Zenji Horita

Principle of equal-channel angular pressing for the processing of ultra-fine grained materials

Considerable interest has centered recently on the processing of materials with ultra-fine grain sizes, typically <100 nm. These materials may be prepared by various techniques including gas condensation and subsequent in situ consolidation under high vacuum, high energy ball milling and sliding wear. Alternatively, it has been shown that submicrometer-grained structures may be produced in a wide range of materials (e.g. pure metals, metallic alloys including superalloys, intermetallics, semiconductors) by subjecting these materials to a very high plastic strain using either equal-channel angular (ECA) pressing or torsion straining under high pressure. These latter procedures usually give grain sizes in the submicrometer range of {approximately}100--200 nm although there are reports of grain sizes as small as {approximately}50 nm. In practice, ECA pressing is an especially attractive processing method because it provides the capability of producing large bulk samples which are free from any residual porosity.

The shearing characteristics associated with equal-channel angular pressing

Equal-channel angular (ECA) pressing is a processing method for introducing an ultra-fine grain size into a material. In practice, it is a procedure that may be used to achieve high total strains by subjecting a sample to repetitive pressings. There is experimental evidence showing that the nature of the microstructural evolution in ECA pressing depends upon whether the sample is rotated between each passage through the die. This paper examines the shearing characteristics associated with ECA pressing for six different processing routes and reaches conclusions concerning the optimum processing procedure and the development of texture.

An investigation of microstructural evolution during equal-channel angular pressing

Abstract Experiments were conducted to investigate the development of an ultra-fine grain size during equal-channel angular (ECA) pressing of high purity aluminum with an initial grain size of ∼1.0 mm. The results show that, under ECA pressing conditions giving a strain of ∼1.05 on each passage through the die, the microstructure is reasonably homogeneous after a single pressing and consists of parallel bands of elongated subgrains, having an average length of ∼4 μm, and these subgrains are further divided by boundaries with very low angles of misorientation. Repetitive pressings were conducted on the same samples, up to a total of 10 passages through the die, with the samples pressed either without rotation (route A) or after rotating through 180° between each pressing (route C). It is demonstrated that the misorientations of the subgrain boundaries increase with repetitive pressings until ultimately both routes lead to a similar equiaxed ultra-fine grain size of ∼1 μm after 10 pressings, but the microstructural evolution is enhanced using route C where there is a more rapid transition into an array of high angle grain boundaries. The results suggest that, at least for high purity aluminum, an ultra-fine microstructure close to optimum may be obtained after only 4 pressings provided the sample is rotated through 180° between each pressing.

Improving the mechanical properties of magnesium and a magnesium alloy through severe plastic deformation

Pure Mg and Mg alloys generally exhibit only limited ductilities at ambient temperatures. Experiments were conducted to evaluate the potential for improving the mechanical properties of pure Mg and an Mg–0.9% Al alloy at room temperature by subjecting these materials to severe plastic deformation through the procedure of equal-channel angular pressing (ECAP). It is shown that ECAP may be applied successfully to these materials at elevated temperatures and this leads to grain refinement due to the occurrence of recrystallization during the pressing process and to significant improvements in the strength and ductility of these materials. Since these improvements are apparent after only a single pass through the ECAP die, it is concluded that the introduction of ECAP provides a simple and effective procedure for improving the ambient temperature mechanical properties of materials, such as hcp metals, where the measured ductilities are generally limited.

Influence of channel angle on the development of ultrafine grains in equal-channel angular pressing

Abstract Equal-channel angular pressing provides a convenient procedure for introducing an ultrafine grain size into a material. Using samples of pure Al, tests were conducted to determine the influence of the channel angle Φ, defined as the angle of intersection of the two channels within the die, on the subsequent microstructure attained by pressing. Experiments were performed using dies having channel angles from 90 to 157.5°. The results show that an ultrafine microstructure of essentially equiaxed grains, separated by high angle grain boundaries, is achieved only when a very intense plastic strain is imposed on the sample in each passage through the die as when using a die having a channel angle of Φ close to 90°

Microhardness measurements and the hall-petch relationship in an Al-Mg alloy with submicrometer grain size

Abstract An Al-3% Mg solid solution alloy was subjected to intense plastic deformation, using either equal-channel angular (ECA) pressing or torsion straining, to produce grain sizes in the submicrometer range. Static annealing at elevated temperatures led to grain growth and average grain sizes of up to > 100 μm. As-fabricated and statically annealed specimens were used to determine the variation in microhardness with grain size, and results confirm that the Hall-Petch relationship persists down to at least the finest grain size examined experimentally (∼90 nm). The results provide no evidence to support the claims of a negative Hall-Petch slope when the average grain size is very small, but there is evidence of a decrease in the slope of the Hall-Petch plot at the very finest grain sizes (

An investigation of grain boundaries in submicrometer-grained Al-Mg solid solution alloys using high-resolution electron microscopy

High-resolution electron microscopy was used to examine the structural features of grain boundaries in Al–1.5% Mg and Al–3% Mg solid solution alloys produced with submicrometer grain sizes using an intense plastic straining technique. The grain boundaries were mostly curved or wavy along their length, and some portions were corrugated with regular or irregular arrangements of facets and steps. During exposure to high-energy electrons, grain boundary migration occurred to reduce the number of facets and thus to reduce the total boundary energy. The observed features demonstrate conclusively that the grain boundaries in these submicrometer-grained materials are in a high-energy nonequilibrium configuration.

Developing superplasticity in a magnesium alloy through a combination of extrusion and ECAP

Abstract A new processing procedure was applied to a cast Mg-9% Al alloy. This procedure involves the sequential application of extrusion and equal-channel angular pressing and is designated EX-ECAP. Experiments show that the Mg-9% Al alloy has an initial grain size of ~50 μm after casting but this is reduced to ~12 μm after extrusion and it is further reduced to ~0.7 μm when the extruded alloy is subjected to ECAP for 2 passes at 473 K. Although the cast alloy exhibits extremely limited ductility and the extruded alloy is only moderately ductile, it is demonstrated that processing by EX-ECAP produces excellent superplastic ductilities including the occurrence of both low temperature superplasticity and high strain rate superplasticity. The EX-ECAP process is less effective when the ECAP step is conducted at 573 K because, although the pressing is then very easy, there is significant grain growth at this higher temperature.

Microstructure of two-phase Al-1.7 at% Cu alloy deformed by equal-channel angular pressing

Abstract The microstructural changes of an aged Al–1.7 at% Cu alloy associated with severe plastic deformation have been studied by transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EF-TEM). θ′ precipitates are almost completely dissolved after eight passes of equal-channel angular (ECA) pressing, and nearly single-phase α with a fine grain size of approximately 500 nm is obtained. When a severely deformed sample is aged for 24 h at 100°C, precipitation of equiaxed θ phase is observed along the grain boundaries, whereas only GP zones are formed in the undeformed sample. The dissolution and precipitation processes in severely deformed Al–1.7 at% Cu alloy have been examined by TEM and energy-filter mapping.

An investigation of microstructural stability in an AlMg alloy with submicrometer grain size

Abstract The microstructural stability of an Al 3%Mg solid solution alloy with a submicrometergrained (SMG) structure (∼ 0.2 μm) was evaluated using both static annealing and transmission electron microscopy over a range of temperatures from 443 to 803 K and differential scanning calorimetry (DSC) up to 773 K. The results show that the SMG structure contains many non-equilibrium grain boundaries but recrystallization occurs at the higher temperatures giving large grains with boundaries having high-angle equilibrium configurations. There are significant differences between the DSC curves of the SMG alloy and a standard cold-rolled Al 3%Mg alloy, due primarily to the advent of significant heat release at low temperatures in the SMG alloy because of recovery at the non-equilibrium grain boundaries. A temperature of ∼ 500 K, close to half the absolute melting temperature, represents an effective upper limit for utilization of the SMG structure in this material.