近年来, 高熵合金, 多种(>=3)元素混合形成的合金, 其独特的化学序赋予了它优异的性能。这一概念也极大地扩宽了合金的开发空间, 不断涌现出具备优异力学性能、抗氢脆和低温的高熵合金, 在深海、 天然气管道运输和极低温等极端服役环境下有很大的应用前景。 然而目前对高熵合金在多场环境中的力学与损伤行为还处于空白阶段, 仍有大量问题有待研究。 因此本研究针对高熵合金在极端环境下的损伤行为进行了一系列多尺度的研究工作。
(1) 通过第一性原理计算了氢在 CoCrFeMnNi 高熵合金的溶解能和扩散系数。结果表明， 高熵合金中独特的晶格畸变导致了局部氢溶解能的分布。 氢的迟滞扩散现象主要源于崎岖的扩散路径、高的势垒和势能面的不对称性。 进一步计算发现氢同时降低不稳定层错能和层错能, 从而促进变形孪晶的形成, 这主要归因于氢和金属原子之间的电子转移。 本研究为揭示高熵氢脆机理和改良抗氢脆性能提
供了理论依据。
(2) 开展了分子动力学模拟和第一性原理计算, 以研究冲击速度和局部化学
序对冲击波载荷下 CrCoNi 中熵合金的层裂行为。随着冲击速度的增加, 微孔成
核位置从晶界向晶内过渡, 以释放多余的外加能量。 在晶界成核过程中, 贫 Cr 区
发生较大的局部非仿射变形, 而产生微孔洞成核。 这主要是因为贫 Cr 区含有较
少的自由电子, 导致该区金属键较弱。 在晶内成核过程中, 有大量位错在狭小的
孪晶片层中产生, 导致位错在此处大量的塞积, 显着增加了局部储能并促进了微
孔洞成核。 这些结果揭示了化学无序的中熵合金的层裂机理。
(3) 通过设计双靶板充氢层裂对比实验惊喜地发现氢抑制层裂现象。进一步采用一系列微观表征手段、开展第一性原理和发展氢环境下跨尺度损伤演化模型,得到了氢抑制层裂机理: 空位被氢钉扎无法参与孔洞的成核, 导致孔洞成核速率降低; 同时氢促进变形孪晶生成, 导致孔洞长大阻力变大, 减少其长大速率。与传统的氢脆认知不同, 这一发现为CoCrFeMnNi合金在极端条件下的应用提供了新的思路。
(4) 本课题组开展了高熵 CoCrFeMnNi 合金在 273、 77 和 4.2K 温度下的拉伸测试, 得到了光滑变形到 4.2 K 的锯齿塑性流动转变过程。基于透射电镜表征结构, 提出了位错相互转换的演化模型, 并且结合位错动力学方程, 得到了描述锯齿流变的模型。 稳定性分析表明, 锯齿流是由位错惯性运动与 LC 锁之间的相互作用引起的, 是由惯性时间尺度和粘性时间尺度之间的竞争导致的。
 

Recently, high-entropy alloys (HEAs), with multiple principal elements (>=3) in equimolar, possesses extraordinary properties due to its unique chemical order. This concept radically expands the development space of alloys, has drawn a surging interest owing to excellent mechanical properties, hydrogen embrittlement resistance and cryogenic property. So HEAs exhibit great application rospects in extreme environments such as deep sea and gas transportation. However, the damage behavior of HEAs in extreme environments is still not clear. In this research, a series of multiscale experiments and simulations on the damage behavior of HEAs in extreme environment have been carried out.
(1) First-principles calculations were performed to investigate the solution and diffusion of hydrogen and its effect on the stacking fault energy of CoCrFeMnNi. It is shown that the unique lattice distortion in HEAs causes a wide distribution of local hydrogen solution energy, and the trapping of hydrogen in low energy sites increases diffusion barriers. The zigzag path and asymmetry of forward and backward diffusion result in the sluggish diffusion of hydrogen. Furthermore, hydrogen reduces unstable and stable stacking fault energies, originated from the transfer of electron between hydrogen and metal atoms, which promotes formation of deformation twins. This
provides a theoretical guidance for designing novel engineering materials with optimal combination of their mechanical properties and hydrogen embrittlement resistance.
(2) Molecular dynamics simulations and first principle calculations is carried out to investigate the effects of impact velocities and local chemical order on spallation micro-void nucleation in CrCoNi medium entropy alloy under shock wave loading. As he impact velocity increases, micro-void nucleation site exhibits a transition from grain
boundary to grain in order to release redundant imposed energy. During intragranular nucleation process, micro-void nucleates in poor-Cr region with large local nonaffine deformation, which is attributed to weak metallic bonds in this position with sparse free electrons. For intergranular nucleation, Franke-like dislocation source forms through
the dislocation reaction, leading to enormous dislocations pile up in a narrow twin stripe, which increases markedly local stored energy and promotes the micro-void nucleation. These results shed light on the mechanism of spallation in chemically complexed medium entropy alloys.
(3) By using a specially designed double-target technique, an unexpected phenomenon of hydrogen-retarded spallation was observed in CoCrFeMnNi HEA under plate impact loading. To reveal the underlying mechanism, a trans-scale statistical damage mechanics model was developed based on microstructural characterization and
first principles calculations. The hydrogen-retarded nucleation of micro-voids is attributed to hydrogen-vacancy complexes with high migration energy, while formation of nano-twins with high resistance reduces their growth rate. These results shed light on the better understanding of hydrogen embrittlement in chemically complex HEAs.
(5) The tensile test of CoCrFeMnNi alloy at 273, 77 and 4.2 K have been carried out, and a transition process from smooth deformation to serrated plastic flow was found. Based on microstructural characterisations at 77 K and 4.2 K, a dislocation evolution model was established to describe the phenomenon of serrated plastic flow.
A stability analysis shows that the jerky flow results from an interaction between dislocation inertial motion with L-C locks. The instability results from a competition between inertial and viscous time scales characterised by a Deborah number.