Ti對於CoCrFeMnNi高熵合金塊材微結構與機械性質之影響

Abstract

金屬材料相關研究在材料科學領域中發展悠久，且普遍應用於人類生活中。然而，傳統以單一元素為主元素的金屬材料已缺乏大幅度突破，並面臨材料設計瓶頸。高熵合金於2004年提出之後，即受到各界學者關注，其特殊的機械性質以及元素組成為金屬材料的發展打開嶄新的研究領域。在諸多高熵合金中，Cantor合金(組成為CoCrFeMnNi)廣被諸多學者研究，該合金具有優異的低溫機械性能與斷裂韌性，然而，其在室溫下的強度還需進一步提升，以滿足更多工業應用的基本要求。本研究以電弧熔煉技術製備CoCrFeMnNi高熵合金塊材，並添加不同含量的Ti，合金設計為CoCrFeMnNiTix (x = 0, 0.2及0.5，x為莫耳比)，觀察 Ti 元素含量的變化對合金之微結構、硬度及拉伸性質的影響；合金並進行冷軋和退火再結晶處理，分析塊材在不同退火溫度下其微結構對機械性質的影響。利用光學顯微鏡 (OM)、電子顯微鏡 (SEM) 觀察塊材顯微組織，電子背向散射繞射分析儀 (EBSD) 探討材料晶粒大小與結晶取向，X光繞射分析儀 (XRD) 進行相分析，同時藉由奈米壓痕試驗分析材料微觀機械行為，並進行拉伸試驗研究材料巨觀機械性質。研究結果顯示，在1173 K下退火可使高熵合金具有極佳的延展性，而添加Ti的高熵合金在x=0.2時，其最大拉伸強度可達到1087 MPa。

High entropy alloys (HEAs) with a face-centered cubic structure have recently received significant attentions due to their outstanding ductility and fracture toughness. In this study, the effects of Ti addition on the microstructures and mechanical properties of CoCrFeMnNi were investigated systematically. Three CoCrFeMnNiTix (x=0, 0.2 and 0.5 in molar ratio) HEAs were prepared by arc melting and studied in as-cast and annealed condition, respectively. It was found that Ti additions in HEAs led to the formation of intermetallic phases and increased the hardness. While Ti0.5 was embrittled due to the presence of hard intermetallic phases, the Ti0 and Ti0.2 ingots were then reduced in thickness of 80% by cold rolling and annealed for 1 h at two annealing temperatures, 1073 and 1173 K. The microstructure evolution, crystallographic texture, hardness and tensile properties were studied. The Ti0 alloy exhibited a single-phase FCC solid solution, while (Cr, Fe)-rich sigma phase and (Co, Ni)3Ti Laves phase precipitated in the Ti0.2 alloy after annealing. With precipitation of the hard yet brittle sigma phase and Laves phase, the yield strength of Ti0.2 alloy increased significantly but with the sacrifice in the tensile elongation. Both grain boundary strengthening and precipitation strengthening were the main strengthening mechanisms of the alloy doped with the Ti element.