Impression Creep Characteristics of a Cast Mg Alloy
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INTRODUCTION
RECENTLY, magnesium alloys have received increasing attention due to a strong desire for reduction of weight and fuel emission. Magnesium offers a good combination of low density, high strength-to-density ratio, and good corrosion resistance. These properties along with low-cost processing via die casting have nominated magnesium as a potential competitor for metals such as aluminum, steel, and polymers in aerospace and automotive applications.[1] However, low mechanical properties at elevated temperatures, especially creep resistance, limit the use of magnesium alloys to applications at low temperatures.[2] Aluminum, as the most favorable alloying element, improves die castability and mechanical properties of magnesium at room temperature without significantly raising its density. As a result, the most commonly used magnesium alloys are those based on Mg-Al, such as AZ91 and AM60, which have covered about 90 pct of the total magnesium market for structural applications. A part of the added aluminum dissolves into the matrix and improves the mechanical properties of magnesium through solid solution hardening. For aluminum contents higher than 2 wt pct, a new b-Mg17Al12 phase could form as particles and precipitates, improving room-temperature mechanical properties of the alloys by hindering dislocation movements. Moreover, the Mg17Al12 phase has a low melting point of about 437 °C, which improves die castability. However, this phase softens readily at high temperatures (T > 130 °C), B. KONDORI, Graduate Student, and R. MAHMUDI, Professor, are with the School of Metallurgical and Materials Engineering, University of Tehran, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted January 7, 2009. Article published online June 16, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
and thus has deleterious effects on the creep properties of Mg-Al based alloys. Finding a way to suppress this drastic drop in mechanical properties would introduce new applications for these alloys in automotive and aerospace industries. Accordingly, improvement of mechanical properties, especially creep resistance of Mg-Al based alloys, has attracted a substantial amount of research work. Recent investigations on creep-resistant magnesium alloys have focused on determining deformation mechanisms,[3,4] studying the effects of alloying elements on the high-temperature deformation behavior,[5–9] and developing models for predictive purposes.[10] It has been found that reduction of Al content and formation of intermetallic phases having high thermal stability are effective ways to develop more creep-resistant Mg-Al alloys.[11] Therefore, the AM60 alloy (Mg-6 wt pct Al-0.3 wt pct Mn) with a lower Al content than the most commonly used AZ91 magnesium alloy offers acceptable room-temperature mechanical properties that can be considered as a suitable candidate for further studies on creep-resistant magnesium alloys. Despite a great number of investigations on magnesium alloys and their properties, there are limited data on high-te
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