【作者】 张海峰；

【导师】 贾楠；

【作者基本信息】 东北大学 ， 材料学， 2022， 博士

【摘要】 近年来,高熵合金(High Entropy Alloys,HEAs)凭借其优异的力学性能、耐腐蚀性和抗辐照性而备受瞩目。然而,这类合金中复杂的化学成分配比导致借助传统的微结构实验表征手段难以构建材料成分、结构与性能三者之间的定量关系。本文通过跨尺度模拟手段对等原子比FeMnCoCrNi高熵合金的微观形变机制和力学行为进行了系统研究。首先,采用第一性原理(First-principles)方法计算了以不同含量的Co和Cr替代Ni时合金的弹性常数、理想拉伸强度和层错能。随后,对此合金单晶和不同取向的双晶微柱在单轴拉伸过程中的力学行为和变形机制进行了分子动力学(Molecular Dynamics,MD)模拟研究,进一步揭示了形变过程中逆相变发生的微观本质以及晶格畸变和层错能对面心立方(Face-centered Cubic,FCC)高熵合金微观形变机制的影响。论文取得的主要研究结果如下:通过第一性原理计算方法对不同Co和Cr含量下FeMnCoCrNi高熵合金弹性常数(Elastic Constant)、理想拉伸强度(Ideal Tensile Strength,ITS)和层错能(Stacking Fault Energy,SFE)的研究表明,随着Co含量增加,面心立方相的机械稳定性、体弹性模量B、杨氏模量E、剪切模量G和理想拉伸强度单调增大;柯西压力CP、Pugh比B/G、泊松比v、Zener各向异性比Az和弹性各向异性比AVR单调减小;层错能降低,对应于其塑性变形机制由位错滑移转变为孪生,再到ε-马氏体相变。随着Cr含量增加,面心立方相的弹性常数和理想拉伸强度变化相对复杂,但Cr元素添加对层错能和塑性变形机制的影响与Co元素一致。Co或Cr的增加导致Fe和Mn的磁矩减小。这是导致晶格常数和层错能随着Co含量的增加而单调下降的原因之一。然而,当用Cr替代Ni时,包括价电子浓度、磁性和化学键等在内的多种因素会引起晶格常数和层错能的改变。对具有不同初始晶体取向的FeMnCoCrNi高熵合金单晶的分子动力学模拟研究表明,在单晶取向与加载方向平行的单轴拉伸过程中,[001]、[111]和[112]取向的单晶中均以层错为主要的形变机制,且各单晶内总位错密度比[110]和[123]取向的高。层错、孪生和ε-马氏体相变共同促进[110]单晶的塑性变形,且孪晶的开动较其它单晶显著。[123]单晶中则以ε-马氏体相变为主导的形变机制。此外,与低熵材料Fe50Ni50和Cu相比,高熵合金中较大的晶格畸变(Lattice Distortion,LD)更易促进位错形核,从而导致其发生弹性向塑性转变所需的临界剪切应力较低。同时,高的晶格畸变导致高熵合金在形变中存储位错的能力最强。对具有不同轴比的[110]取向FeMnCoCrNi高熵合金单晶在单轴拉伸载荷下的力学行为模拟表明,当加载方向平行于[110]晶向时,随着轴比由2降低至0.5,模型的流变应力增高,均匀塑性变形的能力增强。当轴比为0.5时,合金在塑性变形中发生了显著的逆相变,同时表现出优异的塑性。此外,对具有0.5轴比的FeMnCoCrNi、FeCuCoCrNi高熵合金和纯Cu微观形变的对比揭示了层错能是导致三种面心立方材料微观形变机制存在差异的主要因素。高熵合金中较高的晶格畸变未对其形变机制产生重要影响。肖克莱不全位错的滑移是导致三种材料中发生双向相变、孪生和去孪生的本征因素。利用分子动力学模拟方法对具有不同初始取向组合的FeMnCoCrNi高熵双晶微柱在单轴拉伸载荷下的研究表明,对所有双晶模型而言,位错优先在晶界处形核并向两侧的晶粒内滑移。在形变过程中,晶界发生了不同程度的宽化和弯曲。当晶界与拉伸方向垂直时,颈缩易于在晶界处发生,这导致双晶的流变应力随外加载荷增大而降低。而当晶界平行于拉伸方向时,在整个塑性变形过程中模型均保持1 GPa以上的流变应力。相较于其它双晶而言,[111]与[110]取向组合的双晶流变应力波动幅度最大,同时呈现出最高的加工硬化能力。其中应力的下降被归因于大量位错发生了滑移,而高的硬化能力则是由较多的ε-马氏体、层错以及孪晶形成所致。此外,与纯Cu相比,FeMnCoCrNi和FeCuCoCrNi高熵合金中的晶格畸变使得晶界较为粗糙,从而在外加载荷下位错易于形核,且层错能较低的FeMnCoCrNi中形成的ε-马氏体最多。上述研究结果揭示了 FeMnCoCrNi高熵合金在塑性变形中微观形变的物理本质,丰富了不同尺度下材料的形变理论,有助于建立面心立方结构高熵合金中成分、结构与性能之间的关系,并可为设计具有良好强度与塑性匹配的多主元合金提供理论指导。更多还原

【Abstract】 In recent years,high entropy alloys(HEAs)have attracted great attention for their excellent mechanical properties,corrosion and radiation resistance.However,the complex chemical composition ratio in these alloys makes it difficult to construct the quantitative relationship among composition,structure and properties with the help of traditional microstructure experimental characterizations.In this paper,the microdeformation mechanisms and micromechanical behaviors of FeMnCoCrNi HEA with equal atomic ratio were systematically studied by cross-scale simulation.Firstly,the elastic constant,ideal tensile strength(ITS)and stacking fault energy(SFE)are calculated by first-principles method when replcacing Ni with different contents of Co and Ni.Subsequently,the mechanical behaviors and deformation mechanisms of the FeMnCoCrNi single crystal and bicrystal micropillars with different initial orientations during uniaxial tensile deformation were simulated by molecular dynamics(MD).The micromechanisms of the reverse phase transformation and the effects of lattice distortion(LD)and SFE on the microdeformation mechanisms of the face-centered cubic(FCC)HEAs were further studied.The main research results obtained are as follows:The elastic constant,ITS and SFE of FeMnCoCrNi HEA with different Co and Cr contents are studied by first-principles calculations method.The results show that for the alloys with Co substitution for Ni,the elastic stability of the FCC phase,bulk elastic modulus(B),Young’s modulus(E),shear modulus(G)and ITS increase monotonically with increasing Co content.However,the Cauchy pressure(CP),Pugh ratio(B/G),Poisson ratio(v),Zener anisotropy ratio(AZ)and elastic anisotropy ratio(AVR)decrease monotonically.The SFE also decreases with the increase of Co,resulting in the change of plastic deformation mechanisms from dislocation slip to mechanical twinning,and then to ε-martensitic transformation.The Cr substitution for Ni leads to the more complex change of elastic constants and ITS of FCC phase.The increase of Cr shows the similar effect on SFE and deformation mechanisms as that of Co.The increase of either Co or Cr leads to the reduced magnetic moments of Fe and Mn.This could be responsible for the monotonic decrease of both lattice constant and SFE as the Co content increases.However,when replacing Ni with Cr,multiple factors including valence electron concentration,magnetism and chemical bonding ect.,can affect the evolution of lattice constant and SFE.In the uniaxial tensile process where the orientation of single crystal is parallel to the loading direction,the molecular dynamics simulation results of FeMnCoCrNi HEA single crystals with different initial orientations show that the initiation of stacking faults is the main plastic deformation mechanism for the crystals initially oriented with[001],[111]and[112],and the total dislocation densities in these crystals are higher than that with the[110]and[123]orientations.Stacking faults,twinning,and ε-martensitic transformation jointly promote the plastic deformation of the[110]orientation,and twinning in this crystal is more significant than that with other orientations.Deformation in the crystal oriented with[123]is dominated by the ε-martensite transformation.Besides,the comparison in the FeMnCoCrNi HEA and the conventional materials,i.e.FesoNiso and Cu show that the larger LD in the HEA makes dislocation slip tends to be activated more readily,leading to the lower critical shear stress for dislocation nucleation and elastic-plastic transition in the former.Meanwhile,higher LD results in the increase of the dislocation storage capacity of HEAs.In the uniaxial tensile process where the[110]orientation is parallel to the loading direction,the simulation results of mechanical behaviors for[110]-oriented FeMnCoCrNi HEAs with different aspect ratios show that,during deformation,strain-hardening ability of the FeMnCoCrNi HEA increases as the aspect ratio decreases from 2 to 0.5.When the aspect ratio is 0.5,the reverse transformation is more significant compared with other models,while a good plasticity can still be maintained.Besides,the microdeformation comparisons of the FeMnCoCrNi,FeCuCoCrNi HEAs,and pure Cu with the 0.5 aspect ratio show that the SFE plays a major role in the activation of different deformation mechanisms in the three FCC materials.However,the LD in the HEA does not significantly affect the activation of deformation systems.The Shockley partials slip leads to bidirectional phase transition,twinning and de-twinning in the three materials.Molecular dynamics simulation method was used to study the FeMnCoCrNi HEA bicrystal micropillars with different initial orientation combinations deformed under uniaxial tension.The results show that for all models studied,dislocations nucleate preferentially at the grain boundary and slip into the grains on both sides.During deformation,grain boundaries are widened and curved.When the grain boundary is perpendicular to the tensile direction,necking tends to occur at the grain boundary,which leads to the decrease of flow stress with increasing loading.For the model with grain boundary parallel to the deformation direction,the flow stress of the model remains at a level above 1 GPa during the whole plastic deformation.Compared with other models,the bicrystals with a combination of[111]and[110]orientations show the most significant fluctuation of flow stress and the highest work hardening ability.The decrease of stress with deformation is attributed to the slip of a large number of dislocations,while the high strain hardening ability is caused by the formation ofε-martensite,stacking faults and twins.In addition,compared with pure Cu,the larger LD in FeMnCoCrNi and FeCuCoCrNi HEAs makes grain boundaries coarser,which makes dislocations easier to nucleate under loading,and ε-martensite is the most remarkable in FeMnCoCrNi HEA with the lower SFE.The research results above reveal the physical mechanisms of the microdeformation mechanisms of HEAs,enrich the material deformation theories at different scales,help to establish the relationship between composition,structure and property of FCC structured FeMnCoCrNi HEA,and provide theoretical guidance for the design of multi-principal component alloy with advanced mechanical properties.更多还原