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

Electrical and Magnetic Properties of 5-to 6-Component High-Entropy Alloys Made from Al, Co, Cr, Fe, Ni, and Ti

林川翔(Chuan-Hsiang Lin)

指導教授 ： 黃金花 ； 陳瑞凱

國立清華大學/工學院/材料科學工程學系/碩士(2008年)

https://doi.org/10.6843/NTHU.2008.00460

摘要

本研究從Al, Co, Cr, Fe, Ni, Ti中，選取五元及六元，製造高熵合金。經過真空電弧熔煉，形成七種鑄造態塊材。再經1100 °C持溫24小時後水淬，成為七種均質態試片。之後，即進行這些鑄造態及均質態微結構及其低溫導電與導磁性質之探討。由室溫之XRD, SEM, EDS與硬度量測，以及5 K ~ 300 K之SQUID量測ZFC-FC曲線和四點探針方法量測電阻對溫度的關係，得到了以下的結論：
部分鑄造態及均質態試片出現Heusler介金屬相。經歸納得出，當Al與Ti同時存在試片中時，因與其他元素之兩兩元素之間結合焓較負，容易以強鍵結方式，形成Heusler介金屬相。又，若欲在高熵合金中形成Heusler相，推測須滿足，合金系統至少需有兩種過渡金屬、與Heusler相之結合焓須足夠負等多項條件才行。
本研究的高熵合金硬度均高於500 Hv，此特徵與微結構有關。其中，以含高硬度HCP相的鑄造態CoCrFeNiTi合金的硬度，接近900 Hv為最高。大部分均質態試片，硬度均較鑄造態的低。
本研究的14種等莫耳高熵合金的電阻率均比傳統合金的高，此現象，應與高熵合金嚴重晶格扭曲，導電載子有較高漫射效應有關。低溫之大部分試片有類近藤效應（Kondo-like effect），且近藤溫度（TK）較以往研究過的高熵合金的為高。本研究高熵合金之電阻率溫度係數（Temperature coefficient of resistivity, TCR）較現今常用之電阻材料的還低，此特性也與高熵合金嚴重晶格扭曲，聲子與電阻率對溫度的敏感度降低有關。
本研究合金有鐵磁、順磁和反鐵磁等。順磁合金透過平均原子量之假設，即可利用Langevin順磁理論，求得與實驗值吻合的結果。

關鍵字

高熵合金 ； 低溫 ； 電性質 ； 磁性質 ； Heusler合金 ； AlCoCrFeNiTi

並列摘要

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.

並列關鍵字

HASH(0x1bf3da90) ； HASH(0x1bf3db30) ； HASH(0x1bf3dbd0) ； HASH(0x1bf3dc70) ； HASH(0x1bf3dd10) ； HASH(0x1bf3ddb0)

參考文獻

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