An internal variable approach to high temperature deformation and superplasticity of Mg alloys

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Abstract

High temperature deformation behavior of pure Mg and Mg alloys has been investigated within the framework of an internal variable theory for inelastic deformation. To this end, a series of load relaxation tests has been performed for pure Mg single and poly crystals as well as the AZ31 Mg alloys. At 623 K, pure Mg single crystal exhibited a transition in deformation mechanism from dislocation climb at lower strain rates to plastic deformation at higher rates, while the polycrystal showed only the plastic deformation. The AZ31Mg alloy with the fine grain size of d = 10 μm revealed both the grain boundary sliding and plastic deformation under suitable test conditions in terms of temperature and strain rate, while the coarse grain with d = 20 μm exhibited only the plastic deformation. From the constitutive analysis based on the internal variable theory, the role of grain size refinement and process temperature on superplasticity has been clarified precisely as a dividend of present approach.

Introduction

Magnesium alloys have recently emerged as lightweight structural materials to replace aluminum alloys or polymer materials due to their lowest density as well as other excellent physical properties such as relatively high specific strength, superior damping capacity, electromagnetic shielding characteristics and so on. Magnesium and its alloys have hexagonal closed packed structure and thus exhibit very limited ductility at room temperature. Most of the structural magnesium products are therefore fabricated by casting processes, having inferior mechanical properties compared to wrought ones [1], to limit their structural applications so far. In order to overcome the limitations and enlarge the application areas, it is necessary to develop suitable plastic forming processes at elevated temperatures, because the ductility of magnesium is known to drastically enhance by activating additional slip systems at high temperatures. Many of research works have, therefore, focused on the high temperature deformation behavior and superplasticity of magnesium alloys [2], [3], [4], [5], [6].

Superplastic deformation of crystalline solids is now well known to occur by grain/phase boundary sliding (GBS) together with certain geometric accommodation processes at high temperatures [7], [8]. Although crystal superplasticity involves two distinctly separate physical mechanisms, viz. GBS and accommodation, it has generally been described by a single phenomenological power law relation between the applied stress and inelastic strain rate with a strain rate sensitivity parameter m as the power index. The parameter m has widely been used to characterize the superplastic deformation behavior requiring large values of m as a prerequisite condition for superplasticity, which is not necessarily true in many cases [9], [10]. This simple power law relation is therefore deemed not suitable to prescribe the observed deformation behavior and the micromechanical mechanisms, apart from the variation of m over wider strain rate range or the inconsistency of its critical value for superplasticity.

The high temperature deformation behavior and superplasticity of AZ31 magnesium alloy were investigated in this study within the framework of an internal variable theory proposed by the author [11]. The theory has been developed based on dislocation dynamics and applied successfully to describe the high temperature as well as superplastic deformation behavior of various metallic materials [11], [12], [13], [14], [15], [16].

Section snippets

Inelastic deformation of grain matrix

Within the framework of well-known dislocation dynamics, inelastic deformation is treated here as simultaneous processes of dislocation accumulation against strong barriers such as grain boundaries and relaxation to relieve the accumulated internal strain energy. Inelastic grain matrix deformation (GMD) then naturally produces an internal strain a due to accumulated dislocations and the various non-recoverable strain rates such as the plastic strain rate α˙ and the creep rate β˙ to provide the

Experimental procedures

A commercial grade (99.94 wt.%) polycrystalline Mg was die-cast first and then hot rolled with 50% reduction at 623 K prior to annealing and mechanical test. Cylindrical specimens with 27 mm gage length and 6 mm gage diameter were machined from the hot-rolled sheet and then heat treated at 773 K for 3 h under Ar gas atmosphere followed by water quenching to homogenize the elongated microstructures. Equiaxed microstructure with the average grain size about 100 μm was obtained. Single crystals were

Pure magnesium

The flow curves of pure Mg poly and single crystals shown in Fig. 2 were determined from the load relaxation tests at 623 K. The rate-controlling mechanism of creep in polycrystalline Mg has been observed to change from dislocation climb at low temperature to cross slip from basal to prismatic planes at higher temperatures [18], [19]. The flow curves in higher strain rate region can thus be viewed as corresponding to our α˙, since the cross slip is regarded as a typical relaxation process for

Summary

High temperature deformation behavior of pure Mg alloys has been investigated within the framework of an internal variable theory for inelastic deformation. The load relaxation test appears to provide a powerful means to characterize the dominant deformation mechanisms, when combined with an internal variable theory. Pure Mg single crystal exhibited a transition in deformation mechanism from dislocation climb at lower strain rates to plastic deformation at higher rates, while the polycrystal

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