Deep-learning assisted damage observations on the microscale – A new viewpoint on microstructural deformation, fracture and decohesion processes

In recent years, state-of-the-art micromechanical systems have given researchers the ability to observe deformation processes in-situ. While this technology enables a site-specific observation, this very achievement can turn into a major limitation: To deduct conclusions about the relevance of specific processes for the bulk material, a larger field of view than typically possible in microscale observations is often required. Please click Additional Files below to see the full abstract

Mechanical properties and deformation mechanisms of manganese sulphide inclusions

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Plasticity of the C15-CaAl2 Laves phase at room temperature

Magnesium is a promising material for light-weight applications but its application is strongly limited because of its low room temperature ductility and low creep resistance. By alloying with Al and Ca, the cubic CaAl2-, the hexagonal CaMg2- and the Ca(Mg,Al)2- Laves phases form, which positively influence these properties. Due to their complex packing, the macroscopic deformation of these phases at low homologous temperatures is strongly limited. In order to overcome this restriction and to study their mechanical properties and mechanisms of plasticity, nanomechanical testing, such as nanoindentation and micropillar compression were applied by the authors. Please click Download on the upper right corner to see the full abstract

Thermal activation of plasticity in BCC materials investigated by cryo-micropillar compression

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Understanding the damage initiation and growth mechanisms of two DP800 dual phase grades

Dual phase (DP) steels are amongst the most widely used structural steels for automotive applications. It is essential to understand the damage initiation and damage growth in these high strength steels and further shed light on improving mechanical properties. In this work, two DP800 dual phase grades are investigated, which exhibit identical ultimate tensile stress but significantly different elongation in the uniaxial tensile test. To explain the difference in ductility, particularly described by uniform elongation, we investigate the damage initiation and growth mechanisms by analyzing microstructural changes upon deformation, such as voids, dislocation structures and the grain morphology. Furthermore, ferrite micropillars in pre-strained samples are tested in situ to capture the strain hardening capability of ferrite. We found that the DP steel with harder martensite and softer ferrite exhibits more damage initiation sites after deforming to an identical strain. However, void growth is much slower compared to the DP steel grade with fewer initiation sites. We explain the suppressed void growth by significant strain-hardening of ferrite surrounding the voids, which is observed in the micropillar compression experiments. The improved strain hardening of ferrite originates primarily from the difference in chromium content considering the negligible influence of dispersed particles

On the effects of microstructural orientation on fracture toughness in (V,Al)-nitride and -oxynitride thin films

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On the automated characterisation of inclusion-induced damage in 16MnCrS5 case-hardening steel

Manganese sulphide inclusions are commonly found in steels and known to facilitate the formation of deformation-induced damage sites in the form of voids during cold forming. These damage sites either exist as cracks, splitting the inclusion in two parts, or as delamination, separating the inclusion from the surrounding steel matrix. Both negatively influence the longevity of components, especially under cyclic loading. The analysis of damage is inherently scale-bridging, ranging from deteriorated global mechanical properties of the finished part, over the damage behaviour of individual inclusions, to the local description of individual voids. In this work, we set out to devise an analysis approach gathering information on all these scales. To this end, we conducted in-situ tensile tests while acquiring high resolution SEM panoramic images and analysed them with two neural networks, trained for this work, to detect damage sites with respect to the inclusions at which they nucleated. We find that the main damage mechanism during tensile deformation parallel to the length of inclusions is cracking and that damage evolution is equally influenced by void nucleation and void growth in the observed range of deformation. By focussing on the damaging behaviour of different inclusions, we show that the position of inclusions in the microstructure influences the resulting damage evolution and that the vicinity of pearlite bands leads to decreased damage formation