Recovery of AlMg alloys: flow stress and strain-hardening properties
Introduction
Mechanical properties of pure metals or undersaturated solid solutions depend on their grain structure (morphology and size, texture, misorientations) and dislocation structure (density, network, cells or subgrains, deformation or shear bands). The annealing of such defect structure, like recovery and recrystallization, induces a softening of the materials. Recovery, which occurs during annealing at moderate temperatures of severely cold-worked materials, is a continuous transformation, where a given grain and dislocation structure evolve everywhere, through the movement and annihilation of dislocations, towards another structure of lower stored elastic energy.
Some aspects of recovery kinetics and mechanisms, which have been mainly studied in pure metals1, 2, 3, 4, are still unclear or insufficiently documented. This is the case for instance of the estimate of the driving force (the stored energy) and the stability of the dislocation structures formed during cold-rolling and recovery. Basically the difficulty is related to the very large density of dislocations involved (between 1014 and 5×1015/m2) and their heterogeneous distribution, which makes their energy density and their individual (cross-slip, climb, solute drag) or collective behaviour difficult to handle, either analytically using simple elementary dislocation models, or by computer simulation. Experimentally the situation is not simple as well: the elastic stored energy is low (a few tenths of J/g in Al), even for high strains, making its measurement difficult. On the other hand, the observation of microscopic mechanisms in dense dislocation tangles is difficult to perform by transmission electron microscopy (TEM).
The correlation betwen mesoscopic and macroscopic scales, between recovery microstructures and mechanical properties (flow stress, strain hardening, ductility, forming ability…) is of prime importance for industrial applications.
In order to address such basic questions, we have performed a number of physical observations for several recovery conditions on AlMg alloys, the basis of wrought work-hardenable aluminium alloys. Most of the recent studies on these alloys have focused on strain localization[5] and dislocation structures after large deformations are practically unknown6, 7. Due to the above-mentioned difficulties, a large number of property measurements have been performed simultaneously: electrical resistivity, specific mass, differential scanning calorimetry, X-ray diffraction line broadening, conventional and “in-situ” TEM, mechanical properties[8].
We report here part of the study which deals with the microstructural evolution and the related evolution of mechanical properties observed during static recovery at moderate temperatures. We first consider the decrease in the flow stress which occurs during recovery as a function of temperature T and duration t of the annealing treatment (see also Ref.[9] for mechanical test results). We also consider changes in the tensile stress–strain curves at constant strain rate, such as the decrease in strain hardening and the increase in ductility resulting from such heat treatments.
After describing the experimental procedure and the materials, we first summarize the characteristic results obtained by TEM[10]. These observations are compared with the microstructural feature evolution assumptions needed to model the evolution in work-hardening behaviour.
The mechanical behaviour is then described and analysed on the basis of a general model, which makes use of a convenient constitutive law of plasticity where microstructural components are taken into account to describe the dynamical behaviour of the material. Flow stress and strain hardening in constant strain rate tensile tests are also discussed.
Section snippets
Experimental
The alloy Mg content is 2.5 wt%. Aluminium with two different purities has been used: Al (1070) and Al (1199) which essentially differ by the Fe content (0.15 wt% for 1070 and 0.0014 wt% for 1199). For the (1070) base alloy, coarse (1 μm diameter) Al3Fe precipitates are present together with a fine dispersion of small Al6Fe and (AlMgSi) precipitates (150–300 nm diameter).
After solutionizing for 24 h at 560°C, a cold-rolling strain, ranging from 0.1 to 3 equivalent strain, is applied [the equivalent
TEM microstructural data
These TEM observations are made “post-mortem” as a function of the cold-rolled prestrain ε and of the temperature and time (T,t) of the annealing treatment. Only salient results are presented here (see Ref.[10] for a more complete description). The thin foils are parallel and longitudinal transverse sections with respect to the cold-rolling plane and direction.
Fig. 1 shows a transverse section of an (1199+2.5 Mg) alloy cold-rolled to ε=0.5. The dislocations are organized in a cellular structure
Conclusion
This work relates mechanical properties of AlMg alloys, yield stress and strain-hardening behaviour, with several aspects of the dislocation substructure and evolution during recovery heat treatments. In particular the degree of heterogeneity of the dislocation distribution is considered.
A single internal variable, the total dislocation density, appears to be sufficient to describe the time decay of the yield stress (subsequently measured at room temperature) during an isothermal recovery heat
Acknowledgements
The Région Rhone-Alpes is gratefully acknowledged for the doctoral grant to one of us (M.V.) as well as the Pechiney Rhenalu Company for financial support.
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Present address: Los Alamos National Laboratory, Center for Materials Science, Los Alamos, NM 87545, U.S.A.