Fatigue behavior of nanostructured bainite

纳米贝氏体由于其合金设计和加工简单，力学性能优异，达到了贝氏体钢中最高的强度-韧性组合（2.2GPa-30MPa·m1/2），是一种具有工业化初期发展前景的先进钢。在将这种合金应用于轴承、齿轮或铁路系统等先进部件之前，必须对机械疲劳进行研究，即材料对循环载荷的响应。研究微观组织对纳米贝氏体疲劳响应的影响是有原因的。首先，单调性不能解释疲劳性能，增加强度和延性不能改善疲劳性能。其次，基于S-N曲线（应力与循环次数）预测寿命的传统方法没有区分裂纹成核和扩展，这可能导致对寿命的危险高估。最后，同样，在短裂纹状态下，裂纹扩展速率不能用线弹性断裂力学（LEFM）来解释，即扩展速度可能高于长裂纹。局部组织控制疲劳响应的时间是在裂纹成核之前和短裂纹阶段。到目前为止，还没有找到一个全球性的有效晶粒尺寸。与贝氏体块相关的形态参数可能控制着这些组织的固有疲劳极限，但不同应力水平下微观组织对疲劳寿命的影响尚未得到解决。在一定晶界处的裂纹挠度与裂纹止裂之间的关系也尚未建立，而裂纹止裂有助于提高疲劳寿命。第二相，残余奥氏体的作用仍然是未知的。本项目旨在获得关于贝氏体组织性质的新知识，即裂纹成核前和裂纹路径的晶界、机械诱导相变和缺陷演化。变形和损伤机制将结合循环塑性硬化/软化行为进行处理，并与疲劳裂纹扩展的不同阶段相关联。为此目的，将应用和发展以显微镜和衍射为基础的先进实验技术和理论方法。它将能够评估这种不断变化的微观结构对疲劳寿命的相对重要性。因此，在安全性方面改进疲劳设计将成为可能。

Nanostructured bainite, in its first stages of its industrialization, is one promising advance steel because of its simplicity in terms of alloy design and processing, and its outstanding mechanical properties, having achieved the highest strength-toughness combinations ever recorded in bainitic steels (2.2GPa-30MPa·m1/2). Before the potentialapplication of this alloy in advance components such as bearing, gears or railway systems, the study of mechanical fatigue, i.e., material´s response to cyclic loading is essential. There are several reasons motivating a detailed study of the effect of the microstructure on the fatigue response in nanostructured bainite. First of all, monotonic properties fail to explain the fatigue performance, which is not improved by increasing the strength and the ductility. Second, a conventional approach to predict the lifetime based on S-N' curves (stress v. number of cycles) does not make distinctions between cracknucleation and propagation, which may lead to dangerous overestimates of life. Finally, and likewise, at the short crack regime, the crack growth rate is not explained by the Linear Elastic Fracture Mechanics (LEFM), i.e., the propagation speed may be higher thanthat of long cracks. It is before crack nucleation and during short crack regimes when the local microstructure controls the fatigue response. So far a global effective grain size has not been found. Morphological parameters associated to the bainite block might be controlling the intrinsic fatigue limit of these microstructures, but the question aboutthe effect of the microstructure on the fatigue life at different stress levels remains unsolved. The relationship between the crack deflection at certain grain boundaries and the crack arrest, which can contribute to enhance the fatigue life, has not been established either. The role of the second phase, the retained austenite, remains alsounknown. This project is intended to gain new knowledge on the nature of the bainitic structure, i.e., grain boundaries, mechanically induced phase transformation and defects evolution before the crack nucleation and along the crack path. The deformation and damage mechanisms will be treated in combination with the cyclic plastic hardening/softening behavior and correlated with the different stages of the fatigue crack growth. For that purpose, both advanced experimental techniques based on microscopy and diffraction and theoretical approaches will be applied and developed. It will enable the assessment of the relative importance that this evolving microstructure has on the fatigue life. As a result, an improved fatigue design in terms of safety will be possible.