Intermetallic Compounds in the Banded Structure and Their Effect on Mechanical Properties of Al/Mg Dissimilar Friction Stir Welding Joints

Dissimilar friction stir welding（FSW） between aluminum and magnesium alloy was performed, using various tool rotational speed（TRS） at a ？xed travel speed, with tool offset to aluminum to investigate the formation of intermetallic compounds（IMCs） in the banded structure（BS） zone and their effect on mechanical properties. Large quantities of IMCs, in the form of alternating bands of particles or lamellae, were found in the BS zone, where drastic material intermixing occurred during FSW. The BS microstructural characters in terms of the morphology of the bands and the quantity and distribution of IMC particles varied with TRS. All welds exhibited brittle fracture mode with their fracture paths propagating mainly in/along the IMCs in the BS. It is shown that these BS microstructural characters have significant effect on the mechanical properties of the joints. Suggestions on tailoring the BS microstructure were proposed for improving the strength of the BS zone and the final mechanical properties of the Al/Mg FSW joints.

Dislocation Source and Pile-up in a Twinning-induced Plasticity Steel at High-Cycle Fatigue

Dislocation behaviour of a twinning-induced plasticity(TWIP)steel subjected to high-cycle fatigue tests is investigated in the present study.Grain boundaries are the important sources of dislocation generation during fatigue tests,contributing to the increase in dislocation density.Continuous emission of dislocations from grain boundaries is observed in many grains.Inclusions can sustain large dislocation pile-ups at the inclusion interfaces,leading to a high stress concentration and therefore acting as potential sites of microcrack nucleation.In contrast,annealing twin boundaries are relatively weak boundaries for dislocation pile-ups.When the number of dislocations in a pile-up is large,dislocations can crossover twin boundaries and glide inside the annealing twins.The stress concentration at the twin boundary is relatively low so that twin boundaries could not act as the sites for microcrack initiation.

Recent developments and perspectives of advanced high-strength medium Mn steel:from material design to failure mechanisms

Advanced high-strength steels are key structural materials for the development of next-generation energy-efficient and environmentally friendly vehicles.Medium Mn steel,as one of the latest generation advanced high-strength steels,has attracted tremendous attentions over the past decade due to its excellent mechanical properties.Here,the state-of-the-art developments of medium Mn steel are systematically reviewed with focus on the following crucial aspects:(a)the alloy design strategies;(b)the thermomechanical processing routes for the optimizations of microstructure and mechanical properties;(c)the fracture mechanisms and toughening strategies;(d)the hydrogen embrittlement mechanisms and improvement strategies.