多種五元及六元Al, Co, Cr, Fe, Ni與Ti高熵合金之霍爾效應及相關電與磁性質研究

Hall effect and related electrical and magnetic properties in various quinary and senary high-entropy alloys selected from Al, Co, Cr, Fe, Ni, and Ti

朱柏州(Po-Chou Chu)

指導教授 ： 陳瑞凱 ； 葉均蔚

國立清華大學/工學院/材料科學工程學系/碩士(2009年)

摘要

本研究以真空電弧熔煉法，製備由Al, Co, Cr, Fe, Ni與Ti組成的，多種五元及六元等莫合與非等莫耳高熵合金。所得到的鑄造態合金經過1100℃持溫24小時，均質化水淬，得到均質化態合金。鑄造態合金及均質化態合金，作微結構、熱膨脹係數及磁場大至9 T的霍爾效應量測。微結構主要由FCC, BCC, Full Heusler與Sigma相組成，大部分具有磁性。均質化態合金的磁性Invar效應，使合金熱膨脹係數在居禮溫度處下降，此乃磁矩排列由有序變成無序的結果。等莫耳均質化態合金居禮溫度在480 K到750 K之間，非等莫耳均質化態合金居禮溫度在780 K到870 K之間。
本研究許多合金出現異常霍爾效應，與各約一半合金出現類電子與類電洞行為，5 K及300 K載子濃度在10^22 ~ 10^23 cm^-3之間，與純金屬和傳統合金的相似。載子遷移率為0.21 ~ 2.61 cm^2 V^-1 s^-1之間，比純金屬與傳統合金的低，與多元素混合產生的嚴重晶格缺陷，增加電子散射效應，阻礙電子的移動有關。自由電子模型有效自由電子數即價數，在0.171至4.716之間。等莫耳均質化態合金殘餘電阻率比值(RRR)分布在0.988至1.259之間，在1.032至1.056與1.139至1.207兩區出現正霍爾係數，其餘為負。等莫耳均質化態合金殘餘電阻率比值(RRR)分布在1.114至1.174之間，在1.158至1.173區間出現正霍爾係數，其餘為負。霍爾係數的正負值，被解讀成與聲子和晶格缺陷共同作用，導致Brillouin 區與Fermi區形狀隨溫度變化有關。
高熵合金具有簡單的晶體結構，故滿足Bloch定律，可以平均效應的概念，修正並應用Kronig-Penney模型於高熵合金中。

關鍵字

高熵合金 ； 霍爾效應

並列摘要

In this study, selected from Al, Co, Cr, Fe, Ni, and Ti, various quinary equal- and non-equal-mole and senary high-entropy alloys (HEAs) are made by vacuum-arc melting. The as-cast alloys are sequentially homogenized at 1100℃ for 24 h and water-quenched to obtain as-homogenized alloys. Microstructure examination, coefficient of thermal expansion (CTE) measurement, and Hall effect up to 9T are performed. Microstructure is composed of FCC, BCC, full Heusler, and sigma phases. Most alloys are of magnetic. CTE of as-homogenized alloys drops at Curie point due to magnetic order-disorder transition, the so-called magnetic Invar effect. Curie points thus obtained are in the intervals of 480 K to 750 K and 780 K to 870 K for equal- and non-equal-mole as-homogenized alloys, respectively.
Anomalous Hall effect occurs in most alloys, and electron-like and hole-like electrical conduction behaviors emerge in 50 %-50 % ratio of alloys. Similar to their pure elements and conventional alloy counterparts, carrier concentration of HEAs in this study at 5 K and 300 K varies from 10^22 to 10^23 cm^-3, while lower than their pure elements and conventional alloy counterparts, mobility of carriers varies from 0.21 to 2.61 cm^2 V^-1 s^-1, due to multielement mixing-caused heavy imperfection in alloy lattices, making strong electron scattering effect and impeding the electron transport. The effective number of free electrons in the free electron model is the valence number, ranging from 0.171 to 4.716. The residual electrical resistivity ratio (RRR) for equal-mole and non-equal-mole alloys ranges from 0.988 to 1.259, and from 1.114 to 1.174, respectively. The sign of Hall coefficient is positive for RRR intervals of 1.032 to 1.056 and 1.139 to 1.207 for equal-mole alloys, while it is positive for RRR in the interval of 1.158 to 1.173 for non-equal-mole AlxCoCrFeNi alloys. Otherwise, it has negative sign. The sign of Hal coefficient is explained t have something to do with the interaction between phonon and lattice defects that determines the shape of temperature-dependent Brillouin zone and the Fermi surface.
HEAs have simple crystal structure, thus they obey the Bloch theorem. One can apply a modified Kronig-Penney model t them with an idea of “average” effect in potential.

並列關鍵字

high-entropy alloys ； hall effect ； Heusler ； AlCoCrFeNiTi

參考文獻

[4] 林川翔, "從Al, Co, Cr, Fe, Ni, Ti選取五至六元高熵合金之電與磁性質研究," 國立清華大學材料科學與工程學所碩士論文 (2008)

[3] 陳廷傑, "簡單相高熵合金AlxCoCrFeNi(0≦x≦2)之電性質研究," 國立清華大學材料科學與工程學所碩士論文 (2006)

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