The influence of transient deformation conditions on recrystallization during thermomechanical processing of an Al–1% Mg alloy

doi:10.1016/S1359-6454(99)00113-5

Acta Materialia

Volume 47, Issue 8, 22 June 1999, Pages 2377-2389

https://doi.org/10.1016/S1359-6454(99)00113-5 Get rights and content

Abstract

The recrystallization behaviour of an Al–1% Mg alloy has been investigated following hot deformation under varying strain-rate conditions. Significant differences in recrystallized grain size and recrystallization kinetics are observed following a decreasing strain rate, as compared with constant strain-rate tests. These differences reflect the microstructural transients, which are presented in an accompanying paper. It is shown that the data are reasonably consistent with a physically based internal state variable model for recrystallization. A feature of the modelling is the sensitivity analysis conducted, which indicates the degree of uncertainty in predicting recrystallization behaviour, and identifies the critical areas in which experimental effort should be concentrated.

Introduction

Recrystallization during thermomechanical processing of metals is a topic which has been covered by numerous papers during the last two–three decades. Almost all work on hot deformation, with a few exceptions1, 2, has concentrated on situations where the deformation parameters, strain-rate and temperature, have been constant during the deformation process. In industrial practice however, such as rolling, extrusion and forging, transient deformation conditions are the norm. It was shown in the accompanying paper[3] that there is good experimental evidence that varying strain-rate conditions cause significant variations in the internal microstructure, characterized in terms of the subgrain size, δ, misorientation between subgrains, θ, and the dislocation density inside the subgrains, ρ_i. These microstructural features represent the stored energy in a material[4] and consequently the energy which drives the recrystallization process.

Several attempts have been proposed to model the recrystallization process, though until relatively recently these have been mostly empirically based. The physically based models developed in the last five years are based on internal state variables describing the deformation microstructure, and linking this to the nucleation positions and growth behaviour of the recrystallized grains. The results have shown that such models describe the experimental results for different alloys well, though only for uniform processing conditions, while transient deformation conditions have been largely neglected. Since the inputs to these models are based on extensive microstructural measurement, the degree of uncertainty in the modelling is potentially very great. Sensitivity analysis of the effect of this uncertainty is however very lacking in the literature.

The objectives of the present paper are: (i) to examine the consequences for the recrystallization behaviour of the observed differences in deformation microstructure in an Al–1% Mg alloy deformed under transient conditions; (ii) to predict the behaviour using an internal state variable model for recrystallization; and (iii) to evaluate the sensitivity of the predictions to uncertainty in the data.

Section snippets

Experimental work

The material used in this work is an Al–1 wt% Mg alloy with the chemical composition and preprocessing as given in the accompanying paper, which also gives details of the deformation conditions used[3].

Plane strain compression (PSC) tests were used to simulate industrial hot rolling, as for this purpose PSC machines have been demonstrated to give realistic descriptions of microstructure and texture[5]. However, it must be noted that the microstructure in the deformed region of a PSC sample is

Results

For the readability of this paper it is useful to recapitulate the main points from the accompanying paper[3]. Significant differences in microstructure of an Al–1% Mg alloy occurred depending on the deformation history. Three loading histories at constant T=385°C were applied up to a strain of 1 [Fig. 3(a)]:² (i) constant strain-rate $ε ̇ =0.25$ , 2.5 and 25/s; (ii) strain-rate increasing

Summary of recrystallization model

The influence of strain-rate history and strain on the recrystallized grain size d_rex and recrystallization kinetics t₅₀ are modelled by considering the dependence of nucleation site density and stored energy on the deformed microstructure.

Conclusions

The results are stimulating and show the importance of the deformation history in aluminium alloys in controlling the structure evolution which drives subsequent recrystallization. Significant reductions in recrystallized grain size d_rex and recrystallization time t₅₀ were observed when the strain rate was reduced during deformation. There is good evidence that the transients in recrystallization behaviour are controlled by the transients in subgrain size, and to a lesser extent the transients

Acknowledgements

The authors acknowledge the financial support of the UK EPSRC, who supported this collaborative research project at the Universities of Cambridge and Sheffield. The contribution of the other project participants, Qiang Zhu, Mike Ashby and John Whiteman is also acknowledged.

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¹: Present address: Hydro Aluminium a.s., R&D Materials Technology, N-6601 Sunndalsøra, Norway.

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The influence of transient deformation conditions on recrystallization during thermomechanical processing of an Al–1% Mg alloy

Abstract

Introduction

Section snippets

Experimental work

Results

Summary of recrystallization model

Conclusions

Acknowledgements

Acta metall.

Mater. Sci. Engng

Acta mater.

Acta mater.

Phil. Mag. A

Mater. Sci. Forum