The study of phase transformations in CoCrFeNi and CoCrFeNiAl0.5 high entropy alloys

• Main Authors: Kuan-Chieh Fang, 方冠傑
• Other Authors: Wei-Chun Cheng
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/mmh324

碩士 === 國立臺灣科技大學 === 機械工程系 === 107 === 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.