本研究主要目的在於探討Cu0.5CoCrFeNi合金在不同退火溫度下對於其顯微結構演化的機制，並針對存在於基地上析出物的相變化過程進行研究，歸納出多元合金組成所具有的高熵特性，進而瞭解其對機械性質與電化學性質的影響。利用光學顯微鏡、電子顯微鏡觀察Cu0.5CoCrFeNi合金的顯微組織；能量散佈光譜儀與電子微探儀進行組成成份、元素擴散行為及親和力的觀察與分析。X光繞射儀分析顯微組織的組成相與結晶度。微小維氏硬度試驗儀量測其合金硬度值。穿透式電子顯微鏡分析其析出物的相變化過程。此外，針對Cu0.5CoCrFeNi合金的熱穩定性在不同升溫條件下進行熱示差掃描卡量計的熱穩定性的評估。Cu0.5CoCrFeNi合金在不同退火溫度條件下進行3.5wt% NaCl水溶液的電化學性質探討，觀察其參數與顯微組織之關係；並且探討表面生成鈍化膜化學組成。其合金在3.5wt% NaCl水溶液的耐腐性質和蝕孔破壞由極化曲線及電子顯微鏡(SEM)進行評估及觀察；其鈍化膜化學組成由X射線光電子能譜儀(XPS)與低掠角X光繞射儀(XRD)進行分析。
試驗結果顯示，多元合金系統依冷卻速率、成份、各元素間熔點差、原子半徑差與晶體結構差均會影響到顯微結構的結晶性呈現有序排列微結晶相；在利用不同退火溫度促使微結晶相轉變成為無序排列的奈米結晶相，則其過程產生離相分解(spinodal decomposition)，此為多元合金具備高熵特性的基本條件之一。而在顯微組織的觀察，其鑄態及經由低溫至高溫的退火處理，均呈現簡單固溶體相-FCC結構，此為多元合金具備高熵特性的基本條件之二，而構成FCC結構的相組成有基地相、富Cu相及富Cr相。特別值得注意，富Cu相在低溫階段的退火溫度下，呈現短程有序排列結構；則在高溫時，呈現無序排列結構。
微硬度觀察合金在不同退火處理條件下，硬度差異性不太顯著；就原子間之鍵結強度而言，當受到不同程度的熱效應影響促使合金元素原子排列呈現隨機分佈機率增加，而且在配合Cu，Co和Ni的混合焓為正值，會加速原子鍵結強度下降，促使硬度下降。
TEM觀察相變化，發現主要相為L12相，此L12相在鑄態冷卻過程即存在於FCC基地相中，以均質方式整合型式析出的結構相；故該階段的相變化過程為FCC→L12。而經由650、950及1350C 熱處理後，其相組成均為FCC相及L12相；當合金經650℃熱處理後，相組成為FCC相及L12相加上少量的新FCC相析出(B2相)；合金經950C以上的退火溫度後，於TEM的分析中，發現B2相析出，其晶格常數為接近鑄態合金所計算出晶格常數的，經由EDS分析發現，此新析出物的Cu含量非常高，故應為從基地重新析出的富Cu相結構，且亦為整合型的析出物，其晶格常數為基地相L12晶格常數的三倍。此新整合型FCC相的析出物的相變化型式應為FCC→L12→新整合型FCC相(B2相)。
DSC熱穩定性觀察，發現其熱穩定性不佳，主要歸因於離相分解(spinodal decomposition)效應，而且在基地重新析出相結構所造成同時分解及析出行為所致，此為多元合金具備高熵特性的基本條件之三。
在3.5wt% NaCl水溶液中觀察其電化學性質，在低溫階段其合金系統抗腐蝕性較差，隨著退火溫度增加到達1350C，可改善其抗腐蝕性；在抗抗蝕孔的能力部分，主要在其合金表面無法有效形成保護型鈍化膜而造成；而形成腐蝕破壞的形態，隨著富Cu相及富Cr相形貌而決定。此外，腐蝕破壞型態有傾向於沿晶腐蝕及蝕孔的行為；鈍化膜化學組成，在低溫階段以Cu2O、CuO及Cu-為主，隨著退火溫度增加到達1350C，以CrNi、Cr2O3為主，故在高溫階段，其合金具有較佳耐蝕性。

The purpose of this investigation is to experimentally determine the evolution mechanism and the phase transformation of the microstructure and the precipitate of a Cu0.5CoCrFeNi alloy at different annealing temperatures. Research on the multicomponent alloy has revealed that the alloy has high-entropy characteristics in the induction formation direction. In addition, in order to understand the effect of the environment on the annealing temperatures of this alloy, the mechanical and electrochemical behavior of the alloy has been examined. The microstructures of the Cu0.5CoCrFeNi alloy were examined using an optical microscope (OM) and a scanning electron microscope (SEM). The chemical composition, crystalline phase, element diffusing action, and affinity of this Cu0.5CoCrFeNi alloy were studied using an energy dispersive spectrometer (EDS), an X-ray diffractometer (XRD), and an electron probe X-ray microanalyzer (EPMA). The profile of the hardness distribution of the microstructures was obtained using a Vickers microhardness tester. The phase transformations of the precipitate phase were evaluated using a field emission transmission electron microscope (FE-TEM). The heat stability of the Cu0.5CoCrFeNi alloy was evaluated at various heating rates by using a differential scanning calorimeter (DSC). A potentiodynamic method in 3.5% NaCl solution at room temperature was employed to evaluate the electrochemical properties of the specimens at various annealing temperatures. The corrosion resistance behavior and the pitting fracture of the specimens were evaluated using polarization curves and SEM. Compositional and chemical states of the specimen surface were investigated by carrying out a passive film analysis in the 3.5% NaCl solution using glancing incident angle diffraction and an X-ray photoelectron spectrometer (XPS).
Experimental results reveal that the microstructure of the multicomponent alloy is dominant because of the cooling rate, the number of components, the mole percentages of the components, the difference in melting points of the components, the atomic radii, and the crystalline structures; this microstructure is called the ordered microcrystalline phase. The ordered microcrystalline phase changes into a disordered nanocrystalline phase at different annealing temperatures. A spinodal decomposition behavior is observed during the annealing treatment.
The FeCoNiCrCu0.5 alloy the as-cast substrate and annealed substrate exhibits an FCC phase. The FCC phase of the alloy system tends to form a matrix phase, a Cu-rich phase, and a Cr-rich phase during different annealing treatments. In particular, the Cu-rich phase forming a short-range ordered structure is observed at a low temperature. However, this phase exhibits a disordered structure at a high temperature. In this study, the hardness difference of the alloy system is not clearly observed for the element atoms with different bonding strengths. However, the thermal behavior is associated with the positive mixing enthalpies of Cu, Co, and Ni, and this behavior lowers the hardness.
The Cu-rich (L12) phase is the major phase confirmed by the TEM. The L12 phase is the derivative phase from the FCC parent phase in the as-cast state. The mechanism for the phase transformation is FCC phase → L12 during the solidification process. When the annealing temperature is 650, 950, and 1350°C, the microstructure of the alloy exhibits a trend similar to that in the case of annealing conduction. However, at an annealing temperature below 650°C, the major phase is transformed into an FCC phase, L12 phase, and new FCC phase (B2 phase). Therefore, the upper limit temperature for the existence of the B2 phase is between 950 and 1350°C; the B2 phase with a high Cu content precipitates from the matrix. The lattice constant of the B2 phase is similar to that in the as-cast condition because of the coherent precipitated phase of the matrix. The lattice constant of the new FCC phase is three times larger than that of the L12 phase. The mechanism of the phase transformation of the new coherent FCC phase is FCC → L12 → new coherent FCC phase.
The thermal stability is investigated by DSC. For the experiments result it is not stabilize. This was because of the spinodal decomposition effects. In addition, new phases are separated and dissolved from the matrix phases at the same time.
The anticorrosion properties are evaluated by an electrochemical analyzer. The anticorrosion properties are lower after the low-temperature annealed treatments than before. This is because the protective films shielded the alloy surface effectively. The corrosion form difference was forces on the Cu-rich phase or Cr-rich phase from the matrix concentration. The corrosion behavior was fracture on the intercrystalline and pitting two forms. The chemical composition of the protective films is Cu2O、CuO and Cu– after the low temperature treatments. However, the anticorrosion properties improve when the annealing temperature increases to 1350°C, due to the fact that the concentration of the protective films is CrNi and Cr2O3 after high-temperature annealing (1350°C).

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