Precipitates are crucial for crafting mechanically strong metallic materials. In this work, we report the dislocation cutting of B2 (ordered body-centered cubic) nanoprecipitates, typically considered nonshearable intermetallics, in a lightweight compositionally complex steel during cryogenic tensile loading. Shearing is enabled by the high strength level for dislocation glide within the austenitic matrix, attributed to the substantial strengthening from subnanoscale local chemical ordering zones and the pronounced solid solution strengthening from the multiprincipal elements in the matrix. This mechanism not only harnesses the intense strengthening and strain hardening provided by otherwise impenetrable brittle nanoprecipitates but also introduces ductility through their sequential shearing with ongoing deformation. Our steel thus showcases ultrahigh cryogenic tensile strength up to 2 gigapascal at a remarkable tensile elongation of 34%. This study reveals a new strategy for designing high-performance structural materials.

Rare earth (RE) addition to steels to produce RE steels has been widely applied when aiming to improve steel properties. However, RE steels have exhibited extremely variable mechanical performances, which has become a bottleneck in the past few decades for their production, utilization and related study. Here in this work, we discovered that the property variation of RE steels stems from the presence of oxygen-based inclusions. We proposed a dual low-oxygen technology, and keeping low levels of oxygen content in steel melts and particularly in the raw RE materials, which have long been ignored, to achieve impressively stable and favourable RE effects. The fatigue life is greatly improved by only parts-per-million-level RE addition, with a 40-fold improvement for the tension-compression fatigue life and a 40% enhancement of the rolling contact fatigue life. We find that RE appears to act by lowering the carbon diffusion rate and by retarding ferrite nucleation at the austenite grain boundaries. Our study reveals that only under very low-oxygen conditions can RE perform a vital role in purifying, modifying and micro-alloying steels, to improve the performance of RE steels.© 2022. The Author(s), under exclusive licence to Springer Nature Limited.

In order to improve the uniformity of microstructures, and thus enhance the mechanical properties, especially the impact toughness, of 52100 bearing steels, modified quenching heat treatments with different austenitizing processes have been designed. The results show that 740 °C is the optimal holding temperature (approaching Ac₁ = 750 °C) to improve the impact toughness of the steel by 37% after quenching and low-temperature tempering. Microstructure observations reveal that 740 °C holding shortens the nucleation incubation period of austenitization phase transformation, and inhibits the overfast growth of austenite in some regions, which leads to simultaneous nucleation and growth of austenite during austenitizing, and homogeneous dissolution of carbides in different austenite grains. The changes in microstructure evolution improve the microstructure uniformity (carbide and grain size) after quenching. Uniformly distributed carbides and grain size effectively reduce stress concentration and retard crack propagation during impact loading, thus resulting in high impact toughness.

In the light of the property requirements to design microstructures will become an important develop direction of metal materials. Here, a new concept of microstructure tailoring is proposed. The main features of microstructure tailoring include designing mesoscale microstructure, establishing quantitative relation between microstructures and properties, accurately inverse-designing and fabricating microstructures to satisfy the property requirements. It means screening, multi-scale calculation, and quantification of the essential microstructural factors should be performed first. Second, the microstructures are purposefully fabricated after adjusting the thermodynamics and kinetics of phase transformation. Third, the microstructures are assessed and tailored through iterative optimization. Microstructure tailoring must be preceded by purification and homogenization of metals. Only when the purity problem of materials is solved first, the influence of inclusions and impurity elements can be eliminated. Only by eliminating the macro-segregation can the material achieve homogeneity. And then the intrinsic properties of the material be fully reflected. As an example of microstructure tailoring, this study investigates the expected fatigue-life requirements of M50 (G80Cr4Mo4V) steels used for bearings in aircraft engines. By controlling the macro-segregation and purification, it is found that the fatigue-life of M50 steel mainly depends on primary carbides. And then the size, type, and morphology of the primary carbides are quantitatively tailored to fulfill the fatigue-life requirement. With technological developments in the metallurgy industry, microstructure tailoring will become a mainstay of the development of metals. And, applying data science and modeling along with microstructure tailoring technology, the alloy design will be gradually optimized in the future. The expensive metal addition will be reduced gradually, so as to save resources and develop green materials.

根据性能需求设计组织是金属材料的重要发展方向。本工作提出了金属材料组织定制的新学术理念，其内涵是：基于材料物理本质进行介观尺度设计，建立组织与性能的量化关系，通过逆向精准调制并制备材料组织，满足材料服役性能需求。即，根据服役性能需求，首先进行组织筛选、跨尺度计算与组织要素量化，然后基于热力学和动力学调控进行组织制备，最后进行性能考核，迭代优化，实现组织定制。组织定制的前提是保证金属材料的纯净度和均质性，只有解决了材料的纯净性问题，夹杂物、杂质元素的影响作用才能排除；只有消除了宏观偏析缺陷的影响，材料才能实现均质性，材料的本征性能才能充分体现。本工作以航空发动机轴承用M50 (G80Cr4Mo4V)钢为例，介绍了在控制钢的纯净度和宏观偏析基础上，根据疲劳性能目标要求，指出粗大的共晶碳化物是影响疲劳性能的关键组织，进而对碳化物量化设计，最后通过控制碳化物尺寸、类型、形貌及演化行为满足了疲劳性能需求。随着冶金工业的技术进步，组织定制是金属材料学科发展的必然。依托材料计算与数据科学，在组织定制基础上，未来将逐步优化合金成分设计，实现贵金属元素减量化、微量化直至素化，从而节约资源，发展绿色材料。

借助光学显微镜、扫描电镜、透射电镜、拉伸试验机、冲击试验机等研究了热处理工艺对新型中碳合金钢42CrMo4M组织性能的影响。结果表明,经800~900℃淬火油冷后,42CrMo4M钢的淬火态组织主要为板条马氏体,且随着淬火温度的升高,晶粒尺寸逐渐增大,马氏体板条粗化,在860℃淬火时,晶粒度均匀且硬度最高。经860℃淬火后,在580~700℃回火温度范围内,随着回火温度的升高,42CrMo4M钢中的碳化物由长条状向球形转化,并由马氏体边界处不均匀分布演变为基体中弥散分布;当回火温度达到700℃时,42CrMo4M钢中碳化物明显粗化;随着回火温度的升高,42CrMo4M钢的强度先升高后降低,-20℃低温冲击吸收能量先增大后减小。680℃回火时,42CrMo4M钢的冲击吸收能量达到峰值,为99 J。在试验条件下,42CrMo4M钢经860℃淬火及660~680℃回火后具有良好的综合力学性能。

42CrMo medium carbon bearing steel was surface quenched with different temperatures and quenching mediums. Then the microstructure and hardness of different regions of the quenched specimens were tested and analyzed by means of Rockwell hardness tester, metallographic microscope, scanning electron microscope and transmission electron microscope. The results show that after surface quenching treatment, according to the hardness from large to small, the specimen can be divided into three different regions: hardened zone, transition zone and base material. The depth of surface hardened layer increases with the increase of surface quenching heating temperature, and that of oil quenching is significantly reduced compared with that of water quenching. Microstructure analysis shows that the microstructure in the water quenching hardened zones are all martensite, while the oil quenching has a slow cooling rate, which causes that the hardened zone is martensitic+ferrite. The transition zones under different surface quenching conditions are all martensite+tempering sorbite. Furthermore, the base material is the tempered sorbite in the original quenched and tempered state. Differences in the microstructure of hardened zone, transition zone and base material leads to differences in hardness in different regions. In practical applications, appropriate heating temperature for water quenching should be selected according to the required depth of the hardened layer.

对42CrMo中碳轴承钢进行不同温度中频感应加热及淬火介质的表面淬火处理，并使用洛氏硬度计、光学显微镜、扫描电镜及透射电镜对淬火试样不同区域组织及硬度进行测试分析。结果表明，经表面淬火处理后，按硬度由大到小试样可分为淬硬区、过渡区及基体3个区域，随着表面淬火加热温度的升高，表面淬硬层的深度增加，并且相对于水淬，油淬的淬硬层深度显著减少。组织分析表明，水淬淬硬区组织均为马氏体，而油淬工艺由于冷速较慢，淬硬层组织为马氏体+铁素体组织，不同表面淬火工艺条件下过渡区组织均为马氏体+回火索氏体，基体为原始调质态的回火索氏体。淬硬区、过渡区及基体的组织差异导致不同区域的硬度差异。实际应用中应根据所需淬硬层深度选择合适的水淬加热温度。