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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/68124


    Title: CoCrFeMnNi 高熵合金 形變行為之探討;Study on deformation behaviors of CoCrFeMnNi high entropy alloys
    Authors: 杜尚益;Tu,Shang-Yi
    Contributors: 化學工程與材料工程學系
    Keywords: 高熵合金;Hihg entropy alloy
    Date: 2015-07-27
    Issue Date: 2015-09-23 10:45:17 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 等莫耳比的CoCrFeMnNi高熵合金為單一Face Center Cubic相的合金,在Face Center Cubic晶格中Co、Cr、Fe、Mn、Ni五種原子隨機分布其中,而因為五種原子的電負度以及原子半徑等皆不相同,因此會有晶格扭曲的現象,影響其機械性質。本研究利用量測不同溫度時的機械性質以及以即時中子繞射觀察在不同溫度以及不同形變程度下繞射圖譜的變化以研究在各種條件下的晶格應變的不同,再利用穿透式電子顯微鏡(TEM)觀察各溫度下的表面微結構以研究機械性質與微結構之間的關係。
    從巨觀拉伸試驗的結果可知道CoCrFeMnNi高熵合金在低溫甚至常溫下可同時擁有良好的強度以及延展性,非常適合應用在工程材料上[1],而從即時中子繞射的結果可以看出,CoCrFeMnNi高熵合金的表現行為與傳統合金不一樣,高熵合金的彈性係數比平均法則來的小,且對溫度不太敏感。原因來自於晶格扭曲導致原子不在完美的晶格位置上,施加應力後,系統對於應力的反應和一般金屬不同,在高溫熱振動會先抵銷晶格應變,使得彈性係數對溫度不敏感。
    從中子繞射的微觀結果可以看出,其各個晶面的彈性模數隨著溫度上升而會有兩組趨勢,其中(311)(222)(420)等平面為一組,其彈性模數隨著溫度上升而直線下降,(111)(200)(220)(400)(331)面則為另外一組,其彈性模數隨著溫度上升會在200及400oC時先上升,到600OC後才下降,造成此現象的原因為高熵合金晶格扭曲所造成,而當溫度更高時,原子間距變遠,因此晶格扭曲的現象減弱而使彈性模數回到應該有的位置。
    另外,本研究搭配了兩個擬合軟體輔助研究,分別是Elastic Visco-Plastic Self Consistent fitting (EVPSC)以及Convolutional Multiple Whole Profile fitting (CMWP)。從EVPSC可得知CoCrFeMnNi高熵合金內部的滑移機制並非單純只有傳統FCC滑移系統的貢獻,還有其他因素造成。而利用CMWP可以得知CoCrFeMnNi高熵合金的晶粒大小以及位錯差排密度。
    ;Equimolar CoCrFeMnNi High Entropy Alloy is a single phase Face-Centered Cubic alloy. In FCC dendrites, Co, Cr, Fe, Mn and Ni these five atoms are randomly distributed. Since these five atoms have different electronegativities and atomic radii, lattice distortion takes place and hence affects its mechanical properties. This study aims at investigating the variation of lattice strain under different conditions by measuring its mechanical properties at different temperature levels and observing the changes of diffraction profiles under different temperatures and with in-situ Neutron Diffraction. Moreover, by observing the surface microstructure under different temperatures with Transmission Electron Microscope (TEM), the relationship between the microstructure and mechanical properties of the material is investigated.

    From the result of tensile testing, it is known that CoCrFeMnNi High Entropy Alloy has both good strength and ductility at low temperature, even at room temperature, which is suitable for engineering material application [1]. From the result of in-situ Neutron Diffraction, it shows that the behavior of CoCrFeMnNi High Entropy Alloy is different from that of conventional alloys. The elasticity of High Entropy Alloys is usually smaller and the alloy is insensitive to temperature. Due to lattice distortion, the atoms are not at their lattice positions and hence the system experiences a distinct reaction from ordinary metals when stress is applied. Since high temperature thermal vibration cancels off the lattice strain, leading to the low sensation of the elastic modulus towards temperature.

    From the microscopic result of in-situ Neutron Diffraction, we can see that the elastic modulus of each crystal plane increases with the temperature and has two particular types of trends. Plane (111), (200), (311), (400), etc. are in one group as their elastic modulus decreases linearly when the temperature arises while plane (220), (331), (420) and (222) belong to another group as their elastic modulus first increases when the temperature reaches 200oC and 400oC and then decreases after 600OC. This is due to the lattice distortion of High Entropy Alloys: the higher the temperature elevates, the further the distance between the neutrons will be. As a result, the reduction in lattice distortion causes the elastic modulus to go back to its original position.

    Last but not least, this study collaborates with two fitting software applications, which are Elastic Visco-Plastic Self-Consistent model fitting (EVPSC) and Convolutional Multiple Whole Profile fitting (CMWP) respectively. EVPSC is used to identify other factors causing the slip mechanism of inner part in CoCrFeMnNi High Entropy Alloy besides the contribution from conventional alloys. The grain size and dislocation density of CoCrFeMnNi High Entropy Alloy can be determined with Convolutional Multiple Whole Profile fitting (CMWP).
    Appears in Collections:[化學工程與材料工程研究所] 博碩士論文

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