Abstract
This work deals with the microstructural characterization of two equiatomic high-entropy, low-density alloys (HEA), the AlCrTiV and AlCrTiV-Si7.2. These alloys can serve as potential candidates for advanced applications where high strength and enhanced ductility is demanded. For ensuring high ductility the alloys must contain as minimum as possible hard precipitates. As the strength increase is based on both solid solution and precipitation hardening, the laboratory made alloys were investigated in as-cast and heat-treated conditions. For the heat treatment a high soaking temperature of 1200°C for 8 hours was selected to ensure microstructure homogenization. Micrographic observations of the AlCrTiV and AlCrTiV-Si7.2 samples in the as-cast condition indicated the presence of a dendritic microstructure. Furthermore, chemical micro-analysis showed segregation in the matrix in both samples. This is a critical result as this segregation will lead to heavy precipitation at interdendritic regions, it may sensitize these regions and in the worst-case scenario may cause cleavage fracture in the micro scale, which can trigger brittle fracture during cooling even without the application of deformation. However, the selected heat treatment eliminated the segregation phenomena forcing the alloying elements to be uniformly distributed in the matrix. At the center of the heat-treated AlCrTiV-Si7.2 sample the fragmentation and spheroidization of the intermetallic phase Ti5Si3 was observed. For the same sample, at the mold-sample’s interface, the particles Ti5Si3 were shown to dissolve and form aggregates. Both alloys exhibited high hardness values with small differences between the as-cast and heat-treated conditions, which indicates that the AlCrTiV–Si7.2 high entropy alloy presents high yield strength and may operate at high temperatures without deterioration of the mechanical properties nor unexpected failure.
Highlights
There are 2 widely used ways in order to process what a High-Entropy Alloy (HEA) is
In the respective heat-treated sample, the previously mentioned dendritic microstructure in the as-cast condition is replaced by coarse grains (Fig. 1)
Results from EDS analysis showcase the total elimination of segregation in the matrix (Fig. 2) and the average microhardness is slightly decreased at a value of 619 HV0.2, compared to the as-cast AlCrTiV
Summary
There are 2 widely used ways in order to process what a High-Entropy Alloy (HEA) is. The first theory, categorizes HEAs as alloys, which contain at least 5 main alloying elements varying from 5 to 35 at. % and potential additional elements below 5 at. %. The second one is based on the configurational entropy and accepts HEAs as alloys which have: ΔSୡ୭୬ > 1.5R Where: ΔSconf. Where: R = 8.314 J/K mol (gas constant) ci = atomic concentration of element i [1]. In the recent years a new category of lightweight high-entropy alloys (LWHEAs) has emerged with the aim of creating materials with high specific mechanical properties. The AlCrTiV and AlCrTiV-Si5 alloys are distinct examples of successful HEA alloy design, by combining characteristics from both the LWHEAs and refractory high-entropy alloys (RHEAs) categories [2]. The alloys produced in this study can be considered as MediumEntropy Alloys (MEAs)
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