Keiichiro Oh-ishi

Processing of a low-carbon steel by equal-channel angular pressing

Abstract A low-carbon steel containing 0.08% C was processed by Equal-Channel Angular Pressing (ECAP) with the sample rotated by 90° in the same direction between each pass in processing route BC. Samples were pressed through a total of three passes at room temperature. The results reveal an evolution in the microstructure with increasing strain from an array of elongated subgrains separated by boundaries having low angles of misorientation to an array of reasonably equiaxed grains separated by high-angle boundaries. The average grain size after three passes was ∼0.2xa0μm. Processing by ECAP increases the 0.2% proof stress and the ultimate tensile strength (UTS) but there is a corresponding decrease in the elongation to failure. A comparison with earlier published results obtained using processing routes A (no rotation between passes) and C (rotation of 180° between passes) suggests that route BC is preferable because these samples exhibit an extensive region of strain hardening and a reasonably high elongation in tensile testing at room temperature. This difference is attributed to the greater microstructural homogeneity that is achieved when using processing route BC.

Grain boundary structure in Al–Mg and Al–Mg–Sc alloys after equal-channel angular pressing

Samples of an Al-3% Mg alloy and an Al-3% Mg-0.2% Sc alloy were subjected to equal-channel angular pressing (ECAP) to reduce the grain size to approximately 0.2-0.3 im. Some samples of each alloy were also annealed for 1 h at temperatures of either 423 or 673 K, respectively. High-resolution electron microscopy was used to examine the microstructure both before and after annealing. The grain boundaries after ECAP were wavy and faceted and in high-energy nonequilibrium configurations. These results were consistent with earlier observations of materials subjected to severe plastic deformation using high-pressure torsion. In addition, some grain boundaries in the Al-Mg-Sc alloy had a zigzag appearance after annealing at 673 K, where the straight portions of the boundary were identified as low-energy {111} planes. It is suggested these are mobile boundaries lying in a lowest energy configuration where mobility may be restricted by the presence of incoherent Al3Sc particles.

Fabrication and Thermal Stability of a Nanocrystalline Ni-Al-Cr Alloy: Comparison with Pure Cu and Ni

A Ni–Al–Cr alloy with an initial grain size of∼60 mm was subjected to torsion straining to a strain of ∼7 at room temperature, thereby reducing the grain size to ∼34 nm. Similar torsion straining with samples of pure Cu and pure Ni gave grain sizes of∼170 and∼130 nm, respectively. Inspection of the Ni–Al–Cr alloy after torsion straining revealed highly strained regions containing dislocations associated with lattice distortions but with an absence of any Ni 3Al ordered phase. The ultrafine grains in the Ni–Al–Cr alloy were extremely stable at high temperatures, and it was possible to retain a grain size of less than 100 nm after annealing at temperatures up to ∼900 K. By contrast, there was rapid grain growth in the samples of pure Cu and Ni at annealing temperatures in the vicinity of ∼500 K. The stability of the grains in the Ni–Al–Cr alloy is attributed to the formation of a Ni 3Al-based ordered phase after annealing at∼650–700 K. The presence of this phase also leads to an apparent negative slope in the standard Hall–Petch relationship.

Microstructure control using severe plastic deformation

Abstract The process of severe plastic deformation (SPD) was applied to Al alloys in order to demonstrate that it is possible to control the morphology and distribution of second phase particles using SPD. Equal-channel angular pressing (ECAP) is used for the SPD process and transmission electron microscopy is used for the microstructural observations. It is shown that precipitate particles are severely deformed or fragmented by the shear strain introduced by the ECAP process. It is also shown that the particles may even be dissolved in the matrix when the strain becomes intense. However, the change in the particle size and morphology are different depending on the alloy system.

Microstructure and strength of B2-ordered NiAl containing L21-Ni2AlHf precipitates

Abstract Microstructural variations and correlated hardness changes during phase decomposition of B2-ordered NiAl( β ) containing 2 mol% of Hf have been investigated by means of transmission electron microscopy (TEM), hardness and creep tests. At the beginning of aging, fine platelets of L2 1 -type Ni 2 AlHf (Heusler phase) precipitate on the {111} planes of the matrix NiAl phase keeping a perfect lattice coherency. With longer periods of aging, Ni 2 AlHf precipitates lose their lattice coherency and change their morphology to spherical ones surrounded by misfit dislocations. Compressive creep tests clearly indicated that the creep rate decreases appreciably by the fine precipitation of Ni 2 AlHf phase. TEM observations of the interaction between dislocations and spherical precipitates revealed that the dislocations tend to be strongly attracted to the particle interfaces during the creep deformation.

An Investigation of Deformation in Aluminum Single Crystals Using Equal-Channel Angular Pressing

This paper describes experiments in which high purity copper single crystals of two different orientations were processed for one pass by equal-channel angular pressing (ECAP) and the deformed structures were examined using optical microscopy (OM), orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). The first single crystal (0° specimen) was oriented within the entrance channel of the die so that the {111} slip plane and the slip direction were parallel to the theoretical shear plane and shear direction, respectively. The second crystal (20° specimen) was oriented with the {111} slip plane and the slip direction rotated by 20° in a clockwise sense from the theoretical shear plane and shear direction, respectively. For the 0° specimen, after passing through the shear plane there were two crystallographic orientations representing the initial orientation and an orientation rotated by 60° in a counter-clockwise sense from the initial orientation. For the 20° specimen, there was an orientation rotated by 20° in a counter-clockwise sense from the initial orientation after passing through the shear plane.

Microstructure Evolution and Microstructure-Property Relationships in Friction Stir Processing of NiAl Bronze

Friction stir processing (FSP) has been employed for localized modification and control of microstructures in NiAl bronze materials, which are widely utilized for marine components. The thermomechanical cycle of FSP results in homogenization and refinement and the conversion of microstructures from a cast to a wrought condition within stir zones in the material. However, the direct measurement of stir zone temperatures, strains, strain rates and cooling rates is difficult due to steep gradients and transients in these quantities, and this is an impediment in the assessment of FSP-induced microstructures and properties. Quantitative microstructure analyses following FSP of cast NiAl bronze materials have been used to develop estimates of stir zone thermomechanical cycles. The estimation procedures will be reviewed and the microstructure-based estimates will be compared to results from computational models and embedded thermocouples measurements. Stir zone microstructures comprise a mixture of primary α grains and transformation products of the β that formed during processing. Recrystallization in the primary α occurred due to particle-stimulated nucleation in this low stacking fault energy material. Factors that influence the distribution of strength and ductility in the stir zone appear to include the mixture of microstructure constituents and gradients in microstructure due to gradients in processing conditions.

Influence of Processing Parameters on Texture and Microstructure in Aluminum after ECAP

The influence of strain path during equal-channel angular pressing (ECAP) has been evaluated in pure aluminum by orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). The material was examined after four pressing operations by route BC in a 90° die, or eight pressing operations by route BC in a 135° die. The von Mises equivalent strains were essentially the same for these two ECAP procedures. The microtexture data indicate that the distortion during ECAP corresponds to a simple shear in a direction approximately parallel to diechannel exit and on a plane perpendicular to the flow plane. For both procedures the OIM data reveal prominent meso-scale band-like features. Lattice orientations in each band correspond to a texture orientation but the particular combinations of orientations depend upon ECAP die angle. High-angle boundaries in the structure correspond to interfaces between the bands.

Microstructural Control of a Precipitate-Hardenable Al-Ag Alloy Using Severe Plastic Deformation

Departments of Aerospace & Mechanical Engineering and Materials ScienceUniversity of Southern California, Los Angeles, CA 90089-1453, USA, langdon@usc.eduKeywords: Al-Ag alloy, equal-channel angular pressing, precipitate particles, aging, metastablephaseAbstract. An Al-10.8wt%Ag alloy was subjected to aging treatment followed by Equal-ChannelAngular Pressing (ECAP) (designated process AE) or ECAP followed by aging treatment(designated process EA). Hardness measurements were undertaken with respect to the number ofECAP passes for process AE or with respect to aging time for process EA. Microstructures wereexamined by transmission electron microscopy (TEM) including X-ray mapping. It is shown thatage hardening is observed for the ECAP sample due to the precipitation of very fine particles withinthe small grains.IntroductionIt is well known that the process of equal-channel angular pressing (ECAP) is used to reduce thegrain size of metallic materials to the submicrometer range or the nanometer range [1-3]. However,it is not so well established that the ECAP process may also be used to control the morphology anddistribution of second phase particles in two-phase metallic materials. Thus, grain refinement andsecond-phase control are both feasible because severe strain is created through the process of ECAPbased on the principle that a sample is pressed through a channel bent into an L-shape within a dieand this pressing may be repeated without any change in the cross-section of the sample.Fragmentation not only of the grains but also of second-phase particles may occur due to theintroduction of severe strain in the material.Although many reports have now been published for grain refinement using the ECAP process[2,3], there are only a limited number of examples of the application of the ECAP process tosecond-phase control [4,5]. It was shown that the application of ECAP led to dissolution of 0particles precipitated by aging of an Al-3.7wt%Cu alloy [4]. Supersaturation thus occurred in thealloy and subsequent aging gave rise to the formation of a stable 0-phase. The 0-phaseprecipitation was also recognized after aging of a severely deformed Al-3.7wt%Cu alloy usingECAP. An application of ECAP to an A1-0.9wt%Mg

Effect of fine precipitation of coherent disordered phase on creep strength of L12-ordered Co3Ti

Abstract Compressive creep tests were performed under constant stress over a range of temperatures from 923 to 1073 K on precipitation strengthened L1 2 -ordered Co 3 Ti. The creep rates decrease appreciably by the fine precipitation of coherent disordered f.c.c. Co-rich phase when the applied stress is low. An analysis of the creep data is developed taking a threshold stress into account of the creep equations. The significance of the threshold stress is discussed based upon the transmission electron microscope observations of the interaction between dislocations and precipitates. The superdislocations produced during deformation tend to be strongly attracted and dissociate as they meet the coherent disordered precipitates because the anti-phase boundary energy in the disordered phase is zero. The extra stress necessary to pull the dislocation out of the precipitates may play an important role to establish the threshold stress which is expected to be beneficial to the improvement of the creep strength of L1 2 -ordered intermetallics for high temperature structural applications.