Microstructural characterisation and mechanical properties of an extruded nanoquasicrystalline aluminium alloy

• Main Author: Hameed, Asad
• Other Authors: Audebert, Fernando ; Galano, Marina
• Published: University of Oxford 2016
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.730079

Conventional aluminium alloys rapidly lose mechanical strength above 150°C, limiting their elevated temperature usage in engineering applications. The rapidly solidified (RS) nanoquasicrystalline (NQX) Al₉₃Fe₃Cr₂Ti₂ alloy has the potential to be used at elevated temperatures (250-350°C) as it retains high strength at those temperatures. There have been some microstructural studies and limited work on the mechanical properties of the alloy. However, a detailed study of both the microstructure and the mechanical properties (elastic constants, tensile behaviour, and fatigue behaviour) has not been carried out for the bulk alloy. RS NQX Al₉₃Fe₃Cr₂Ti₂ alloy powder in 25-50μm fraction size was consolidated using extrusion to produce bars. The microstructure of the extruded bars was characterised using X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, scanning transmission electron microscopy, and electron backscattered diffraction. The microstructure was observed to contain approximately 44% volume fraction of nano size quasicrystalline icosahedral (I-phase) particles embedded in submicron size aluminium grains. The alloy bar samples were annealed at elevated temperatures (250, 300, 350, 400, and, 450°C) for long hours (100h) and the microstructural evolution was studied using aforementioned characterisation techniques. As a result of the annealing at 350°C for 100h, the I-phase starts to transform into other stable and metastable intermetallics which grow with the increase in the annealing temperature. Room temperature mechanical properties such as hardness, Young's modulus, and shear modulus were measured, and tensile tests for the alloy samples were carried out in the 'as extruded' and 'annealed' conditions. The alloy retains its microstructure and mechanical properties after annealing at 300°C for 100hrs. The alloy results are exceptional, as conventional Al alloys suffer >45% strength loss after the same heat treatment. Orowan and Hall-Petch mechanisms were found the dominant strengthening mechanism in the alloy at room temperature. Tensile tests were performed at 23°C as well as elevated temperature (150-500°C) at four strain rates (10^-2 to 10^-5 s^-1). Tensile strength values at ambient temperature are comparable to the high strength conventional Al alloys; however, elevated temperatures (~300°C) tensile strength of the NQX Al₉₃Fe₃Cr₂Ti₂ alloy is 3 to 8 times higher than conventional Al alloys. High values of the apparent stress exponent and the apparent activation energy of plastic flow were observed for all test temperatures. At intermediate temperatures (150-250°C), several manifestations of dynamic strain ageing were observed in the alloy. Plastic flow models were used to explain the elevated temperature behaviour, and it was deduced that 250-350°C temperature range experienced a transition from power law break down to power lawcreep, whereas a climb controlled mechanism was operative at 425-500°C temperature range. Fatigue testing was carried out using conventional and ultrasonic frequencies at roomtemperature and only conventional frequency at 300°C. The alloy exhibits type II fatigue behaviour. The room temperature fatigue performance is comparable with the high strength conventional alloys, and far superior to any conventional Al alloy at 300°C.