Fatigue behavior of nanostructured bainite

纳米结构贝氏体是一种具有发展前景的先进钢，其合金设计和加工工艺简单，力学性能优异，达到了贝氏体钢的最高强度-韧性组合（2.2GPa-30 MPa·m1/2）。在将这种合金应用于轴承、齿轮或铁路系统等先进零部件之前，材料对循环载荷的响应至关重要。有几个原因促使详细研究纳米结构贝氏体中微观结构对疲劳响应的影响。首先，单调特性不能解释疲劳性能，疲劳性能不能通过增加强度和延展性来改善。第二，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，这可能导致危险的寿命高估。最后，同样地，在短裂纹区域，裂纹增长速率不能用线性弹性断裂力学（LEFM）来解释，即，扩展速度可能比长裂纹的扩展速度高。在裂纹形核之前和短裂纹区域，局部微观结构控制疲劳响应。到目前为止，还没有发现一个全局有效晶粒尺寸，与贝氏体块体相关的形态参数可能控制着这些组织的本征疲劳极限，但在不同应力水平下，组织对疲劳寿命的影响问题仍然没有解决。在某些晶界处的裂纹偏转和裂纹止裂之间的关系，这可以有助于提高疲劳寿命，也没有建立。第二相，残余奥氏体的作用仍然是未知的。该项目旨在获得有关贝氏体组织性质的新知识，即，晶界、机械诱导相变和裂纹形核前及沿着裂纹路径的缺陷演化，结合循环塑性硬化/软化行为处理变形和损伤机制，并与疲劳裂纹扩展的不同阶段相关联。为此，将应用和发展基于显微镜和衍射的先进实验技术以及理论方法。这将使评估的相对重要性，这种演变的微观结构对疲劳寿命。因此，在安全性方面改进疲劳设计将是可能的。

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.