高熵合金(High-entropy Alloys,HEAs)是使用多種元素混合之合金，每個主元素原子百分比介於5%至35%，而次要元素則小於5%。在元素比例上做適當之調配，可獲得耐腐蝕、耐磨性與高強度等優越性質。本論文延續先前探討成分為CoCrFeNi與CoCrFeNiAl0.5之二種高熵合金，係研究經高溫固溶處理與100小時低溫恆溫處理之相變化過程。經固溶處理後之CoCrFeNi高熵合金，其主要結構為FCC沃斯田體(γ)單相組成，而經長時間之低溫恆溫處理後，於晶界處有富含鉻元素之碳化物產生。經固溶處理後之CoCrFeNiAl0.5高熵合金，其主要結構由FCC沃斯田體(γ)相與、BCC肥粒體(α)相及B2相組成，而經過低溫恆溫處理後，發現σ相析出，並發生spinodal相分離與有序化相變化，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’’)，其中含高溶質之γ’’相於更低溫時，再經由有序化反應而相轉變為L12相，觀察發現其晶粒尺寸小於CoCrFeNi高熵合金。總反應式為：γ→γ’+γ’’→γ’+L12。

HEAs are defined as those alloys containing at least five principal elements, each having the atomic percentage between 5% and 35%. The atomic percentage of each minor element, if any, is less than 5%. These alloys show some unusual features, such as excellent corrosion resistance, wear resistance, high strength, and some of them are yet to be discovered. However, the development of the HEAs requires the knowledge of the phase transformations that occur during the alloy making processes. The present study is a continuation of the previous work. and aims to understand the microstructure and phase transformations of the alloys, Alloy A (CoCrFeNi) and Alloy B (CoCrFeNiAl0.5) after solution treatment for 1 hour and then quenched followed by prolonged annealing for 100 hours isothermally at various temperature. Alloy A shows single phase FCC austenite (γ) after solution treatment in as quenched condition whereas Alloy B shows multiphase consisting of FCC austenite (γ) phase, BCC ferrite (α) phase, and B2 phase after solution treatment in as quenched condition.
Chromium rich carbides precipitates at the grain boundaries after prolong annealing of 100 hours at various studied temperature for Alloy A. The results of our study show that the spinodal decompositions occur in CoCrFeNiAl0.5 alloys after heating and cooling from 1050°C, 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 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 Alloy B can be written as follows:γ→γ’+γ’’→γ’+L12

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