Megumi Kawasaki

Different models of hardness evolution in ultrafine-grained materials processed by high-pressure torsion

High-pressure torsion (HPT) is an attractive processing method in severe plastic deformation techniques involving the application of high compressive pressure with concurrent torsional straining. Excellent grain refinement is anticipated when using this technique to average grain sizes of the submicrometer or even nanometer ranges. Because of the significant microstructural changes during processing, there are numerous reports showing evolution in local hardness toward homogeneity throughout a disk diameter with increasing numbers of revolutions. The achieved hardness after HPT is mostly much higher than that in the as-received condition because of exceptional grain refinement although there are a limited number of metals and alloys showing softening or weakening after HPT processing. This paper was initiated to review recent discoveries in the experimental results on hardness evolution toward homogeneity during HPT processing and discuss the different models of hardness developments with respect to imposed equivalent strain by HPT processing for a range of metals and alloys. Moreover, recent results of hardness homogeneity and heterogeneity through thicknesses of the processed disks are discussed toward a complete understanding of hardness evolution in the UFG metals processed by HPT.

Review: Overcoming the paradox of strength and ductility in ultrafine-grained materials at low temperatures

Ultrafine-grained (UFG) materials with grain sizes in the submicrometer or nanometer range may be prepared through the application of severe plastic deformation (SPD) to bulk coarse-grained solids. These materials generally exhibit high strength but only very limited ductility in low-temperature testing, thereby giving rise to the so-called paradox of strength and ductility. This paradox is examined and a new quantitative diagram is presented which permits the easy insertion of experimental data. It is shown that relatively simple procedures are available for achieving both high strength and high ductility in UFG materials including processing the material to a very high strain and/or applying a very short-term anneal immediately after the SPD processing. Significant evidence is now available demonstrating the occurrence of grain boundary sliding in these materials at low temperatures, where this is attributed to the presence of non-equilibrium grain boundaries and the occurrence of enhanced diffusion along these boundaries.

Twenty-five years of severe plastic deformation: recent developments in evaluating the degree of homogeneity through the thickness of disks processed by high-pressure torsion

The processing of disks by high-pressure torsion leads to an inhomogeneous distribution in strain with a high strain around the perimeter of the disk and a zero strain in the center. Despite this apparent inhomogeneity, there are now many experiments showing that the hardness values on the surfaces of disks gradually evolve with increasing strain to give a reasonably high level of homogeneity. Experiments were conducted to determine whether this high level of homogeneity extends also through the thickness of the disks or whether inhomogeneities occur in the axial direction. Results are presented for high-purity aluminum and a magnesium AZ31 alloy as two representative materials showing different hardness characteristics.

Atomic-scale investigation of interface-facilitated deformation twinning in severely deformed Ag-Cu nanolamellar composites

We report an atomic-scale investigation of interface-facilitated deformation twinning behaviour in Ag-Cu nanolamellar composites. Profuse twinning activities in Ag supply partial dislocations to directly transmit across the Ag-Cu lamellar interface that promotes deformation twinning in the neighbouring Cu lamellae although the interface is severely deformed. The trans-interface twin bands change the local structure at the interface. Our analysis suggests that the orientation relationship and interfacial structure between neighbouring Ag-Cu lamellae play a crucial role in such special interface-facilitated twinning behaviour.

Applied stress controls the production of nano-twins in coarse-grained metals

We present evidence that the level of the applied stress plays a critical role in deformation twinning in face-centred cubic alloys. While conventional cold rolling of a face-centred cubic structure produces a microstructure with a high-density of extended dislocations, increasing the applied stress using high-pressure torsion gives a nano-twinned coarse-grained structure. This suggests the existence of a critical stress for deformation twinning which thereby delineates an approach for the production of nano-twinned microstructures in coarse-grained materials with superior mechanical properties.

Using deformation mechanism maps to depict flow processes in superplastic ultrafine-grained materials

Deformation mechanism maps are a very useful tool for displaying deformation mechanisms as a function of the three fundamental parameters of high temperature flow: the applied stress, the testing temperature and the grain size of the material. These maps are used extensively in the field of high temperature creep but there has been very little use with ultrafine-grained (UFG) metals. This article reviews the principles of deformation mechanism maps, presents examples of maps for some representative metals processed by equal-channel angular pressing or high-pressure torsion and then describes a simple procedure for constructing maps for UFG materials.

The significance of self-annealing in two-phase alloys processed by high-pressure torsion

The Zn-22% Al eutectoid alloy and the Pb-62% Sn eutectic alloy were processed by high-pressure torsion (HPT) over a range of experimental conditions. Both alloys exhibit similar characteristics with significant grain refinement after processing by HPT but with a reduction in the hardness values by comparison with the initial unprocessed conditions. After storage at room temperature for a period of time, it is shown that the microhardness of both alloys gradually recovers to close to the initial unprocessed values. Electron backscatter diffraction (EBSD) measurements on the Pb-Sn alloy suggest that the self-recovery behaviour is correlated with the fraction of high-angle grain boundaries (HAGBs) after HPT processing. Thus, high fractions of HAGBs occur immediately after processing and this favours grain boundary migration and sliding which is important in the self-annealing and recovery process. Conversely, the relatively lower fractions of HAGBs occurring after annealing at room temperature are not so conducive to easy migration and sliding.

Characteristics of High Temperature Creep in Pure Aluminum Processed by Equal-Channel Angular Pressing

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.

Superplasticity in a lean Fe-Mn-Al steel

Superplastic alloys exhibit extremely high ductility (>300%) without cracks when tensile-strained at temperatures above half of their melting point. Superplasticity, which resembles the flow behavior of honey, is caused by grain boundary sliding in metals. Although several non-ferrous and ferrous superplastic alloys are reported, their practical applications are limited due to high material cost, low strength after forming, high deformation temperature, and complicated fabrication process. Here we introduce a new compositionally lean (Fe-6.6Mn-2.3Al, wt.%) superplastic medium Mn steel that resolves these limitations. The medium Mn steel is characterized by ultrafine grains, low material costs, simple fabrication, i.e., conventional hot and cold rolling, low deformation temperature (ca. 650 °C) and superior ductility above 1300% at 850 °C. We suggest that this ultrafine-grained medium Mn steel may accelerate the commercialization of superplastic ferrous alloys.Research in new alloy compositions and treatments may allow the increased strength of mass-produced, intricately shaped parts. Here authors introduce a superplastic medium manganese steel which has an inexpensive lean chemical composition and which is suited for conventional manufacturing processes.

The evolution of homogeneity during processing of aluminium alloys by HPT

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.