316LN不锈钢在极低温环境下的锯齿状塑性屈服实验观测与机理研究

Stainless steel type 316LN (SS316LN) has excellent mechanical properties under extremely low temperature conditions, such as the yield strength of 1100Mpa, high tensile strength up to 1500MP, more than 40% of the breaking elongation and its 200Gpa Young's modulus, thus making it has broad prospects in cryogenic applications and especially nuclear fusion project. At the same time, SS316LN has shown different stress-strain curves compared with traditional stainless steels at very low temperature (below 20k). Especially in plastic stage, the discontinuous plastic flow (serrated yielding) occurs, and the stress oscillation amplitude becomes greater at lower temperature. At present, researchers have speculated that the phase transformation (austenitic to martensitic) leads to the discontinuous plastic flow. But magnetic martensite phase can become a significant influence on the distribution of the superconducting magnet magnetic field, which brings potential safety hazard to superconducting devices. This project is focused on the SS316LN which is widely used in the ITER device, and it aims to give credible experiments and set models to its mechanical behaviors including the discontinuous plastic flow at low temperature. Furthermore, the project will reveal the mechanical mechanism of the discontinuous plastic flow and provides reference for further study of the phase transformation behavior of materials at extremely low temperature environment.

316LN不锈钢在极低温环境下具有优异的力学性能，如在4.2K环境中，其屈服强度、断裂延伸率、抗拉强度和杨氏模量分别可高达1100MPa、40%、1500MPa和200GPa，使得这类材料在低温工程特别是在核聚变工程中具有非常广泛的应用前景。这种材料在极低温环境中(35K以下)，其应力应变曲线表现出有异于传统不锈钢材料的特征，体现在塑性阶段呈现出不连续的锯齿状屈服行为，并且温度越低，这种锯齿状屈服的幅值越大。目前，研究人员均猜测低温下应力驱动的奥氏体和马氏体相变导致了这种锯齿状的塑性屈服，而磁性马氏体相的产生将对超导磁体磁场的分布带来显著影响，给超导装置带来安全隐患。本项目面向ITER装置中广泛使用的316LN不锈钢，对其在极低温环境的力学行为特别是锯齿状塑性屈服进行实验和理论建模研究，弄清这种材料在低温下出现锯齿状塑性屈服的力学机制，为进一步研究极低温环境中材料应力驱动相变行为提供参考。

316LN不锈钢在低温工程特别是核聚变工程中有广泛的应用前景，是目前最大的国际合作项目ITER中超导磁体核心部件CICC导体的铠甲材料，这种材料在低于35K的环境温度下，会表现出一种有别于传统材料的锯齿状塑性屈服，且温度越低，这种锯齿状塑性屈服行为愈加显著。一般认为这种锯齿屈服的行为主要由应变驱动的奥氏体向马氏体发生的相变所导致，这种低温下的相变行为将对整个磁场装置的磁场分布产生影响，因此揭示这种材料的这一独特力学行为具有重要的科学研究价值和工程应用前景。项目的主要研究成果是：1构建一套中子环境的低温加载装置，开展316LN不锈钢在低温下的原位中子实验；2，发展一套全场的温度测试方法；3，建立基本的力学模型揭示其力学机理。在完成上述工作基础上，项目负责人将这种低温原位方法扩展至超导材料的原位测试，发现了“力致失超”新现象，同时发展了一种全新的超导材料3D打印的制备工艺，相比传统制备方式具有高效，形状可控，高性能等优点。在项目执行期内共发表学术论文30余篇，其中第一/通讯/共同通讯学术论文29篇(第一标注)，包括高水平的国际期刊Nature Communications, Superconductor Science and Technology、Experimental Mechanics、Advanced Functional Materials、Acta Mechanica Sinica等，显著的提升了我国低温材料及超导研究的国际影响力。