Effects of high-density pulse current on mechanical properties and microstructure in a rolled Mg–9.3Li–1.79Al–1.61Zn alloy
Introduction
Recently, Mg–Li alloys, the lightest metallic material, have drawn extensive research interest in alloying [1], [2], [3], [4], conventional plastic deformation such as extrusion [2], [3], [4], [5], [6], [7] and rolling [8], [9], [10], and severe plastic deformation such as equal channel angular extrusion [11], double change channel pressing [12], and high-pressure torsion [13] due to their extremely low density, high specific stiffness, good damping properties, and electromagnetic shielding capability. Owing to these advantages, Mg–Li alloys have great potential for applications in aerospace, weapons, 3C electronic components, and automobile manufacturing. As shown in the Mg–Li binary phase diagram [14], when Li content is below 5.7 wt%, α phase, a Mg-rich hexagonal closed-packed (HCP)-structured solid solution, forms; when Li content is greater than 11 wt%, β phase, a Li-rich body-centered cubic (BCC)-structured solid solution, forms; when Li content is between 5.7 and 11 wt%, α+β dual phases form. α phase binary alloys are slightly brittle [15]. β Phase binary alloys have excellent ductility but poor strength [16]. Dual-phase alloys have better comprehensive mechanical properties than single-phase alloys at room temperature [17]. They even have superplasticity at elevated temperatures [18], [19], [20], [21], [22]. Al and Zn are the most commonly used alloying elements in strengthening Mg–Li alloy matrix. However, there appears to be little information available on fabricating Mg–9Li–2Al–2Zn (designated as LAZ922) sheets by casting and rolling and investigating its mechanical properties at room temperature in terms of our literature survey.
Electroplastic effect (EPE) is the material plasticity-enhancement phenomenon induced by current during plastic deformation. Electric pulse current, an approach of EPE, is applied to the solidification process of immiscible alloys and results in the refinement of the microstructure [23]. EPE has found extensive application in metal forming field [24], [25]. It is used for the investigation of the mechanical behaviors of a wide range of materials such as 5052-H32 aluminum alloy [26], shape-memory Ti–Ni alloy [27], high strength steels [28], and Mg alloy [29]. Results have shown that the stress decreases and the elongation increases due to EPE [25], [28]. However, microstructural investigation is rather limited. To the best of our knowledge, no report is available regarding the effects of electric pulse current on the mechanical properties and microstructure in an ultralight Mg–Li–Al–Zn alloy. It is necessary to investigate the material science aspects of EPE in this alloy.
The present study had four goals. First, LAZ922 alloy sheets were fabricated by casting and rolling. Second, the mechanical properties of the present alloy with a current and without a current were investigated. Third, corresponding deformation microstructures were studied. Fourth, the dislocation density and number of dislocations inside the grain in consideration of the pulsed current were modeled. Their relationships with the experimental results were elucidated. It is expected that this first report may stimulate interest in the investigation into the electro-forming behavior of Mg–Li alloys.
Section snippets
Material preparation
The flux of LiCl+LiF with a mass ratio of 3:1 was melted in a cast iron crucible. After the flux melted, Mg, Al, and Zn blocks whose purity was more than 99.9 mass% were melted at the temperature of 983 K in an argon environment. Then the crucible was taken out of the furnace and the melt was cooled to 933 K. Li strips covered by aluminum foil was pressed into the melt and melted at 933 K, and the melt was heated to 983 K again. After the surface flux of the liquid metal was removed, the melt was
Initial microstructures and mechanical properties
The microstructures before tensile deformation are shown in Fig. 2. The optical microstructures of etched Mg–Li alloy samples are well documented. In most studies, HCP-structured α-phase is shown in white and BCC-structured β-phase is shown in gray [30], [31], [32]. The microstructures in Fig. 2 consist of mostly white Mg-rich HCP-structured α solid solution phase and gray Li-rich BCC-structured β solid solution phase. Fig. 2(a) shows the as-cast microstructure. White rod-shaped α-phase grains
Causes of the decrease in stress and the increase in elongation
When high density pulse current is applied to the sample, numerous electrons flow through the sample and form electron wind. This electron wind causes a force that acts on the dislocations, called electron wind (EW) force. The EW force or the force per unit dislocation length exerted by an electric current, , obeys the following relation [33]:where is the resistivity, is the dislocation density, e is the electron charge, is the electron density, and j is the current
Conclusions
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The ultimate tensile strength of 209 MPa and the elongation to failure of 24% were demonstrated in the LAZ922 alloy sheets processed by casting and rolling. Variation in true stress with true strain showed that stress decreases and elongation to failure increases under high density pulsed current. The stresses at the current density of 1.44×103 and 2.11×103 A/mm2 were 57% and 82% less, respectively, than the stress without a current. The elongations to failure at this current density were 33.2%
Acknowledgements
The authors gratefully acknowledge the financial support of National Natural Science Foundation of China under Grant nos. 51334006 and 51205376 and National Basic Research Program of China under Grant no. 2011CB012803. We also appreciate the help from graduate student Zhou Bijin during the preparation of the manuscript.
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Present address: Suzhou Suxin Special Steel Co., Ltd., Suzhou 215151, PR China.