This paper presents the results of a study of the microstructure and thermal stability of an aluminum alloy Al–5,7 Cr–2,5 Fe–1,3 Ti (wt.%) obtained by powder metallurgy. The aim of the study was to identify the effect of the alloying elements Cr, Fe, and Ti on the structure and thermal stability of the alloy at elevated temperatures. As a result of the conducted studies, the main phases formed in the alloy were determined, as well as their morphology and distribution were studied. Thermal stability was evaluated by annealing at temperatures up to 500 °C, followed by analysis of changes in microstructure and hardness. The results obtained allow us to evaluate the potential of using this alloy under conditions of high temperatures and mechanical loads.
Aluminum alloys are widely used in various industries, including aviation, automotive and mechanical engineering, due to their high specific strength and lightness characteristics. To increase the heat resistance and improve the mechanical properties of aluminum alloys, additives of transition metals such as chromium, iron and titanium are often used. The powder metallurgy (PM) method makes it possible to obtain alloys with a more uniform microstructure and a high degree of alloying, which is difficult to achieve with traditional casting methods. Alloys obtained by the PM method can show excellent characteristics at high temperatures, due to the formation of fine hardening phases. Studying the structural characteristics and thermal stability of such alloys is important for optimizing their composition and processing modes in order to achieve optimal performance properties.
Alloy Al-5,7 Cr-2,5 Fe-1,3 Ti (wt.% ) was obtained by powder metallurgy. Elemental powders of aluminum, chromium, iron, and titanium were mixed in a planetary ball mill. The powder mixture was pressed into cylindrical blanks. Then the blanks were subjected to sintering in a vacuum furnace at a temperature close to the melting point of aluminum. Further, the sintered samples were subjected to hot extrusion to achieve maximum density and grind the structure.
The microstructure of the alloy was studied by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). X-ray phase analysis (XRD) was used to identify the phases. The hardness of the samples was measured by the Vickers method. The thermal stability of the alloy was evaluated by annealing at 300 °C, 400 °C, and 500 °C for 100 hours, followed by microstructure and hardness analysis.
Studies of the microstructure of the initial alloy obtained by the PM method showed the presence of small, evenly distributed secondary phases in the aluminum matrix. SEM and TEM data showed that these phases have a complex composition containing Cr, Fe, and Ti. X-ray diffraction revealed the presence of intermetallic compounds of the Alx(Cr,Fe,Ti)y type. Thanks to the PM method and subsequent hot extrusion, it was possible to achieve a high degree of grinding of the structure and a uniform distribution of strengthening phases.
After annealing at 300 °C, no significant changes in the microstructure were observed. However, as the annealing temperature increased to 400 °C and 500 °C, some secondary phases were enlarged. At the same time, the alloy hardness decreased slightly after annealing at 400 °C and more noticeably after annealing at 500 °C. An increase in the size of secondary phases at high temperatures leads to a decrease in their strengthening effect and, as a consequence, to a decrease in hardness. TEM data showed that at annealing temperatures above 400 °C, small particles coalesce, which leads to the formation of larger, less effective hardeners.
Initially, the alloy contained fine intermetallides of the Alx(Cr,Fe,Ti)typey. During annealing, their morphology and composition changed. X-ray diffraction analysis has shown that at high annealing temperatures, new phases can form or the stoichiometry of existing ones can change. The exact composition of these phases requires further investigation using analytical microscopy techniques. However, the general trend is that during annealing, the alloying elements are redistributed and the particles are enlarged, which leads to a decrease in the thermal stability of the alloy.
Studies have shown that the Al–5,7 Cr–2,5 Fe–1,3 Ti alloy obtained by powder metallurgy has a finely dispersed structure with uniformly distributed secondary phases containing Cr, Fe, and Ti. The thermal stability of the alloy decreases during annealing at temperatures above 400 °C, which is associated with the coarsening of the strengthening phases and, possibly,with a decrease in the strength of the with a change in their phase composition. Despite the decrease in hardness at high temperatures, the alloy shows a fairly high stability, which allows us to consider it as a promising material for use in conditions of elevated temperatures. Further studies are needed to optimize the technological modes of alloy manufacturing and increase its thermal stability by introducing additional alloying elements or using other processing methods.
Author: D. Vojtěch, A. Michalcová, J. Pilch, P. Šittner b, J. Šerák a, P. Novák
Institute:Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republic, Institute of Physics ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic