Yanxia Chen

Bio: Yanxia Chen is an academic researcher from Northwestern Polytechnical University. The author has contributed to research in topics: High-resolution transmission electron microscopy & Scanning transmission electron microscopy. The author has an hindex of 8, co-authored 17 publications receiving 219 citations.

Crystal substructures of the rotation-twinned T (Al20Cu2Mn3) phase in 2024 aluminum alloy

Abstract: The substructures in rotation-twinned T (Al 20 Cu 2 Mn 3 ) particles were investigated by means of high resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) in the present work. A flattened hexagonal structural subunit with 20 atomic columns was proposed. The stacking mode of these subunits in non-defective T phase was proved to be tessellation of many flattened hexagonal subunits with the same orientations, while the stacking modes near anti-phase boundary (APB) and twin boundary (TB) were tessellations of two differently oriented flattened hexagonal subunits. The transition region at twin domain junctions has hybrid structure and perfect or imperfect pentagram structure. Centered with the perfect pentagram transition structure, a rotation twin with ten fan-shaped domains and constituted by five twin variants can be deduced.

HRTEM and HAADF-STEM tomography investigation of the heterogeneously formed S (Al2CuMg) precipitates in Al–Cu–Mg alloy

Abstract: The distribution of variants and three-dimensional (3D) configurations of the heterogeneously formed S (Al2CuMg) precipitates at dislocations, grain boundaries and the Al20Cu2Mn3 dispersoid/Al interfaces were studied in this research. By means of high resolution transmission electron microscopy, we systematically investigated the orientation relationships (ORs) between these heterogeneously formed S precipitates and the Al matrix, and further unraveled that the preferred orientation of S variants at grain boundaries and at dispersoid/Al interfaces are respectively associated with the OR between the precipitate habit plane and the grain boundary plane, and the OR between the precipitate habit plane and the interface plane. The inherent characteristic of the crystal structure of the S phase, i.e. the symmetry of the pentagonal subunit, was considered to be the fundamental factor determining the preference of the variant pair. By using high angle annular dark field scanning transmission electron microscopy t...

Surface gradient nanostructures in high speed machined 7055 aluminum alloy

Abstract: The high speed machining induced surface deformation layer in a 7055 aluminum alloy was investigated by means of transmission electron microscopy (TEM) and precession electron diffraction (PED) assisted nanoscale orientation mapping. The gradient nanostructures were composed of equiaxed and lamellar nanograins and ultrafine grains decorated by coarse grain boundary precipitates (GBPs). The presence of low angle dislocation boundaries, the recrystallized nanograins and ultrafine grains showed direct evidence that dislocation activities and dynamic recrystallization are two dominant grain refinement approaches, while the large size and density differences between GBPs and grain interior precipitates (GIPs) unraveled a prominent precipitate redistribution, which can be accomplished via the thermally and mechanically induced precipitate dissolution, solute diffusion and reprecipitation. The quantitative prediction of solute diffusion in the current machining condition agreed well with the TEM observation results. The crystallographic texture of the surface nanostructured layer was proved to be a mixture of brass, cube and weak rotated cube, the severe but diversified thermomechanical effect of high strain, high strain rate and high temperature shear deformation during high speed machining is responsible for texture development.

Microstructure, microtexture and precipitation in the ultrafine-grained surface layer of an Al-Zn-Mg-Cu alloy processed by sliding friction treatment

Abstract: Precipitate redistribution and texture evolution are usually two concurrent aspects accompanying grain refinement induced by various surface treatment. However, the detailed precipitate redistribution characteristics and process, as well as crystallographic texture in the surface refined grain layer, are still far from full understanding. In this study, we focused on the microstructural and crystallographic features of the sliding friction treatment (SFT) induced surface deformation layer in a 7050 aluminum alloy. With the combination of transmission electron microscopy (TEM) and high angle angular dark field scanning TEM (HAADF-STEM) observations, a surface ultrafine grain (UFG) layer composed of both equiaxed and lamellar ultrafine grains and decorated by high density of coarse grain boundary precipitates (GBPs) were revealed. Further precession electron diffraction (PED) assisted orientation mapping unraveled that high angle grain boundaries rather than low angle grain boundaries are the most favorable nucleation sites for GBPs. The prominent precipitate redistribution can be divided into three successive and interrelated stages, i.e. the mechanically induced precipitate dissolution, solute diffusion and reprecipitation. The quantitative prediction based on pipe diffusion along dislocations and grain boundary diffusion proved the distribution feasibility of GBPs around UFGs. Based on PED and electron backscatter diffraction (EBSD) analyses, the crystallographic texture of the surface UFG layer was identified as a shear texture composed of major rotated cube texture {001} 〈110〉 and minor {111} 〈112〉, while that of the adjoining lamellar coarse grained matrix was pure brass. The SFT induced surface severe shear deformation is responsible for texture evolution.

Influence of equal-channel angular pressing on aging precipitation in 7050 Al alloy

Abstract: 7050 Al alloy was successfully processed by equal-channel angular pressing (ECAP) at room temperature (RT). The effect of ECAP on the subsequent aging precipitation behavior was investigated by using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The results reveal that the kinds, spatial distribution and sizes of precipitates in the unECAPed and the ECAPed samples are different. ECAP accelerates the process of aging precipitation and results in the broadening of precipitate size distribution. ECAP can produce not only deformation heat but also internal defects such as excess vacancies and high density of dislocations when the sample passes through the main deformation zone. Deformation heat can lead to pre-precipitation, forming a small amount of GPII zones during ECAP processing. Strain-induced excess vacancies make solute segregation along dislocations by the mechanism of nonequilibrium segregation. High density dislocations mainly accelerate the process of aging precipitation. Besides, dislocations also induce the competition between homogeneous precipitation and heterogeneous precipitation on dislocations due to the flow of solutes and vacancies towards dislocations.

The Diffusion of Solids

Abstract: IN view of the interest attaching to the vaporisation and diffusion of solids, the following observations may be worthy of record.

Enhanced Mechanical Properties of Graphene (Reduced Graphene Oxide)/Aluminum Composites with a Bioinspired Nanolaminated Structure

Abstract: Bulk graphene (reduced graphene oxide)-reinforced Al matrix composites with a bioinspired nanolaminated microstructure were fabricated via a composite powder assembly approach. Compared with the unreinforced Al matrix, these composites were shown to possess significantly improved stiffness and tensile strength, and a similar or even slightly higher total elongation. These observations were interpreted by the facilitated load transfer between graphene and the Al matrix, and the extrinsic toughening effect as a result of the nanolaminated microstructure.

Inoculation treatment of an additively manufactured 2024 aluminium alloy with titanium nanoparticles

Abstract: Considerable studies on metal selective laser melting (SLM) have proved the necessity to refine microstructure parts fabricated by SLM in order to eliminate property anisotropy, hot-tearing and to increase the SLM-processability. In the present work, Ti nanoparticles, at the first time, were discovered to be an extremely effective inoculant for an SLMed 2024 aluminium alloy. 0.7 wt% addition of Ti nanoparticles was capable of substantially eliminating the hot-tearing cracks and columnar structure, and refining the grains in the SLMed 2024 alloy in a broad processing window. The substantial grain refinement in the Ti-inoculated 2024 alloy was attributed to the in-situ formation of Al3Ti nanoparticles with a L12 ordered structure, which formed a coherent interface with Al matrix and therefore significantly promoted the heterogeneous nucleation of the α-Al during solidification of melt pools in the SLM process. After a conventional T6 heat treatment, this SLMed alloy exhibited a superior balance of strength and ductility (tensile strength was up to 432 ± 20 MPa and elongation of 10 ± 0.8%), which was comparable to its wrought counterpart. This work can be considered as a breakthrough in research of fabricating high-strength aluminium alloys using SLM.

Inoculation Treatment of an Additively Manufactured 2024 Aluminium Alloy with Titanium Nanoparticles

Abstract: Considerable studies on metal selective laser melting (SLM) have proved the necessity to refine microstructure parts fabricated by SLM in order to eliminate property anisotropy, hot-tearing and to increase the SLM-processability. In the present work, Ti nanoparticles, at the first time, were discovered to be an extremely effective inoculant for an SLMed 2024 aluminium alloy. 0.7 wt.% addition of Ti nanoparticles was capable of substantially eliminating the hot-tearing cracks and columnar structure, and refining the grains in the SLMed 2024 alloy in a broad processing window. The substantial grain refinement in the Ti-inoculated 2024 alloy was attributed to the in-situ formation of Al3Ti nanoparticles with a L12 ordered structure, which formed a coherent interface with Al matrix and therefore significantly promoted the heterogeneous nucleation of the α-Al during solidification of melt pools in the SLM process. After a conventional T6 heat treatment, this SLMed alloy exhibited a superior balance of strength and ductility (tensile strength was up to 432 ± 20 MPa and elongation of 10 ± 0.8%), which was comparable to its wrought counterpart. This work can be considered as a breakthrough in research of fabricating high-strength aluminium alloys using SLM.

The effects of heat treatment on 7075 Al cold spray deposits

Abstract: High-pressure cold spray was used to deposit 7075 aluminum powder onto 7075-T6 substrates. We investigated the effects of post deposition heat treatments on the microstructure and mechanical properties of the deposits. For this purpose, both low-temperature and high-temperature treatments were carried out on specimens excised from the deposits. Microstructures of the as-deposited and heat treated samples were characterized via different microscopy techniques and mechanical properties were evaluated by microtensile and hardness tests. The results were then correlated with the observed microstructures in different conditions. The strength and ductility of the cold sprayed 7075 deposits increased after both low- and high-temperature treatments, which resulted in precipitation of strengthening phases and increased inter-particle bonding. Because of a change in bonding mechanism, heat treatment at high temperature yielded markedly greater ductility than all other conditions. Diffusion and microstructural sintering at the particle-particle interfaces were proposed to cause the change in bonding mechanism from mechanical interlocking to metallurgical bonding and lead to the ductile characteristics of these samples. The understanding gained from this research should lead to optimization of and pre- and post-processing treatments for cold spray deposits.