高熵合金CoCrFeMnNi於高壓下相變與機制探討

Abstract

等莫耳比高熵合金CoCrFeMnNi在常溫常壓下為單一Face-centered cubic (FCC)相的合金，近期研究相信該高熵合金有低疊差能，低溫下因奈米雙晶有優秀的機械性質，但未觀察到相變現象。本研究利用角度解析式X-光繞射(Angular-dispersive X-ray diffraction ,ADXRD)高壓實驗，結果發現該高熵合金在7.1GPa時觀察到FCC至Hexagonal close packing (HCP)相的高壓相變，兩相持續共存到實驗最大壓力20GPa，卸載回常壓後仍有殘存HCP相，此過程為不可逆高壓相變。本研究也計算兩相的晶格常數、相比例與半高寬，也確認該高熵合金在高壓下非受到靜水壓力，受到非等向性的壓縮，材料在變形中將有對應的織構與滑移系統產生，與文獻比對發現高熵合金轉為HCP相後的織構分布與鋅在高壓下的結果相似，確認最終應轉變為單一HCP相，最後此研究也對高壓相變的機制進行探討。

An equal-molar CoCrFeMnNi high-entropy alloy has the cubic crystal system of face-centered-cubic (FCC) at room temperature and atmospheric pressure. The recent research believed that the high-entropy has the property of low stacking fault energy, and excellent mechanic property because of the structure of nanocrystalline in low temperature. However, there was no phase-changing observed. This research used Angular-dispersive X-ray Diffraction (ADXRD) under high-pressure, pressurized the CoCrFeMnNi high-entropy alloy system to 20GPa. After analyzing diffraction data, there was phase transformation from FCC to Hexagonal Close Packing (HCP) when the pressure reached 7.1GPa. Both phases existed until the maximum pressure of 20GPa. When the pressure was unloaded to atmospheric pressure, there are remaining HCP-phase in the alloy, which shows the phase transformation is a non-reversible phenomenon. Besides observing phase transformation under high-pressure and the remaining HCP phase, this research will also calculate the lattice constant, ratio and full width at half maximum (FWHM) of both phases. Then, the result of the analysis will be compared with other theses, and to ensure that high-entropy alloy will not be affected by hydrostatic pressure in a high-pressure environment and non-isotropic compression. During the transformation of the material, there was corresponding texture and slip system. After comparison with other thesis, we discovered that when the phase of that alloy was transformed to HCP, the texture distribution is similar with Zine under pressurized. And confirmed the final phase of the alloy should be a uniform single phase. Finally, this research will investigate the phase transformation mechanism under high-pressure environment.