【作者】 盛洪飞；

【导师】 彭良明；

【作者基本信息】 中国科学技术大学 ， 材料力学与设计， 2014， 博士

【摘要】 多主元高熵合金(简称高熵合金)是近新兴的合金材料,它打破了传统合金中主要组成元素为一种或两种的合金设计理念。多主元高熵合金是由至少五种以上的主要元素构成,而且每种元素原子百分比不超过35%。合金主元增多产生的高熵效应,使晶体易于形成简单体心或简单面心结构,并可能伴有晶间化合物以及纳米晶,从而达到固溶强化、沉淀强化和弥散强化效果。通过合金成分优化设计可以使高熵合金在性能上比传统合金具有更大的优势,例如高硬度、高强度、耐高温氧化、耐腐蚀等。本文利用真空电磁感应熔炼合成了AlxCoCrCuFeNi (x=0.5,1.0,1.5)(简记为Alx)六主元高熵合金,同时,为进一步提高多主元高熵合金的综合性能,我们通过原位自生合成反应制备了TiC颗粒增强Al0.5高熵合金基(Al0.5CoCrCuFeNi-y vol.%TiC (y=5,10,15),简记为Al0.5-TiCy)复合材料,并对以上材料进行了不同温度的高温时效热处理；利用X射线衍射、高分辨率扫描电镜、高分辨率透射电镜、MTS力学试验系统及显微硬度计结合,对微观组织、力学性能进行了分析讨论。此外,对高熵合金潜在应用于高温环境的特点,本文亦对六主元Alx(x=0.5,1.0,1.5)高熵合金及Al0.5-TiCy (y=5,10,15)复合材料的高温氧化行为进行了讨论。研究发现,对于Alo.5CoCrCuFeNi高熵合金,自然冷却(缓冷)形成的树枝晶结构由简单面心立方固溶体组成,而浇铸(快速冷却)形成的等轴晶结构中生成了少量体心立方结构相。经600℃、24h退火热处理后,树枝晶结构Al0.5高熵合金中有少量体心立方结构相生成。同时大量针状富Cu纳米相在枝晶间区域析出,退火后压缩屈服强度由487MPa增至600MPa。等轴晶结构的Al0.5高熵合金的退火前后组织结构无显著变化,铸态屈服强度相比树枝晶大幅提高了11.9%。不同Al元素含量等轴晶结构AlxCoCrCuFeNi六主元高熵合金研究结果显示,Al含量均匀分布于基体及晶间区域,对合金中元素分布无显著影响,Co、Cr、Fe、Ni元素主要分布于基体中,而Cu元素偏聚于晶间区域。然而由于原子尺寸效应,合金中固溶体晶体结构随着Al含量的增加发生晶格畸变,基体主晶相由面心立方结构变为体心立方结构,且构成基体的BCC相发生调幅分解。当Al元素摩尔比由0.5增加至1.0时,合金基体内伴随有大量板条状和球形富Cu纳米相析出,致使Alx高熵合金获得优异的综合力学性能,其断裂强度高达1739.3MPa,同时压缩率高约12.1%。Al含量继续增加,合金硬度和强度得以进一步提高,但其塑性下降较明显,断裂形式由韧性断裂变为准解理断裂。结合多种经典强化理论与实验数据综合分析发现,Alx高熵合金的主要强化方式为固溶强化与沉淀析出强化。实验所制Al0.5-TiCy (y=5,10,15)复合材料基体为树枝晶,TiC颗粒均匀分布其中。随着TiC增强相体积分数的增加,TiC陶瓷颗粒尺寸由Al0.5-TiC5的200nm长大至Al0.5-TiC15的3um。Al0.5-TiCy (y=5,10,15)复合材料室温压缩屈服强度分别为740MPa、709MPa及680MPa。原位自生TiC颗粒增强Al0.5高熵合金基复合材料的力学性能较之基体有很大提高,以Al0.5-TiC5为例,其室温压缩屈服强度相对于Al0.5高熵合金基体提高近50%。Alx (x=0.5,1.0,15)高熵合金及Al0.5-TiCy (y=5,10,15)复合材料的高温氧化试验分别在850、950、1050℃与950℃下大气环境中进行。实验结果表明,Alo.5高熵合金在850、950℃氧化动力学相同,均符合抛物线规律,但当温度升至1050℃时,其氧化动力学曲线基本呈线性,氧化层开裂和剥落严重,导致出现严重的内氧化。与此不同的是,高Al含量的Al1.0CoCrCuFeNi和Al1.5CoCrCuFeNi合金在以上三个温度下的长时氧化行为均符合抛物线规律,,且氧化程度相对较轻。原因在于Al含量增加致使大量Al2O3致密氧化膜生成从而提高了合金的高温抗氧化性能。相比Al0.5高熵合金基体,TiC颗粒增强复合材料高温氧化增重明显减小,氧化速率显著降低,其中Al0.5-TiC5材料950℃下经100h氧化后增重仅为0.5mg/cm2。更多还原

【Abstract】 Multi-elemental high-entropy alloy (HEA) has drawn extensive attentions in the past decade. As a new type of alloy, HEA is composed of at least five principle elements with the content of each component less than35%, which has gone beyond the designing concept of traditional alloys that basing on only one or two major elemental compositions. The high entropy effect in HEA induced by the increased species of major components results in the easily formation of simple body centered (BCC) or face centered (FCC) structures, and probably accompanied by the formation of inter-crystalline compounds or nano-crystals. Therefore, three strengthening effects can be achieved:solid solution strengthening, precipitation strengthening and dispersion strengthening. Through optimizing the composition, HEAs are capable of possessing higher hardness, higher strength, and better high temperature oxidation and corrosion resistance than traditional alloys.In the present study, AlxCoCrCuFeNi (x=0.5,1.0,1.5)(denoted as Alx in the following section) high entropy alloys were prepared using the medium frequency electromagnetic induction method. With the aim of improving the overall performance of HEA, TiC particle-reinforced Al0.5CoCrCuFeNi alloy based composites with different volume fractions of TiC, Al0.5CoCrCuFeNi-y vol.%TiC (y=5,10, and15, denoted by Al0.5-TiCy), were synthesized by in situ reaction. Furthermore, aging procedure was performed on the above materials at different temperatures. The microstructure and mechanical properties of these self-produced alloys were investigated using high resolution transmission electron microscope, scanning electron microscope, material testing system and hardness tester. In addition, the high temperature oxidation behavior of Alx alloys and Al0.5-TiCy composites were also examined as HEAs are potential in high-temperature applications. The findings and conclutions have been drawn as follows.Al0.5CoCrCuFeNi alloy with dentrite crystal structures formed with lower cooling rate is composed of simple FCC solid solution. However, small amount of BCC structured phases were generated in the equiaxed polygrain structured Al0.5alloy prepared by induction melting and casting (rapid silidification). Intriguingly, small amount of phases with BCC structure can be also produced in the dentrite structured Al0.5alloy after subjected to annealing at600℃for24h. Moreover, large amount of Cu-rich nano-precipitations existed in the interdendritic regions after annealing treatment, and the compressive yield strength was increased from487MPa to600MPa. In contrast, no obvious variation of phase composition was found in the Al0.5alloy with equiaxed structure under annealing, and its compressive yield strength was enhanced by11.9%compared to the alloy with dendrite structure in as-cast state.Microstructural investigations of the dentrite structured Alx (x=0.5,1.0and1.5) alloys revealed that Co, Cr, Fe and Ni are majorly distributed in grain matrices while Cu is riched in grain boundary regions. It indicates that the molar fraction of Al has insignificant effects on the element distribution of Alx series of HEAs. However, lattice distortion occurred in the crystal structure of solid solutions with increasing Al content due to the obvious atomic size difference. As a result, the main FCC phase transformed into BCC accompanied by spinodal decomposition of the latter. It is worth noting that large amount of strip and spherical Cu-rich nano-phases were precipitated with x increasing from0.5to1.0, leading to significant improvement in the overall mechanical properties of the alloy. The ultimate strength of Al1.0alloy reached1739.3MPa and the extension rate was more than12%. The hardness and strength of the alloy can still be enhanced via further increasing Al content. Nevertheless, remarkable degradation of plasticity occurred, and the Alx alloys exhibited variations in the failure modes from ductile fracture to cleavage fracture. The results indicated that solid solution strengthening and precipitation strengthening are the prominent strengthening mechanism of Alx series of HEAs.The matrices of in situ synthesized Al0.5-TiCy (y=5,10and15) composites exhibited dentrite structured with TiC particles distributed homogeneously. With the increase of the TiC volume fraction, the average size of TiC ceramic particles increases from200nm of Alo.5-TiC5to3um of Al0.5-TiClO. The compressive yield strength of Al0.5-TiCy (y=5,10and15) composites reached740MPa,709MPa and680MPa, respectively. The mechanical properties of TiC particle-reinforced composites have been improved remarkably compared with those of Al0.5HEA. Take Al0.5-TiC5for an example, the compressive yield strength is almost50%higher than that of Al0.5alloy matrix.The high temperature oxidation behavior of Alx alloys and Al0.5-TiCy (y=5,10, and15) composites were examined between850and1050℃in still atmosphere. Results revealed that the oxidation dynamics for Alx alloy exposed at850and950℃are similar, abiding by parabolic law. However, the oxidation kinetic curve becomes almost linear when oxidated at1050℃, with serious cracking and spalling of oxide layer and severe internal oxidation. However, oxidation dynamics for Al1.0and Al1.5alloys with higher Al content deviated from the parabolic raw with much less oxidization. The formation of dense Al2O3film as Al content increases prevented the inner part of the sample from further oxidation. Compared to Al0.5CoCrCuFeNi alloy, the oxidation rate was decreased dramatically in the composites. In particular, the oxidation weight is only0.5mg/cm2for Alo.5-TiC5after oxidized at950℃for100h.更多还原