鈷鉻錳鎳體系高熵合金的實驗和計算熱力學研究

EXPERIMENTAL AND COMPUTATIONAL THERMODYNAMIC STUDIES OF CoCrMnNi, CoCrCuMnNi AND AlCoCrMnNi HIGH-ENTROPY ALLOYS

宇宙(K, Guruvidyathri)

指導教授 ： Murty, B. S. ； HariKumar, K. C. ； 葉均蔚

國立清華大學/工學院/材料科學工程學系/博士(2019年)

摘要

高熵合金（HEAs）是具有五種或更多種等量元素的新型材料。 HEA研究的一個主要挑戰是缺乏相穩定性信息。由於缺乏高階相圖，成分選擇是另一大挑戰。兩種主要類型的方法用於組合物選擇，兩者都具有某些挑戰。一種是簡單的相位預測方法，另一種是穩健的Calphad（PHAse Diagram的計算）方法。簡單的相位預測方法具有顯著的不准確性，並且沒有一種方法直接使用吉布斯能量。在Calphad方法中，不可用或不准確的Gibbs能量數據庫是問題。本研究試圖解決這些問題。
研究了CoCrMnNi，CoCrCuMnNi和AlCoCrMnNi高熵合金的相穩定性。將真空電弧熔化的樣品在1273K下熱處理24小時，然後在673,873和1073K下熱處理240小時。使用XRD和SEM表徵微結構。
為了檢查Calphad方法的成功，使用Thermo-Calc軟件中的TCHEA數據庫進行計算，並將結果與實驗微結構進行比較。 1273 K微結構與計算相匹配。在大多數長期熱處理情況下，在所有三種合金中都觀察到富含Cr，Co和Mn的σ相。計算在預測σ相穩定性方面不成功。為了解決這種差異，需要在Co-Cr，Cr-Mn，Co-Mn和Co-Cr-Mn系統中開發σ相的新吉布斯能量函數。熱力學評估用於此目的。輸入生成是通過文獻數據收集，算法計算和涉及平衡合金和擴散偶的實驗。在熱力學評估之後，新數據用於替換TCHEA數據庫中的相應功能。使用TCHEA（改進）數據庫進行計算。新數據成功地提供了更準確的結果，因為σ相出現在673 - 1073 K的計算中，與實驗相匹配。 TCHEA（改進的）數據將極大地提高Calphad對HEAs應用的準確性，例如使用高通量方法快速篩選有用的組合物。
除了相穩定性研究和改進的Calphad研究外，本文還提出了一種簡單的組合設計方法。在用於組成設計的簡單方法中，沒有一種方法能夠同時正確地預測當前三種合金中的相。提出了一種使用二元吉布斯能量 - 成分（G-x）圖來預測相位的新方法。它只需要單獨的二元吉布斯能量函數而不是多組分數據庫。它適用於設計單相作為目標微結構的HEA。此外，它也為多相形成提供了重要的見解。它成功地預測了目前合金中的相以及文獻中報導的少量合金。它提供了二元相圖檢查方法和多組分Calphad研究之間的折衷。由於它只需要對單個二進制系統進行熱力學描述，因此可以通過HEA社區更廣泛地使用可用的二進制Calphad描述。

關鍵字

高熵合金 ； 計算相圖模擬 ； 合金設計 ； 相穩定分析

並列摘要

High-entropy alloys (HEAs) are promising new class of materials having five or more elements in equiatomic amounts. A major challenge in HEA research is lack of phase stability information. Composition selection is another big challenge due to the lack of higher order phase diagrams. Two major types of methods are used for composition selection, both with certain challenges. One type is simple phase prediction methods and the other one is the robust Calphad (CALculation of PHAse Diagram) approach. Simple phase prediction methods have significant inaccuracies and none of the methods use Gibbs energy directly. In the Calphad approach, unavailable or inaccurate Gibbs energy databases are the issues. The present study is an attempt to address these issues.
Phase stability of CoCrMnNi, CoCrCuMnNi and AlCoCrMnNi high-entropy alloys is investigated. Vacuum arc melted samples were heat treated at 1273 K for 24 h followed by heat treatment at 673, 873 and 1073 K for 240 h. Microstructures were characterized using XRD and SEM.
In order to check the success of the Calphad method, calculations using TCHEA database in Thermo-Calc software were performed and the results were compared with the experimental microstructures. 1273 K microstructures were matching with the calculations. In most of the long-term heat treatment cases, σ-phase rich in Cr, Co and Mn was observed in all the three alloys. Calculations were not successful in predicting the σ-phase stability. In order to address this discrepancy, development of new Gibbs energy functions of σ-phase in Co-Cr, Cr-Mn, Co-Mn and Co-Cr-Mn systems were required. The thermodynamic assessment was used for the purpose. Input generation was through data collection from literature, ab intio calculations and experiments involving equilibrated alloys and diffusion couple. After the thermodynamic assessment, the new data are used to replace the corresponding functions in TCHEA database. With the TCHEA (Improved) database calculations were performed. The new data was successful in providing more accurate results, since σ-phase appeared in calculations at 673 – 1073 K, matching with the experiments. The TCHEA (Improved) data will greatly improve the accuracy in applications of Calphad to HEAs, like rapid screening of useful compositions using high-throughput methods.
Besides phase stability studies and improved Calphad studies, a simple method for compositional design is also proposed in the present thesis. Among the simple methods for compositional design, none of the methods were able to simultaneously predict phases in the present three alloys correctly. A new approach of using binary Gibbs energy - composition (G-x) plots for predicting the phases is proposed. It requires only individual binary Gibbs energy functions instead of a multicomponent database. It is suitable for designing a HEA with a single-phase as the target microstructure. Besides, it gives important insights into multiphase formation as well. It successfully predicts the phases in the present alloys as well as few alloys reported in the literature. It offers a compromise between binary phase diagram inspection methods and multicomponent Calphad studies. Since it requires only thermodynamic descriptions of individual binary systems, it will enable wider usage of available binary Calphad descriptions by the HEA community.

並列關鍵字

High-entropy alloys ； Calphad ； Compositional design ； Phase stability studies

參考文獻

Allibert C., Bernard C., Valignat N., and Dombre M. (1978), Co-Cr binary system: Experimental re-determination of the phase diagram and comparison with the diagram calculated from the thermodynamic data. J. Less-Common Met. 59, 211–228.

Bell H.B., Hajra J.P., Putland F.H., and Spencer P.J. (1973), The Determination of the Thermodynamic Properties of Cobalt-Chromium Alloys Using Solid-Electrolyte EMF and High-Temperature Calorimetric Techniques. Met. Sci. J. 7, 185–190.

Bracq G., Laurent-Brocq M., Perrière L., Pirès R., Joubert J.M., and Guillot I. (2017), The fcc solid solution stability in the Co-Cr-Fe-Mn-Ni multi-component system. Acta Mater. 128, 327–336.

Brif Y., Thomas M., and Todd I. (2015), The use of high-entropy alloys in additive manufacturing. Scr. Mater. 99, 93–96.

Cantor B., Chang I.T.H., Knight P., and Vincent A.J.B. (2004), Microstructural development in equiatomic multicomponent alloys. Mater. Sci. Eng. A 375–377, 213–218.