Using high-pressure torsion for metal processing: Fundamentals and applications

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Abstract

High-pressure torsion (HPT) refers to the processing of metals whereby samples are subjected to a compressive force and concurrent torsional straining. Although the fundamental principles of this procedure were first proposed more than 60 years ago, processing by HPT became of major importance only within the last 20 years when it was recognized that this metal forming process provides an opportunity for achieving exceptional grain refinement, often to the nanometer level, and exceptionally high strength. This review summarizes the background and basic principles of processing by HPT and then outlines the most significant recent developments reported for materials processed by HPT. It is demonstrated that HPT processing leads to an excellent value for the strength of the material, reasonable microstructural homogeneity if the processing is continued through a sufficient number of torsional revolutions and there is a potential for achieving a capability for various attractive features including superplastic forming and hydrogen storage. The review also describes very recent developments including the application of HPT processing to bulk and ring samples and the use of HPT for the consolidation of powders.

Introduction

The grain size of polycrystalline materials plays a major role in dictating many critical properties including the strength and resistance to plastic flow. In general, materials with small grain sizes have several advantages over their coarse-grained counterparts because they have higher strength and other favorable properties including a potential for use in superplastic forming operations at elevated temperatures. This significance was recognized several years ago and led to the concept of nanocrystalline materials which were discussed in detail in an early review [1].

In order to place the present review in perspective, it is first necessary to define some basic terms. Using the definitions developed earlier [2], bulk ultrafine-grained (UFG) materials are defined as bulk materials having fully homogeneous and equiaxed microstructures with average grain sizes less than ∼1 μm and with a majority of the grain boundaries having high-angles of misorientation. These UFG structures divide into materials having submicrometer grain sizes where the grains are within the range of 0.1–1 μm and true nanocrystalline materials where the grain sizes are <100 nm. As described elsewhere [2], X-ray analysis often reveals that the UFG microstructures with submicrometer grain sizes may additionally contain domains having sizes of the order of ∼40–50 nm which are associated with localized distortions of the crystalline lattice and substructure. Accordingly, these observations led to the introduction of the more general term “bulk nanostructured materials”. Many early reports described new and important results that were obtained using materials processed in different ways to produce UFG structures [3], [4].

It is now established that materials with UFG microstructures may be fabricated using two different approaches which are generally termed the “bottom-up” and “top-down” procedures [5].

In the “bottom-up” procedure, the bulk solids are fabricated through the assembly of individual atoms or nanoparticulate solids. Examples of this approach include inert gas condensation [6], electrodeposition [7] and ball milling with subsequent consolidation [8]. These approaches have the capability of producing materials with exceptionally small grain sizes but they have disadvantages because the sizes of the finished products are always very small, there may be some contamination introduced during processing (for example, from the ball milling) and there is invariably at least a low level of residual porosity.

The “top-down” approach avoids the introduction of either contaminants or porosity by taking a bulk solid with a relatively coarse grain size and then processing it to refine the grain size to at least the submicrometer level. There are now several possible procedures for processing these bulk solids but all procedures rely upon the imposition of heavy straining and thus upon the introduction of a very high dislocation density. Since these processes introduce severe plastic deformation (SPD) into the materials, it is convenient to describe all of these operations using the general term SPD processing. A detailed description of SPD processing was presented in an earlier review [9] and a formal definition was introduced recently [2]. Specifically, SPD processing is defined as any method of metal forming under an extensive hydrostatic pressure that may be used to impart a very high strain to a bulk solid without the introduction of any significant change in the overall dimensions of the sample and having the ability to produce exceptional grain refinement. It is important to note that the shape of the sample is retained in SPD processing by the use of special tool geometries which effectively prevent free flow of the material and thereby produce a significant hydrostatic pressure which leads to a high density of lattice dislocations and consequent grain refinement.

Several “top-down” processes are now available but the three procedures receiving the most attention at the present time are equal-channel angular pressing (ECAP) [10], accumulative roll-bonding (ARB) [11] and high-pressure torsion (HPT) [12]. These three procedures are fundamentally different but they each produce exceptional grain refinement to at least the submicrometer level.

This review is concerned with the production of UFG materials using HPT. Although the general principles of this processing method were first proposed more than 60 years ago, the procedure has become of general scientific interest only within the last 20 years. Furthermore, it is only within approximately the last 5 years that numerous extensive reports documenting the processing and properties of materials fabricated by HPT have started appearing in the scientific literature. This very recent interest, and the large number of recent publications, was the impetus for this report. Accordingly, the present review is designed to introduce the fundamental principles of HPT processing in the next section and the following sections summarize many of the important scientific results that have become available when using this processing method.

Section snippets

An historical perspective

The principle of achieving high strength and superior properties in metallic alloys through the application of severe plastic deformation has its origins more than 2000 years ago in the metal-working procedures developed during the Han dynasty (200 B.C.) of ancient China [13]. Nevertheless, although this ancient type of processing has similarities with the modern procedures of ECAP and ARB, it was not a true precursor of HPT because it failed to incorporate any torsional straining. By contrast,

Variation in homogeneity across an HPT disk

An important limitation in HPT is that the imposed strain varies across the sample and, in principle at least, the strain is reduced to zero at the disk center. As a consequence of this variation, it is reasonable to anticipate that the microstructures produced by HPT will be extremely inhomogeneous. Nevertheless, the experimental data available to date suggests there is a potential in many materials for achieving a gradual evolution into a reasonably homogeneous microstructure. Much of the

Microstructures in pure metals processed by HPT

In most early studies, dating back to the mid-1980s, microstructural evolution was investigated only in the general vicinity of the central portion of disks processed by HPT [12], [18]. For example, the TEM microstructure and microhardness were studied in (0 0 1)[0 1 0] single crystals of pure nickel and copper subjected to HPT to different total strains at room temperature under a load of 6 GPa and the region of investigation was located at a distance of ∼2 mm from the centers of each disk [12]. An

Characteristic properties in simple metallic alloys

Although numerous reports are now available describing the application of HPT processing to a wide range of materials, many of the published results relate to the processing of aluminum-based alloys. It is appropriate, therefore, to consider an example of these alloys because the results are generally reasonably representative of a wide range of metallic alloys.

A recent report described the processing of disks of an Al–3% Mg–0.2% Sc alloy by HPT [33]. The initial grain size was  0.5 mm but HPT

Processing of intermetallics by HPT

Several reports are now available describing the application of HPT processing to intermetallics. For example, processing of the stoichiometric binary TiAl intermetallic compound by HPT led to a highly cold-worked microstructure despite its intrinsic brittleness [103]. Increasing the applied strain by increasing the numbers of revolutions under an increased pressure gave some disordering of the TiAl compound and then a partial transformation of the ordered phase, L10, into the disordered hcp

The extension of HPT to bulk samples

An important limitation with conventional HPT is that the samples are extremely small. Typically, the processed specimens are in the form of disks having diameters of ∼1 cm and thicknesses less than 1 mm. In an attempt to overcome this limitation, exploratory tests were undertaken using bulk samples in the form of small cylinders with diameters of 1 cm and well-controlled heights of 8.57 mm [124]. This height was calculated because of the special configuration of the HPT facility as shown

Mechanical behavior of HPT metals at room temperature

It is well established that the mechanical behavior of crystalline solids at low temperatures, below ∼0.5Tm where Tm is the absolute melting temperature of the material, is dependent upon the Hall–Petch relationship where the yield stress, σys, is given by [146], [147]σys=σ0+kyd-1/2,where σ0 is a friction stress, ky is a constant of yielding and d is the grain size. From this relationship, it is readily apparent that the yield stresses of materials processed by HPT should be very high because

The relative advantages and disadvantages of processing by HPT

The refining of grains to the submicrometer or nanocrystalline level provides opportunities for producing materials having many superior properties including high strength at ambient temperatures and, if the grains exhibit reasonable thermal stability, a superplastic forming capability at elevated temperatures. As outlined briefly in Section 1, two complementary approaches have been developed for the fabrication of nanostructured materials [5].

In the first procedure, known as the “bottom-up”

Developments in HPT with potential industrial significance

Processing through the application of severe plastic deformation has developed extensively over the last decade. Furthermore, it is now recognized that the polycrystalline materials produced by various SPD techniques exhibit unique and attractive characteristics including excellent strength at ambient temperature. Accordingly, materials processed by SPD are now receiving serious consideration for potential use in a wide range of products including in commercial aerospace and automotive

Summary

The classic text of Bridgman, published in Journal of Applied Physics in 1943 with the title “On Torsion Combined with Compression” [14], set the scene for drawing attention to the significance of combining a compressive load with concurrent torsional straining in the processing of materials having exceptional mechanical properties. Although this research received only modest attention at the time of publication, in later years it was appreciated that the same approach, when adapted to the form

Acknowledgements

Our knowledge of HPT owes much to the many researchers around the world who have undertaken and reported significant advances in this field. We take this opportunity to thank all of our colleagues in the field of SPD processing, and HPT in particular, for their many insightful contributions and especially we thank our scientific collaborators who have contributed to our publications in this area: many of these papers are cited in the reference section of this report.

We thank Dr. Megumi Kawasaki

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