理論計算探討鋁鈷鉻鐵鎳高熵合金相穩定度與高壓下相轉變的合理性

Abstract

本篇論文運用第一計算原理進行AlCoCrFeNi的相穩定性分析以及高壓相轉變模擬分析，進而了解到此高熵合金的最終穩定相，包含B2-AlNi、B2-FeCo BCC-Cr三相，確實可從AlxCoCrFeNi (0≤x≤0.7)延伸到Al1.0CoCrFeNi，也能對各種穩定相之間的析出先後關係做初步推論，其中B2-AlNi相是最低結構能量，包含慢速退火與高速退火的AlCoCrFeNi高熵合金。透過這些晶相在不同壓力環境下，模擬分析其能量狀態、計算XRD波形，甚至模擬不同晶格扭曲的XRD波形，與參考的實驗文獻中0.3 GPa至30.4 GPa的XRD圖譜波形進行比對，藉以分析此高熵合金的晶相轉變合理性，試著了解一定壓力範圍內相轉變趨勢(tendency)與晶格扭曲的機制與結構演變，清楚第一原理計算對結構分析的貢獻。

In this work, first principle calculation is used in the analysis of phase stability and high pressure phase transformation of AlCoCrFeNi alloy. By phase stability analysis on AlCoCrFeNi alloy, it is found that the most thermodynamically stable phases are the three phases including B2-AlNi, B2-FeCo and BCC-Cr, as predicted from the argument based on the phase stability analysis on AlxCoCrFeNi (0≤x≤0.7). The introductory inference of the sequence of precipitation is also deducted. B2-AlNi is the binary phase with the lowest energy, which is favored to exist in both low speed quench and high speed quench AlCoCrFeNi alloys. Energies and XRD profiles of phases with or without lattice distortion under different pressure are simulated, and the reasonability of phase transformation of AlCoCrFeNi alloy is analyzed by comparing the simulated and experimental XRD profiles from 0.3 GPa to 30.4 GPa. The phase transformation tendency and mechanism of lattice distortion and structural evolution of AlCoCrFeNi alloy under certain range of pressure is attempted to understand, in order to understand the contribution of first principle calculation on structure analysis.