Study of the Isochronal Annealing of High Pressure Die-Cast Magnesium Alloy AZ91 by Positron Annihilation Lifetime Technique

Abstract

Key words: Positron lifetime; isochronal annealing; AZ91 alloy; microhardness; microstructure INTODUCTION Magnesium alloys (light alloys) are of increasing attention for transport applications in the automotive and aerospace industry. The most popularly used magnesium alloys are those based essentially on the Mg-Al system, such as AZ91, AM60B and AM50A. In addition, the poor elevatedtemperature properties of low-cost magnesium alloys have now become a critical issue for widespread applications of magnesium alloys [1] . Several investigations have been performed using positron annihilation lifetime (PAL) for studying defects in magnesium and magnesium alloys [2-6]. Positron Annihilation Lifetime (PAL) is a specific technique for the detection of open volume defects such as vacancies, vacancy clusters, dislocations, grain boundaries in materials (metals and alloys) and free volumes for polymers Mg, 9%Al -and 1%Zn (AZ91) alloy is the most widely used commercial Magnesium alloy and has a good combination of castability, mechanical strength and ductility. Currently, AZ91 alloy is used mainly in the high pressure die-casting (HPDC) form for structural components in the automobile industry. HPDC is a well established process, which is of high efficiency and low cost. However, the HPDC components contain a substantial amount of porosity due to gas entrapment during die-filling and hot tearing during the solidification in the die cavity. Such porosity not only affects mechanical properties, but also denies the opportunity for property enhancement by subsequent heat treatmen