A newly proposed multi-directional forging (MDF) was successfully applied to a commercially pure titanium (CP Ti). Severe plastic deformation would result in significant and complex changes of microstructure and mechanical properties, so microstructure characterization and a mechanical test of CP Ti were conducted after different cycles of MDF. The results demonstrated that dynamic recrystallization was the dominant grain refinement mechanism of MDF CP Ti. With increasing the cycles of MDF, grain size, fraction of low angle grain boundaries and dislocations density decreased due to grain refined. After three cycles of MDF, the mean grain size was about 200 nm. The values of tensile strength and hardness increased significantly from zero cycles to one cycle of MDF, but increased slowly after one MDF cycle. Numerous dimples and tear ridges were present, but the dimples were smaller and shallower with increasing cycles of MDF.

In this paper, a low-cycle-fatigue (LCF) crack initiation life prediction approach that explicitly distinguishes nucleation and small crack propagation regimes is presented for ultrafine-grained (UFG) aluminum alloy by introducing two fatigue indicator parameters (FIPs) at the grain level. These two characterization parameters, the deformation inhomogeneity measured by the standard deviation of the dot product of normal stress and longitudinal strain and the microscale multiaxial strain considering the non-proportional cyclic additional hardening and mean strain effect, were proposed and respectively regarded as the driving forces for fatigue nucleation and small crack propagation. Then, the nucleation and small crack propagation lives were predicted by correlating these FIPs with statistical variables and cyclic J-integrals, respectively. By constructing a microstructure-based 3D polycrystalline finite element model with a free surface, a crystal plasticity finite element-based numerical simulation was carried out to quantify FIPs and clarify the role of crystallographic anisotropy in fatigue crack initiation. The numerical results reveal the following: (1) Nucleation is prone to occur on the surface of a material as a result of it having a higher inhomogeneous deformation than the interior of the material. (2) Compared with the experimental data, the LCF initiation life of UFG 6061 aluminum alloy could be predicted using the new parameters as FIPs. (3) The predicted results confirm the importance of considering the fatigue behavior of nucleation and small crack propagation with different deformation mechanisms for improving the fatigue crack initiation life prediction accuracy.

7075 aluminum alloy has been widely used because of its excellent mechanical properties. The fatigue crack acceleration induced by compressive overloading is detrimental to 7075 aluminum alloy in service, and the corresponding mechanisms are complex. To further reveal these mechanisms, the effect of microstructure on fatigue damage response ahead of the crack tip is simulated based on the damage-coupled crystal plasticity model for 7075 aluminum alloy. Results show that the ratchetting strain ahead of the crack tip at the compressive overload ratio of −3 is greater than that at the compressive overload ratio of −2, which is attributed to the fact that higher compressive overloading induces greater tensile residual stress ahead of the crack tip. The proportion of material points with no less than four activated slip systems increases after compressive overloading, indicating the number of activated slip systems ahead of the crack tip increases because of the compressive overload effect. This causes the dislocation multiplication rate to be faster, thereby accelerating the fatigue damage accumulation after compressive overloading. Furthermore, the effect of compressive overload changes the slip system activity, and this change in slip system activity depends on grain orientation. It is also found that for a grain ahead of the crack tip, the acceleration of fatigue damage accumulation caused by the effect of compressive overload is influenced by the orientation of its nearest neighbor grains. The findings provide further insight into the acceleration mechanisms induced by the compressive overload effect from a microscopic perspective.

Ultrafine-grained aluminum alloys (UFG AA) show great potential in the design of fatigue-resistant lightweight alloys, and the methodology to assess low-cycle fatigue (LCF) life remains to be studied. In this work, a micromechanics-based LCF life prediction model is presented by conducting crystal plasticity finite element simulation (CPFEM). The fatigue indicator parameter (FIP) of maximum accumulated equivalent plastic strain energy, modulated by triaxiality, is developed to assess the material damage in the microstructure. Particularly, a new multiaxial strain parameter is proposed by considering the combined influence of the mean strain and non-proportional cyclic additional hardening effect, and then directly embedding into the cyclic J-integral. Finally, the reformulated Manson-Coffin relationship is theoretically constructed by correlating the crack tip opening displacement to the crack propagation equation. The results show the scatter fatigue life of UFG AA6061 is not only related to the inhomogeneous evolution of plastic deformation but also to the local stress state. Since the proposed approach considers both the deformation mechanisms at the micro-scale and the corresponding macroscopic responses, it can predict the LCF life of UFG AA with reasonable accuracy.

In the serviced components of a 7075 aluminum alloy, the propagation of fatigue crack can be retarded because of the overload effect; however, the corresponding retardation mechanisms are complex. To provide further insights into the retardation mechanisms of 7075 aluminum alloys, this study addresses the crack tip damage response of a cracked 7075 aluminum alloy under an overload effect. Based on the dual-scale modeling approach and the damage-coupled crystal plasticity model, the effect of the microstructure of a 7075 aluminum alloy on the damage behavior ahead of the crack tip under an overload was studied. The factors affecting fatigue damage accumulation ahead of the crack tip, such as dislocation density, the variation in the activities of slip systems, and the orientation effect of the nearest neighbor grains, are described. The results show that for the 7075 aluminum alloy, the compressive residual stress induced by the overload effect not only decreases the number of activated slip systems, but also lowers the rate of increase in dislocation density. This causes a decrease in fatigue damage accumulation during deformation. Moreover, the overload effect decreases the slip system activity as well as the resultant plastic slip; however, the decrease in plastic slip varies with the grain orientation, indicating that the overload effect depends on the grain orientation. It can also be found that both the damage strain energy release rate and lattice strain are influenced by the orientation of the nearest neighbor grains, which can eventually affect the overload effect. These findings contribute to understanding the retardation mechanisms from a microscopic perspective and provide guidance on improving the material design of a 7075 aluminum alloy to some extent.