The effect of mechanical alloying (MA) and preliminary heat treatment (PHT) on phase transformations in Al–Cu–Fe compacts during subsequent microwave annealing (MWA) was studied. The initial powders of Al, Cu, and Fe were mixed in a stoichiometric ratio close to the composition of the quasicrystalline phase Al62Cu25.5Fe12.5 and subjected to mechanical alloying in a high-energy ball mill.
ML resulted in gradual particle refinement, formation of a nanocrystalline structure and partial dissolution of the elements in each other. X-ray diffraction (XRD) showed peak broadening indicating a decrease in crystallite size and an increase in microstrains. Powder morphology, examined by scanning electron microscopy (SEM), demonstrated the formation of agglomerates and lamellar structures.
Alloys consisting of aluminum, copper and iron are of considerable interest due to their availability, non-toxic composition and ability to form a quasi-crystalline phase. Due to a unique combination of properties such as a low coefficient of friction, increased hardness, thermal expansion comparable to metals, moderate thermal and electrical conductivity, these materials are widely used. Alloys of this group are capable of forming amorphous, crystalline and quasi-crystalline structures. Quasi-crystalline (QC) structure is most often observed in alloys based on Al, Zr, Zn, Mg, Ti, while Al-based alloys are considered the most popular due to the variety of methods for their production. Obtaining a quasi-crystalline phase in Al-Cu-Fe alloys has always been an important task for researchers, since quasicrystals demonstrate different physical, mechanical and chemical characteristics compared to crystalline materials. The unique property set of QCs makes them promising for a variety of applications including catalysts for steam reforming of methanol, coatings and strengthening additives for composite materials.
Among the various methods for obtaining such materials, mechanical alloying (ML), as a nonequilibrium method, allows creating structures with a uniform composition at relatively low temperatures. This method is characterized by its relative speed and is well suited for obtaining phases that are stable in a narrow range of compositions and at low temperatures. The occurrence of all reactions in the solid state allows avoiding the difficulties associated with solidification in traditional casting. In the process of mechanical alloying, the final structure of the system is affected by the initial composition of the components, the duration of grinding, heat treatment, grinding intensity and other parameters.
Some of the mechanically alloyed powders were subjected to preliminary heat treatment at temperatures from 400°C to 600°C to relieve internal stresses and partial crystallization. After PHT, the compacts were annealed in a MVP at various temperatures and holding times. The use of a MVP significantly reduced the annealing time compared to traditional methods.
XRD and transmission electron microscopy (TEM) analysis showed that the phase composition and microstructure of the compacts depend significantly on the parameters of ML, PHT, and annealing in the MVP. Optimum ML and PHT modes promoted the formation of a stable icosahedral phase (i-phase) during annealing in the MVP. It was found that PHT promotes the formation of i-phase nuclei, which then rapidly grow during microwave heating. Microwave treatment also leads to selective heating and diffusion, which accelerates phase transformations and improves the homogeneity of the microstructure.
Author: Vahid Aghaali, Touradj Ebadzadeh, Zahra Karimi, Asghar Kazemzadeh, Ehsan Marzbanrad
Institute: School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Narmak, 16844, Tehran, Iran, Materials and Energy Research Center, Imam Khomeini Blvd, Meshkindasht, P.O. Box 31787-316, Karaj, Iran, University of Waterloo, 200 University Ave W, Waterloo, Ontario, N2L 3G1, Canada