Mechanical alloying (MA) is an effective method for synthesizing metastable phases, including quasicrystals (QC). In this paper, the formation process and stability of the icosahedral quasicrystalline phase (i-phase) in the Al-Cu-Fe-Si system by MA are investigated.
The initial powders of Al, Cu, Fe and Si were mixed in different proportions corresponding to the nominal composition of Al62Cu25Fe12Si1. The resulting mixture was mechanically processed in a planetary ball mill in an argon atmosphere. The phase composition and microstructure of the powders were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM).
The XRD results showed that after a certain ML time, the i-phase is formed in the powders. Increasing the ML time leads to an increase in the proportion of the i-phase and a decrease in the crystallite size. SEM images demonstrate that the ML process results in particle grinding and mixing at the nanoscale.
The thermal stability of the obtained i-phase was studied using differential scanning calorimetry (DSC). The results showed that the i-phase remains stable up to a certain temperature, after which it decomposes into equilibrium crystalline phases. The addition of Si helps to increase the thermal stability of the i-phase.
The effect of silicon (Si) addition on the formation of the quasicrystalline phase in Al-Cu-Fe-Si alloys produced by mechanical alloying was studied using X-ray diffraction and scanning and transmission electron microscopy. To assess the effect of the e/a ratio on the sequence of phase formation during milling, two compositions with 10 at.% Si content were selected: Al58.5Cu18Fe13.5Si10 (e/a = 1.98) and Al53.5Cu19.5Fe17Si10 (e/a = 1.75). In both alloys, after ten hours of milling, the quasicrystalline icosahedral phase (i-phase) was recorded, represented by nano-quasicrystals with a size of 10 to 20 nm.
The introduction of Si contributed to the stabilization of the quasicrystalline phase, making it predominant after long-term milling. This differed from the behavior of the reference ternary powder Al65Cu20Fe15, which, in addition to the quasicrystalline phase, contained the cubic phase β-Al(Cu, Fe). The thermal stability of the quasicrystalline phase in powders subjected to 10 hours of milling was assessed after annealing at 800 °C for 4 hours.
The i-phase was partially retained in the Al53.5Cu19.5Fe17Si10 powders and the Al65Cu20Fe15 reference powders (both with e/a=1.75), coexisting with the β-Al(Cu, Fe) and Al13Fe4 or α-Al55Si7Cu25.5Fe12 and Al2Fe3Si3 phases, respectively. In the case of the Al58.5Cu18Fe13.5Si10 powders (e/a = 1.98), annealing resulted in a complete transformation of the i-phase into the cubic α-Al55Si7Cu25.5Fe12.5 approximant with the formation of crystallites ranging in size from 100 to 300 nm.
Thus, ML is an effective method for synthesizing the i-phase in the Al-Cu-Fe-Si system. By varying the ML parameters and the composition of the initial mixture, it is possible to control the phase composition and microstructure of the resulting materials. The addition of Si helps to increase the thermal stability of the i-phase, which opens up prospects for its application in various fields.
Author: Mikołaj Mitka,Anna Goral,Lidia Lityńska-Dobrzyńska
Institute: Institute of Metallurgy and Materials Science, Polish Academy of Sciences, ul. Reymont 25, 30-059, Krakow, Poland