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This study examines strain distribution occurring in the high-pressure sliding (HPS) processing for rods of pure Al and an AZ61 alloy. The strain distribution is evaluated through the optical micrographs and hardness measurements on the cross sections at the positions of the front, center, and rear along the longitudinal axis with respect to the shearing direction. The advent of superplasticity is also used for the evaluation of sufficient introduction of strain and thus grain refinement. The strain distribution is also measured on the longitudinal cross section by marking square grids and measuring the change in grid shapes after the HPS processing. This direct measurement on the longitudinal cross section is compared with an FEM simulation. It is shown that strain is preferentially introduced in an oblique direction. Reciprocation of the sliding direction is a good solution for this inhomogeneous strain introduction. Rotation of the rod around the longitudinal axis after each sliding pass is also effective to improve the homogeneous strain distribution on the vertical cross section of the rod. It is suggested that reciprocation of the sliding direction together with rotation of the rod around the longitudinal axis promotes the homogeneity of the strain distribution.

It is well known that severe plastic deformation (SPD) produces ultrafine-grained structures in bulk metallic materials. The SPD process becomes more versatile when it is performed under high pressure as high-pressure torsion (HPT) and high-pressure sliding (HPS). Not only the grain size is more refined but also the process is applicable to hard-to-deform materials such as intermetallics, semiconductors and ceramics, leading to enhancement of functional properties as well as structural properties. The major drawback is that the sample size is small so that the applicability is limited to a laboratory scale and it is an important subject to increase the sample dimensions. This paper presents an overview describing efforts devoted thus far to deal with this upscaling issue.

The high-pressure sliding (HPS) process was applied for grain refinement of a pipe form of an Al-3wt%Mg-0.2wt%Sc alloy by developing two types of straining techniques (called in this study anvil sl...

Sheets of coarse-grained S304H austenitic steel were processed by high-pressure sliding (HPS) at room temperature and a ultrafine-grained microstructure with a mean grain size of about 0.14 µm was prepared. The microstructure changes and creep behavior of coarse-grained and HPS-processed steel were investigated at 500–700 °C under the application of different loads. It was found that the processing of S304H steel led to a significant improvement in creep strength at 500 °C. However, a further increase in creep temperature to 600 °C and 700 °C led to the deterioration of creep behavior of HPS-processed steel. The microstructure results suggest that the creep behavior of HPS-processed steel is associated with the thermal stability of the SPD-processed microstructure. The recrystallization, grain growth, the coarsening of precipitates led to a reduction in creep strength of the HPS-processed state. It was also observed that in the HPS-processed microstructure the fast formation of σ-phase occurs. The σ-phase was already formed during slight grain coarsening at 600 °C and its formation was enhanced after recrystallization at 700 °C.

In this study, the multi-pass high-pressure sliding (MP-HPS) process was applied for grain refinement of rods with an upsized dimension. To achieve a homogeneous microstructure throughout the cross section of the rod, the rod samples are rotated with 60° around the longitudinal axis (MP-HPS-R) or are shifted upward and downward from the mid-height on the cross-sectional plane (MP-HPS-S). A homogeneous microstructure with an average grain size of ~ 200 nm for an Al-3 wt%Mg-0.2 wt%Sc alloy and ~ 160 nm for Inconel 718 were developed through both of the MP-HPS processes. The rods of the Al-3 wt%Mg-0.2 wt%Sc alloy and Inconel 718 after MP-HPS processing exhibited superplasticity with total elongations more than 600% and 400%, respectively. A finite-element method (FEM) was used to simulate the magnitude of the strain including its distribution on the cross section of the rod through processing by MP-HPS-R. It was demonstrated that microstructural homogeneity was developed in the rod processed by the MP-HPS-R but inhomogeneity was left by the MP-HPS-S.

Martensitic creep-resistant P92 steel was deformed by different methods of severe plastic deformation such as rotation swaging, high-pressure sliding, and high-pressure torsion at room temperature. These methods imposed significantly different equivalent plastic strains of about 1&ndash, 30. It was found that rotation swaging led to formation of heterogeneous microstructures with elongated grains where low-angle grain boundaries predominated. Other methods led to formation of ultrafine-grained (UFG) microstructures with high frequency of high-angle grain boundaries. Constant load tensile creep tests at 873 K and initial stresses in the range of 50 to 300 MPa revealed that the specimens processed by rotation swaging exhibited one order of magnitude lower minimum creep rate compared to standard P92 steel. By contrast, UFG P92 steel is significantly softer than standard P92 steel, but differences in their strengths decrease with increasing stress. Microstructural results suggest that creep behavior of P92 steel processed by severe plastic deformation is influenced by the frequency of high-angle grain boundaries and dynamic grain coarsening during creep.

This study updates a process of high-pressure sliding (HPS) recently developed as a severe plastic deformation process under high pressure for grain refinement of sheet samples. The updated version, which we call the incremental feeding HPS (IF-HPS), consists of sliding for SPD and feeding for upsizing the SPD-processed area so that, without increasing the capacity of processing facility, it is possible to cover a much larger area with an SPD-processed ultrafine-grained structure with a grain size of ~ 120 nm. For the IF-HPS processing, anvils with flat surfaces but without grooves are used in an unconstrained condition, and the feeding distance is set equal to the deformed width. A Ni-based superalloy (Inconel 718) is processed by the IF-HPS under 4 GPa at room temperature, and it is possible to obtain an SPD-processed sheet with dimensions of approximately 100 × 100 × 1 mm3. Strain distribution and evolution were examined by hardness measurement and simulation using a finite element method. Tensile tests were conducted using tensile specimens extracted from the IF-HPS-processed sheet. Advent of high strain rate superplasticity with the total elongation of more than 400 pct was confirmed by pulling the tensile specimens with an initial strain rate of 2.0 × 10−2 s−1 at a temperature as low as 1073 K. The formability of the IF-HPS-processed sheet was confirmed by successful cup forming. It was also confirmed that the restoration after the superplastic deformation was feasible by subjecting to conventional heat treatment used for Inconel 718.

Inconel 718 was successfully processed by multi-pass high-pressure sliding (MP-HPS) for production of ultrafine-grained structures in rods with upsized dimensions. An ultrafine-grained (UFG) structure with an average grain size of ~ 140 nm was developed through rotation of 45°. Tensile testing at 1073 K with an initial strain rate of 2x10-2 s−1 led to superplastic elongations more than 400%. A relation is established between hardness and elongation to failure so that it should be possible to predict the advent of superplasticity only from the hardness measurement.

This study reports a production of a superplastic Ni-based superalloy (Inconel 718) using a process of severe plastic deformation through high-pressure sliding (HPS). The grain size of the alloy was reduced to ~120 nm by operating the HPS process under 4 GPa at room temperature with a recently upscaled facility. The ultrafined-grained structure was well retained even after annealing at 1173 K for 1 h. Tensile tests were conducted in air at a testing temperature in the range of 973–1173 K with an initial strain rate of 5.0×10 −4 –2.0×10 −2 s −1 . Superplastic elongation more than 400% were attained at all testing conditions except at 973 K. High-strain rate superplasticity (defined with strain rates higher than 1×10 −2 s −1 ) was achieved at temperatures higher than 1073 K. Electron back scatter diffraction analyses revealed that a preferential orientation of the grains was developed by the HPS processing but it was randomized with tensile deformation. Evaluation of the strain rate sensitivity and the activation energy for the superplastic deformation confirmed that the superplasticity of Inconel 718 was controlled by grain boundary sliding through lattice diffusion.

The process of high-pressure sliding (HPS) is a method of severe plastic deformation developed recently for grain refinement of metallic materials under high pressure. The sample for HPS is used with a form of sheet or rod. In this study, an HPS facility with capacities of 500 tonnes for vertical pressing and of 500 and 300 tonnes for horizontal forward and backward pressings, respectively, was newly built and applied for grain refinement of a Mg alloy as AZ61, Al alloys such as Al-Mg-Sc, A2024 and A7075 alloys, a Ti alloy as ASTM-F1295, and a Ni-based superalloy as Inconel 718. Sheet samples with dimensions of 10 to 30 mm width, 100 mm length, and 1 mm thickness were processed at room temperature and ultrafine grains with sizes of ~200 to 300 nm were successfully produced in the alloys. Tensile testing at elevated temperatures confirmed the advent of superplasticity with total elongations of more than 400 pct in all the alloys. It is demonstrated that the HPS can make all the alloys superplastic through processing at room temperature with a form of rectangular sheets.

High-pressure sliding (HPS) was successfully applied for consolidation of aluminum powders at room temperature. A full densification was attained by the HPS processing under 2 GPa through 3 mm sliding (equivalent strain of 2.3). The consolidated samples showed ultrafine-grained structures with average grain sizes of ~0.85 μm and ~1.0 μm for the powder sizes of 45 μm and 75 μm, respectively. When the consolidated samples were annealed at a temperature in the range of 473–673 K, grain growth was significantly reduced by the presence of oxide particles originated from the surface layers on the powders. The hardness and tensile strength increased with increasing imposed strain and decreased with annealing but they remained higher in the consolidated samples than that in the bulk sample. The electrical conductivity increased with straining and saturated to a value of 50%IACS in a manner similar to the density. The strengthening mechanisms were evaluated in terms of particles dispersions as well as grain refinement. This study demonstrates that the HPS processing provides a useful approach to consolidate metallic powders into bulk form for application of electrical wires with higher tensile strength and enhanced thermal stability.

The austenitic stainless steel 304 L was subjected to severe plastic deformation using high-pressure torsion technique at room temperature. The severe plastic deformation led not only to the grain size reduction but also to the transformation of austenite into deformation-induced martensite. At high imposed strains the martensitic boundaries disappeared and misorientation distribution exhibited nearly random misorientation distribution. The thermal exposure and tensile creep tests under constant load performed at 923 K and stresses ranging from 50 to 150 MPa revealed that austenitic microstructure contains σ phase. The ultrafine-grained microstructure coarsens during creep testing but the mean grain size is significantly finer than the predicted stationary subgrain. The results demonstrate that during creep testing new grains at precipitates of σ phase are formed and texture changes. The formation of new grains during creep keeps the microstructure of being ultrafine-grained and thus grain boundary mediated processes are enhanced in this steel.

A Ni-based superalloy (Inconel 718) was successfully processed by high-pressure torsion (HPT) under a pressure up to 6 GPa at room temperature. The grain size after the HPT processing was reduced to ~100 nm. Tensile testing at 1073 K led to the advent of superplasticity with a maximum elongation of 1190% at a strain rate of 2×10−2 s−1 which is well in the range of high-strain-rate superplasticity. This superplastic flow was accompanied by a significant decrease in the flow stress.