AlxCoCrFeNi (0 ≤ x ≤ 2)高熵合金之導熱、熱膨脹及導電研究

Thermal conductivity, thermal expansion and electrical conductivity in AlxCoCrFeNi ((0 ≤ x ≤ 2) high-entropy alloys

周暄苹(HASH(0x1bf8bbf0))

指導教授 ： 張一熙 ； 陳瑞凱

國立清華大學/工學院/材料科學工程學系/碩士(2008年)

摘要

為了研究高熵合金的基礎性質，本實驗選用Al、Co、Cr、Fe與Ni等5個元素，以Al為變量配製非等莫耳AlxCoCrFeNi (0 ≤ x ≤ 2)高熵合金，探討微結構對於導熱(298 K ~ 573 K)，熱膨脹(298 K ~ 1073 K)及導電(298 K ~ 400 K)等性質的影響，並了解這些性質的相互關係及高熵合金內原子鍵結排列的情形。
XRD及SEM的結果顯示，Al變量x增加，使鑄造態合金從FCC結構(0 ≤ xFCC ≤ 0.375)逐漸轉為BCC結構(xBCC ≥ 0.875)，中間為兩相區(0.5 □ xduplex □ 0.75)。經均質化水淬後，兩相區的範圍擴大(0.375 < xduplex < 1.25)。由DSC結果可知均質化試片從室溫到773 K間沒有明顯相變化發生，微結構顯示此合金由於高熵效應而有較高的溶解度。
AlxCoCrFeNi合金的XRD峰值偏低顯示FCC及BCC晶格對XRD的漫射效應較大，電子與聲子間散射較多，相對於純金屬，具有較低的熱傳導係數及導電率。由導熱係數對導電率的比值可知，聲子對熱傳導的貢獻足以與電子的貢獻相匹配；晶格扭曲造成非簡諧震盪偏離中心位置的程度加大，使得溫度升高時晶格膨脹係數變大。
本研究顯示微結構對許多性質造成影響，Al的原子半徑比Co、Cr、Fe與Ni大了約14.4 %，Al變量增加，合金的XRD峰值降低，漫射效應增大，同時影響單相區及兩相區的合金性質。均質化水淬試片之熱傳導係數及導電率與Al含量的關係大致上分成三區，在單相FCC與單相BCC結構區時，分別皆隨Al增加而降低，主要受電子與晶格的散射效應影響，而兩相區相界多，載子在傳遞過程中比單相結構容易受到阻礙，合金熱傳導係數及導電率均下降。
鑄造態合金及均質化水淬態合金硬度皆隨Al含量增加而增加，顯示合金鍵結隨Al含量增加而增強，硬度與成分關係大致可分成單相FCC結構區、兩相區及單相BCC結構區。在單相區中硬度雖略有增加，但大抵維持常數；而在兩相區中，隨BCC比例增加，合金硬度幾乎呈線性增加。實驗亦顯示，均質化水淬態合金之BCC相硬度較鑄造態合金之BCC相硬度略微升高，應與均質化水淬態處理過程中，發生之離相分解(Spinodal decomposition)有關。
均質化水淬態合金之熱膨脹係數隨Al增加而降低，與鍵結強度有關，磁性效應與相變化互相作用，使熱膨脹係數在某一溫度區間產生下降峰。在單相區中隨Al量增多，居禮溫度些微增加(正比於分子場Molecular field強度)。在兩相區中，居禮溫度則有一最低值。

關鍵字

熱傳導 ； 電阻率 ； 熱膨脹 ； 合金

並列摘要

In order to investigate fundamental properties of high-entropy alloys, this study has selected 5 elements, such as Al, Co, Cr, Fe and Ni, to prepare various AlxCoCrFeNi alloys for 0 □ x □ 2, and to explore the effects of microstructure on the heat conductivity, thermal expansion and electrical conductivity of the alloys in the temperature ranges of 298 – 573 K, 298 – 1073 K, and 298 – 400 K, respectively. Besides, the relation between properties and atomic bonding among elements in the alloys is also explored in this study.
Experimental results from XRD and SEM show that the microstructure of these alloys is single FCC, duplex FCC + BCC, and single BCC for as-cast alloys (C-alloys) in the composition ranges of 0 □ xFCC □ 0.375, 0.5 □ xduplex □ 0.75, and xBCC □ 0.875, respectively, while the duplex phase area xduplex extends from 0.375 to 1.25 and xBCC □ 1.25 for the homogenized and quenched alloys (H-alloys). DSC analyses show no evidences of phase transformation in the temperature range of 298 to 773 K, indicating that the high-entropy effect makes the alloys more soluble among components in the alloys.
The XRD peak intensities for H-alloys are lower than those of the pure component elements indicates that FCC and BCC structures in H-alloys have larger scattering effect for x-ray diffraction and more electron-phonon scattering and hence H-alloys have lower thermal and electrical conductivity. The ratio of thermal conductivity to electrical conductivity shows the contribution of phonon is comparable to that of electron in thermal conductivity. The anharmonic oscillation for atoms due to lattice distortion is large. As temperature increases the thermal expansion coefficient increases accordingly.
This study shows various aspects of microstructural influence on the properties of the alloys. Since the atomic radius of Al is approximately 14.4 % greater than the radii of Co, Cr, Fe and Ni, the increasing amount of Al addition to the alloys decreases the XRD intensities of the alloys. This in turn increases the x-ray scattering in the alloys and influences properties of both single phases and duplex phase of the alloys. The relation of both thermal conductivity and electrical conductivity as a function of the amount of Al addition is seen to divide in three regimes just as that in the case of microstructure, i.e., FCC, FCC + BCC, BCC regimes. In both single-phase regimes, both thermal conductivity and electrical conductivity decrease as the amount of Al, x, increases. In duplex FCC/BCC regime both thermal conductivity and electrical conductivity are smaller than those in single-phase regimes. This is because of the additional scattering effect of FCC-BCC phase boundaries.
Hardness increases monotonically with x for both C- and H-alloys, indicating that the atomic bonding strength also increases with x. In both single-phase regimes although the hardness increases slightly with x, the hardness keeps roughly constant in single-phase regimes, while the hardness of alloys in the duplex regime increases linearly with x. In BCC regime the hardness for H-alloys is slightly higher than that for C-alloys. This is attributed to the spinodal decomposition during homogenization of H-alloys at 1100 oC.
The thermal expansion coefficient of H-alloys decreases with x is also attributed to the increase in bond strength as x increases. There are two phase-transformation temperatures for H-alloys in both thermal expansion measurements and DTA analyses. One is for ferromagnetic-to-paramagnetic transition (i.e., Curie temperature,) the other is for □–NiCoCr precipitation that is characterized by HTXRD and DTA. Curie temperature for single-phase H-alloys increases slightly with x, while there is a lower point for Curie temperature for duplex H-alloys. Since Curie temperature is proportional to molecular field in the Weiss theory of magnetism, the molecular field is also closely related to bond strength and microstructure of the alloys.

並列關鍵字

HASH(0x1bf8adf0) ； HASH(0x1bf8ae90) ； HASH(0x1bf8af30) ； HASH(0x1bf8afd0)

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