Alpha titanium (α-Ti) is a promising material for making high-performance components for applications in aerospace, marine, energy and healthcare fields. The excellent strength-ductility synergy has been observed for α-Ti at cryogenic temperature. Twinning is generally considered a key mechanism of outstanding cryogenic ductility. The dislocation-grain boundaries (GBs) interaction and void nucleation usually play crucial roles in the plastic deformation of polycrystalline materials, but their effects on the cryogenic ductility of α-Ti are rarely considered. To eliminate this confusion and gain an in-depth insight into the mechanism of the cryogenic strength-ductility synergy of α-Ti, in this work, a series of characterization experiments and molecular dynamics (MD) simulations were designed and carried out. 1) From uniaxial tension tests of the coarse-grained α-Ti sheets at the temperature from 25 to -180 °C, the uniform elongation and post-necking elongation were increased by 92 % and 20 %, respectively. The material maintained a larger strain hardening rate within a greater range of strain at cryogenic temperature compared with room temperature. 2) Via microstructure and fractography observations and the analysis of slip and geometrically necessary dislocation (GND) activities, the uniform plastic deformation was mainly accomplished by prismatic slip, whether at room temperature or at cryogenic temperature. The significantly increased uniform elongation is mainly attributed to the more uniform distribution of GND pile-ups at cryogenic temperature. 3) The MD simulations revealed that cryogenic temperatures made the GBs present a stronger barrier effect on dislocation transmission compared with that at room temperature, contributing to the more uniform distribution of GNDs and lower densities of GND pile-ups. The GBs at cryogenic temperature show a greater ability to resist void nucleation due to the decreased accumulation rate of excess potential energy and increased energy required to void nucleation. The larger strains were thus required to increase the densities of GND pile-ups to induce large stress concentrations for driving void nucleation. This made the uniform elongation of α-Ti increase significantly at cryogenic temperature. This study revealed that the enhanced barrier effect of GBs on dislocation transmission and the improved ability of GBs to resist void nucleation are key mechanisms besides twinning governing the cryogenic strength-ductility synergy of α-Ti. The understanding developed in this work can be useful for the development of new high-performance materials and the precise forming of complex components with high quality.

中文翻译：

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通过实验和原子模拟研究α-Ti低温强塑协同机制

α钛（α-Ti）是一种很有前途的材料，可用于制造航空航天、海洋、能源和医疗保健领域的高性能部件。在低温下观察到 α-Ti 具有优异的强度-延展性协同作用。孪生通常被认为是出色的低温延展性的关键机制。位错-晶界（GB）相互作用和空洞成核通常在多晶材料的塑性变形中起着至关重要的作用，但它们对α-Ti低温延展性的影响却很少被考虑。为了消除这种困惑并深入了解α-Ti低温强度-延展性协同作用的机制，在这项工作中，设计并进行了一系列表征实验和分子动力学（MD）模拟。 1)粗晶α-Ti板材在25至-180°C温度范围内的单轴拉伸试验表明，均匀伸长率和颈缩后伸长率分别提高了92%和20%。与室温相比，该材料在低温下在更大的应变范围内保持更大的应变硬化率。 2）通过微观结构和断口观察以及滑移和几何必要位错（GND）活动的分析，无论是在室温还是在低温下，均匀塑性变形主要是通过棱柱滑移来完成的。均匀伸长率的显着增加主要归因于低温下 GND 堆积的分布更加均匀。 3）MD模拟表明，与室温相比，低温使得GBs对位错传输呈现出更强的势垒效应，从而导致GND分布更加均匀和GND堆积密度更低。由于过剩势能的积累速率降低以及空洞成核所需的能量增加，低温下的GB表现出更强的抵抗空洞成核的能力。因此需要更大的应变来增加 GND 堆积的密度，以诱导大的应力集中来驱动空穴成核。这使得α-Ti在低温下的均匀伸长率显着增加。这项研究表明，除了孪晶之外，晶界对位错传输的势垒效应增强和晶界抵抗空洞成核的能力提高是控制α-Ti低温强度-延展性协同作用的关键机制。这项工作中形成的理解可用于开发新型高性能材料和高质量复杂部件的精确成型。