China Science

In spite of being sheared along the so-called pseudo-twinning direction, ?-TiAl undergoes true twinning under zero pressure or hydrostatic tension by means of a specific combination of , and shears in two consecutive (1 1 1) matrix planes allowing the adjacent twin to thicken over one (1 1 1) atomic layer. The corresponding total shear strain of is four times as large as that generated by conventional deformation twinning or during the L10 to L11 transformation by or shears, respectively. This shear is substantially more effective in accommodating stress concentration and high strain rate than conventional deformation twinning. The conditions under which twinning by dislocations operates are interpreted based on a modified gamma-surface and discussed in terms of zonal partial dislocations.

Dongsheng XuaEmail:dsxu@imr.ac.cn?Hao Wanga?Rui Yanga?Patrick Veyssièreb
[a]Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China;[b]LEM, CNRS-ONERA, BP 72, 92322 Chatillon, France

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Two-point statistics describe the first-order spatial correlations between the constituent distinct local states in the internal structure of the material. These are usually recovered by randomly throwing vectors of all sizes and orientations into the material microstructure. Building on very recent advances in this emerging field, it is demonstrated in this paper that the complete set of 2-point correlations carry all of the information needed to uniquely reconstruct an eigen microstructure to within an translation and/or an inversion. For this purpose, novel algorithms based on phase-recovery methods used in signal processing have been developed and successfully implemented. The computational speed and the versatility of these new mathematical procedures are demonstrated through reconstruction of several two- and three-dimensional microstructures from their 2-point statistics.

David T. Fullwooda?Stephen R. Niezgodab?Surya R. KalidindibEmail:skalidin@coe.drexel.edu
[a]Department of Mechanical Engineering, Brigham Young University, Provo, UT 84602, USA;[b]Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA

In this paper, a micromechanically based formulation of the cooperative model is proposed for the yield behavior of semi-crystalline polymers. The semi-crystalline polymer is considered as a two-phase material where the yield processes involved in the amorphous phase and in the crystalline phase are considered separately. By using homogenization methods, the yield behavior of polyethylene and poly(ethylene terephthalate) has been modeled. The Takayanagi model is used to compute an effective activation volume and an effective activation energy which is then implemented in the cooperative model. The predicted results are in good agreement with the experimental results of Truss et al. [Truss RW, Clarke PL, Duckett RA, Ward IM. Polym Phys Ed 1984;22:191] for polyethylene over the temperature range ?30–85 °C, and of Lim et al. [Lim JY, Donahue HJ, Kim SY. Macromol Chem Phys 2003;204:653] for the poly(ethylene terephthalate) over the temperature range 20–125 °C.

O. Gueguena?J. Richetonb?S. Ahzia Email:ahzi@imfs.u-strasbg.fr?A. Makradia
[a]Université Louis Pasteur, IMFS-UMR7507, 2 Rue Boussingault, Strasbourg, France;[b]Momentive Performance Materials GmbH, Building R20, D-51368 Leverkusen, Germany

In Part 1 of this paper, a model was described to simulate the dynamic shape evolution of Ni superalloy rings during spray forming, concentrating on the effects of droplet splashing and redeposition. In this part, a companion model is presented that simulates the heat flow and solidification of the Ni superalloy rings during spray forming. In this model, generic algorithms of (1) coupling of droplet mass and enthalpy at a deposition surface, and (2) data mapping between time-evolving computational domains were developed and implemented. The effects of (1) droplet redeposition, and (2) changes in the convective heat transfer coefficients and their distributions on the resulting ring preform heat flow and solidification were studied; and simulations were again compared with experiments. The model was applied to investigate the effects of key processing parameters on the internal heat flow and solidification of large diameter IN718 alloy rings.

J. Mi aEmail:jiawei.mi@materials.ox.ac.uk?P.S. Granta
[a]Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK

??????????????????? Ni ???????????????????(1 ) ????????????????????????(2 ) ???????????????? / ??????????(3 ) ???????????????? rede ????????????????? IN718 ???????????????????????????????????? IN718 ????????????????????????????????????????????????????? 2 ????
A numerical model has been developed to simulate the dynamic shape evolution of Ni superalloy rings during spray forming. The model comprises: (1) a droplet primary deposition model, simulating droplet primary deposition at a deposition surface; (2) a droplet splashing model, simulating the droplet splashing/scattering behaviour; and (3) a droplet redeposition model, simulating the redeposition of the scattered droplets onto the deposition surface. The model has been validated against experiments of spray forming large diameter IN718 alloy rings, and for the first time, the effects of droplet splashing and redeposition on the dynamic shape evolution of spray forming IN718 alloy rings and the deposition yields have been investigated and quantified. The model serves as the basis for a thermal dynamic model that is described in Part 2 of this publication.

J. Mi aEmail:jiawei.mi@materials.ox.ac.uk?P.S. Granta
[a]Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK

Mesoscale simulations of the shock-compression response of nickel and aluminum powder mixtures have been performed to investigate the influence of particle configuration (size, shape and distribution) on the micromechanical processes of deformation, mass flow and mixing. Real microstructures were constructed from scanning electron microscopy montages of the starting mixtures pre-pressed at various densities [Eakins D, Thadhani NN, J Appl Phys, 2007;101:043508]. The travel of the high-pressure disturbance was used to determine the equation-of-state of the mixture material, and compared to the results of real-time experiments for validation. Observations of particle-level processes were used to formulate an understanding of the wide range of macroscopic behaviors exhibited as a function of particle morphology and distribution, porosity, and dissimilarities in properties of the two constituents. The results reveal the effects of particle heterogeneity on the physical, chemical and micromechanical response of shock-compressed Ni + Al powder mixtures, and their role in the initiation of shock-induced chemical reactions.

D.E. Eakinsa?N.N. Thadhani aEmail:naresh.thadhani@mse.gatech.edu
[a]Georgia Institute of Technology, 771 Ferst Drive NW, Love Building, Room 288, Atlanta, GA 30332, USA

Molecular dynamics (MD) simulations are presented to investigate the rate of infiltration of liquid Cu through a channel in crystalline Ni. Two temperatures, T = 1750 K and 1500 K, are studied using two types of simulations: non-dissolutive (ND), where Ni atoms are held fixed, and dissolutive (D), where Ni atoms relax according to MD equations of motion. At T = 1500 K the penetration rate agrees well with theoretical models based on capillary forces, regardless of Ni dissolution behavior. At T = 1750 K data cannot be explained based solely on capillarity; however, this discrepancy is remedied by including an additional driving force for infiltration that is directly proportional to dissolution rate. A model for dissolution rate as a function of liquid composition and temperature is presented. For Ni dissolving into pure Cu(l) the dissolution rate exhibits Arrhenius temperature dependence and this is used to explain differences in infiltration behavior at the two temperatures studied.

E.B. Webb IIIa Email:ebwebb@sandia.gov?J.J. Hoytb
[a]Sandia National Laboratories, Albuquerque, NM 87185, USA;[b]Department of Materials Science; Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7

Discrete dislocation dynamics simulations in three dimensions are performed on micro-sized bending beams and the results are compared with experiments. A strong size dependence of the flow stress ?f (or bending moment) is found. The flow stress scales approximately inversely with the beam thickness t. The simulations show that the dislocation structure exhibits pronounced pile-ups around the neutral plane of the beam. The back stress from these pile-ups on the dislocation sources is analyzed by means of an analytical pile-up model. It is shown that the scaling behavior ?f?t-1 can be explained by a combination of pile-up and source size limitation. Subsequently, the applicability of strain gradient plasticity models on micro-bending is discussed.

C. MotzaEmail:motz@unileoben.ac.at?D. Weyganda?J. Sengera?P. Gumbscha
[a]IZBS, University of Karlsruhe [TH], Kaiserstrasse 12, 76131 Karlsruhe, Germany;[b]Erich Schmid Institute, Austrian Academy of Sciences, Jahnstrasse 12, 8700 Leoben, Austria