2017/08/24 星期四

林俊铭:Effect of Al addition on properties and microstructure of refractory high-entropy AlxHfNbTaTiZr (x=0, 0.3, 0.5, 0.75, 1.0) alloys

2017-03-02, 星期四

报告时间:2017年3月2日(星期四)16:00—17:30

报告地点:格物楼二楼报告厅

报 告 :林俊銘助理教授,博导

工作单位:台北科技大学

举办单位: 机械工程学院

个人简介

     林俊铭,台北科技大学机械工程学系助理教授,博士生导师。2010年6月毕业于台湾科技大学机械工程学系获得工学博士学位。长时间从事焊接冶金基础研究及其技术应用、先进合金材料设计之开发及特性探讨。获授权专利1项,发表科技论文50多篇。其中SCI论文37,多篇发表于MSEA,JAC,MD,SCT,Measurement及Intermetallics期刊。同时,林博士兼任多家SCI期刊审稿人、台湾焊接学会学术编辑及出版委员。

报告内容

   This study developed a series of refractory AlxHfNbTaTiZr high-entropy alloys (HEAs) with an aim to improve strength, and reduce density of the very ductile base alloy HfNbTaTiZr. Despite the diversity of crystal structures among the constituent elements, all the HEAs are single solid solution phase with body-centered cubic (BCC) structure. The addition of Al significantly improves the strength but reduces the ductility due to large solution hardening. The linear relation between yield strength and atomic percentage of Al suggests that the strengthening effect of a certain element in a single-phase HEA alloy can be explained based on quasi-binary alloy concept. Crack formations in deformed AlHfNbTaTiZr alloy with the lowest fracture strain are mainly along the boundaries between dendrite and interdendrite. This agrees with its large deviation of Al content and thus strength between dendrite and interdendrite. In addition, A system of refractory high-entropy alloys (HEAs) is proposed as a family of candidate materials for structural applications at high temperatures. The primary aim of this study is to design refractory AlxHfNbTaTiZr HEAs and investigate such attractive properties as microstructure and oxidation behavior. In the as-cast condition, these alloys exhibit a non-homogeneous microstructure with pronounced dendritic and interdendritic regions. In terms of oxidation behavior, the addition of Al to all of the HEAs significantly improves their oxidation resistance resulting in oxidation properties very similar to conventional nickel based superalloy. This is because aluminum oxide (i.e., oxide scales) provides an effective barrier to oxidation between 700 oC and 900oC. At a higher temperature of 1300oC exhibits the highest weight gain and hence the worst oxidation resistance, because HEA compositions can cause the oxide scale layers to loosen, which exposes an unprotected layer of the other oxide. Thus, the oxide scales cannot provide a sufficient barrier.

以五种以上主要元素设计的新型耐火高熵合金,已受国内外关注。美国空军实验室近三年开发以耐火金属元素所组成的耐火高熵合金,并证实耐火高熵合金W-Nb-Mo-Ta和W-Nb-V-Mo-Ta其高温强度优于镍基超合金,至1600°C仍有400 MPa的强度,但室温为脆性断裂。此外,在室温下, Hf-Nb-Ta-Ti-Zr合金压缩延迟大于50%,但其高温软化较快。Ti-Zr较轻具有极佳的室温延性,但却有耐火元素先天抗氧化性不佳的缺点,限制其高温的应用性,因此本研究对此合金添加不同含量的铝,以改善此缺点。除研究抗氧化性外,将探讨微结构及量测室温、高温压缩性质。结果发现铝添加0至1.0份间,晶体结构皆简单BCC相。此种合金都呈现高温析出硬化,无铝的HfNbTaTiZr合金,1100°C时效有Hf-Zr相析出于晶界处,添加10at以上铝的合金,在900°C以上时效有Al3(Ti,Zr,Hf) C时铸造的偏析相回溶至基地相,虽无析出强化但却有明显的固溶强化,使硬度上升。无铝的合金在700?900°C氧化有害现象,但添加铝即可,铝含量增加,抗氧化性增加,但900°C以上,高铝的抗氧化效果明显变差。铝的添加,对常温至中温的压缩降压应力有提升,但测试温度升高至900°C以上,呈现明显软化。前者是由第二相的析出强化,后者则由于添加铝,合金理论熔点降低所致。

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