目的 揭示镍基高温合金（GH4169）在激光冲击诱导极端应变率下拉伸变形力学响应的变化规律,以及材料微观组织结构的演变机制。方法 采用分子动力学建立极端应变率条件下不同合金元素组分及晶体结构的拉伸变形仿真分析模型。结果 在109 s-1应变率条件下拉伸,3种合金元素组分单晶模型的弹性模量分别为240、237、217 GPa,极限抗拉强度分别为28.3、27.1、17.6 GPa;对于8种合金元素组分的模型,在108、109、1010 s-1等3种应变率条件下拉伸,单晶模型的弹性模量均为217 GPa,极限抗拉强度分别为17.1、17.6、18.7 GPa;多晶模型的弹性模量分别为213、220、226 GPa,极限抗拉强度分别为9.6、10.3、11.8 GPa。结论 8种合金元素组分对应单晶模型的弹性模量和极限抗拉强度与实验结果更接近。在相同拉伸变形条件和特定晶体取向前提下,晶体类型对材料弹性模量的影响较小。另外,单晶模型在极端应变率条件下表现出更高的塑性变形抗力。多晶模型因晶粒尺寸低于10 nm,在变形过程中其位错形态及其分布均表明存在逆Hall-Petch效应,且该效应的形成与材料在极端应变率条件下发生塑性变形时晶粒内部出现的位错增殖导致位错塞积,以及位错滑移至晶界附近并被其吸收这2种现象的进行速度之间的竞争机制有关;在极端应变率拉伸变形过程中,弹性模量对应变率变化的敏感度不高。极限抗拉强度随着应变率的增加而增加,表现出明显的应变率强化现象。此外,当应变率达到1010 s-1时,因位错滑移时间较短,单晶与多晶模型晶粒内部均出现了位错增殖和由位错墙组成的亚晶界。多晶模型因晶粒内部位错增殖速度大于滑移至晶界被吸收的速度,这在一定程度上削弱了逆Hall-Petch效应导致材料塑性变形抗力降低的影响。
Abstract
Laser shock peening (LSP) is a promising surface strengthening technology. Since the deformation with extreme strain rates occurs in the surface material of reinforced components under the action of tensile stress waves induced by LSP, spallation defects are prone to form. In this study, a molecular dynamics model based on the "EAM+L-J" hybrid potential function was developed to investigate the variation law of tensile deformation mechanical response of nickel-based superalloy (GH4169), and the microstructure evolution of the material was also analyzed for different alloy compositions, crystal structures and strain rates. At a strain rate of 109 s-1, the Young's modulus of three different single crystal models with varying alloy compositions was found to be 240, 237, and 217 GPa, while their ultimate tensile strength was 28.3, 27.1, and 17.6 GPa, respectively. For the single crystal model with eight alloying elements, at strain rates of 108, 109, and 1010 s-1, the Young's modulus remained constant at 217 GPa, while the ultimate tensile strength increased with the strain rate, reaching 17.1, 17.6, and 18.7 GPa, respectively. In the polycrystalline model, the Young's modulus was measured as 213, 220, and 226 GPa under the different strain rate conditions, whereas the ultimate tensile strength values were 9.6, 10.3, and 11.8 GPa, respectively. The results indicated that the Young's modulus and ultimate tensile strength obtained from single crystal model with eight alloy compositions were closer to the experimental results and under identical tensile deformation conditions and specific crystal orientation, the Young's modulus of the single crystal model (217 GPa) and the polycrystalline model (220 GPa) showed minimal differences, suggesting that crystal structure had a relatively small impact on the Young's modulus. In addition, the single crystal model exhibited higher plastic deformation resistance under extreme strain rate condition through the comparison with the polycrystalline model. The polycrystalline model showed the existence of inverse Hall-Petch effect in terms of dislocation morphology and distribution during deformation due to the fact that the grain size was less than 10 nm. This effect arose from the competition between dislocation pile-up within grains and their absorption at grain boundaries during plastic deformation under extreme strain rates. As the grain size decreased, the proportion of grain boundaries increased, enhancing their ability to absorb dislocations, reducing dislocation pile-up, and lowering the probability of crack initiation within grains. Moreover, during extreme strain rate tensile deformation, the Young's modulus was insensitive to the changes in strain rates, showing an obvious strain rate strengthening phenomenon. Furthermore, dislocation multiplication and sub-grain boundaries composed of dislocation walls were observed in both single and polycrystalline models because of the extremely short dislocation slip time when the strain rate reached 1010 s-1. Additionally, in the polycrystalline model, the dislocation multiplication rate surpassed the rate of which dislocations slid to and were absorbed by grain boundary within the grains, partially weakening the impact of inverse Hall-Petch effect on the reduction of material plastic deformation resistance. This study can provide valuable theoretical references for analyzing the spallation formation mechanism induced by LSP for nickel-based superalloy.
关键词
激光冲击 /
镍基高温合金 /
逆Hall-Petch效应 /
极端应变率 /
分子动力学
Key words
laser shock /
nickel-based superalloy /
inverse Hall-Petch effect /
extreme strain rate /
molecular dynamics
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基金
国家自然科学基金（51575117）;湖南省自然科学基金（2019JJ50519）;湖南省教育厅项目（20C1601,20C1607）
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