mechanisms and by plotting a deformation mechanism map to provide a visual representation of the creep properties.

Abstract: High purity aluminum (Al) and copper (Cu) single crystals of different crystallographic orientations were processed for one pass by equal-channel angular pressing (ECAP). The deformed structures were examined using orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). This paper examines the experimental results in terms of the values of the shear factors based on simple shear theory.

Abstract: The effect of the combination of natural aging and severe plastic deformation (SPD) produced by Equal-Channel Angular Pressing (ECAP) on the microstructure, the strength, as well as the ductility of age-hardenable AlZnMg alloys was investigated. A strategy is proposed for the processing of these “difficult-to-work” alloys at room temperature. Several advantages such as strengthening, precipitation-accelerating and ductility-improving effects of ECAP at room temperature are also shown and discussed in this work.

Abstract: The processing of bulk metals through the application of severe plastic deformation provides an opportunity for achieving exceptional grain refinement to the submicrometer or even the nanometer range. This paper examines the characteristics of metals processed by equal-channel angular pressing with special emphasis on the levels of ductility that may be attained. It is shown that the amount of ductility is dependent not only upon the composition of the material but also, and to a major extent, upon the testing temperature. Specifically, the ductilities are often low at ambient temperatures where the strength of the as-processed material is relatively high but, by contrast, exceptionally high superplastic ductilities may be achieved over short ranges of strain rate when testing at elevated temperatures.

Abstract: Processing through the application of severe plastic deformation (SPD) provides an opportunity for achieving very significant grain refinement in bulk metals. Since the occurrence of superplastic flow generally requires a grain size smaller than ~10 µm, it is reasonable to anticipate that materials processed by SPD will exhibit superplastic ductilities when pulled in tension at elevated temperatures. This paper summarizes the fundamental principles of SPD processing and describes recent results demonstrating the occurrence of exceptional superplastic flow in these ultrafine-grained materials.

Abstract: The evolution of texture and deformation in the grains during one pass of equal-channel angular pressing (ECAP) was examined for fine grained high strength and low strength Al alloys and a coarse grained low strength Al alloy. The materials were analysed using electron back-scatter diffraction (EBSD). The results are consistent with the materials responding to the intense macroscopic shear stress by deformation of individual grains through movement of dislocations on one or more of the slip crystallographic slip planes {hkl} that are favourably oriented, combined with the rotation of grains to directions that bring main crystallographic slip planes parallel to the macroscopic shear direction and crystallographic slip directions parallel to two main shear directions. Contrary to reports claiming up to 4 slip systems are activated, it was observed that only the {111}<110> and {001}<110> shear systems are activated. Macroscopic shear deformation occurs on two shear planes: the main shear plane (MSP), equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP.

Abstract: This work is focused on the effect of the combination of natural aging and severe plastic deformation (SPD) produced by Equal-Channel Angular Pressing (ECAP) on the microstructure and strength of supersaturated AlZnMg alloys. Following a solution heat-treatment and quenching into water at room temperature, samples were naturally aged for different time periods and then processed by ECAP. The microstructure and mechanical properties of these samples are described and discussed. This investigation leads to proposing an interesting application of ECAP for supersaturated alloys. Using the shear bands created by ECAP in only one pass and applying appropriate subsequent aging treatments, composite-like microstructures can be achieved in conventional age-hardenanble Al alloys.

Abstract: Billets of a commercial purity aluminium Al-1050 alloy were processed by equal-channel angular pressing (ECAP) for up to a maximum of 6 passes. Following processing, the billets were sectioned and hardness measurements were recorded on both longitudinal and transverse sections. These measurements showed the hardness increases significantly in the first pass and continues to increase by small amounts in subsequent passes. Initially, there are regions of lower hardness running in bands near the top and bottom surface of each billet. The region of lower hardness near the upper surface disappears with increasing numbers of passes but near the bottom surface the lower hardness remains even after 6 passes. The results show that, neglecting the small region near the bottom of the billet, there is an excellent potential for achieving microstructural homogeneity within the Al-1050 alloy after pressing through a sufficient number of passes in ECAP.

Abstract: Tensile tests were carried out at 473 K with initial strain rate of 10-4 s-1 in samples of a ZK60 alloy (Mg-5.5% Zn-0.5% Zr) processed by different number of passes using Equal-Channel Angular Pressing (ECAP). The measured superplastic elongations ranged from ~930% to a record of ~3050%. The flow behavior was found to vary with the number of passes of ECAP. It is shown that strain-hardening due to grain growth and the evolution of the strain rate sensitivity with the strain determines the flow behavior and final elongations. The results are consistent with theories of plastic flow in tensile testing.

Abstract: High-pressure torsion (HPT) is an important processing technique in which a disk is subjected to a high pressure with concurrent torsional straining. In principle at least, the imposed strain is zero at the center of the disk and a maximum at the outer edge. This difference suggests, therefore, that materials processed by HPT will exhibit considerable inhomogeneity. This paper describes the results obtained in a series of experiments which were designed to evaluate the evolution of homogeneity during the processing of two materials by HPT. It is demonstrated that it is possible to achieve a reasonable level of homogeneity in both materials but there are important differences which reflect the dependence of the microstructure on the occurrence of dynamic recovery.