以機械合金法探討高熵合金熱動力學之行為

Study on the Thermodynamics and Kinetics of High-entropy Alloys under Mechanical Alloying

陳育良(Yu-Liang 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/博士(2009年)

https://doi.org/10.6843/NTHU.2009.00380

摘要

本研究利用機械合金法來探討高熵合金的熱動力學行為。首先，利用機械合金法配置CuNiAlCoCrFeTiMo二元至八元合金，觀察在球磨過程中，合金粉末的結構變化。研究結果發現，在機械合金化的過程中元素之間有互相競爭的現象，有先後不同的合金化順序，藉由分析二至八元合金的XRD及EDS mapping，可以判斷出此合金系統中各元素的合金化順序為Al → Cu → Co → Ni → Fe → Ti → Cr → Mo。此外，配置反向元素添加順序的二至七元合金加以驗證，也確認相同的合金化順序。由冶金因素的探討，發現此順序跟元素熔點有很高的相關性，因為熔點關係到元素是否容易被延展破碎及分散，亦關係到擴散速率的快慢。
在相轉變方面，此系列合金的機械合金化與熔煉法製備的高熵合金一樣，會形成BCC和FCC的固溶相。屬傳統合金的二元和三元合金，即使經過六十小時的球磨仍維持結晶態；但四元至八元合金經過球磨先形成結晶固溶相而後再形成非晶相，其中八元合金則有Mo元素殘留。此一非晶化過程屬於Weeber和Bakker依二元合金觀察所歸納的第一類非晶化。此第一類非晶化的原因經探討乃由於元素數目增加、原子尺寸差及機械合金引入的大量變形效應所共同作用的結果。
將四至七元合金所得的非晶粉末作熱分析加熱及不同溫度退火，觀察穩定相的形成，並和熔煉法所得相結構作比較。結果顯示所有非晶粉末在100°C時開始有回復的現象，約250至280°C發生結晶化，在更高的溫度有第二相析出及晶粒成長發生。由於穩定態下的相仍為固溶相，所以再次肯定先前所提高熵促進固溶相的效應。對於非晶化的準則，發現Egami的準則可以延伸至高熵合金，得到合理的預測及解釋。然而，此些非晶合金升溫下並未呈現玻璃轉換溫度，證明Inoue 12%的尺寸差異仍是玻璃轉換溫度存在的必要條件。
除八元合金系統外，本研究亦利用機械合金法配置一組完全由HCP元素所組成的合金系統BeCoMgTi和BeCoMgTiZn，觀察是否和其他高熵合金一樣容易形成BCC或FCC的固溶相。結果顯示，此HCP合金系統在完全非晶化之前，並不會形成任何的結晶態固溶相或介金屬化合物，而屬於Weeber和Bakker所提出的第二類非晶化，亦即機械合金直接形成的是非晶固溶相。此迥異於前述八元系統的第一類非晶化。經探討發現介金屬化合物的抑制係由於元素間化學的適合性、高熵效應與球磨變形效應促進互溶的作用。而直接形成非晶固溶相乃由於此HCP合金系統具有比八元合金系統更大的原子尺寸差。

關鍵字

高熵合金 ； 機械合金 ； 熱力學 ； 動力學

並列摘要

In this thesis, we investigate the thermodynamic and kinetic behaviors of high-entropy alloys via mechanical alloying. At first, a series of Cu-Ni-Al-Co-Cr-Fe-Ti-Mo alloys from binary to octonary one were prepared to examine the structure evolution during mechanical alloying process. Results reveal the alloying competition between elements during mechanical alloying. That is, not all the elements are alloyed at the same time but in a specific sequence. By examining the XRD patterns and EDS mapping images of the binary to octonary alloys the alloying sequence of this octonary alloy system is determined as Al → Cu → Co → Ni → Fe → Ti → Cr → Mo. This alloying sequence is also confirmed by another series of alloys with inverse element adding sequence. The alloying rate correlates best with the melting point among metallurgical factors. The mechanism for this correlation is explained through the effect of melting point on solid-state diffusion rate and mechanical disintegration which are both critical for the final alloying.
As for the phase evolution, these alloys prepared by mechanical alloying also form BCC and FCC solid solution phases as that prepared by melting route. However, the binary and ternary alloys still remain crystalline structure even after milling for 60 h. The quaternary to octonary alloys finally transformed into amorphous structure after sufficient milling times. Only the octonary one has residual Mo after milling for 60 h due to its high melting point. The amorphization of these alloys is belong to type I as classified by Weeber and Bakker. The reasons include the increased number of elements, atomic size difference, and the continued lattice distortion imposed by milling.
The amorphous quaternary to septenary alloy powders were further examined by thermal analysis and with annealing to observe the phase transformation during heating. It shows that recovery begins at 100°C, crystallization occurs in the range of 250 to 280°C, new phase forms or grain growth occurs at even higher temperatures. Simple phases are found to exist in the equilibrium state. This confirms again that high entropy effect enhances the formation of solid solution phases. However, glass transition temperature as that found in bulk amorphous alloys is not observed for the present amorphous alloys. Egami’s criterion for amorphization based on topological instability concept can also be applied to high-entropy alloys to explain the amorphization tendency. However, even larger size difference as that required by Inoue’s rule is still crucial to the existence of glass transition temperature.
Furthermore, two equimolar alloys entirely composed of HCP elements were also prepared by mechanical alloying to investigate if they are also easy to form solid solution phases. However, no crystalline solid solutions and compounds form before full amorphization. The amorphization processes of these two alloys thus conform to type II. The inhibition of intermetallic compounds before amorphization is due to chemical compatibility among the constituent elements in company with high entropy effect and deformation effect which enhance the mutual solubility. Direct formation of the amorphous solid solution phase instead of the crystalline one attributes to their large range of atomic size.

並列關鍵字

high-entropy alloys ； mechanical alloying ； thermodynamics ； kinetics

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