Different ferrite-martensite dual-phase (DP) steel microstructures were subjected to controlled electrochemical hydrogen charging followed by tensile deformation. An increase in hydrogen content enhanced strengthening and dynamic recovery, but lowered non-uniform elongation. Hydrogen embrittlement originated from the severity of microscopic post-necking strain localizations in the ferrite phase, and preferential/accelerated damage initiation at the ferrite-martensite boundaries. Atomistic diffusion-based hydrogen concentrations, at different microstructural features, were captured by multi-scale molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations. The hydrogen content, in particular, reduced with an increase in martensite tetragonality. This, in turn, enhanced the phase boundary hydrogen concentrations leading to interface decohesion.

The microscopic hole expansion ratio (mu HER) of a ferritic-martensitic (similar to 10 %) dual phase steel was measured using a novel in-situ scanning electron microscope based miniature hole expansion setup. The effect of extrinsic parameters such as specimen thickness and machining conditions, and intrinsic parameters such as hardness differential (Delta H =H-alpha'-H-alpha) between the soft ferrite matrix and hard martensite islands on mu HER values was studied. The miniature HER setup allowed site-specific measurement of microscopic strain localizations in the DP microstructure through high resolution digital image correlation under the triaxial state of stress. The results from these experiments were juxtaposed against another triaxial state of stress ahead of a crack tip, in a fracture toughness (J(Ic)) test using the single edge notched tensile (SENT) geometry for the same thickness and microstructural conditions of DP steel. It was found that the tempered DP specimen with lower Delta H resulted in a similar to 45 % higher mu HER as compared to the as-received DP600, although both specimens exhibited similar J(Ic) values. This apparent discrepancy between the trends in mu HER and J(Ic) values was explained in terms of the differences in failure modes, triaxiality and plastic zone evolution in the two conditions.

Diamond-reinforced metal matrix composites (DMMC) have great potential for wear-resistance applications due to the superior hardness imparted by diamond. Atmospheric plasma spraying involving axial injection of suitable feedstock is a convenient pathway to fabricate DMMC coatings for tribological applications. In this paper, thick DMMC coatings were deposited by plasma spraying Ni–P clad diamond particles under varying spray conditions. It was found that the phase characteristics of DMMC coatings as well as extent of diamond retention and fragmentation were significantly influenced by spray conditions such as, stand-off distance (SOD) and carrier gas flow rate (CGFR). Mechanical characterization (by micro-indentation) on all DMMC coatings developed in this work showed that coatings sprayed with longer SOD and higher CGFR has relatively higher hardness than other two coatings. However, on nanoindentation, the diamond hardness was found overestimated due to effect of diamond roughness on fragmentation. Ball-on-disc wear testing showed excellent tribological properties in all cases, with enhanced wear performance being noted when more diamond is retained in the coating.

The hole expansion ratio (HER) test is used to determine the stretch-flangeability of materials. Standard HER tests are performed on specimens sized a few tens of centimeters, termed macro-HER tests. This leads to significant material wastage due to the destructive nature of the tests. No information at the microstructure length scale is obtained, and the results suffer from user uncertainty in the identification of the through-thickness crack. This paper presents a novel miniature HER setup (termed micro-HER test), in which miniature specimens are tested inside a scanning electron microscope (SEM). The deformation is imaged from the top using the secondary electron detector installed in the SEM, coupled with the digital image correlation (DIC) technique, allowing for measurement of full field strains at the microstructural scale and identifying their deformation/fracture mechanisms. As a case study, six different steel grades were tested to measure their micro-HER values and compare them with the corresponding macro-HER values. The latter were found to be higher for the more ductile grades of steel. Late detection of through-thickness cracks and thicker samples leading to a higher volume of plastic deformation could contribute to this overestimation of values in macro-HER tests. DIC results from micro-HER tests on a ferrite-martensite (10% volume fraction) dual-phase steel showed high magnitudes of strain localization at the ferrite-martensite interfacial regions, indicating that such interfaces might be hotspots for failure under triaxial stress states. The challenges and errors associated with the measurements are also discussed.

Microscopic strain localizations were explored with correlative characterization and J2 plasticity modeling. The former involved electron backscattered diffraction (EBSD), nano-indentation mapping and microscopic digital image correlation. These ‘mapped’ the evolution of strain localizations during controlled tensile deformation of dual phase (DP) steel microstructures. Though the initial multi-phase microstructures were similar, they had different hardness differentials (ΔH) between ferrite and martensite. It was shown both experimentally and from the numerical model that ΔH controlled the strain partitioning between the phases, which triggered the microscopic strain localizations and determined the macro-tensile behavior. For example, highest ΔH provided most severe strain localizations in the ferrite phase, ultimately leading to micro-cracking in the martensite. Further, the experimental microstructures were used in our plasticity modeling, and the state of stress and strain were simulated. In the regions of interest in the present study, both interface decohesion and cracking inside martensite was observed, although the martensite interior experienced negative stress triaxiality. This was possibly triggered by severe strain localizations at the interface. This study thus provided a niche combination of correlative characterization with plasticity modeling, which were adopted to explore the mechanical behavior of complex multi-phase microstructure(s).

Laser-welded fully austenitic stainless steel (AISI 316LN) weldments revealed the presence of micro-fissures. They appeared at the grain boundaries, near the center of the welds. The grain boundaries, however, did not contain second phase or micro-segregation. The relative presence of micro-fissures decreased with increasing weld heat input. Spatial locations of the micro-fissures and their relative presence were associated with local misorientations and, in particular, with grain reference orientation deviation. Microstructural and microtextural studies indicated that solidification shrinkage was the origin of the micro-fissures in the laser welds. The use of sub-size tensile specimens (of 5- and 1-mm gauge length) with digital image correlation (DIC) related the presence of micro-fissures with mechanical property degradation by the appearance of strain localizations. This was further confirmed by analytical solutions and finite element analysis of critical flaw size and critical stress for fracture.

Instrumented indentation is gaining ground as a tool for generating mechanical phase maps in composite structures such as dual phase (DP) steels. The plastic zone evolution dictates the indentation parameters in such measurements and needs to be estimated accurately. This study uses finite element modeling to simulate nanoindentation responses on a ferrite–martensite DP steel to directly quantify the plastic zone in the heterogeneous microstructure. The polycrystalline tensile deformation response of individual phases of the composite structure are fed as input to a micro-mechanical finite element model to determine the indentation response. The effect of extrinsic parameters, such as tip radius and geometry, and intrinsic microstructural parameters, like the martensite volume fraction and hardness on the plastic zone evolution and the hardness derived thereof, is established. The model correctly predicts the trend in hardness of individual phases as well as the two-phase composite structure. More importantly, it allows for direct visualization of the plastic zone and the stress triaxiality underneath the complex stress state, enabling prediction of failure modes in these microstructures. This offers a complementary tool to the expensive process of locating and cross-sectioning the indents through site-specific micromachining tools to assess the damage zone under them.

Development of non-conventional mechanical testing techniques was primarily driven by the requirement to measure mechanical properties at smaller length scales with increasing miniaturization of devices, as well as the need for microstructure design from bottom up. This review covers the techniques involved in determining the small-scale deformation and fracture response of materials under different stress states. This is an attempt to provide a summary of choices and test protocols to potential users based on the property of interest to them. It begins with the basics of test instrumentation and sample preparation, followed by a short introduction to modeling tools that accompany testing, and later gets into the details of individual tests and their advantages and limitations. Selected applications from recent published works are presented to provide a flavor of material systems whose behaviour differs significantly from the macro-scale due to their size and/or architecture. At the end fallacies in data interpretation and a roadmap to standardization followed by ideas and future scope for non-conventional small-scale testing are given.

This study uses a computational modeling framework for studying the structure-property correlations in Dual Phase (DP) steels. The microstructure-sensitive, dislocation density-based, J2 plasticity finite element framework simulates deformation in the constituent ferrite and martensite phases and the interplay of various deformation mechanisms. The framework accounts for the effects of the input microstructure, specifically the spatial heterogeneity of phase distribution, strength differential, and grain size on the evolution of deformation in terms of the local and aggregate mechanical properties. Experimental characterization of the deformed microstructure has been performed using in-situ Digital Image Correlation (DIC). The model predictions are validated with the DIC measurements on DP 600 as well as available data from literature. The simulations reveal significant impact of varying martensite volume fraction and phase strength differential, but not that of martensite island size. New DP microstructures having enhanced strength and reduced strain partitioning are then predicted. Processing routes to achieve such microstructures are proposed.

A Machine Learning (ML)-based surrogate modeling framework is developed to predict the heterogeneous deformation behavior of dual phase microstructures. The deformation is first simulated using a dislocation density-based J2 plasticity Finite Element (FE) model, whose results form the basis for surrogate model training and validation. Long Short Term Memory (LSTM)-based ML models, with different architectures, are employed to predict the spatio-temporal evolution of three output variables: effective strain, von Mises effective stress, and the stress triaxiality ratio. Two metrics, the mean average error (MAE) and the coefficient of determination, R2, are used to assess the performance of the models and different architectures. Based on our analysis, the LSTM model is generally found to predict the spatio-temporal deformation fields with reasonable accuracy, even for untrained microstructures with varying microstructural attributes and random instantiations. The LSTM model is also used to predict aggregate properties, such as the stress–strain response and the strain partitioning in the dual phase microstructures.

This study involved a commercial hot-rolled dual-phase (DP) steel consisting of martensite (~10 pct) and ferrite phases. The harder lath martensite was located at the grain boundaries and triple junctions of the equiaxed ferrite grains. Tempering and high-pressure torsion (HPT) were used to alter the phase hardness differential ∆H (where $$ \Delta H = H_{\text{Martensite}} - H_{\text{Ferrite}} $$ ) of the DP. The relationship between ∆H and non-uniform elongation, εNU, or post-necking ductility under tensile deformation, was then explored. Tempering softened predominantly the martensite, while HPT increased the ferrite hardness. Both led to a reduction in ∆H. A drop in ∆H in the tempered DP resulted in a steady increase and eventual saturation in εNU. On the other hand, a ∆H decrease in the HPT specimens showed an initial increase in εNU followed by a drop. Strain analysis, with optical digital image correlation during tensile deformation of the tempered DP samples, clearly related the formation of strain localization with ∆H. In particular, severity of strain localization during necking scaled linearly with ∆H. This study thus brought out a potential relationship among the phase hardness differential (∆H), severity of strain localizations and post-necking ductility (εNU).