An internal variable approach to high temperature deformation and superplasticity of Mg alloys
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 and the creep rate 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 , 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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Modelling studies of sheet metal formability of friction stir processed Mg AZ31B alloy under stretch forming
2012, Materials and DesignCitation Excerpt :They measured the ductility using tensile tests at a temperature of 250 °C at three strain rates, and demonstrated that the enhanced ductility and lower forming loads are due to the decrease in the grain size. Lee et al. [11] developed an internal variable approach to high temperature deformation and super plasticity of Mg alloys through a series of load relaxation tests at room and elevated temperatures. They established different types of deformation mechanisms for different Mg alloys at different processing conditions.
Effect of initial texture on deformation behavior of AZ31 magnesium alloy sheets under biaxial loading
2012, Materials Science and Engineering: ACitation Excerpt :They are now well known to exhibit various deformation modes such as 〈a〉 basal, 〈a〉 non-basal, and 〈c + a〉 pyramidal slip as well as twinning. Deformation modes other than the usual basal slip are, in general, difficult to activate near room temperature (RT) due to their higher critical resolved shear stress (CRSS) [2–4]. Although twinning behavior could assist plastic deformation providing additional deformation mode at RT, wrought magnesium alloys still exhibit limited room temperature ductility due to insufficient number of active slip systems required for a general plastic deformation, inherent in most of the HCP crystals near room temperature [5].
The effect of texture and strain conditions on formability of cross-roll rolled AZ31 alloy
2011, Journal of Alloys and CompoundsCitation Excerpt :However, Mg alloys often exhibit relatively low strength and poor formability at room temperature [3]. The limited ductility is attributed to a strong planar anisotropy, where the basal plane (0 0 0 2) tends to be distributed parallel to the rolling direction, since the basal slip mode provides the only two independent slip systems [4–6]. However, it has been reported that both ductility and texture of Mg alloys during rolling process largely depend on rolling conditions such as rolling temperature, rolling direction and plastic deformation [7].
Nano grained AZ31 alloy achieved by equal channel angular rolling process
2011, Materials Science and Engineering: ACitation Excerpt :Mg alloys are known as lightweight structural materials to replace aluminum alloys or polymer materials due to their excellent properties such as low density and high specific strength [1].
Deformation behavior of as-rolled and strip-cast AZ31 magnesium alloy sheets
2011, Materials Science and Engineering: ACitation Excerpt :Magnesium is now well known to have less than five independent slip systems required for general plastic deformation of polycrystalline materials. Namely, the basal slip systems with the lowest critical resolved shear stress (CRSS) can only provide the two independent slip systems [3–5]. Thus, the deformation twinning is an important behavior to accommodate effectively plastic strain due to limited slip systems of polycrystalline magnesium alloys [6].
An internal variable approach of orientation dependent deformation mechanism of rolled AZ31 magnesium alloy
2011, Metals and Materials International