高熵合金是由多種溶質元素且無主要溶劑元素所組成的合金。高熵合金經由適當的熱處理，可獲得極佳的機械與物理性質，例如：耐高溫氧化、高硬度與耐磨性能等。本論文探討兩種成分約為Al0.5CoCrFeNi2的高熵合金，經由高溫熱處理後所發生之spinodal相分離與有序化反應。本團隊自行熔鑄、熱鍛與冷軋，取得高熵合金薄板，進行1050℃固溶處理，與低溫的時效處理。此二種高熵合金經1050℃固溶處理後的主要相結構皆由沃斯田體(γ)與少量的B2晶粒所組成。此二種組成相於冷卻過程中，皆發生spinodal相分離與有序化相變化。其中沃斯田體相的相變化形式如下：高溫沃斯田體於冷卻過程中分解為二個低溫沃斯田體(γ’ + γ”)，其中含高溶質的γ”相於更低溫時，經由有序化反應而相轉變為L12相。其總反應式為：γ → γ’ + γ” → γ’ + L12。而於B2晶粒的相變化形式如下：高溫B2相於冷卻過程中分解為二個低溫B2相(B2’ + B2”)，其中含高溶質的B2”相於更低溫時，經由有序化反應而相轉變為D03相。其反應式為：B2 → B2’ + B2” → B2’ + D03。另於低溫時效處理時發現Cr23C6碳化物以及(γ + L12)層狀組織。

High-entropy alloys (HEAs) are alloys composed of at least five principal elements. These alloys show some unusual features, such as excellent mechanical and high temperature properties, and some of them are yet to be discovered. However, the development of the HEAs requires knowledge of the phase transformations that occur during the alloy making processes. Phase transformations of two Al0.5CoCrFeNi2 HEAs have been study. Our methods of studying the phase transformations of the Al0.5CoCrFeNi2 HEAs include water-quenching and/or air-cooling to room temperature after heating the steel at 1323 K. The alloys are dual phase of major austenite and minor B2 phase after cooling from high temperature. The as-quenched specimens also isothermally held at low temperatures. The results of our study show that the spinodal decompositions occur in the HEAs after heating and cooling from 1323 K, the high temperature austenite (γ) decomposes through the spinodal mechanism into low temperature solute-lean austenite (γ’) and solute-enriched austenite (γ”). The solute-enriched austenite phase also transforms into the L12 phase via the ordering reaction upon cooling to lower temperature. Therefore, fine particles of L12 phase precipitate homogeneously in the austenite grains. The occurrence of spinodal decomposition and ordering reaction in the austenite phase of the HEA can be written as follows. γ → γ’ + γ” → γ’ + L12. In addition, the minor B2 phase has also undergone spinodal decomposition and ordering reaction. The reactions are as follows. After cooling from high temperature, the high temperature B2 phase decomposes through the spinodal mechanism into low temperature solute-lean B2’ and solute-enriched B2”. The solute-enriched B2” also transforms into D03 via the ordering reaction upon cooling to lower temperature. Therefore, fine particles of D03 phase precipitate homogeneously in the B2 grains. The occurrence of spinodal decomposition and ordering reaction in the B2 phase of the HEA can be written as follows. B2 → B2’ + B2” → B2’ + D03. After annealing the HEAs at low temperatures, we have observed that Cr23C6 carbide precipitates in the austenite and lamellae of austenite and L12 form.

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