| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 96 === Al, Co, Cr, Fe, Ni, and Ti are used to make six 5-component and one 6-component high-entropy alloys in this study. As-cast state of these seven bulk alloys is from smelting them in a vacuum arc remelter, while as-homogenized state is to treat the as-cast bulk alloys in a furnace at 1100 °C for 24 h.
After the alloys are made, characterization, such as microstructure, electrical and magnetic properties of these 14 samples, is performed. According to results from room-temperature XRD, SEM, EDS and hard-ness measurements and results from 5 K ~ 300 K SQUID for ZFC and FC magnetization curves and 4-probe resistivity vs. temperature measure-ments, we obtain conclusions stated below.
Part of as-cast and as-homogenized samples emerges the Heusler intermetallic phase. It has induced that as both Al and Ti contained in the alloys, the total formation enthalpy counted all of the bonds between Al or Ti and one of other elements in the system (i.e., Co, Cr, Fe, and Ni) needs to be so negative that Heusler phase can be formed. That is, in this high-entropy alloy system, both conditions mentioned above suffice the formation of Heusler phase. In other words, one is the presence of both Al and Ti. The other is sufficient negative formation enthalpy.
Hardness for all alloys in this study is higher than 500 Hv. This is closely connected with the microstructure. As-cast CoCrFeNiTi, which contains hard HCP phase in microstructure, has the highest hardness of 900 Hv among all alloys in this study. Almost all as-homogenized alloys are harder than their as-cast counterparts.
The resistivity for all 14 equi-molar high-entropy alloys in this study is higher than that for conventional binary and ternary alloys. This phenomenon is believed to have something to do with heavy lattice distortion that causes the high diffuse scattering effect of charge carriers. Most samples at low temperatures show the Kondo-like effect. TK of the alloys in this study is higher than that of the high-entropy alloys in previous studies. The temperature coefficient of resistivity of the alloys in this study is the lowest among those have been found at the moment. This is also believed to be due to the heavy lattice distortion of the high-entropy alloys, which causes the lower sensitivity of phonon and resistivity with respect to temperature.
High-entropy alloys in this study show ferromagnetic, paramagnetic, and antiferromagnetic properties. By use of the assumption of “average atomic weight” (AAW) for a paramagnetic alloy in Langevin relation for magnetic susceptibility, one can have excellent fitting between Langevin relation and experimental results.
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