簡單相高熵合金AlxCoCrFeNi (0 ≦ x ≦ 2) 之電性質研究

Electrical Properties of Simple-Phase High-Entropy AlxCoCrFeNi (0 ≦ x ≦ 2) Alloys

陳廷傑(Ting-Jie Chen)

指導教授 ： 葉均蔚 ； 陳瑞凱

國立清華大學/工學院/材料科學工程學系/碩士(2006年)

https://doi.org/10.6843/NTHU.2006.00484

摘要

先前的研究發現，高熵合金具有形成簡單固溶相的能力，以及獨特的電與磁性質。因此本研究利用電弧鎔鍊法鑄造 AlxCoCrFeNi (0 ≦ x ≦ 2) 結晶態塊材，並進行均質化與滾壓成型處理，來比較不同成分與不同處理狀況下的電性質。
從X射線繞射、掃描式電子顯微鏡和硬度量測的觀察中發現，此高熵合金的微結構會隨鋁的添加而逐漸從FCC相轉變成BCC相。在均質化過後，FCC和BCC兩相共存的區域會擴大，且在兩相共存區和BCC區中皆可觀察到因離相分解而產生的富鎳鋁的有序BCC相。
磁性量測發現均質化後的AlxCoCrFeNi高熵合金皆為軟磁性，且其飽和磁化量因高熵效應而降低。在5 K下的飽和磁化量可用微結構討論並解釋之，其居禮溫度也能被估計出來。此外，在低溫下，特定成分的AlxCoCrFeNi似乎存在「再進入自旋玻璃」狀態。
AlxCoCrFeNi皆具有相對高的電阻率，並且從晶格常數的比較中可以發現此合金具有較高程度的電子與聲子交互作用。造成高熵合金高電阻率的原因來自於高熵效應造成的高溶解度，使得晶格嚴重扭曲。在低溫下，電阻率表現兩種行為：殘留電阻率和類近藤電阻率。而藉由觀察電阻率曲線變化，可以了解聲子、磁性、和類近藤效應對各個溫度區間電阻率的影響。
霍爾效應量測中發現，均質化後的高熵合金隨成分的不同會出現電子或電洞兩種形式的導電載子。經過計算後，得知此合金的載子濃度介於每立方公分十的19次方到22次方之間，類似一般金屬或合金的載子濃度。高磁場下的低載子遷移率，則同樣也是嚴重晶格扭曲所致。本研究中的合金皆具有鐵磁性，因此在居禮溫度之下可以發現異常霍爾效應，且發現較強的異常霍爾效應伴隨著較高的飽和磁化量。異常霍爾效應的機制推測可能是side-jump effect。

關鍵字

高熵 ； 晶格扭曲 ； 電阻率 ； 磁性 ； 近藤效應 ； 霍爾效應

並列摘要

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.

並列關鍵字

high entropy ； lattice distortion ； resistivity ； magnetic ； Kondo effect ； Hall effect

參考文獻

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