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，經由不同的合金元素設計，可獲得極佳的耐高溫氧化、高硬度與耐磨性能等。本研究探討成份為CoCrFeNi與CoCrFeNiAl0.5高熵合金經1050°C固溶處理與低溫恆溫處理的相變化機制。本論文自行熔鑄、熱鍛與冷軋，取得高熵合金薄板。經固溶處理後CoCrFeNi中主要結構為沃斯田體( γ)單相，經700°C恆溫處理後於晶界處有富鉻析出物的產生。CoCrFeNiAl0.5高熵合金中觀察晶粒尺寸比先前研究過的高熵合金還小，其原因可能為B2相過多抑制晶粒成長。合金B晶高溫冷卻後沃斯田體內發生spinodal相分離與有序化相變化，其反應如下：高溫沃斯田體(γ)於冷卻過程中分解為二個低溫沃斯田體(γ'+γ")，其中含高濃度的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。

High-entropy alloys (HEAs) are alloys composed of at least five principal elements. These alloys show some specific 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 producing. Two HEAs, CoCrFeNi and CoCrFeNiAl0.5 were studied on the effect of cooling process including air cooling and water cooling after heating alloys at 1050°C. The as-quenched specimens also isothermally held at 800°C to 600°C at 50°C interval. The results of our study show that CoCrFeNi is of single phase austenite after cooling from high temperature. In CoCrFeNi alloy, Cr-rich phase precipitates at the grain boundary around 700°C. CoCrFeNiAl0.5 is a dual phase 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 CoCrFeNiAl0.5 alloys after heating and cooling from 1323 K, the high temperature austenite ( γ) decomposes 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. 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.

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