高熵合金(High Entropy Alloys, HEAs)是由四種或是四種以上元素所組成的合金，並且各元素原子百分比介於5%到35%。高熵合金經由適當的熱處理，可獲得極佳的材料性質，例如：耐高溫氧化、高強度與抗腐蝕等。本論文探討由真空感應熔煉的Co2CrFeNi (合金A)和Al0.4Co2CrFeNi (合金B)的兩種高熵合金，觀察從1050℃固溶處理與恆溫處理後的相變化以及其微觀結構改變，發現Al0.4Co2CrFeNi經由熱處理後析出B2相和L12所發生之spinodal相分離與有序化反應。高熵合金A經1050℃固溶處理後之相結構由單相的沃斯田體(γ)所組成，此組成相於長時間的恆溫處理後，並無發現明顯析出物。而高熵合金B經固溶處理後之相結構為單相的單相的沃斯田體(γ)所組成，而經過恆溫處理後，發現B2相，另於較低溫之恆溫處理時發現L12相以及M23C6碳化物。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。

High-entropy alloys (HEAs) are defined as those alloys containing at least four principal elements, each having an atomic percentage between 5% and 35%. With the proper heat treatments, those HEAs can possess some excellent material properties, for example, oxidation resistance, high strength, high corrosion resistance, etc; however, certain knowledge of phase transformations is required during the alloy-making processes. Phase transformations of both Alloy A (Co2CrFeNi) and Alloy B (Al0.4Co2CrFeNi) HEAs have been studied throughout this study. The results show that the Alloy A exhibits a single Face-centered Cubic (FCC) phase after solution treatment for an hour and annealing treatments for 100 hours. On the other hand, alloy B transformed to dual-phase of major austenite and minor B2 phase after annealing treatments for 100 hours at certain temperatures ranging from 650℃ to 950℃. The outcome of our study shows that spinodal decompositions occurred in the HEAs after heating at and cooling from 1323 K, the high-temperature austenite (γ) decomposed through the spinodal decomposition into low-temperature solute-lean austenite (γ′) and solute-enriched austenite (γ″). The solute-enriched austenite phase also transformed into the L12 phase via the ordering reaction upon cooling to lower temperatures. The occurrence of spinodal decomposition and ordering reaction in the austenite phase of the HEA can be written as following, γ → γ′ + γ″ →γ′ + L12.

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