Correlation of Microstructure with Tensile, Creep and Corrosion Behaviour of Az91 Mg Alloy Fabricated by Three Different Casting Techniques

Abstract

In the present investigation an attempt has been made to correlate microstructure with tensile, creep and corrosion behaviour of the AZ91 Mg alloy produced by three different casting techniques. All the as-cast alloys consist of primary Mg (α-Mg) and β-Mg17Al12 phases. The volume fraction of the β-Mg17Al12 phase is the highest in the ingot casting (IC), intermediate in the squeeze-casting (SC) and the lowest in the high pressure die-casting (HPDC). The tensile tests are performed at ambient, 150 and 200°C temperatures. The best tensile properties are exhibited by the SC alloy at all the temperatures employed in the present investigation owing to the presence of negligible porosity and relatively finer grain size. Fracture surfaces of the broken tensile specimens reveal quasi-cleavage fracture. The creep tests are performed in compression at a stress of 70 MPa and temperature of 150ºC. The best and worst creep resistance are exhibited by the SC and HPDC alloys with the IC alloy exhibiting the intermediate creep resistance. Poor creep resistance of the HPDC alloy is attributed to the presence of relatively higher amount of porosity that allows easy crack initiation and growth during creep tests. On the contrary, presence of negligible porosity and continuous network of eutectic phase contributes to the superior creep resistance of the SC alloy. Electrochemical corrosion tests of all the alloys are carried out at ambient temperature in 0.5 wt.% NaCl aqueous solution with neutral pH. The best and worst corrosion resistance are exhibited by the HPDC and IC alloys. The superior corrosion resistance of the HPDC alloy is attributed to the relatively finer grain size and higher percentage of Al dissolved in α-Mg phase. Thus, the SC alloy exhibits the best tensile and creep properties with intermediate corrosion resistance among the alloys employed in the present investigation.