| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 94 === Electrical properties of simple-phase AlxCoCrFeNi (0 ≦ x ≦ 2) high-entropy alloys (HEAs) have been studied in cast, homogenized, and deformed conditions. All samples are tested by XRD, SEM, hardness, and electrical resistivity. Homogenized alloys are further tested by SQUID and Hall effect measurement. XRD patterns, SEM images, and hardness for both as-cast and as-homogenized samples show microstructures transform from FCC to BCC as x increases. After homogenization, the FCC + BCC transition region expands, and there is spinodal decomposition in both the transition region and BCC region that is revealed by the wall-like structures in these two regions. All BCC phase whatever in the transition or in BCC region contains an AlNi-rich phase with ordered BCC structure.
The results of SQUID indicate AlxCoCrFeNi HEAs are soft-ferromagnetic alloys, and reentrant spin glass state seems to exist at low temperatures. High-entropy effect disturbs ordering of spin and causes lower saturation magnetization. It also shows that Ms can be explained with obtained microstructures. Curie temperatures are determined and estimated for these alloys.
Relatively high electrical resistivities are measured and related to the high degree of electron-phonon interaction that is closely associated with lattice constants of the alloys. Relatively high residual resistivity can be explained by lattice distortion in HEAs. At low temperatures, there are two types of behavior; one is residual resistivity and the other Kondo-like resistivity. Contributions from various factors, such as phonon, magnon, and Kondo-like effect, can be determined via fitting curves.
Both carrier types, electrons and holes, appear in H-x, and the carrier concentration is in the order of 10 to the power of 19 ~ 22 per cubic centimeter. This means that the alloys in this study have the same carrier concentration as that of the conventional metals and alloys. The mobilities are small under high magnetic field because HEAs have high resistivity due to lattice distortion. Anomalous Hall effect (AHE) is seen below Curie temperature, and higher Ms leads to stronger AHE. The mechanism for AHE is probably the side-jump effect.
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