【作者】 阳喜元；

【导师】 胡望宇；

【作者基本信息】 湖南大学 ， 材料科学与工程， 2013， 博士

【摘要】 镍基单晶高温合金为典型的Ni/Ni3Al两相结构，具有良好的抗蠕变、抗断裂、抗氧化和耐腐蚀等性能，是制造航空发动机和工业燃气轮机叶片的关键材料。本文结合改进分析型嵌入原子方法(MAEAM)，利用分子动力学方法(MD)探讨Ni/Ni3Al纳米丝中合金化、界面和尺寸等对纳米丝形变机制的影响，为高温合金材料的设计及其应用提供理论指导。本文首先研究Ni、Al和V纳米丝的尺寸效应，结果表明由于自由表面的变化导致纳米丝的力学和热学性能随着纳米丝尺寸减小而线性减小。纳米丝熔化机制随着尺寸减小而发生变化，相应的临界尺寸分别为0.476nm、0.526nm和0.625nm。对于小于临界尺寸的纳米丝，其熔化主要由原子振动非谐效应所致，相反，表面预熔及其熔化在纳米丝熔化过程中占主导地位。对比分析Ni/Ni3Al(γ/γ’)纳米丝和块体材料的表观界面能，发现块体材料的表观界面能为负，而纳米丝的表观界面能为正，其数值随着界面过渡区(ITR)厚度减小而线性减小。通过研究ITR厚度对原子平均能量的影响关系，得出块体材料ITR的临界厚度约等于1.7nm。探讨了γ’-相体积分数对临界厚度界面区域表观界面能的影响，发现含有(50-70)%γ’-相体积分数的Ni/Ni3Al界面结合最好。对于Ni/Ni3Al纳米丝，通过研究尺寸对纳米丝在一定厚度界面区域能量与其平均能量极限值之差的影响，确定纳米丝的ITR临界厚度约为6.20nm。分析了Ni/Ni3Al纳米丝的延-脆性断裂转变机制，发现Ni/Ni3Al纳米丝的延-脆性断裂转变临界尺寸处于(2.3～3.5)nm之间。研究了合金化元素(Re、Ru、Co、Ta)在Ni3Al合金中的位置取代情况，结果表明Co趋向于置换其中的Ni位，其置换形成能为正，相反，Re、Ru和Ta倾向于取代Al位，且其置换形成能都负。分析了这些合金化元素及其掺杂方式对Ni3Al合金的微观结构和力学性能影响，发现当Re、Ru和Ta等元素置换第一、第三、第一和三近邻Ni原子以及第四近邻以内的所有基体原子时，合金化原子显著增强了Ni3Al合金力学性能，而当这三种元素置换第二、第四、第二和四近邻的Al原子时，对Ni3Al合金力学性能的强化效应不明显。而对于Co元素，无论用哪种置换方式，对Ni3Al合金力学性能强化效应不明显。详细分析各溶质元素对合金力学性能的强化机制，发现溶质原子的尺寸及其与基体原子间相互作用是力学性能增强的主要因素。研究了溶质(Re、Ru、Co、V和Ta)团簇对Ni/Ni3Al纳米丝的力学性能影响，发现纳米丝体模量随溶质团簇尺寸变化关系与其在块体材料中变化趋势不相同。其中，Re团簇增强纳米丝的力学性能，而且这种增强效应随着尺寸增大而变得更加明显，Ru和Co团簇对Ni/Ni3Al纳米丝的力学性能基本没有影响， V和Ta团簇的加入有降低合金纳米丝力学性能的趋势，且这种趋随着团簇尺寸增大变得更为明显。Ni/Ni3Al纳米丝在外加载荷条件下的尺寸效应研究表明，合金纳米丝的屈服强度和弹性模量随尺寸增大而变大，而合金纳米丝的屈服应变随着尺寸的增大而减小，三者随着尺寸不断变化而逐渐接近于一极限值，进一步研究发现层错滑移是纳米丝形变产生的主要原因。通过对Ni/Ni3Al纳米丝拉伸应变温度效应的研究，发现其形变主要归结于层错滑移和原子振动非谐效应，随着温度升高，纳米丝形变主要因素由以层错滑移为主逐步转向原子振动的非谐效应为主。研究合金化(Re、Ru、Co、Ta和V)团簇对Ni/Ni3Al纳米丝的力学性能及其机制的影响。除Co团簇外，其他合金化团簇的加入明显改变了纳米丝的力学性质。讨论合金化(Re、Ru和Ta)团簇对50%γ’-相Ni/Ni3Al纳米丝的形变行为及其微观机制的影响，研究表明，低温下，含Re和Ru团簇纳米丝的形变主要由位错滑移所致，且合金化团簇强烈地阻碍位错运动，使滑移现象局限在其周围，并在此出现颈缩现象。而在高温条件下，由于原子振动非谐效应，Re团簇无法有效地阻止位错滑移，而Ru团簇仍然保持完好结构，有效地阻止体内位错滑移，与其在低温形变机制类似。而含Ta团簇的纳米丝形变却不同，形变范围仅限于Ta团簇周围，其形变机制主要是由Ta原子尺寸及其非谐效应所致。更多还原

【Abstract】 Due to the good creep, fracture resistance, antioxidation and corrosionresistances, Ni-based single crystal superalloy with a typical two-phase structure ofNi/Ni3Al, is a key material to produce the blade of aero-engines and industrial gasturbines. In the dissertation, molecular dynamics (MD), together with the modifiedanalytic embedded-atom method (MAEAM), is used to explore the effects of thealloying elements, interface and their influence on the deformation mechanisims ofNi/Ni3Al nanowire (NW), which can provide theoretical guidances for the designs andapplications of advanced high-temperature alloy materials.In the dissertation, the size effect of Ni, Al and V NWs are firstly studied. Theresults obtained indicate that the mechanical and thermodynamic properties decreaselinearly with the size decreasing as result of the existence of the free surface. We findout that the melting mechanism of Ni, Al and V NWs also changes with the sizedecreasing and the corresponding critical size of these NWs is about0.476nm,0.526nm and0.625nm, respectively. For the small-sized NWs, the melting behaviorresults mainly from the anharmonic effect of atomic vibration, on the contrary, for thelarge-sized NWs, the free surface plays a predominant role in the surface premeltingand melting transition of NWs.We calculate the apparent interfacial energy (AIE) of the bulk and NWs. The AIEvalues of the bulk are negative; in contrast, those of NWs are positive. And the AIEmagnitudes of the bulk and NWs decrease linearly with the decreasing thickness ofthe interfacial transition region (ITR). For the bulk materials, the critical thickness ofITR is about1.7nm by examining the thickness dependence of the average energy ofatom. We discuss the γ’-phase volume (γ’-VF) effect on the apparent interfacial energyof ITR with the critical thickness. The results show that the value of AIE is the lowestas γ’-VF ranges from50%to70%, which indicates that the bonding strength of theNi/Ni3Al interface with these γ’-VFs is the most excellent. For the Ni/Ni3Al NWs with50%γ’-VF, we estimate that the critical thickness of ITR is about6.20nm bymonitoring the size dependence of the differentia between the average energy of ITRand its limited value. Successively, the ductile-brittle fracture transition mechanism ofNW is in detail discussed. We discover that the critical size of discriminating betweenthe ductile-brittle transition behaviors is in the range of (2.3～3.5) nm. The site preference of the alloying elements (Re, Ru, Co and Ta) in the Ni3Alalloy is studied, which shows that the Co atom prefers to substitute the Ni site and itssubstituting formation energy is positive, conversely, the Re, Ru and Ta atoms preferto occupy the Al site and the substituting formation energies are less than zero. Westudy the effect of the alloying elements, together with the substituting manners, onthe microstructure and the mechanical properties. When these alloying elements (Re,Ru and Ta) substitute the1st,3rd,1stand3rdnearest neighboring Ni atoms or all matrixatoms within the4thnearest neighboring distance, the mechanical property of Ni3Alalloy is significantly enhanced. In contrast, as the alloying elements replace the2nd,4th,2ndand4thnearest neighboring Al atoms, the strengthening effect of the solutedoping on the mechanical property is obscure. In addition, no matter what types of thedoping manners, the strengthening effect of the Co element is not obvious. Thedetailed analysis is focused on the strengthening mechanism of the alloying elementson the mechanical property, which indicate that there are two main factors, namely thesize of the solute atom and the solute-solvent interaction, to control the strengtheningeffect. According to the solute (Re, Ru, Co, V and Ta) cluster having an effect on themechanical properties of Ni/Ni3Al NWs, we find out that the evolution tendencies ofthe bulk modulus varying with the incremental size are apparently different fromthose of the bulk counterpart. The mechanical properties of Ni/Ni3Al NWs with Realloying cluster increase apparently with the cluster size increasing. The sizes of theRu and Co alloying clusters have a weak effect on the improvement in the mechanicalproperties. And for the V and Ta cluster, their addition can reduce the mechanicalproperties, the trends of which are more and more prominent as the cluster sizeenlarges.The size effect on the mechanical properties of Ni/Ni3Al NW is studied with agiven external load. The obtained results indicate that the yield strength and theelastic modulus increase exponentially and the yield strain descreses exponentiallywith the increasing size. And three of them are gradually close to the limit value asthe wire size reaches a critical one. Then, we investigate the deformation mechanismof Ni/Ni3Al NW and find out that the fualt slip is the decisive factor. The temperaturedependence of the stress-strain process of Ni/Ni3Al NW is in detail discussed, whichindicates that the deformation of NW is mainly ascribed to the anharmonic vibrationand dislocation slip. The dominant function of the above factors is different with thetemperature increasing. Under the condition of a loading strain, we study theinfluence of the Re, Ru, Co, Ta and V clusters on the mechanical propert ies of Ni/Ni3Al nanowire. The doping solute clusters except the Co cluster can soften themechanical properties of NW.Finally, taking Ni/Ni3Al NW with the Re, Ru and Ta cluster locating in theγ’-phase as an example, we analyze the temperature dependence of the mechanicalproperties of nanowire and detailedly discuss the deformation behavior and itsinherent mechanism of wire. For NW with the Re cluster, the deformation results fromthe dislocation slip at the lower temperatures. Because the cluster impedes thedislocation motion, the slip phenomenon occur around the Re cluster and then thenecking generate here. The similar deformation mechanism has been also found inNWs with the Ru cluster at the lower temperatures. At the higher temperatures, thedeformation of NW has yet originated from the dislocation slip. But the Re cluster hasno influence on the dislocation motion for the stronger anharmonic effect. However,the deformation mechanism of NWs with the Ru cluster is in disagreement. Becausethe Ru cluster with a perfect lattice can effectively hold back the dislocation spread,similar to that of NW with the Ru cluster at the lower temperatures. The causes ofdeformation of NWs with the Ta cluster are different. The deformation behavior isalmost confined to the cluster vicinity. Therefore, the deformation mechanism of NWwith the Ta cluster originates mainly from the solute size and the anharmonic effect.更多还原