Structural and thermal properties of nanocrystalline Alx(SiFeCoNi)100-x medium entropy alloys

Abstract

Medium entropy alloys (MEAs) are pretty attractive for metallurgical engineers due to their distinctive crystal structures, microstructures and exceptional properties for different applications. Researchers are very keen to study aluminum-based alloys because these alloys find extensive applications in aerospace, space, and engine block applications, etc. This research work reports the design of aluminum-based non-equiatomic MEAs for the first time. Alx(SiFeCoNi)100-x {x = 30, 40, 50} MEAs and their complementary equiatomic quinary AlSiFeCoNi high entropy alloy (HEA) was synthesized via mechanical alloying procedure. In order to compare the structural properties of Alx(SiFeCoNi)100-x MEAs and their complementary AlSiFeCoNi HEA, the microstructure was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystal structure was studied by x-ray diffraction (XRD) and scattered area electron diffraction (SAED) patterns. Moreover, a correlation between experimental results of crystal structure and that predicted by thermodynamic analysis was also studied. SEM and elemental mapping show irregular morphology in 60 h milled alloy powder particles and a complete as well as homogeneous mixing of all constituent elements. TEM shows that 60 h milled alloy powders are agglomerates of nanocrystalline particles. XRD results reveal that all the alloys exhibit the superb capacity to form single phase solid solutions upon mechanical alloying for 60 hours. Moreover, scattered area electron diffraction (SAED) patterns identify that crystal structure present in single-phase solid solutions are mainly simple body-centered cubic (BCC). Thermal stability of alloys assessed by differential scanning calorimetry (DSC) depicts that aluminum-based non-equiatomic Alx(SiFeCoNi)100-x MEAs are thermally more stable than complementary equiatomic quinary AlSiFeCoNi HEA.