Mechanical alloying (MA) has become a generally accepted method for producing metastable alloys and composites, including quasicrystalline (QC) materials. In Al-Cu-(Fe, Cr) alloys obtained by MA, subsequent high-temperature treatment stimulates the growth of QC grains, significantly affecting the microstructure and properties of the material.
Initially, during the ML process, powder mixtures are subjected to intense deformation and welding, which leads to the formation of nanostructured composites. Subsequent high-temperature heating provides the atoms with the necessary kinetic energy for diffusion and rearrangement, initiating the growth of the QC phase. At the initial stages of heating, small QC crystallites are formed, surrounded by an amorphous or nanocrystalline matrix. With increasing temperature and heating time, these crystallites grow, absorbing the surrounding matrix.
Despite the difficulties caused by peritectic reactions (separation of chemical elements in liquid and solid phases), single-phase quasicrystals are often created from melts using the Czochralski method and other approaches. Solid-phase reactions are used to partially solve these problems, allowing narrow regions of homogeneity to be achieved and, ultimately, a single-phase quasi-crystalline state to be obtained. However, solid-phase methods lead to the formation of granular intermetallics, and they have not yet succeeded in producing single-phase millimeter-sized quasicrystals.
The formation of a single-phase icosahedral quasicrystalline grain structure was also recorded in Al65Cu20Fe15 and Al65Cu20Cr15 alloys obtained by melt spinning or by conventional heat treatment of the solidified alloy.
Mechanical alloying (MA), as well as its combination with subsequent heat treatment, leads to the formation of quasi-crystalline phases. However, to compensate for incomplete mixing or contamination, it is necessary to carefully regulate the chemical composition of the initial powder mixtures, otherwise the final product, in addition to quasi-crystals, will contain a number of secondary phases with a similar chemical composition. When annealing mechanically alloyed Al65Cu23Fe12 powder at 800 °C, a single-phase icosahedral quasi-crystalline intermetallic was obtained.
The mechanism of CC grain growth in Al-Cu-(Fe, Cr) alloys after ML and heating includes the diffusion of aluminum, copper, iron, and chromium atoms through the grain boundary of the CC phase. The grain growth rate is determined by the temperature, heating time, alloy composition, and the degree of deformation achieved during the ML process. The presence of chromium as an alloying element has a stabilizing effect on the CC phase, slowing down its degradation at high temperatures.
A detailed study of the growth of QC grains allows us to optimize the parameters of ML and subsequent heat treatment to obtain Al-Cu-(Fe, Cr) alloys with specified microstructural characteristics and improved properties, which opens up prospects for their application in various fields of technology.
Author: AI Salimon, AA Stepashkin, VV Tcherdyntsev, LK Olifirov, MV Klyueva, SD Kaloshkin
Institute: National University of Science and Technology MISiS, Leninsky Prospekt, 4, 119049, Moscow, Russia