Variations in microstructure and texture during high temperature deformation of Al–Mg alloy

Abstract

The effect of strain rate on the superplastic deformation of a fine-grained aluminium–magnesium alloy has been investigated. Microstructural transformations were quantified by crystallographic texture, the study of the size of the grains, their morphology and their spatial distribution. At low strain rate a progressive reduction of the initial texture is observed, resulting from the predominance of grain boundary sliding. But even for large strains the texture randomisation is not complete, which confirms the role played by dislocation creep in superplasticity of the alloy. At high strain rate textural components of plasticity develop and the grains become elongated along the tensile direction. Nevertheless, transmission electron microscopy observations and a calculation of the expected variation with strain of the shape of the grains, suggest that continuous recrystallisation is likely to take place during deformation.

Introduction

Superplasticity of high strength aluminum alloys (like Al–Zn–Mg 7000 alloys) has been widely studied in the past [1], [2], [3], [4], [5], [6] due to their use for structural parts in aeronautics. Important efforts were carried out to produce very fine microstructures, typically with mean grain sizes of about 10 μm or less and with a narrow distribution around this mean value. In recent years superplasticity of aluminum–magnesium alloys has been increasingly studied [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] due to the weldability, the corrosion resistance and the strength at room temperature (from about 150 to 300 MPa) of these alloys. In the group of Al–Mg alloys, the 5083 alloy, of mean composition Al–4.5Mg–0.7Mn (in this paper all concentrations are given in wt.%) has been preferentially investigated. In order to get superplastic properties a moderately fine grained structure (mean grain size between 10 and 15 μm) is generally produced before deformation. Due to the relatively large amount of manganese the 5083 Al alloy contains a significant distribution of second phases, which facilitates the production and the stability of the fine grains. The largest second phase present through the material act as sites for nucleation of recrystallisation while the smallest pin the migrating grain boundaries and consequently hinder grain growth. A special grade of alloy is frequently used for superplasticity, in which the silicon and iron contents are reduced to limit the number of large intermetallic particles.

The temperature domain in which superplastic properties of the 5083 Al alloy are obtained is generally between 500 and 560°C with optimum strain rates generally from 10⁻⁴ s⁻¹ and 10⁻³ s⁻¹ [8], [10], [11], [12], [13], [14], [16], [19], [20]. Nevertheless, at higher strain rates, relatively large tensile ductilities may be maintained (elongations to fracture ≈200%) [11], [19], [20] resulting from values of the strain rate sensitivity parameter, m, close to 0.3. It is associated with deformation by dislocation creep in which magnesium atoms strongly interact with dislocations [7], leading to a glide control of deformation by a solute-drag process. Such quite large ductilities may still be obtained even in the case of large grained Al–Mg alloys [21], [22], [23], [24].

Under the superplastic conditions moderate values of m were reported, typically between 0.4 and 0.6 [8], [9], [10], [11], [12], [13], [14], [15], [16], lower than those obtained in the case of Al–Zn–Mg alloys [1], [3]. Due to these moderate values of m, the role of dislocation creep (DC) during superplastic deformation of Al–Mg alloys has been investigated [9], [13], [14], [15], [17], [18]. Data were generally obtained from transmission electron microscopy (TEM) observations [9], [15] and variations of grain morphology [13], [14], [17], [18] in samples deformed at 550°C. Dislocation microstructures resulting from superplastic deformation were carefully studied using a technique which includes quenching of the samples and aging under constant stress in order to preserve the strained microstructures [9], [15]. TEM observations of the deformed specimens suggest that intragranular slip makes a significant contribution to macroscopic deformation even in optimum superplastic conditions. This was confirmed from the variation of grain intercept lengths during uniaxial superplastic deformation, of a fine grained Al–4.7Mg–0.7Mn–0.4Cu [17]. In this paper the authors studied the variation with strain of the aspect ratio and the grain size and distinguish grain elongation associated with dislocation creep from strain-induced grain growth. Grain boundary sliding (GBS) and DC are supposed to occur independently and the DC strain rate $\text{ε}\text{̇}{}_{\mathrm{<mtext>DC</mtext>}}$ is deduced from the macroscopic variation of flow stress with strain rate. From the corresponding value of $\text{ε}\text{̇}{}_{\mathrm{<mtext>DC</mtext>}}$, grain elongation due to DC is calculated and compared to the experimental grain elongation measurements. From these results the authors concluded that, in the optimum superplastic range, the contribution of GBS to macroscopic strain decreased from a value higher than 80% at the beginning of deformation to about 60% when fracture occurs. In other words, intragranular dislocation creep appears as a significant straining process during superplastic flow. However, it must kept in mind that intercept lengths depend not only on the size and the morphology of the grains but also on their spatial orientation in reference to the axes of measurement. In consequence, they cannot be considered as intrinsic parameters and it has been demonstrated that their variation with strain may be difficult to interpret, in particular in the case of an initial random distribution of not fully equiaxed grains [25]. Moreover, in this paper the authors assume that the variation of the shape of the grains can result only from intraganular deformation of grain growth. In other words, they do not take into account a possible variation of the shape associated with a dynamic recrystallisation process.

Complementary information about the contribution of DC to macroscopic strain can be obtained from the variation of the crystallographic texture with strain. However, in the case of aluminum alloys most studies on the variation with superplastic deformation of crystallographic textures dealt with materials containing a large fraction of low-angle boundaries when deformation starts [26], [27], [28], [29]. In this case the first steps of deformation induce continuous recrystallisation by increasing the average boundary disorientation between subgrains. In the case of recrystallised Al alloys before deformation, very limited results have been reported about the effect of superplastic deformation on texture. The texture variation during superplastic deformation of a recrystallised Al–5Ca–5Zn alloy was recently investigated [30] but this alloy has a two-phase microstructure consisting in an aluminum matrix containing approximately 20 vol.% Al₃CaZn particles. Before deformation microtextures analysis indicated that most boundaries admitted high disorientation angles (≥15°) and texture measurements showed that copper and brass components were present. After superplastic deformation performed at 550°C for which m≈0.5 and even for large strains (ε>1.5), only a slight decrease in texture intensity was detected in reference to the initial one. Their results support the idea that DC can play a significant role since if superplasticity is assumed to be due only to GBS, strain is expected to continuously reduce the initial texture as a result of the movements of grains.

The aim of this study was to obtain information about the effect of strain rate on the mechanisms of deformation in a 5083 aluminum alloy tested at 525°C. The investigated experimental domain ranged from the optimum superplastic conditions associated with a maximum value of m to conditions for which m≈0.3. Data were essentially obtained from the variation with strain of the crystallographic texture, the size of the grains and their morphology.