Mechanical alloying (MA) is an effective method for obtaining metastable phases, including quasicrystals (QC). In this paper, the formation of icosahedral quasicrystalline phase (i-phase) in Al65Cu25Fe15 alloy by MA is investigated.
The initial powders of Al, Cu and Fe with high purity were mixed in stoichiometric ratio and subjected to mechanical alloying in a planetary ball mill in an argon atmosphere. The structural changes occurring during the ML process were monitored using X-ray diffraction (XRD). The morphology of the powder particles was studied by scanning electron microscopy (SEM).
The powder mixture consisting of elements with a given ratio of Al65Cu20Fe15 was subjected to a mechanical milling process for 8 hours. Then, the obtained powders were subjected to heat treatment at different temperature conditions ranging from 600 to 800 °C for 4 hours. A set of methods including X-ray diffractometry, differential scanning calorimetry, as well as transmission and scanning electron microscopy were used to analyze the microstructure of the ground powders both before and after annealing. The icosahedral quasicrystalline (QC) phase was not formed immediately after the milling process. The formation of the QC phase was achieved from the unmilled powder by annealing at a temperature exceeding 700 °C for 4 hours. However, a combination of mechanical milling and subsequent annealing can promote the formation of the QC phase at lower temperatures, starting from 600 °C.
Quasicrystals (QC) are a special state of matter that differs from classical crystalline and amorphous structures. They are characterized by long-range orientational order, but without translational symmetry. QCs have such valuable properties as high hardness and rigidity, low electrical and thermal conductivity, and low surface energy. These properties make QCs promising for use in catalysis and as coatings. Icosahedral QCs in the ternary Al–Cu–Fe system attract attention due to the ease of their preparation, non-toxicity, and thermal stability. Mechanical alloying (MA) is an effective method for the synthesis of QCS, allowing the production of significant volumes of powder for subsequent use. However, mechanical milling of elemental AlCuFe powder does not always lead to the formation of the QC phase. Subsequent annealing is often required to synthesize a stable QC phase, since quasicrystals must have a high degree of perfection of the crystal lattice. The aim of this work is to investigate the influence of the combined action of mechanical processing and heat treatment on the formation of quasicrystals in Al65Cu20Fe15.
X-ray patterns obtained at different stages of ML showed a gradual broadening of peaks corresponding to the initial elements, which indicates grain refinement and an increase in the defectiveness of the crystal lattice. After a certain ML time, diffuse peaks characteristic of the amorphous state appeared on the X-ray patterns. Further increase in ML time led to crystallization of the amorphous phase and formation of the i-phase.
SEM images showed that the ML process changes the morphology of powder particles from lamellar to spherical. It was found that the formation of the i-phase in the Al65Cu25Fe15 alloy by ML occurs through the amorphization stage. The obtained results demonstrate the efficiency of ML as a method for synthesizing quasi-crystalline materials.
Author:. Yong, IT Chang, IP Jones
Institute: Department of Metallurgy and Materials Science, Faculty of Engineering, University of Birmingham, Birmingham B15 2TT, UK