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ABSTRACT: The subgrain size and the spacing of high angle boundaries are important parameters used to describe the microstructure in metals deformed to large strains. How the flow stress depends on the boundary spacing is discussed here and it is argued that the best way this is treated is in the work hardening model developed by Nes and co-workers (Progr. Mater. Sci., 1998, 41, 129 – 193; Mater. Sci. Tech., 2001, 17, 376 – 387; Mater. Sci. Eng., 2002, A 322, 176 – 193). The theoretical arguments given are supported by experimental observations.
Materials Science and Technology 10/2004; 20(11):1377-1382. · 0.77 Impact Factor
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ABSTRACT: Particle distributions play a major role in the processing response of aluminum alloys. While large constituent particles play an important role in the nucleation of recrystallization, small particles may heavily restrict the growth of recrystallized grains. In the present investigation, a two-dimensional (2-D) tesselation procedure has been used to characterize the particle distribution in commercial aluminum alloys and its relevance to nucleation of recrystallization. This procedure enabled the quantification of the degree of particle clustering in samples rolled to different strain levels. A characteristic aspect seems to be a transition from a rather nonuniform spatial distribution at low rolling strains, toward a more or less random distribution at high strains. Nucleation kinetics has been found to be site saturated, indicating that all nucleation events effectively occur at the start of recrystallization. A simple model is proposed, which explains the development of the spatial particle distribution as a function of rolling strain.
Metallurgical and Materials Transactions A 11/2003; 34(12):2705-2715. · 1.54 Impact Factor
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ABSTRACT: A new approach to the modelling of work hardening during plastic deformation of pure fcc metals has recently been proposed by the present authors. The model is based on a statistical approach to the problem of athermal storage of dislocations. By combining the solution for the dislocation storage problem with models for dynamic recovery of network dislocations and sub-boundary structures, a general internal state variable description is obtained. In the present work this model is further developed by including the effects resulting from grain boundaries, elements in solid solution, and the presence of non-deformable precipitate particles. The result is a work hardening model and associated computer code, capable of providing the stress–strain behaviour for a given solid solution alloy under any combination of constant strain rate and temperature. The model has been applied to the problems of work hardening and flow stress saturation in Al–Mg alloys. It is demonstrated that the model predictions, in terms of microstructure evolution and associated strengthening, are in good agreement with experimental observations.
Materials Science and Technology 03/2001; 17(4):376-388. · 0.77 Impact Factor
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ABSTRACT: The Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory for recrystallisation has been modified by using an analytical approach and three dimensional computer simulations. A non-constant growth rate, the result of either spatial variations in the net driving force for the recrystallisation reaction, or simultaneous recovery during the reaction, has been considered. Either may account for considerable negative deviations from classical JMAK kinetics. The effect of recovery on transformation kinetics in the case of site saturated nucleation is analysed in detail. An inhomogeneous distribution of nucleation sites may also have a significant effect on the kinetics; leading to non-constant Avrami exponents usually much lower than the classical values of 3 and 4for three dimensional homogeneous site saturation and Johnson-Mehl nucleation kinetics, respectively. It is suggested that the near log-normal size distributions of grain section areas which are commonly observed experimentally may reflect a non-random distribution of nucleation sites. The modelling results are discussed in terms of experimental observations.MST/1294
Materials Science and Technology 10/1990; 6(11):1093-1102. · 0.77 Impact Factor
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ABSTRACT: A new approach to the modeling of work hardening during plastic deformation of f.c.c.-metals and alloys has been recently proposed by the present authors. The model is based on a statistical approach to the problem of athermal storage of dislocations. By combining the solution for the dislocation storage problem with models for dynamic recovery of network dislocations and sub-boundary structures, a general internal state variable description is obtained. The model includes effects due to variations in: (i) stacking fault energy, (ii) grain size, (iii) solid solution content, and (iv) particle size and volume fraction. The result is a work hardening model, which in principle is capable of providing the stress–strain behavior for a given metal or solid solution alloy under condition ranging from deformation in the ambient temperature range to high temperature creep. It will be demonstrated that the model predictions, in terms of microstructure evolution and associated properties, in general, are in good agreement with experimental observations.
Materials Science and Engineering: A.
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ABSTRACT: The non-isothermal formation of Sc- and Zr-containing dispersoids in Al–Zr–Sc ternary alloys has been investigated by atom probe tomography (APT). In the early stages of precipitation, a high number density of Sc-rich clusters form. These clusters subsequently transform into Al3Sc particles with a L12 structure. When Zr diffusion becomes significant, Zr atoms are found to segregate to Al3Sc/α-Al matrix interfaces. Further annealing at 748 K gives rise to a duplex core/shell structure.
Journal of Alloys and Compounds 470:107-110. · 2.29 Impact Factor
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ABSTRACT: In order to develop aluminium alloys with a high structural stability, dispersoid-forming elements are added. Examples of such elements are scandium (Sc) and zirconium (Zr). In this work Hf, which is a relatively untried element with respect to recrystallisation resistance, was added to pure aluminium in combination with Sc and Zr. The results revealed that twinned columnar grains (TCG's) formed during solidification at a high Hf- content (1.1 wt%) in both Al-Hf-Zr and Al-Hf-Sc. However, when the Hf-content was kept low (0.2 wt%) a more equiaxed structure was obtained. The effect of TCG's on the recrystallisation resistance after cold rolling was also examined. However, despite the presence of twinned columnar grains a remarkably high recrystallisation resistance was found for Al-Hf-Sc.
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ABSTRACT: Three dimensional atom-probe (3DAP) has been applied in order to study the nucleation of small, spherical and coherent Al3(Sc,Zr)-dispersoids. The results indicate that Al3(Sc,Zr) in the beginning of the nucleation process mainly consist of Al and Sc, while Zr enters the dispersoids at a later stage, i.e. relatively Zr-rich shells seem to form around Sc-rich cores.
Scripta Materialia.
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ABSTRACT: The mechanical properties of aluminum alloys with grain sizes in the range from less than a micron (ultra-fine) to hundreds of microns have been modelled within the framework of the multi-parameter microstructural work hardening model developed by Nes and co-workers [E. Nes, Prog. Mater. Sci. 41 (1998) 129–194; K. Marthinsen, E. Nes, Mater. Sci. Technol. 17 (2001) 376–388; E. Nes, K. Marthinsen, Mater. Sci. Eng. A322 (2002) 176–193; E. Nes, K. Marthinsen, B. Holmedal, Mater. Sci. Technol. B 20 (2004) 1377–1382]. The effect of grain size on the flow stress and work hardening, including a deviation from the Hall–Petch grain size dependency for ultra-fine grain sizes, is well accounted for by the model. A characteristic feature of the stress–strain behaviour of Al–Mg-alloys and ultra-fine grained variants is a sharp yield point followed by Lüders-band elongation. A mechanism for such an elastic–plastic transition is suggested.
Materials Science and Engineering: A.
