High-Temperature Mechanical Behavior of Extruded Mg-Y-Zn Alloy Containing LPSO Phases
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INTRODUCTION
ENERGY saving and material recyclability requirements have motivated the search for new magnesium alloys as structural materials on account of their low density and good mechanical properties. Powertrain applications, such as transmission cases and engine blocks, are a major growth area for magnesium alloys in the automotive sector. Such applications require good high-temperature performance at service conditions ranging from 423 K to 473 K (150 C to 200 C) and stresses ranging from 50 to 70 MPa.[1] As commercial magnesium alloy systems cannot deliver this required high-temperature performance, there is growing interest to develop new magnesium alloys with higher creep resistance at intermediate temperatures. A recent study of the creep resistance of a cast Mg97Y2Zn1 (at. pct) alloy showed promising creep properties in the required stress range compared with magnesium alloys based on complex Rare-earth element compositions.[2] The two-phase microstructure of this alloy is characterized by magnesium dendrites and a Long Period Stacking Ordered structure (LPSO) phase distributed in the interdendritic regions. The stress dependence of the creep rate presents two different regions. For low temperatures and/or high strain rates, ELVIRA ON˜ORBE, Ph.D., GERARDO GARCE´S and PABLO PE´REZ, Tenured Scientists, and PALOMA ADEVA, Professor, are with the Department of Physical Metallurgy, National Centre for Metallurgical Research, CENIM-CSIC, Av. Gregorio del Amo no. 8, 28040 Madrid, Spain. Contact e-mail: [email protected] FERDINAND DOBES, Professor, is with the Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zˇizˇkova 22, 616 62 Brno, Czech Republic. Manuscript submitted February 21, 2012. Article published online February 26, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
creep behavior shows a high stress exponent and high activation energy. The alloy behaves as a metal matrix composite, where the magnesium matrix transfers a certain stress to the LPSO phase which has a higher Young’s Modulus.[3–5] At high-temperature and/or low stresses, creep is controlled by nonbasal dislocation slip. During creep, the original 18R structure of the LPSO phase is transformed into a 14H structure. Furthermore, Y and Zn atoms diffuse in the magnesium grains and precipitate dynamically on the nonbasal dislocations, inhibiting their movement. In magnesium alloys, the phenomena observed during thermomechanical processes are mainly dynamic recrystallization and dynamic recovery, which occur simultaneously. This fact causes even the coexistence of different kind of microstructures along the extrusion profile with fully recrystallized and elongated unrecrystallized grains.[6–8] Recently, Yamasaki et al.[9] have developed extruded Mg97Y2Zn1 (alloys combining high yield stress (around 350 MPa) and appreciable ductility (around 8 pct) by controlling the extrusion microstructure, which is characterized by three different regions: fine dynamically recrystallized a-Mg grains with random orientation, hot wor
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