China Science

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

A trace (0.1 at.%) addition of Ge to a base Al–1.7Cu–0.3Mg alloy resulted in considerable refinement of the ?? (Al2Cu) precipitate dispersion as well as stimulating the formation of 0 0 1 lath-shaped precipitates in a fine and uniform distribution. These precipitates contain mainly Ge and Mg. The formation of the S(Al2CuMg) and ?(Al5Cu6Mg2) phases was suppressed, presumably due to the stronger interaction between Ge and Mg over Cu and Mg. Significantly, the trace addition of Ge produced an internal structure within the ?? precipitate phase with its habit plane parallel to the {1 0 1}?? planes. Our investigations reveal that this unusual internal structure is secondary precipitation within ?? precipitation of a Mg–Ge rich phase thought to be an allomorph of Mg2Ge, and this influences both the nucleation and growth of ?? precipitates. This internal structure is designated as K phase, possessing a close packed hexagonal structure (a = 0.405 nm, c = 0.607 nm) and oriented such that, (0 0 0 1)K//(1 0 1)?? and ]K//[1 0 0]??. Prolonged ageing at 200 °C led to replacement of the 0 0 1 lath-shaped precipitates through their dissolution and reprecipitation of a more stable form of Mg2Ge (face-centred cubic, Fmm, a = 0.638 nm).

S.P. RingeraEmail:s.ringer@usyd.edu.au?K.S. Prasada?G.C. Quanc
[a]Australian Key Centre for Microscopy; Microanalysis, The University of Sydney, Madsen Building F09, Sydney, NSW 2006, Australia;[b]ARC Centre of Excellence for Design in Light Metals, The University of Sydney, NSW 2006, Australia;[c]Wescast Industries Inc., Brantford, ON, N3T 5L8, Canada

Data on lattice rotations and dislocation structures induced in aluminium by tensile deformation are analysed together in order to extract the active slip systems. The analysis falls in two steps: (i) from the combination of lattice rotation and dislocation structure data, the grain orientation space represented by the stereographic triangle is subdivided into regions with the same active slip systems; and (ii) the active slip systems calculated from the lattice rotations are compared with those known to be active based on the dislocation structure. For the entire stereographic triangle active slip systems which are in good agreement with both lattice rotations and dislocation structures are identified, showing that the grain orientation is the primary factor controlling the slip systems.

Grethe Winther aEmail:grethe.winther@risoe.dk
[a]Center for Fundamental Research: Metal Structures in Four Dimensions, Materials Research Department, Ris DTU – National Laboratory for Sustainable Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark

The yield stress and work hardening behaviour of precipitation hardening alloys is directly related to the nature of the interaction between mobile dislocations and precipitates. In commercial systems such as aluminium alloys, the understanding of this problem is complicated by the overlap between various mechanisms and the interplay between the volume fraction and size of precipitates and the residual solid solution content. In this study a model Al–2.8 wt.% Mg–0.16 wt.% Sc alloy has been chosen for examination since precipitation involves simple spherical precipitates, the absence of metastable phases and the solid solution effect is dominated by the magnesium content. Using a previously development precipitation model, it has been possible to develop an integrated yield stress/work hardening model in which the shearable/non-shearable transition and the size distribution of precipitates are explicitly accounted for. The agreement between the model and the experiments is excellent and the shearable/non-shearable transition radius is consistent with experimental observations.

F. Fazelia?W.J. Poolea?C.W. Sinclair aEmail:chad.sinclair@ubc.ca
[a]Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4