【作者】 李毅；

【导师】 钟云波；

【作者基本信息】 上海大学 ， 钢铁冶金， 2021， 硕士

【摘要】 共晶高熵合金是一种新型的两相多主元金属复合材料,具有良好的液态流动性和铸造性,近年来引起了材料工作者极大的研究兴趣。然而,目前已报道的各种共晶高熵合金中,具有优异拉伸性能的共晶体系非常有限,而且对其微观变形和强化机制的研究仍有待于进一步深入。此外,通过微调成分法去制备近共晶高熵合金是否可以带来更优异的力学性能,值得材料研究者们的细致探索。本文开发制备了一种Al-Fe-Co-Ni系共晶高熵合金,并通过微调共晶成分衍生出两种近共晶高熵合金。通过多尺度的表征手段和定量的拉伸测试,对它们的微观结构、力学性能、变形行为和强化机制进行了系统的研究和对比。本论文的主要工作内容如下:1)Al_19.25Fe_18.36Co_18.86Ni_43.53为共晶高熵合金,呈现出典型的共晶层片组织,由交替排列的FCC和BCC两相组成。两相间通过半共格界面连接,取向关系为Kurdjumov-Sachs。Al_19.25Fe_18.36Co_18.86Ni_41.53为亚共晶高熵合金,凝固组织中存在大量尺寸和空间分布不均匀的BCC初生相,化学成分与共晶层片里BCC相一致。Al_17.25Fe_18.36Co_18.86Ni_43.53为过共晶高熵合金,凝固组织中存在大尺寸的枝状FCC初生相,元素组成与共晶层片里FCC相的成分一致。2)Al-Fe-Co-Ni系（近）共晶高熵合金具有优异的拉伸性能和应变硬化能力:Al_19.25Fe_18.36Co_18.86Ni_43.53共晶高熵合金的屈服强度～486 MPa,抗拉强度～956 MPa,延伸率～10%。亚共晶Al_19.25Fe_18.36Co_18.86Ni_41.53高熵合金的力学性能与共晶高熵合金基本相同,大量BCC初生相并没有显著地降低性能。过共晶高熵合金Al_17.25Fe_18.36Co_18.86Ni_43.53里具有较多的FCC初生相,导致了强度的下降（屈服强度～405 MPa,抗拉强度～884 MPa）,但延伸率相比亚共晶高熵合金提升了～50%。3)共晶高熵合金在拉伸过程中,FCC层片首先发生屈服变形。随着变形的进行,FCC层片内部产生了高密度的多重位错滑移,诱导一种连续的微结构细化,因而实现了动态的Hall-Petch强化。亚共晶高熵合金中的FCC共晶层片以及过共晶高熵合金中的FCC初生相、FCC共晶层片中同样出现了这种动态的微结构细化和强化,最终带来了稳定持续的应变硬化能力和优异的力学性能。然而,BCC共晶层片和BCC初生相拥有较强的抗变形能力,即使在断后的拉伸样中,也只能观察到少量的变形痕迹。4)共晶高熵合金中由于两相变形能力的不同,FCC层片被拉长变细,BCC层片变形较少,所以断口形貌呈现沟槽状。亚共晶高熵合金中由于难以变形的BCC层片和初生相,所以断口呈现出沟槽状和平面台阶状混合的形貌。在过共晶高熵合金的拉伸变形过程中,初生FCC相不仅提供了强的应变硬化能力,也抑制了共晶组织的过早开裂,因而提高了整体拉伸塑性,断口呈现出大量韧窝和初生相被变形拉长形成的山脊状形貌。更多还原

【Abstract】 The eutectic high-entropy alloy（EHEA）is a new type of two-phase multi-principal-element metallic composite with good liquid fluidity and castability,which has attracted great research interest of material workers in recent years.However,the eutectic systems with excellent tensile behaviors are very limited among the various EHEAs reported so far,and the research on the micro-deformation and strengthening mechanisms of EHEAs still needs to be further studied.Furthermore,whether the near EHEAs prepared through fine-tuning composition method can result in better mechanical properties is worthy of careful exploration by material researchers.In this paper,an Al-Fe-Co-Ni-type EHEA was developed and prepared,and two kinds of near EHEAs were derived by fine-tuning the eutectic composition.Their microstructures,mechanical properties,deformation behaviors and strengthening mechanisms were systematically studied and compared by means of multi-scale characterizations and static tensile tests.The main work contents of this paper are as follows:1)Al_19.25Fe_18.36Co_18.86Ni_43.53 is a kind of EHEA,which exhibits a typical eutectic lamellar structure and consists of FCC and BCC phases arranged alternately.The two phases are connected by a semi-coherent interface and the orientation relationship is Kurdjumov-Sachs.Al_19.25Fe_18.36Co_18.86Ni_41.53 is a hypo-EHEA.In the solidified microstructure,there are a large number of the BCC primary phases with heterogeneous size and spatial distribution,and their chemical composition is consistent with that of the BCC phase in eutectic lamellae.Al_17.25Fe_18.36Co_18.86Ni_43.53 is a hyper-EHEA.In the solidified microstructure,there are many large-scale dendritic primary FCC phases,and their chemical composition is consistent with that of the FCC phase in eutectic lamella.2)The Al-Fe-Co-Ni-type（near）EHEAs have excellent tensile properties and strain hardening capability.The yield strength,tensile strength and elongation of Al_19.25Fe_18.36Co_18.86Ni_43.53 EHEA are 486 MPa,956 MPa and 10%,respectively.The mechanical properties of Al_19.25Fe_18.36Co_18.86Ni_41.53hypo-EHEA are basically the same as that of EHEA,and a large number of BCC primary phases do not significantly reduce the performance.There distribute more FCC primary phases in hyper-EHEA Al_17.25Fe_18.36Co_18.86Ni_43.53 that result in the decrease of strength（yield strength of～405MPa and tensile strength of～884 MPa）,but the elongation is increased by～50%compared with the hypo-EHEA.3)During the tensile process of EHEA,the yield deformation of the FCC lamellae firstly occurs.As the deformation progresses,high-density multiple dislocation slips are promoted in the FCC lamellae,which induces a continuous microstructure refinement,thus realizing the dynamic Hall-Petch strengthening.This dynamic microstructure refinement and strengthening also occur in the FCC eutectic lamellae of the hypo-EHEA and in the FCC primary phases and eutectic lamellae of the hyper-EHEA,which ultimately leads to stable and sustained strain hardening ability and excellent mechanical properties.However,the BCC eutectic lamellae and primary phases feature strong resistance to deformation,and only a small amount of deformation traces can be detected even in the post-fractured tensile samples.4)Due to the different deformation ability of two phases in EHEA,the FCC lamellae are elongated and becomes thin,while the BCC lamellae are less deformed,so the fracture appears groove shape.Owning to the hard-to-deform BCC lamellae and primary phases in the hypo-EHEA,the fracture presents a mixed morphology of groove and plane step.During tensile deformation process of the hyper-EHEA,the primary FCC phases not only provide strong strain-hardening ability,but also inhibit the premature cracking of the eutectic microstructure,thus improving the overall tensile plasticity.The fracture shows massive dimples and ridge morphology formed by deformation and elongation of the primary phases.更多还原