Search

As-cast Mg-Al-Ca alloys are among the most promising alloys for elevated temperature applications (≤200 °C) due to their superior creep properties when compared to conventional AZ or AM series Mg alloys. The microstructures of Mg-Al-Ca alloys consist of a soft α-Mg phase reinforced with hard interconnected Laves phases. These interconnected Laves phases are the main reason for the good creep resistance of these alloys as they impede creep deformation. The volume fraction, type and morphology of Laves phases can be controlled through the Ca/Al ratio. Consequently, the Ca/Al ratio can be used to manipulate the mechanical properties of this alloy system in order to achieve optimum creep resistance. We show here that a higher Ca/Al ratio results in i) higher volume fraction of intermetallic Laves phases in the microstructure, ii) improvement in the yield strength (YS), and iii) enhancement in creep resistance at a stress level of 50–70 MPa and a temperature of 170 °C of the as-cast alloys. Moreover, the local strain distribution and partitioning at the microstructural level occurring during high temperature tensile deformation (at ∼170 °C) was measured using quasi in-situ DIC in SEM revealing stress localisation at the α-Mg Laves phase interfaces.

Materials and design 193, 108801 (2020). doi:10.1016/j.matdes.2020.108801, Published by Elsevier Science, Amsterdam [u.a.]

Strain heterogeneity at the microstructural level plays a vital role in the deformation and fracture behaviour of dual or multi-phase materials. In the present work, the strain heterogeneity, localization and partitioning arising at the sub-micron scale during elevated temperature (170 {\deg}C) tensile deformation of an Mg-5Al-3Ca alloy was investigated using quasi in-situ {\mu}-DIC experiments. The results reveal that the strain is mainly carried by the {\alpha}-Mg phase, while the intermetallic Laves phase plays a critical role in that strain concentrations build up at the {\alpha}-Mg matrix and Laves phase interfaces, hence, reducing the overall deformability of the alloy. In quasi in-situ and bulk material analysis at elevated temperature, cracks were observed to nucleate in the Laves phase, at i) the intersection points of slip lines in the {\alpha}-Mg matrix with the Laves phase and ii) the twin intersections with {\alpha}-Mg/Laves phase interfaces and iii) twin transmissions across {\alpha}-Mg/Laves phase interfaces. Euler number analysis has shown that the (inter-)connectivity of the Laves phase decreases with deformation. Finally, cracks grow preferentially along the Laves phases until the material fractures., Comment: 13 figures

In a variety of modern, multi-phase steels, damage evolves during plastic deformation in the form of the nucleation, growth and coalescence of voids in the microstructure. These microscopic sites play a vital role in the evolution of the materials’ mechanical properties, and therefore the later performance of bent products, even without having yet led to macroscopic cracking. However, the characterization and quantification of these diminutive sites is complex and time-consuming, especially when areas large enough to be statistically relevant for a complete bent product are considered. Here, we propose two possible solutions to this problem: an advanced, SEM-based method for high-resolution, large-area imaging, and an integral approach for calculating the overall void volume fraction by means of density measurement. These are applied for two bending processes, conventional air bending and radial stress superposed bending (RSS bending), to investigate and compare the strain- and stress-state dependent void evolution. RSS bending reduces the stress triaxiality during forming, which is found to diminish the overall formation of damage sites and their growth by the complimentary characterization approaches of high-resolution SEM and global density measurements., Metals;9(3)

In order to improve the understanding of damage evolution in mechanically heterogeneous microstructures, like the ones of dual-phase steels, the influence of the applied stress state is a key element. In this work, we studied the influence of the globally applied stress state on the evolution of damage in such a microstructure. Classical damage models allow predictions of damage during deformation based on considerations of the material as an isotropic continuum. Here, we investigate their validity in a dual phase microstructure that is locally dominated by its microstructural morphological complexity based on a statistical ensemble of thousands of individual voids formed under different stress states. For this purpose, we combined a calibrated material model incorporating damage formation to assess the local stress state in samples with different notch geometries and high-resolution electron microscopy of large areas using a deep learning-based automated micrograph analysis to detect and classify microstructural voids according to their source of origin. This allowed us to obtain both the continuum stress state during deformation and statistically relevant data of individual void formation. We found that the applied plastic strain is the major influence on the overall number, and therefore the nucleation of new voids, while triaxiality correlates with the median void size, supporting its proposed influence on void growth. In contrast, coalescence of voids leading to failure appears related to local instabilities in the form of shear band formation and is therefore only indirectly determined by the global stress state in that it determines the global distribution, density and size of voids.