A192 تاثير عمليات حرارتي بر ريز ساختار و خواص مكانيكي آلياژ

EFFECT OF HEAT TREATMENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AZ91 ALLOY

Variations of microstructure and tensile mechanical properties in AZ91 Alloy (Mg-9Wt.%Al-1Wt.%uZn) have been investigated under natural (T4) and artificial ageing (T6) conditions. The temperature of artificial ageing (T6) is 165°C. Hardness and yield strength increase and elongation decreases with age- ing. Also, ultimate tensile strength is increased by ageing but decreases during the initial times of T6 heat treatment (below 40h) compared with T4 conditions. The microstructure was studied by OM and SEM. Precipitation in this alloy occurs in two forms: continuous precipitates that nucleate and grow in grains and discontinuous precipitates that form in grain boundaries. During ageing, the length of continuous precipitation increases, therefore, interprecipitate distance decreases. Formation of discontinuous precipitates causes initiation and propagation cracks in grain boundaries and weakens boundaries. This is the principal reason behind ultimate strength reduction during the primary times of T6 heat treatment in comparison with the T4 condition.