Nb含量对Al_(0.3)CoCrFeNi高熵合金组织与性能的影响

Microstructure and Properties of Al_(0.3)CoCrFeNi High Entropy Alloy with Different Nb Contents

采用真空电弧炉制备了Al_(0.3)CoCrFeNiNb_(x)(x为摩尔比,x=0,0.1,0.3,0.5,0.7,1.0)高熵合金棒材,用X射线衍射(XRD)、光学显微镜(OM)、扫描电镜(SEM)、能谱仪(EDS)、显微维氏硬度计和万能试验机对合金的微观组织和性能进行了研究。结果表明,Al_(0.3)CoCrFeNi合金的微观组织为单相面心立方(fcc);添加Nb后,合金由fcc相和Laves相组成,并随着Nb含量的增加,Laves相体积分数逐渐增多;至Al_(0.3)CoCrFeNiNb_(1.0)时,基体相从fcc相变为Laves相。Al_(0.3)CoCrFeNiNbx合金组织随Nb含量变化经历了亚共晶到过共晶的转变,同时衍射峰(111)fcc先向小角度移动后向大角度移动,晶格常数先增大后减小。Nb元素的增加可以有效提高Al_(0.3)CoCrFeNiNb_(x)合金的强度和硬度,硬度呈近似线性增加,从Al_(0.3)CoCrFeNi合金的HV152提高至Al_(0.3)CoCrFeNiNb_(1.0)合金的HV 745,提高了390%;Al_(0.3)CoCrFeNi合金和Al_(0.3)CoCrFeNiNb_(0.1)合金呈现出优异的压缩塑性(不断裂);随着Nb含量的继续增加,Al_(0.3)CoCrFeNiNb_(x)合金的抗压强度先提高后降低,而塑性降低。Al_(0.3)CoCrFeNiNb_(0.5)合金具有最优的综合力学性能,其抗压强度和维氏硬度分别为2397 MPa和HV 613。

High entropy alloys have broken the traditional alloy design concept and expanded the research fields.Due to the interaction between the multi-principal component atoms,high entropy alloys exhibit excellent properties,such as high strength,hardness and wear resistance,good corrosion resistance and oxidation resistance,which has a diversified industrial application prospect.Compared with conventional alloys,the microstructure of high entropy alloys exhibits single face-centered cubic(fcc)phase,single bodycentered cubic(bcc)phase or dual phases(fcc+bcc).Al-Co-Cr-Fe-Ni system is a relatively mature system for researchers at home and abroad.The phase composition of Alx CoCrFeNi alloy changes from single phase fcc,dual phase and single-phase bcc with increasing Al content.Among them,Al_(0.3)CoCrFeNi alloy with single phase fcc exhibit good plasticity and corrosion resistance,but low hardness.The negative mixing enthalpy between Nb and other elements and maximum atomic size in Al_(0.3)CoCrFeNiNb can result in strong precipitation strengthening or eutectic structure.Therefore,Al_(0.3)CoCrFeNiNb_(x)(mole ratio,x=0,0.1,0.3,0.5,0.7,1.0)high entropy alloy rods were prepared by vacuum arc melting.The microstructures were analyzed by X-ray diffraction(XRD),optical microscope(OM)and scanning electron microscope(SEM),and meanwhile the micro Vickers hardness and compression property at room temperature were tested.The effect of Nb content on the microstructure and properties of Al_(0.3)CoCrFeNi alloy were studied.Based on the research results,the optimal composition was obtained.The results showed that the microstructure of Al_(0.3)CoCrFeNi alloy was single phase fcc,and then transformed into eutectic structure composed of fcc and Laves phases after adding Nb.With increasing Nb content,the volume fraction of Laves phase increased.The lattice constant results showed that the diffraction peak the diffraction peak fcc(111)moved first to small angle,and then to large angle,and the lattice constant also first increased and then decreased.Because Nb atom exhibited significantly larger radius than other atoms,it would inevitably lead to lattice distortion when Nb atoms entered fcc solid solution lattice.With increasing Nb content,Nb atoms would be precipitated from fcc solid solution to form Laves phase,so the lattice constant decreased.In addition,Al_(0.3)CoCrFeNi Nb_(0.7)alloy and Al_(0.3)CoCrFeNiNb_(1.0)alloy exhibited significantly lower diffraction peaks intensity than Al_(0.3)CoCrFeNi alloy and Al_(0.3)CoCrFeNi Nb_(0.1)alloy.The results of OM and SEM images showed that the microstructure of Al_(0.3)CoCrFeNiNb_(x) alloy were dendritic,and the microstructure changed from hypoeutectic to hypereutectic.When x=0.5,the microstructure of the alloy changed obviously,and most of the regions were typical eutectic structure composed of fcc and Laves.The two-phase mixed structure was evenly distributed in the cell,and the strip eutectic structure was radial to the periphery.Notably,the primary phase changed from fcc to Laves.EDS analysis showed that Nb was enriched in Laves region,while Al and Ni were enriched in fcc region.Ni and Al atoms exhibited similar fcc structure and were easy to replace each other in the lattice position.While,Nb atom exhibited the largest atomic radius and most negative mixing enthalpy with other atoms,and tended to form Laves phase.Addition of Nb could effectively improve the strength and hardness.With increasing Nb content,the hardness linearly increased from HV 152for Al_(0.3)CoCrFeNi alloy to HV 745 for Al_(0.3)CoCrFeNiNb_(1.0)alloy.The compressive strength first increased,and then decreased.Al_(0.3)CoCrFeNiNb_(0.5)alloy exhibited the best comprehensive mechanical properties,and its compressive strength and Vickers hardness were 2397 MPa and HV 613,respectively.