| 摘要: | 相較於傳統合金，高熵合金通常擁有五種或以上等莫耳元素組成，且合金中擁有較高的混和熵。高熵合金在高亂度、嚴重的晶格扭曲和緩慢的擴散效應等影響下，導致合金可在特定的冷卻條件下形成穩度的單相結構。因此合金能展現出更好的機械與化學性質，例如較高的強度、熱穩定性和抗腐蝕性等等。目前典型的高熵合金主要由鐵、鈷、鎳、鉻、錳等過渡元素組成 其密度通常落於8.1 g/cm3。而密度介於 3-5 g/cm3為中低密度合金，低於3 g/cm3則稱低密度合金。在此三年研究中，中低密度高熵合金旨在設計和表徵為相的演變、微結構、熱性質、常溫及升溫的機械性質。該系統將從TiAlNb及TiAlV三元合金開始，逐步添加其他的元素如Li、Zr、Cr、Y、Sc等。合金設計由實驗鑄造經驗及Calphad法共同進行。將使用真空感應熔解鑄造來製備合金及採用商業化軟體如Pandat或ThermalCal進行熱力學計算模擬。由於高熵合金通常要求表現出緩慢的擴散，導致其熱穩定性及耐熱性質。本研究還將探討新設計高熵合金的熱擴散、熱導率、熱容量及熱膨脹性質。此研究的重點將放在新設計的中低密度高熵合金的潛變特性，以便應用於耐熱的結構上。為了瞭解每個FCC相及BCC相對於抗潛變的貢獻，將使用背向電子繞射(EBSD)定位此具潛力的高熵合金的每個FCC相及BCC相的方向，像是[111]、[110]、[100]，而在測試各個方向的FCC及BCC相的潛變性質上，將使用新型的Hysitron奈米壓痕系統搭配Berkovich或micropillar loading，實驗溫度為300-800℃。利用潛變速率及擴散速率來檢驗熱機械穩定性與抗潛變能力。潛變指數、活化體積及活化能等數據則用以研究潛變的機制。其中活化體積及活化能也可幫助評估緩慢擴散在多元高熵合金中的可行性。 利用有限元素分析 (FEM) 、分子動力學 (MD)及動力學蒙地卡羅 (KMC) 的模擬計算來證實及比較在奈米尺度下室溫及高溫之機械性質，特別是抗潛變能力的分析。整體來說，這項聯合計畫會從四大方面進行：(i) 合金材料製備與微結構控制、(ii) Calphad相圖計算與建構、(iii) 常溫/高溫材料機械性質、(iv) 有限元素分析與分子動力模擬計算。三年的研究將從三元高熵合金材料出發，後續預期接著四元、甚至多元高熵合金，調整其晶體結構、顯微組織、熱力和熱機械 ;High entropy alloys (HEAs) usually possess about five principal elements and demonstrate high entropy of mixing. The effects of high-degree chaos, extensive lattice distortion, and sluggish cooperative diffusion made the HEAs preferentially to form single phase microstructure. As a result, HEAs could exhibit promising mechanical and chemical properties involving of high strength, high thermal stability, and high corrosion resistance. Current typical HEAs are made mainly by transition elements, such as Fe, Co, Ni, Cr, Mn. The density is typically in the range around 8.1 g/cm3. Density levels in the range from 3.0 to 5.0 g/cm3 are classified as medium-low density (MLD) and those below 3.0 g/cm3 are considered to be low density.In this three-year study, MLD HEAs are intended to be designed and characterized in terms of phase evolution, microstructure, thermal nature, and room temperature and elevated temperature mechanical properties. The systems start from TiAlNb and TiAlV three-component alloys, followed by adding more elements. The alloy design will be conducted from both experimental casting experience and Calphad approaches. Vacuum induction melting and casting will be used to prepare the alloys. Since HEAs are typically claimed to exhibit sluggish diffusion, resulting in thermal stability and heat resistant properties. This study will also explore the thermal diffusion, thermal conductivity, heat capacity, and thermal expansion characteristics for the newly designed medium-low density HEAs. For the application at heat resistant environment, the elevated temperature creep response for the newly designed MLD HEAs will be a focus in this study. To understand the creep resistance contribution from each FCC or BCC phase, the promising HEAs will be indexed by electron backscatter diffraction (EBSD). A new Hysitron nanoindentation system will be adopted to measure the creep response from each phase and each orientation by using Berkovich or micropillar loading. The temperature range will cover from 300 to 800oC. The creep rate and diffusion rate will be extracted to examine the thermomechanical stability and creep resistance. The creep exponent, activation volume and activation energy will all be extracted to understand the creep mechanisms. The activation volume and activation energy can also help to evaluate the feasibility of sluggish diffusion for multiple-element HEAs. Finite element method (FEM), molecule dynamics (MD), and kinetic Monte Carlo (KMC) simulation and calculation will also be incorporated to support and compare the nano-scaled room and elevated temperature mechanical response, especially the creep resistance. Overall, this joint project will proceed from four aspects, namely, (i) alloys preparation and microstructure control end, (ii) Calphad thermodynamics simulation aspect end, (iii) room and elevated temperature mechanical response end, and (iv) FEM and MD/KMC simulation and calculation end. The three-year work load will be launched from ternary, to quaternary and more element HEAs, adjusting their phases, microstructures, thermal and thermomechanical properties. For the last year, these newly designed MLD HEAs will be moved toward industry applications via the help from MIRDC and ITRI. Also, an international collaboration team has also been established, to work concurrently over the next three years. |
