Variation in mechanical properties with MnO2 content in cast and forged in-situ Al-8Mg-MnO2 composites

Particulate Al-8Mg-MnO2 composite is synthesized using stir cast method by adding 3, 5 and 8 wt% of MnO2 powder in Al-8Mg melt and subsequently hot forged at 400 °C. It shows intermetallic precipitates containing Al, Mg and Mn at dendrite boundaries and embedded alumina and MnO2 particles in the matrix. A significant improvement in hardness is observed as the percentage of MnO2 particles increases from 3 to 8%. Similar trend is noticed in the forged samples. An increase in tensile and yield strength is also there as the percentage of MnO2 increases in cast composite. Forging has enhanced the yield and tensile strength further keeping the trend similar to that of as cast composite. Increase in MnO2 percentage has adverse impact on percentage elongation. Forged samples also show a similar trend of decrease in percentage elongation. Whereas forging has improved the initial fracture toughness JIC. There is a decrease in JIC when the percentage of particles increases in both the cast and the forged composites. 3 wt% MnO2 in composite gives significant advantage in growth toughness without impairing initiation fracture toughness over that of the alloy. Increase in particle addition beyond 3 wt% of MnO2 decreases both the initiation fracture toughness and the growth toughness rapidly.