Papers by Author: Terence G. Langdon

Abstract: The processing of metals through the application of severe plastic deformation provides the potential for achieving exceptional grain refinement in bulk solids. Several SPD methods are now available but processing by high-pressure torsion (HPT) has attracted much attention over the last five years. Numerous reports are now available describing the application of HPT to a range of pure metals and simple alloys and excellent grain refinement were achieved using this process with the average grain size often reduced to the nanoscale range. However, in order to make this technique more practical, the nature of the sample characteristics immediately after conventional HPT must be considered in order to understand the fundamental principles of HPT processing. This report examines the procedure with special emphasis on the evolution in hardness homogeneity in both high-purity aluminum and a Zn-22% Al eutectoid alloy processed by HPT.

Abstract: Disks of as-extruded Mg-4Nd alloy were processed by high-pressure torsion (HPT) through ¼ to 5 turns at room temperature. The first 1/4 turn of HPT induces large numbers of twins and some dislocation tangles in the center region of the disk. With increase of torsional straining, the twinning is inhibited gradually and the dislocation density increases relating to the formation of dislocation substructures and ultimately transforming to high fractions of equiaxed gains which have an average grain size of ~200 nm and high-angle boundaries. HPT significantly improves the values of microhardness of this alloy. The hardness values in both the central and edge regions show a sharp rise after HPT for 1/4 turn and exhibit nearly saturation after 1/2 turn although there is a trend of a slight increase with increasing numbers of turns. The experimental results suggest more homogeneous microstructures may be produced by larger numbers of turns in the HPT process.

Abstract: Disks of a commercial purity aluminium Al-1050 alloy and Al-1%Mg alloy were processed by high-pressure torsion (HPT) at room temperature for up to a maximum of 5 turns under a pressure of 6 GPa. Following processing, hardness measurements were recorded across the surfaces of the disks. These measurements showed low values of hardness at the center and high values near the edges of the disks and the hardness increased in both alloys with increasing numbers of turns. The evolution of homogeneity in hardness was rapid in Al-1050 compared to the Al-1%Mg alloy. After 5 turns of HPT under a pressure of 6 GPa, the hardness was fully homogeneous across the total surface of the Al-1050 disk whereas there was a region of lower hardness around the center of the Al-1%Mg disk. The results reveal the significant difference between both alloys where the higher rate of recovery in the Al-1050 alloy leads to a rapid evolution of the hardness homogeneity.

Abstract: Disks of an AZ31 magnesium alloy were processed by High-Pressure Torsion (HPT) at 463 K to different numbers of rotations. The grain structure was evaluated along the cross-section of the disks using optical microscopy. Significant heterogeneities in the average grain size were observed in areas of the disks which were located at similar distances to the center but at different distances from the surface. Moreover, different grain structures were observed in neighboring areas and shear bands occurred at several locations in the disks. Microhardness tests revealed differences in the strength of the material as a function of the distance to the surface. An analysis of the grain structure and hardness distribution suggests the occurrence of flow localization in HPT processing.

Abstract: The processing of bulk metals through the application of severe plastic deformation provides the opportunity for introducing significant grain refinement into bulk solids. In the present investigation, an aluminum alloy (Al-6061) was processed by high-pressure torsion (HPT) at room temperature under an applied pressure of 6.0 GPa up to a total of 5 turns. Detailed measurements after processing revealed the occurrence of continuous grain refinement and material strengthening with increasing imposed strain. The average grain size of the alloy was reduced from ~150 m to a grain size in the range of ~500 nm through processing by HPT. Although there was a difference in the average grain size of samples processed to different levels of imposed strain, careful inspection showed that the structures became similar after annealing at 250°C for 5 min. This suggests that the additional grain refinement introduced at large amounts of deformations is less stable at high temperatures. The results of this investigation, including the distributions of the grain sizes after annealing, are consistent with the predictions of a model based on the occurrence of continuous recrystallization in aluminum alloys having fine grain structures, large fractions of high-angle grain boundaries and where there is a large amount of deformation.

Abstract: The effect of grain size on the deformation twinning and de-twinning in a nanocrystalline Ni-Fe alloy was investigated using transmission electron microscopy. Specimens with different grain sizes were obtained by severely deforming an electrochemically deposited nanocrystalline Ni-20wt.% Fe alloy using high-pressure torsion, which resulted in continuous grain growth from an average grain size of ~ 21 nm in the as-deposited material to ~ 72 nm for the highest strain applied in this study. Results show that deformation de-twinning occurs at very small grain sizes while deformation twinning takes place when the grain size is larger than ~ 45 nm. The mechanism of the observed grain size effect on twinning and de-twinning is briefly discussed.

Abstract: Experiments were conducted to evaluate the feasibility of using a new ECAP die configuration in which there are arcs of curvature at the intersection of the two channels at both the outer and inner points. The experiments used a configuration in which the radii of curvature were identical at both the outer and inner points since calculations suggest this corresponds to an optimum configuration. High-purity aluminum was pressed using this special die and hardness measurements were taken on cross-sectional planes after pressing. The results show the new die configuration leads to less homogeneity than with conventional ECAP dies.

Abstract: It is well established that processing through the application of severe plastic deformation (SPD) provides opportunities for achieving exceptional grain refinement to the submicrometer or even the nanometer level. This paper provides a brief summary of the developments in SPD processing from the earliest days in ancient China to research at the present time. It is shown that SPD research and bulk nanostructured materials have had a significant impact on the field of Materials Science. Using citation data and examining the publication records for five major journals, the analysis shows research in these areas accounts for a large fraction of the top ten all-time most cited papers in these journals.

Abstract: The evolution of the microstructure during processing by equal-channel angular pressing (ECAP) in silver having extremely low stacking fault energy was studied up to 16 passes. It was shown that at high strains the contribution of twinning to deformation increased at the expense of dislocation-controlled processes. It was also found that during storage at room temperature (i.e. at the temperature of ECAP) there was a self-annealing of the severely deformed microstructure after 1 month and its degree was revealed to have a strong dependence on the number of passes.

Abstract: The origin of true laboratory-scale superplasticity may be traced to a publication appearing seventy-five years ago in 1934. This overview examines the subsequent major developments in obtaining a fundamental understanding of superplastic flow and then looks to the future to summarize very new developments that provide the potential for invigorating the field of superplastic research.