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Abstract
The Effect of the solidification conditions and subsequent extrusion of a Mg-3.0Zn-0.5Y (at. %) alloy containing quasicrystalline icosahedral (i-) phase was studied. Solidification was carried out by three methods using a chill casting mold, a conventional steel mold and a water-cooled mold. Subsequently, castings were extruded in the temperature range of 235–270 ∘ C at an extrusion ratio of 25:1. The solidification molds showed different characteristics. The water-cooled mold was most effective in cooling through the walls, but least effective at the center of the mold. The conventional cast mold was the most effective in cooling at the mold center. All the castings had an interdendritic eutectic structure of the i-phase, and a supersaturation of the matrix in zinc. As a result, all the extrusions had similar grain size close to 1 μ m and very fine nano-size precipitation. Yield strengths in tension were in the range of 376 and 404 MPa, and from 300 to 330 MPa in compression. All elongations to fracture were about 13%. It is concluded that supersaturation of the matrix during solidification is the main factor, resulting in the dynamic precipitation of very fine precipitates and fine grain size during extrusion.
Highlights
Being the lightest of all metals for structural applications, magnesium and its alloys have a huge potential for making lighter structures with the aim of energy savings
At the center of the mold the fastest cooling was in case of the conventional mold, followed by chill casting
Three different types of mold were used to cast a Mg-3.0Zn-0.5Y alloy, and all the alloys were extruded at a temperature close to 250 ◦ C at an extrusion ratio of 25:1, into rods of 8 mm diameter
Summary
Being the lightest of all metals for structural applications, magnesium and its alloys have a huge potential for making lighter structures with the aim of energy savings. Of all the possible strengthening methods, grain refinement is especially effective in the case of magnesium and its alloys [1]. A convenient method of grain refinement is by dynamic recrystallization during wrought processing, such as rolling and extrusion. Wrought processing introduces a basal texture, due to the hexagonal crystal structure of the magnesium alloys, which introduces an anisotropy of mechanical properties. The very fine grained structure obtained by dynamic recrystallization enhances the strength of the alloy, but is characterized by a weaker texture. One of the widely studied alloy systems of magnesium is Mg-Zn-Y, which contains such interesting ternary phases as a long range ordered phase of magnesium along its c-axis (long period stacking ordered, or LPSO, Mg12 ZnY) and an icosahedral quasicrystal (i-phase , Mg3 Zn6 Y) [2,3]
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