Abstract
The development of high performance aluminium and magnesium based materials is a subject investigated within the European Commision project ExoMet, that started in the summer of 2012. Novel grain refining and nanoparticle addition in combination with physical melt treatment using external fields is expected to improve mechanical properties of aluminium and magnesium alloys in terms of strength and ductility, but also creep resistance. External fields applied during mixing are electromagnetic, ultrasonic, and mechanical. In this study, magnesium alloy Elektron21 was melted and AlN nanoparticles were added to the melt. The melt was mixed by simultaneously applying mechanical and ultrasonic stirring to avoid agglomeration. A modified permanent mould indirect chill casting process was selected for casting the materials, because this process results in very dense casting, free of pores and blowholes. The resulting materials were examined by metallography, electron microscopy, hardness, compression creep and mechanical compression strength methods. Microstructure investigation, including TEM, shows that most of the AlN-particles are pushed in the intermetallic phase (i.e. in interdendritic regions) during solidification, whereas few of them are trapped in the magnesium-matrix, not far from the intermetallic regions. Compression creep tests at a temperature of 240 °C and constant stresses between 70 and 200 MPa were performed and the minimum creep rate was determined from these tests. Comparison of creep rates of nanocomposite and unreinforced alloy Elektron21 show that the addition of 1 wt. -% AlN nanoparticles improves creep strength significantly. From the results stress exponents are calculated to obtain information about the rate controlling deformation mechanisms during creep.
