Haiming Wen

High-pressure torsion-induced grain growth and detwinning in cryomilled Cu powders

Two mechanisms for deformation-induced grain growth in nanostructured metals have been proposed, including grain rotation-induced grain coalescence and stress-coupled grain boundary (GB) migration. A study is reported in which significant grain growth occurred from an average grain size of 46 nm to 90 nm during high pressure torsion (HPT) of cryomilled nanocrystalline Cu powders. Careful microstructural examination ascertained that grain rotation-induced grain coalescence is mainly responsible for the grain growth during HPT. Furthermore, a grain size dependence of the grain growth mechanisms was uncovered: grain rotation and grain coalescence dominate at nanocrystalline grain sizes, whereas stress-coupled GB migration prevails at ultrafine grain sizes. In addition, detwinning of the preexisting deformation twins was observed during HPT of the cryomilled Cu powders. The mechanism of detwinning for deformation twins was proposed to be similar to that for growth twins.

Dynamic balance between grain refinement and grain growth during high-pressure torsion of Cu powders

High-pressure torsion (HPT) was applied to unmilled coarse-grained (CG) Cu powders with low initial dislocation density and cryomilled nanocrystalline (nc) Cu powders with high initial dislocation density, with identical processing parameters. HPT of unmilled CG Cu powders resulted in exceptional grain refinement and increase in dislocation density, whereas significant grain growth and decrease in dislocation density occurred during HPT of cryomilled nc Cu powders. Equilibrium structures were achieved under both conditions, with very similar stable grain sizes and dislocation densities, suggesting dynamic balances between deformation-induced grain refinement and grain growth, and between deformation-induced dislocation accumulation and dislocation annihilation. The equilibrium structures are governed by these two dynamic balances.

Stress-enhanced grain growth in a nanostructured aluminium alloy during spark plasma sintering

In this study, we report on the influence of high pressure on the microstructure evolution of cryomilled nanostructured Al alloy powders during spark plasma sintering (SPS). Our experimental results suggest that the particular mechanism that governs grain growth during SPS depends on the magnitude of the applied pressure. In the case of material consolidated at a high pressure (e.g. 500 MPa), grain coarsening occurs via a combination of thermally activated grain boundary (GB) migration, stress-coupled GB migration and grain rotation-induced grain coalescence. In contrast, in the case of the material consolidated at a low pressure (50 MPa), grain growth occurs primarily via thermally activated GB migration.

Advanced electron microscopy study of fission product distribution in the failed SiC layer of a neutron irradiated TRISO coated particle

Tristructural isotropic (TRISO) coated particle fuel has been designed for application in hightemperature gas-cooled reactors (HTGR). TRISO particles for the HTGR fuel development effort underway at Idaho National Laboratory (INL) and Oak Ridge National Laboratory (ORNL) consist of a two-phase uranium oxide-uranium carbide (UCO) fuel kernel, a carbon buffer layer, an inner pyrolytic carbon (IPyC) layer, a SiC layer, and an outer PyC (OPyC) layer [1]. The first in a series of irradiation experiments (AGR-1) clearly shows release of certain metallic fission products, e.g., Ag and Pd, through intact TRISO coatings, with Cs generally well retained [1]. No significant chemical interaction was observed between Pd and SiC for UCO TRISO coated particles, which retained Cs [2].

An efficient and cost-effective method for preparing transmission electron microscopy samples from powders

The preparation of transmission electron microcopy (TEM) samples from powders with particle sizes larger than ~100 nm poses a challenge. The existing methods are complicated and expensive, or have a low probability of success. Herein, we report a modified methodology for preparation of TEM samples from powders, which is efficient, cost-effective, and easy to perform. This method involves mixing powders with an epoxy on a piece of weighing paper, curing the powder-epoxy mixture to form a bulk material, grinding the bulk to obtain a thin foil, punching TEM discs from the foil, dimpling the discs, and ion milling the dimpled discs to electron transparency. Compared with the well established and robust grinding-dimpling-ion-milling method for TEM sample preparation for bulk materials, our modified approach for preparing TEM samples from powders only requires two additional simple steps. In this article, step-by-step procedures for our methodology are described in detail, and important strategies to ensure success are elucidated. Our methodology has been applied successfully for preparing TEM samples with large thin areas and high quality for many different mechanically milled metallic powders.

Effects of the Tempering and High-Pressure Torsion Temperatures on Microstructure of Ferritic/Martensitic Steel Grade 91

Grade 91 (9Cr-1Mo) steel was subjected to various heat treatments and then to high-pressure torsion (HPT) at different temperatures. Its microstructure was studied using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Effects of the tempering temperature and the HPT temperature on the microstructural features and microhardness in the ultrafine-grained (UFG) Grade 91 steel were researched. The study of the UFG structure formation takes into account two different microstructures observed: before HPT in both samples containing martensite and in fully ferritic samples.

Atom-Probe Tomographic Study of Precipitation in an Ultrafine-Grained Al-Zn-Mg-Cu Alloy (Al 7075)

Precipitation-hardened alloys can be further strengthened by reducing the grain size down to the ultrafine regime (<1 μm) and thereby incorporating significant grain boundary (GB) strengthening [1, 2]. The ultrafine-grained (UFG) structure is expected to induce an influence on the precipitation behavior, because of the significantly reduced length scale. Nevertheless, there have been very limited studies on the effect of length scale on precipitation [3]. The present study was undertaken to obtain quantitative information on the differences in precipitation behavior between UFG structure and the traditional coarse-grained (CG) counterpart, and to provide fundamental insights into the underlying mechanisms. Al 7075 alloy, with major alloying elements of Zn, Mg and Cu, was selected for study because of its technological importance and the existing extensive studies on precipitation phenomena in CG Al 7075.