Grain size and texture effects on deformation behavior of AZ31 magnesium alloy
Introduction
Magnesium alloys are promising candidates for structural application, especially in automotive industry due to the low density and high specific strength [1]. However, the use of magnesium alloys has been greatly limited by their low ductility and poor formability at room temperature [2]. Due to the limited slip systems available at room temperature, basal slip system (which is easily activated in magnesium alloys) alone cannot accommodate general plastic deformation and other non-basal dislocation slip systems or twinning systems are needed to improve room temperature ductility and formability. Microstructure and texture modification can be used to activate non-basal slip systems [3].
Grain refinement has been a good option for enhancing strength of magnesium alloys, but it is generally accompanied by reduction in ductility, especially when the grain size is refined to the ultrafine regime [4], [5], [6]. Texture modification of AZ magnesium alloys by severe plastic deformation has shown high room-temperature ductility due to the formation of basal texture, which orientated the easy basal slip system to preferred orientation [7], [8], [9], [10], [11]. Koike et al. [8] observed non-basal slip at room temperature in equal channel angular extruded fine grain AZ31 magnesium alloy using transmission electron microscopy (TEM), and attributed the high ductility to the non-basal slip and dynamic recovery. Agnew and Duygulu [12] reported activation of non-basal dislocation slip at room temperature during tensile deformation of a rolled AZ31 magnesium sheet both by TEM observations and theoretical simulation.
The importance of texture on deformation behavior has been highlighted by the strain paths [13], [14]. In addition to dislocation slip, deformation twining is an important deformation mechanism that can accommodate strain in c-axis [15]. It was recently demonstrated that extension twinning persists as an important contributor to the strain accommodation to at least 200 °C and does not inhibit formability [16]. Barnett addressed the likelihood of enhancing uniform elongation through extension twinning in tensile test of extruded AZ31 [17].
Friction stir processing (FSP) has become one of the effective severe plastic deformation tools to refine metals and alloys [18], [19]. Submicron and even nanoscale grain structures have been reported [20], [21], [22], [23]. It also introduces very strong basal texture into the processed magnesium alloys [24], [25], [26], [27]. Previous study has shown significant tensile anisotropy in FSP ultrafine grained AZ31 due to the effect of texture [27], which sheds light on the importance of texture on deformation behavior of processed magnesium alloys. The current study aims to (a) achieve a variant with high ductility and another variant with high strength and moderate ductility in AZ31 magnesium alloy through microstructure and texture modification and (b) explore the influence of grain size and texture on deformation behavior of highly textured magnesium alloy.
Section snippets
Experimental procedure
A commercial grade AZ31-H24 magnesium alloy having a thickness of 2.0 mm was used. To make friction stir passes, a tool with 12 mm diameter concave shoulder and 1.5 mm conical pin was employed. FSP of AZ31 was carried out at various tool rotation rate/traverse speed combinations, 900 revolutions per minute (rpm)/50.8 mm per minute, 600 rpm/50.8 mm per minute and 600 rpm/152.4 mm per minute. A two-pass FSP, with the second pass of 600 rpm/203.2 mm per minute fully overlapping on the first pass of 900
Microstructure and texture
Fig. 2 presents the inverse pole figure maps of FSP AZ31 with various grain sizes: coarse grain (CG), fine grain 1 (FG1), fine grain 2 (FG2) and ultrafine grain (UFG). Each color in the map indicates one crystallographic orientation, with the red color representing the basal texture component, i.e., the c-axis is normal to cross-section plane. High angle grain boundaries with misorientation angles larger than 15° and the low angle grain boundaries with misorientation angles below 15° are
Yielding anisotropy
The significance of strain paths on mechanical property of highly textured magnesium alloys was highlighted from the remarkable difference in tensile strength between PD and TD. For all four grain sizes, the PD testing exhibited much lower YS compared to that of TD testing for each grain size and the YS increases as the grain size reduces in both PD and TD. These were interpreted as classic grain boundary strengthening but different deformation mechanisms taking part at yielding between PD and
Conclusions
- 1.
Friction stir processing of AZ31 magnesium alloy resulted in fine and ultrafine grained structures with strong basal fiber texture having the basal pole tilted about 35–55° towards the processing direction.
- 2.
Processed material exhibited remarkably different tensile behavior in the PD and TD for various grain sizes. Specimens deformed mainly by basal slip with additional extension twinning (in the PD) exhibited low tensile yield strength and high degree of work hardening, and fractured before
Acknowledgments
The authors gratefully acknowledge the support of the National Science Foundation through Grant NSF-EEC-0531019.
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