S.D. Wu

Influence of Grain Size and Texture on the Yield Asymmetry of Mg-3Al-1Zn Alloy

The yield asymmetry between compression and tension of magnesium alloy Mg-3Al-1Zn (AZ31) with different grain sizes and textures has been studied by tensile and compressive testing of as-cast, as-extruded and equal channel angular pressed (ECAPed) specimens. The significant yield asymmetry (the ratio of yield strength between compression and tension sigma(yc)/sigma(yt) is similar to 0.44) was found in as-extruded specimens and the corresponding microstructure evolution during deformation revealed that {1012} tensile twinning is the underlying reason for the large yield asymmetry. Strong texture and grain size are influential factors for large yield asymmetry. The separate contributions of grain size and texture on yield asymmetry were investigated.

High strength and utilizable ductility of bulk ultrafine-grained Cu-Al alloys

Lack of plasticity is the main drawback for nearly all ultrafine-grained (UFG) materials, which restricts their practical applications. Bulk UFG Cu–Al alloys have been fabricated by using equal channel angular pressing technique. Its ductility was improved to exceed the criteria for structural utility while maintaining a high strength by designing the microstructure via alloying. Factors resulting in the simultaneously enhanced strength and ductility of UFG Cu–Al alloys are the formation of deformation twins and their extensive intersections facilitating accumulation of dislocations.

Significance of stacking fault energy on microstructural evolution in Cu and Cu-Al alloys processed by high-pressure torsion

Disks of pure Cu and several Cu–Al alloys were processed by high-pressure torsion (HPT) at room temperature through different numbers of turns to systematically investigate the influence of the stacking fault energy (SFE) on the evolution of microstructural homogeneity. The results show there is initially an inhomogeneous microhardness distribution but this inhomogneity decreases with increasing numbers of turns and the saturation microhardness increases with increasing Al concentration. Uniform microstructures are more readily achieved in materials with high or low SFE than in materials with medium SFE, because there are different mechanisms governing the microstructural evolution. Specifically, recovery processes are dominant in high or medium SFE materials, whereas twin fragmentation is dominant in materials having low SFE. The limiting minimum grain size (d min) of metals processed by HPT decreases with decreasing SFE and there is additional evidence suggesting that the dependence of d min on the SFE decreases when the severity of the external loading conditions is increased.

Effect of CO2 on conversion of inulin to 5-hydroxymethylfurfural and propylene oxide to 1,2-propanediol in water

The CO2–water system has the potential to serve as a substitute for mineral acids for some reactions in acidic media. In this work, two reactions under hydrothermal conditions with and without CO2 were studied – the conversion of inulin to 5-hydroxymethylfurfural (5-HMF), and the hydrolysis of propylene oxide to 1,2-propanediol (1,2-PDO). The effects of CO2 pressure, reaction temperature and reactant concentration on the yield of 5-HMF and 1,2-PDO were examined. It was demonstrated that CO2 could increase the yields of 5-HMF and 1,2-PDO considerably under optimized conditions. The methods to prepare 5-HMF and 1,2-PDO are greener, in that conventional acids are not required and the solution is neutralized automatically after depressurization.

Recrystallization in fatigued copper single crystals under electropulsing

Fatigued copper single crystal was treated by electropulsing, it leads to the formation of two kinds of recrystallized grain with finer-grain size, Due to the homogeneous distribution of the vein structures, the recrystallized grains formed in this region have no preferential direction. However, the recrystallized grains formed around persistent slip bands show bigger grain size along slip direction

Formation mechanism of nanostructures in austenitic stainless steel during equal channel angular pressing

An ultra-low carbon austenitic stainless steel was successfully pressed from one to eight passes by equal channel angular pressing (ECAP) at room temperature. By using X-ray diffraction, optical microscopy and transmission electron microscopy, the microstructural evolution during ECAP was investigated to reveal the formation mechanism of strain-induced nanostructures. The refinement mechanism involved the formation of shear bands and deformation twins, followed by the fragmentation of twin lamellae, as well as successive martensite transformation from parent austenitic grains with sizes ranging from microns to nanometres through the processes γ(fcc) → ε(hcp) → α′(bcc). After pressing for eight passes, two types of nanocrystalline grains were achieved: (a) nanocrystalline austenite with a mean grain size of ∼31 nm and (b) strain-induced nanocrystalline α′-martensite with a size of ∼74 nm. The formation mechanisms are discussed in terms of microstructural subdivision via deformation twinning and martensite transformation.

Enhancing the selectivity of the hydrogenation of naphthalene to tetralin by high temperature water

Enhancing selectivity and an efficient use of feedstocks in chemical reactions using greener methods is an important aspect of green chemistry. In this work, we conducted the hydrogenation of naphthalene to produce tetralin catalyzed by a cheap Fe–Mo based catalyst with and without high temperature water (HTW). The effects of various factors on the reaction, such as density of water, reaction temperature, reaction time and amounts of catalyst, were investigated. It was demonstrated that the addition of water could increase the yield of tetralin and suppress formation of coke effectively. The reaction in the presence of D2O indicated that H/D exchange occurred during the reaction process.

Low-cycle fatigue-cracking mechanisms in fcc crystalline materials

The low-cycle fatigue (LCF) cracking behavior in various face-centered-cubic (fcc) crystalline materials, including Cu single crystals, bicrystals and polycrystals, Cu–Al and Cu–Zn alloys, ultrafine-grained (UFG) Al–Cu and Cu–Zn alloys, was systematically investigated and reviewed. In Cu single crystals, fatigue cracking always nucleates along slip bands and deformation bands. The large-angle grain boundary (GB) becomes the preferential site in bicrystals and polycrystals. In addition, fatigue cracking can also nucleate along slip bands and twin boundaries (TBs) in polycrystalline materials. However, shear bands and coarse deformation bands are observed to the preferential sites for fatigue cracking in UFG materials with a large number of GBs. Based on numerous observations on fatigue-cracking behavior, the fatigue-cracking mechanisms along slip bands, GBs, TBs, shear bands and deformation bands were systematically compared and classified into two types, i.e. shear crack and impingement crack. Finally, these fatigue-cracking behaviors are discussed in depth for a better understanding of their physical nature and the transition from intergranular to transgranular cracking in various fcc crystalline materials. These comprehensive results for fatigue damage mechanisms should significantly aid in obtaining the optimum design to further strengthen and toughen metallic materials in practice.

Water as an additive to enhance the ring opening of naphthalene

Use of water as a reaction medium or additive to enhance reaction efficiency is an important topic in green chemistry, and ring opening and contraction reactions of aromatics are crucial for upgrading diesels. In this work, we investigated the effect of water on the yields of ring opening and contraction reactions of naphthalene. A series of catalysts, such as Rh2O3/HY zeolite, Mo–Ni oxide and their physical mixtures, were used as the catalysts. The influences of the amount of water, hydrogen pressure, reaction temperature and reaction time on the yields of the ring opening and contraction products (ROCP) were studied. It was found that Rh2O3/HY and Mo–Ni oxide showed an excellent synergistic effect for catalyzing the reaction, and water could be used as a green and efficient additive for enhancing the yield of the ROCP. At the optimized conditions, the yield of the ROCP could be as high as 63.3%. The mechanism for the effect of water on the reactions was discussed on the basis of control experiments.

Hydrocracking of Anthracene to Ethyl Biphenyl Promoted by Coupling Supercritical Water and Cracking Catalysts

Efficient conversion of polynuclear aromatic hydrocarbons (PAHs) into liquid fuels is crucial for the hydrocracking processes of heavy oils. In this work, we found that the hydrocracking of anthracene catalyzed by NiFe/HZSM‐5 could produce ethyl biphenyl (EB), which realizes high efficiency of hydrocracking of anthracene analogous to its central‐ring hydrocracking. More interestingly, addition of water could increase the yield of EB significantly, and the yield of EB could be as high as 34 %. A pathway for hydrocracking of anthracene to EB and the mechanism for water to enhance the yield of EB are proposed on the basis of control experiments. Furthermore, our work demonstrated that water could also increase the liquid yields and reduce the gaseous yields in the hydrocracking of the Daqing residue oil in the same reaction system. This work not only provides new chemical knowledge on hydrocracking of PAHs, but also provides very important information for improving the hydrocracking processes of heavy oils to obtain more liquid fuels with less hydrogen consumption.