This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation.
The present paper deals with the study of the mechanical behaviour of sandwich beams under monolithic and cyclic bending. The strains and displacements were monitored by in situ surface displacement analysis (SDA) software. The sandwich beams consisted of an aluminium foam core covered by metallic face sheets. For a certain sandwich geometrical configuration, the operative failure mode is the same under both monolithic and cyclic bending conditions. Indentation (ID) and core shear (CS) are the basic failure modes, which depend entirely on the geometry of the sandwich beam. The SDA results show that ID is localised compression of the beam adjacent to the inner rollers, where the displacement and strain are at a maximum. As for the CS failure mode, failure happens within the core between the inner and outer rollers, where shear crack initiation and growth correspond to the maximum shear strain, accompanied by discontinuous displacements in both vertical and horizontal directions. Owing to constraint of the weak foam core imposed by the strong metallic face sheets in a sandwich beam, the core can bear stresses higher than the yield strength of monolithic foam by a factor of 1 . 6-3 . 2 for both CS and ID failure modes. This factor depends mainly on the geometry of the sandwich beam. Similarly, the fatigue limit of the core in the sandwich beam is also higher than the fatigue strength of monolithic foam. The stronger is the constraint, the higher is the fatigue limit of the core. The fatigue limit of a sandwich beam also relates to the failure mode. At fatigue ratio R50, the fatigue limit of sandwich beams failing by the ID mode is higher than that of beams which fail by the CS mode.
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