摘 要
本論文針對成份為Al0.5CoCrFeNi2之多元高熵合金，觀察此合金的結晶結構。本實驗先觀察此多元合金於鑄造固化狀態的結晶結構；另外，再將合金以1200℃高溫熱鍛後冷軋，再經1050℃熱處理及後續之1000、900、850、800、700、600及500℃熱處理，研究合金經熱處理後所發生的相變化情形。
合金在鑄造固化後，於金相顯微結構的觀察，其金相顯微結構有柱狀組織；於XRD分析中，發現結晶組織以FCC相為主；但於TEM的觀察中，發現主要相為L12相，此L12相為於冷卻過程中於FCC母相中，以均質方式整合型式析出的結構相。相變化過程為FCC→L12。
合金經1050℃熱處理1小時後，以水淬或爐冷方式處理。經水淬處理之試片，其相結構是FCC相與少量的B2相；經爐冷處理之試片，其結晶結構為L12相與少量的B2相；且發現B2於FCC基地中成長時，與FCC相有K-S方位關係，此K-S方位關係為：[10 ]FCC // [11 ]BCC，(111) FCC // (101) BCC。
當合金經1050℃熱處理後，再經1000、900及850℃熱處理後，相組成均為FCC相及少量的B2相；當合金經800℃熱處理後，相組成為L12相加上少量的B2相。故L12結晶相存在之上限溫度介於800至850℃之間。合金經700及600℃的溫度熱處理後，於TEM的分析中，發現新FCC相析出，其晶格常數為0.6785 nm，經由EDS分析發現，此新析出物的Al含量非常高，然而此新FCC析出相的晶格常數大於純Al相的晶格常數，且與具立方晶系的氧化鋁的晶格常數不同，故懷疑此新FCC析出相為未被發現的新結晶相。合金在600℃熱處理後，L12析出物內尚有另一非均質成核成長但為整合型的析出物，此析出物具有FCC的結晶結構，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。

Abstract
We have studied the microstructures of the constituent phases of a Al0.5CoCrFeNi2 multi-component alloy in two conditions which one was in the as-cast condition and the other wais in hot forged, cold rolled, annealed at 1050℃, then heat treatment at lower temperatures.
For the alloy in the as-cast condition, the dendrites appeared in the OM observation, and only FCC peaks were observed from the XRD analysis; however, L12 phase was the major phase confirmed by the TEM study. The L12 phase was the derivative phase from the FCC parent phase. The mechanism for the phase transformation was FCC → L12 during the solidification process.
For the alloy in the as-quenched condition, the major phase of the alloy is FCC. The minor B2 phase precipitated from the FCC matrix and had the K-S orientation relationship with the FCC matrix: [10 ]FCC // [11 ]B2，(111) FCC // (101) B2. For the alloy in the annealing conditions at 1000, 900, and 850℃, the microstructure of the alloy is similar to that in the as-quenched condition. However, for the annealing temperature below 800℃ we found the matrix phase had changed from FCC to L12 phase. Therefore the upper limit temperature for the existence of the B2 phase is between 800 and 850℃. For the annealing temperature at 700 and 600℃, we found a new FCC phase with high Al content precipitated from the L12 matrix. The lattice constant of the new FCC phase is a = 0.6785 nm which is higher than that of pure Al and different from those of Al2O3. For the annealing temperatures at 600 and 500℃, we found another new coherent FCC phase precipitated from the L12 matrix. The lattice constant of the new FCC phase is three times larger than that of L12 phase. The mechanism of the phase transformation of the new coherent FCC phase is FCC→L12→new coherent FCC phase. Both new FCC phases have never been observed in the multi-component alloy systems.

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