An Mg‐Al‐Zn Alloy with Verry High Specific Strength and Superior High‐strain‐rate Superplasticity Processed by Reciprocating Extrusion

Abstract

Magnesium alloys are considered to be the material that has most importantly influenced our life in the 21st century. They are light, recyclable and moderate in physical and mechanical properties, so they are growing in the applications for automobiles, bicycles, hand tools and 3C products. Most such products are made by die-casting and semisolid thixomolding because Mg alloys with hcp structures do not have good ductility and workability. Moreover, they are not sufficiently strong to justify their being used in place of highstrength Al alloys. New magnesium alloys with superior properties must be developed to enable solid-state forming processes, such as extrusion, forging, stamping and press forming, to be used to make parts by mass production, and thus expand the application of wrought products.According to the Hall-Petch relation, [1,2] the flow stress of a metal depends on the grain size:where r is the flow stress of a metal; r 0 is the flow stress in the absence of resistance to slip across grain boundaries; K is a constant and d is the grain size. The constant K in magnesium alloys is well known to exceed those of fcc metals, such as aluminum alloys. [3,4] That is, the flow stress of magnesium alloys rises strongly as grain size decreases. Furthermore, reducing the grain size also improves ductility and superplasticity of magnesium alloys. [3,5±7] Therefore, controlling the grain size is crucial not only in augmenting the strength and ductility but also in obtaining high-strain-rate superplasticity of magnesium alloys.Recently, several studies of modified magnesium alloys, such as Mg-1Zn-2Y [8] and Mg-Al-Ga, [9] have been conducted with rapid solidification, revealing favorable tensile properties and high-strain-rate superplasticity. However, magnesium powder is extremely dangerous and must be processed with many extra steps in an environment of a protective gas. The addition of elements, Ga and Y, is also expensive, narrowing their range of applications.The Mg-Al-Zn magnesium alloy system is the most popular to be used in casting, thixomolding and wrought processes. Previous studies have shown that Mg-Al-Zn alloys exhibit good superplasticity at optimum strain rates and temperatures. [7,8,10,11] The microstructure of Mg-Al-Zn alloys essentially consists of a main a phase of Mg solid solution and a minor b phase of intermetallic Mg 17 Al 12 , whose volume fraction increases with the aluminum content. Although high aluminum and zinc contents improves the strength and corrosion resistance with a slight increase in density from 1.78 to 1.90 g/cm 3 , [12] the b phase is inherently brittle, rendering the alloy brittle. Daloz et al. [13] proposed that the aluminum and zinc contents should be less than 15 and 3 at%, or 14.5 and 2.46 wt%, respectively for feasible applications. Attractive COMMUNICATIONS 948

Superplastic behavior of Al-coated Mg alloy sheet

“…However, the testing temperature, 573 K, for the present alloy was only about 0.62T m of pure Mg, much lower than the eutectic temperature of a Mg-Al binary alloy system, 710 K [39]. Thus, there is little probability of the viscous flow resulted from incipient melting in the present alloy.…”

“…Formation of the filaments is known as one of the micro-superplasticity phenomena [28]. This phenomenon was observed in a number of superplastically deformed materials [28][29][30][31][32][33][34][35][36][37][38][39][40][41].…”

“…The micro-superplasticity was suggested to correlate with several possible mechanisms: (1) viscous flow due to the existence of liquid-like or semi-liquid phase [29][30][31][32][33][34][35][36][37]; (2) single crystalline plasticity [38,39]; (3) severe elongation of cavities under the tensile stress [40]; (4) diffusion creep and the filament growth during the superplastic deformation [41]. However, the testing temperature, 573 K, for the present alloy was only about 0.62T m of pure Mg, much lower than the eutectic temperature of a Mg-Al binary alloy system, 710 K [39].…”

A novel process for fabricating electrical contact SnO2/Ag composites by reciprocating extrusion

“…Twenty-pass extrusion was sufficient to homogeneously disperse the SnO 2 particles in the continuous Ag matrix, as shown in Figure 4(d). In traditional powder metallurgy techniques, it is hard to prevent particle agglomeration when the size of the reinforcing particles is less than 2 m, [14] while in the present method using reciprocating extrusion, the SnO 2 particles with a size of 0.1 to 1.0 m were uniformly distributed within the silver matrix.…”

Microstructural evolution and superplasticity of Al-5.8Mg-0.23Mn alloys processed by reciprocating extrusion

“…Small work pieces are preferred for the same reason. To overcome these drawbacks, the proposed method, "Reciprocating Extrusion," was developed not only to directly process conventional billets of either low or high strength, but also to refine the microstructure and improve the mechanical properties, [12][13][14][15][16][17][18][19] since the method operates at a high processing temperature near the recrystallization temperature and with a high plastic strain in each extrusion pass to induce dynamic recrystallization.…”