China Science

The cyclic plastic response of a single crystalline thin beam subject to combined cyclic tension and bending is analyzed using two-dimensional discrete dislocation plasticity. In this contribution, special attention is paid to the difference in the inherent mechanism of the size effect for different cyclic loads. Results show that the cyclic plastic response has a strong size effect for both cyclic pure tension–compression and pure bending. However, the inherent mechanisms are different. The dislocation starvation mechanism dominates the cyclic tension–compression while the geometrically necessary dislocation dominates the cyclic pure bending. When the combined cyclic tension and bending are applied to the thin beam, the cyclic moment–rotation response shows strong size effect while the stress–strain response shows weak or even no size effect. In addition, it is also found that the cyclic loading paths have considerable influences on the shape of the cyclic stress–strain loops

Chuantao Houa?Zhenhuan Li aEmail:zhli68@263.net?Minsheng Huanga?Chaojun Ouyanga
[a]Department of Mechanics, Huazhong University of Science; Technology, 1037 Luoyu Road, Wuhan 430074, China

An empirically determined measure of the solute drag force called the drag factor is derived and defined. The drag factor is the derivative of mobility with respect to grain size, and describes well the drag effect of solute in the six different aluminas measured. A normalized drag factor allows direct comparison of different dopants, and validation of theoretically predicted trends. This construction is used to verify that the role of magnesia and rare-earth dopants in reducing the grain-growth rate is due to solute drag from the intrinsic mobility. These dopants segregate to the core of the grain-boundary, which differs from classical solute drag models that derive the drag effect from solute in the near-boundary lattice. The solute drag factor is also used to understand the role of drag in grain-boundaries that have mobilities that are enhanced relative to the pure material. This new approach for analyzing grain-growth advances the understanding of microstructural evolution and its relationship to properties.

Shen J. DillonaEmail:sjd6@lehigh.edu?Shantanu K. Beheraa?Martin P. Harmera
[a]Center for Advanced Materials; Nanotechnology, Lehigh University, Bethlehem, PA 18015, USA

Duplex grain structure consisting of coarse-cell and fine-cell dendritic grains is frequently found in the central portion of direct-chill cast billets and ingots. Coarse-cell grains are usually considered as free-floating crystals settled to the bottom of the billet sump. These grains are assumed to be solute-lean and contribute to the negative centerline segregation. In this paper the contribution of coarse-cell and fine-cell grains to macrosegregation is for the first time studied experimentally by direct measurements of their composition. It is shown that the coarse-cell, floating grains are depleted of solute and the areas of their accumulation contribute to the negative macrosegregation. The areas of fine-cell grains can be either enriched in solute or be close to the nominal composition. It is argued that their composition results from the interplay between thermo-solutal and shrinkage-induced flows. The roles of casting speed and grain refining are also under scrutiny in this paper.

D.G. EskinaEmail:d.g.eskin@tudelft.nl?R. Nadellaa?L. Katgermanb
[a]Netherlands Institute for Metals Research, Mekelweg 2, 2628CD Delft, The Netherlands;[b]Delft University of Technology, Department of Materials Science; Engineering, Mekelweg 2, 2628CD Delft, The Netherlands

Dehydriding and rehydriding properties of well-crystallized Mg(BH4)2 were systematically investigated by thermogravimetry (TG) and pressure–composition–temperature (PCT) measurements. The dehydriding reaction of Mg(BH4)2starts at approximately 500 K, and about 14.4 mass% of hydrogen is desorbed according to the following multi-step reaction:Mg(BH4)2 ? some intermediate compounds ? MgH2 + 2B + 3H2 ? Mg + 2B + 4H2The apparent enthalpy change in the dehydriding reaction from Mg(BH4)2 to MgH2 is estimated to be 57 ± 5 kJ mol?1 H2 based on the result of the PCT measurement. It is proved that approximately 6.1 mass% of hydrogen can be reversibly stored for the sample of Mg(BH4)2 after the dehydriding reaction, through the formation of a possible intermediate compound such as MgB12H12.

H.-W. Lia?K. Kikuchia?Y. Nakamoria?N. Ohbab?K. Miwab?S. Towatab?S. OrimoaEmail:orimo@imr.tohoku.ac.jp
[a]Institute for Materials Research, Tohoku University, Sendai 980–8577, Japan;[b]Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480–1192, Japan

Microstructural developments during sintering in 2 and 3 mol% Y2O3-stabilized tetragonal zirconia polycrystals (2Y- and 3Y-TZPs) and 8 mol% Y2O3-stabilized cubic zirconia (8Y-CSZ) were systematically investigated in the sintering temperature range of 1100–1500 °C. Above 1200 °C, grain growth in 8Y-CSZ was much faster than that in 2Y- and 3Y-TZPs. In the grain-boundary faces in these specimens, amorphous layers did not exist; however, Y3+ ions segregated at the grain boundaries over a width of 10 nm. The amount of segregated Y3+ ions in 8Y-CSZ was significantly less than in 2Y- and 3Y-TZPs. This indicates that an increase in segregated Y3+ ions retards grain growth. Therefore, grain growth behavior during sintering can be reasonably explained by the solute-drag mechanism of Y3+ ions segregating along the grain boundary. The segregation of Y3+ ions, which directly affects grain growth, is closely related to the driving force for grain-boundary segregation-induced phase transformation (GBSIPT).

K. MatsuiaEmail:k_matui@tosoh.co.jp?H. Yoshidab?Y. Ikuharac
[a]Tokyo Research Laboratory, Tosoh Corporation, 2743-1, Hayakawa, Ayase, Kanagawa 252-1123, Japan;[b]National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan;[c]Institute of Engineering Innovation, School of Engineering, The University of Tokyo, 2-11-16, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan

The transformation of the grain boundary fct ?-MnNi precipitates to thermodynamically stable austenite was investigated in an Fe–Mn–Ni alloy using transmission electron microscopy (TEM). During aging of Fe–Mn–Ni alloys, fine bands began to develop in the ?-MnNi intermetallic particles. The HAADF images revealed these thin bands to be stacking faults and twins that were formed by the glide of -type Shockley partial dislocations on {1 1 1} planes. The presence of iron in the twin bands was detected by electron energy-loss spectroscopy (EELS) analysis. These observations supported the conclusion that the crystal structure of the twin bands was transformed to that of face-centered cubic (fcc) austenite by iron diffusion into the twin bands. A further transformation of the ?-MnNi particle to austenite was proceeded by the development of new austenite bands rather than by the broadening of the existing bands. The mechanism of this transformation was discussed based on the crystal structure of both phases.

Yoon-Uk Heoa?Miyoung Kima?Hu-Chul LeeaEmail:huchul@snu.ac.kr
[a]School of Materials Science; Engineering, Seoul National University, Seoul 151-744, Republic of Korea

The diffuse scattering of body-centered cubic ?-Ti–V was measured using high-energy synchrotron X-rays and two-dimensional detectors. The study included in situ experiments of the equilibrium ?-phase and room temperature measurements of the quenched metastable state. The kinematical X-ray scattering revealed details in reciprocal space that could not be detected by the electron diffraction employed in previous studies. The signal was analyzed using a statistical thermodynamic approach based on physically motivated parameters. The characteristic features attributed to an ?-like structure or a “diffuse ?-phase” in the past are explained by static lattice distortions due to atomic size mismatch.

I.B. RamsteineraEmail:ramstein@seas.harvard.edu?O. Shchygloa?M. Mezgera?A. Udyanskya?V. Bugaeva?S. Schdera?H. Reicherta?H. Doscha
[a]Max-Planck-Institut für Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart, Germany;[b]European Synchrotron Radiation Facility (ESRF), 38043 Grenoble, France

The high-temperature strength and deformation behavior of ?/?? two-phase Co–Al–W-base alloys have been studied with polycrystalline and single-crystal materials. The ternary, quaternary and higher-order alloys containing Ta, Cr and/or Re exhibit flow stress anomalies above 873 K due to slip of pairs of 1/21 1 0 superpartial dislocations on {0 0 1} planes, in addition to {1 1 1} planes, in the ?? precipitates. Compression tests on the single-crystal specimens reveal a true anomalous peak temperature of 1073 K for both ternary and Ta-containing quaternary alloys. Above the peak, the ternary alloy exhibits a rapid decrease in strength with temperature, as 1/21 1 0 dislocations bypass the ?? precipitates without significant shearing. Conversely, the Ta-containing quaternary alloy sustains strength to higher temperatures due to the activation of 1/31 1 2 partial dislocation slip that introduces a high density of stacking faults in the ?? precipitates.

Akane SuzukiaEmail:suzukia@ge.com?Tresa M. Pollockb
[a]GE Global Research, One Research Circle, Niskayuna, NY 12309, USA;[b]Department of Materials Science; Engineering, University of Michigan, 2300 Hayward Street, Ann Arbor, MI 48109, USA