China Science

Pb(Mg1/3Nb2/3)O3–0.32(PbTiO3), PMN–0.32PT, single crystals have been characterized under combined stress and electric field loading [McLaughlin EA, Liu T, Lynch CS. Relaxor ferroelectric PMN–32%PT crystals under stress and electric field loading: I-32 mode measurements. Acta Mater 2004;52:3849, McLaughlin EA, Liu T, Lynch CS. Relaxor ferroelectric PMN–32%PT crystals under stress, electric field and temperature loading: II-33-mode measurements. Acta Mater 2005;53:4001] [1], [2] and [3]. This approach is extended to PMN–0.26PT single crystals to explore the effect of composition on field driven phase transformations and to PMN–0.32PT ceramic specimens to compare with polycrystalline behavior. Electric displacement and strain were measured as a function of combinations of stress and both unipolar and bipolar electric fields. The single-crystal results indicate that compositions further from the morphotropic phase boundary require higher driving forces for field induced phase transformations. Evidence of these transformations is not apparent in the results from the ceramic specimens.

Kyle G. Webbera?Ruzhong Zuob?Christopher S. LynchcEmail:cslynch@seas.ucla.edu
[a]The George W. Woodruff School of Mechanical Engineering, The Georgia Institute of Technology, Atlanta, GA 30332, USA;[b]Institute of Materials Science, Darmstadt University of Technology, 64287 Darmstadt, Germany;[c]Department of Mechanical; Aerospace Engineering, The University of California Los Angeles, 36-146 Engineering IV, 420 Westwood Plaza, Los Angeles, CA 90095-1597, USA

Different iron–chromium alloys (4, 8, 13 and 20 wt.%Cr) were nitrided in a NH3/H2 gas mixture at 580 °C for various times. The nitrided microstructure was characterized by X-ray diffraction, light microscopy and hardness measurements. Composition depth profiles of the nitrided zone were determined by electron-probe microanalysis. Residual stress–depth profiles of the nitrided specimens were measured using the (X-ray) diffraction sin2 ? method in combination with cumulative sublayer removals and correction for corresponding stress relaxations. Unusual, nonmonotonous changes of stress with depth could be related to the microstructure of the nitrided zone. A model description of the evolution of the residual stress as function of depth and nitriding time was given.

N.E. Vives Díaza?R.E. Schacherlb Email:r.schacherl@mf.mpg.de?L.F. Zagonela?E.J. Mittemeijera
[a]Max Planck Institute for Metals Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany;[b]Institute for Physical Metallurgy, University of Stuttgart, Germany ;[1] On leave from: Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, Unicamp, P.O. Box 6165 Campinas, SP, 13083-970, Brazil; Present address: CEA-DSM/DRECAM-SPCSI, CEA-Saclay, 91191, Gif-sur-Yvette, France.

The transformation of Al3Zr (L12) and Al3(Zr1?xTix) (L12) precipitates to their respective equilibrium D023 structures is investigated in conventionally solidified Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys aged isothermally at 500 °C or aged isochronally in the range 300–600 °C. Titanium additions delay neither coarsening of the metastable L12 precipitates nor their transformation to the D023 structure. Both alloys overage at the same rate at or above 500 °C, during which spheroidal L12 precipitates transform to disk-shaped D023 precipitates at ca. 200 nm in diameter and 50 nm in thickness, exhibiting a cube-on-cube orientation relationship with the ?-Al matrix. The transformation occurs heterogeneously on dislocations because of a large lattice parameter mismatch of the D023 phase with ?-Al. The transformation is very sluggish and even at 575 °C coherent L12 precipitates can remain untransformed. Mechanisms of microstructural coarsening and strengthening are discussed with respect to the micrometer-scale dendritic distribution of precipitates.

Keith E. KniplingaEmail:knipling@anvil.nrl.navy.mil?David C. Dunandb?David N. Seidmanb
[a]Naval Research Laboratory, Code 6356, 4555 Overlook Avenue, SW, Washington, DC 20375-5320, USA;[b]Department of Materials Science; Engineering, Northwestern University, 2220 Campus Dr., Evanston, IL 60208-3108, USA;[c]Northwestern University Center for Atom-Probe, Tomography (NUCAPT), Northwestern University, 2220 Campus Dr., Evanston, IL 60208-3108, USA

The morphology evolution of binary blends subjected to a two-step quench in the presence of external surfaces, such as a static substrate or immobile fillers, was studied using the cell dynamical system (CDS) simulation method. When the binary blends were first annealed at a shallow quench for a long time followed by a second further deep quench into a two-phase region, transient lamellar structures were formed in the blends with both critical and off-critical compositions. The occurrence of the lamellar structure was found to be correlative with the compositional difference between the separated phases formed during the first shallow quench as well as the quench depth of the second quench. For blends contacted with a selective substrate or fillers, well-defined layer structures or target-like compositional waves which were paralleled to the external surfaces were formed during the two-step quench. In these systems, the bulk lamellar structure was affected by the concentration of components and fillers in the blend. The simulation by CDS demonstrated that the combination of various quench conditions and external surfaces play an important role in the pattern formation of binary blend, and provide a new approach in tailoring the morphologies of binary blends in addition to the other available methods.

Yajiang Huanga?Rui Wanga?Yajie Suna?Guangxian LiaEmail:nic8601@scu.edu.cn?Guangling Chena?Qi Yanga
[a]College of Polymer Science; Engineering, Sichuan University, Chengdu 610065, PR China;[b]State Key Laboratory of Polymer Material; Engineering, Sichuan University, Chengdu 610065, PR China

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

A special mechanism of dislocation nucleation in deformed nanocrystalline metals and ceramics is theoretically described. The mechanism represents non-local homogeneous nucleation of a nanoscale loop of “non-crystallographic” partial dislocation whose Burgers vector magnitude continuously grows during the nucleation process. The dislocation loop nucleation is accompanied by nucleation and evolution of a generalized stacking fault bounded by the loop. It is shown that the special mechanism can effectively produce nanoscale loops of lattice dislocations in nanocrystalline metals (Al, Ni) and ceramics (3C–SiC) deformed at high mechanical stresses achieved in shock-wave and indentation load regimes.

M.Yu. Gutkin aEmail:gutkin@def.ipme.ru?I.A. Ovid’koa
[a]Institute of Problems of Mechanical Engineering, Russian Academy of Sciences, Bolshoj 61, Vasilievskii Ostrov, St. Petersburg 199178, Russia

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

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