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The as-cut microstructures and the subsequent microstructural deformation characteristics of dual-phase steel specimens were analyzed using in-situ biaxial Marciniak tests, microscopic digital-image-correlation and nano-indentation, for two industrially relevant cutting processes: laser cutting and blanking. Interestingly, the strain-to-failure of the former is almost twice that of the latter, even though microstructural damage initiates twice as early (at 8% strain) in the ∼60 µm-thick, fully-martensitic surface layer of the laser-cut affected zone. However, its ∼145 µm-thick, tempered-martensite sub-surface layer provides the toughness to delay micro-damage propagation, arrest the crack growth, and ultimately provide the high strain-to-failure. These observations reveal guidelines to avoid cut-edge failure.

Lath martensite is important in industry because it is the key strengthening component in many advanced high strength steels. The study of crystallography and chemistry of lath martensite is extensive in the literature, however, mostly based on fully martensitic steels. In this work, lath martensite in dual phase steels is investigated with a focus on the substructure identification of the martensite islands and microstructural bands using electron backscattered diffraction, and on the influence of the accompanied tempering process during industrial coating process on the distribution of alloying elements using atom probe tomography. Unlike findings for the fully martensitic steels, no martensite islands with all 24 Kurdjumov-Sachs variants have been observed. Almost all martensite islands contain only one main packet with all six variants and minor variants from the remaining three packets of the same prior austenite grain. Similarly, the martensite bands are typically composed of connected domains originating from prior austenite grains, each containing one main packets (mostly with all variants) and few separate variants. The effect of tempering at similar to 450 degrees C (due to the industrial zinc coating process) has also been investigated. The results show a strong carbon partitioning to lath boundaries and Cottrell atmospheres at dislocation core regions due to the thermal process of coating. In contrast, auto-tempering contributes to the carbon redistribution only in a limited manner. The substitutional elements are all homogenously distributed. The phase transformation process has two effects on the material: mechanically, the earlier-formed laths are larger and softer and therefore more ductile (as revealed by nanoindentation); chemically, due to the higher dislocation density inside the later-formed laths, which are generally smaller, carbon Cottrell atmospheres are predominantly observed.

For both single-phase and multiphase metallic materials, it is necessary to understand the mechanical behavior on the grain-size scale in detail to get information that is not obtainable from macro-scale mechanical characterizations. This paper presents a methodology for uniaxial tensile testing of micro-specimens isolated from a bulk material. The proposed concept of multiple parallel micro-tensile specimens at the tip of a macro-sized wedge reduces the alignment work and offers an easy way for specimen handling. The selection of site-specific specimens is based on detailed microstructural and crystallographic characterization. Three kinds of representative specimens are presented to illustrate the wide range of application of the methodology for a variety of materials. Accurate, reproducible measurement of force (2.5 μN resolution) and displacement (~10 nm resolution) is demonstrated, while accurate alignment (in-plane rotational and out-of-plane tilt misalignment of

Well-defined uniaxial micro-tensile tests were performed on lath martensite with different types of substructure boundaries (block, sub-block and lath boundaries) tilted with respect to the loading direction. A characteristic deformation mechanism of lath martensite is hereby identified, i.e. sliding along the substructure boundaries. This boundary sliding can occur at all types of boundaries at relatively low stresses. Internal boundaries not only strengthen lath martensite, as well established in literature, but under favorable orientations also mitigate plastic deformation. The overall plastic deformation results from the competition of crystallographic slip with boundary sliding.

In hard coating development there is a need for fast and efficient screening methods to assess the influence of changes in composition and structure evolution on the mechanical behavior. Nanoindentation constitutes a method that allows fast and efficient probing of mechanical properties. For another important mechanical characteristic of hard coatings, the fracture toughness, no quick benchmark test to compare coatings with various compositions and/or morphologies, exist. Therefore, the goal of this investigation was to determine if a tensile test setup allows for fast and efficient qualitative screening of fracture toughness trends in hard coatings compared to accurate but time consuming micro-bending beam experiments. TiAlN and VAlN, each deposited on ductile Cu-substrates, were chosen for this investigation. In situ scanning electron microscopy tensile tests were performed. Here, the strain at crack initiation of the coating was utilized as the experimental parameter, which is most representative for the fracture toughness. The experiments indicate that TiAlN exhibits a higher fracture toughness than VAlN. This was confirmed using time demanding in situ micro-bending beam fracture experiments. Hence, it is established that under the given conditions macroscopic in situ SEM tensile tests can be used as a fast and efficient screening method for fracture toughness trends of hard coatings.

Transient liquid phase bonding offers one option to generate a robust lead-free die attach with sound thermal and electrical conductivity in microelectronic packages. However, it remains a challenge to characterize the microstructure of the bonding layer because of its ultra-thin thickness and its nano-sized constituents. In this work, we characterized a Au-Sn soldering system of transient liquid phase bonding process in detail using various advanced techniques which successfully identified the nano-constituents and mapped their distribution in the sub-layers. On top of that, the mechanical behavior of the bonding layer has been investigated by micro-cantilever bending testing.

Transient liquid phase bonding offers one option to generate a robust lead-free die attach with sound thermal and electrical conductivity in microelectronic packages. However, it is a challenge to characterize the microstructure and mechanical behavior of the bonding layer because of its ultra-thin thickness and its nano-sized constituents. We show that microstructure s and local mechanical properties of such a modern solder systems are accessible with state-of-the-art methods including transmission electron microscope, synchrotron X-ray nano-beam diffraction as well as micro-scale mechanical testing. Three sub-regions with different morphologies have been identified within the bonding area, and their contained phases have been mapped. Two of the sub-regions contain nano-sized intermetallic compounds (IMC) while the third one is mostly composed of an FCC Au–Cu solid solution with a Cu concentration gradient. On top of that, micro-cantilever bending testing has been conducted to investigate the mechanical behavior of the bonding region. The two IMC sub-regions show brittle behavior while the Au–Cu sub-region is ductile.

Lath martensite is widely present in advanced high strength steels as the key strengthening phase. Unexpectedly high ductility of lath martensite has been reported in the literature in both single-phase and multi-phase steels, however, without systematic identification of the underlying plasticity mechanisms. In this study, first, well-defined micro-tensile tests are carried out on fully martensitic steel with a clean large substructure and a variety of substructure boundary orientations with respect to the loading direction. Two deformation mechanisms of lath martensite were identified, namely, intra-lath crystallographic slip and apparent substructure boundary sliding, that compete with each other to carry the overall plasticity. The condition under which these two mechanisms are active has been clarified. It is found that boundary sliding is more easily activated than intra-lath crystallographic slip. In addition, for dual phase steel, as an example of multi-phase steels, the probability for sliding of lath martensite boundaries was estimated by boundary orientation characterization and micro-tensile tests. The results suggest that the apparent boundary sliding is also important for lath-martensite-containing multi-phase steels, which would explain prior reports in the literature of unexpectedly high local strains in the martensite regions. Keywords: Lath martensite, Substructure boundaries, Apparent boundary sliding, Micro-tensile testing

International audience; The present study unravels details of the micromechanical behavior of a microspecimen made of IF-steel. A triangular prism is machined via focused ion beam (FIB) and contains two ferritic grains. Four experimental tools are integrated to identify the material's crystal parameters: i) an optical confocal microscope captures height profile images, ii) an in-situ tensile stage prescribes the loading history to the macrospecimen, iii) a global Digital Image Correlation (DIC) algorithm measures the 3D surface displacement fields, and iv) an extension of Integrated-DIC for 3D displacement fields is implemented to assess the micromechanical behavior. It is demonstrated that with this methodology the identification of the boundary conditions and crystal plasticity parameters is successfully achieved.

Well-defined uniaxial micro-tensile tests were performed on lath martensite single block specimens and multi-block specimens with different number of block boundaries parallel to the loading direction. Detailed slip trace analyses consistently revealed that in the {110} slip system with the highest Schmid factor is activated. Both block and sub-block boundaries act as barriers to dislocation motion, whereby a Hall-Petch like behavior is observed. Sub-block boundary strengthening appears to be only slightly less effective than block boundary strengthening, even though fracture analyses indicates that dislocation motion can cross sub-block boundaries but not block boundaries.

An original experimental approach is presented to automatically determine the average phase distribution around damage sites in multi-phase materials. An objective measure is found to be the average intensity around damage sites, calculated using many images. This method has the following benefits: no phase identification or manual interventions are required, and statistical fluctuations and measurement noise are effectively averaged. The method is demonstrated for dual-phase steel, revealing subtle unexpected differences in the morphology surrounding damage in strongly and weakly banded microstructures.

Well-defined uniaxial micro-tensile tests are performed on single-crystal ferrite specimens with three different orientations. All specimens reveal a highly reproducible plastic behavior. The {110} and {112} slip systems equally contribute to the deformation, while all other (complex) slip traces can be identified as cross-slip and ‘pencil glide’. No {123} slip system traces were observed. The critical resolved shear stresses of the two active slip systems are close to each other, i.e. CRSS { 110 } = ( 1.0 ± 0.1 ) × CRSS { 112 } . In all the tested specimens, the activation of the primary slip systems (e.g. systems that activate first) follows the Schmid's law. At first glance, the activation of secondary slip systems does not seem to comply with the highest Schmid factor. However, detailed investigation supported by crystal plasticity simulations reveals that the boundary constraints acting on the primary slip direction triggers an increase of the Schmid factors of the activated secondary slip systems, i.e. Schmid's law correctly justified all observed slip traces. Application of the found ferrite slip parameters in crystal plasticity simulations of ferrite-containing steels shows that correct input values are crucial for obtaining meaningful macroscopic predictions.