Nanoquasicrystalline/Al2O3 nanocomposite: the ball milling process

F. AUDEBERT; M. GALANO; W. XU

Nara

Simposio; XXIII International Symposium on Metastable and NanoMaterials; 2016

Aluminium matrix nanocomposites have been of research interest for several years due to the high strength to weight ratio. The work described in this abstract aims at (1) manufacturing of homogeneously distributed nanocomposites maintaining the metastable quasicrystalline phase (2) analysing the effects of the quasicrystalline phase and nano size alumina on the microstructure (3) investigating the relationship between the microstructure and the mechanical property of the nanocomposites in powder form. A quasicrystalline nanocomposite consisting of pure Al93Fe3Cr2Ti2 (at.%) quasicrystalline alloy powder with 12.9vol.% alumina was processed by mechanical milling. The pure Al93Fe3Cr2Ti2 (at.%) quasicrystalline alloy powder was also ball milled under the same conditions to be able to compare the results obtained. 10g powder were ball milled at a time with ball to powder ratio of 10:1, milling time 3, 5, 10, 15, 20, 25, 30hs and ball milling speed 200-250rpm. The effects of mechanical milling on the microstructure and the alumina distribution were investigated by transmission electron microscope and focused ion beam. The crystallite size was determined by means of Williamson-Hall plot from X-ray diffraction patterns and the quasicrystal phase decomposition was studied using differential scanning calorimetry The Vickers microhardness of the powder was also measured. From the FIB images the composites milled with 250rpm can reach an even distribution after 10 hours of milling. When the milling speed was reduced to 200rpm, the composite needed 20 hours milling to reach the same distribution level. It is suggested that the whole milling process consisting in flattening, cold welding, and fracturing mechanisms are crucial in blending the alumina into the quasicrystalline matrix. However, the quasicrystalline phase is decomposed after xxx hs of milling of the pure alloy and after xxx hs of milling of the composite. The quasicrystal phase decomposition during the milling process is developed in two steps. It was also observed the Al crystallite size decreases with increasing the milling time. After 30 hours of milling the smallest Al crystallite sizes of the milled pure alloy and the nanocomposite are 20nm and 14nm respectively. For milling times longer than 10 hours, the alumina reinforced nanocomposite has smaller Al grain size than the milled pure alloy. The alumina improved the Al crystallite refinement after homogeneously mixed into the matrix. An equation model is built to estimate the microhardness of the milled composites based on the grain refinement and the rule of mixtures. The measured microhardness and the estimated microhardness of fifteen nanocomposites powder with milling time ranging between 5-30 hours and milling speed 200/250rpm were compared showing a good match of the estimated values using our model with the measured values. Finally, a nanocomposites consisting of a homogeneous alumina distribution in a nanoquasicrystalline matrix preserving the quasicrystal phase were obtained with different milling processing conditions. The nanocomposites obtained showed a very Vickers microhardness which can be explained with the Al crystalline size and rule of mixture.