Maraging steel (MS1)-tool steel (P20) bimetals additively manufactured using the laser powder bed fusion technique were studied under different heat treatment cycles and loading conditions. The hardening of P20 and aging of MS1 were performed sequentially on the hybrid samples. The interfacial characteristics and microstructural evolution of the bulk materials were studied using various advanced electron microscopy techniques. The post-processing procedures successfully produced a uniform martensitic structure throughout the MS1-P20 hybrid steels, leading to a less detectable interface under electron backscatter diffraction (EBSD) imaging. The mechanical performance of heat-treated hybrid steels was evaluated using complex loading conditions. 3-point and 4-point bending tests were performed to assess the impact of heat treatments on the mechanical performance of the hybrid steels. The heat-treated samples exhibited higher strength with relatively homogeneous hardness variations and deformed more uniformly in bending conditions.

In this work, the effect of heating rate on the phase transformation temperatures was investigated using dilatometry analysis. Continuous heating and isothermal holding above Ac₃ temperature on microstructural evolutions in additively manufactured (AM) parts of Fe-Cr-Ni maraging stainless steel were studied. The microstructural features developed within the heating processes were characterized employing electron backscatter diffraction and transmission electron microscopy. Austenite reversion was found to take place in two steps for the AM parts by a diffusive mechanism as well as the precipitation reactions. Although grain refinement occurred during the austenite reversion of the continuously heated samples, the microstructure showed a coarser grain size after isothermal heating. The crystallographic orientations developed after the heating processes were different from those of the initial ones implying the absence of the austenite memory effect.

The deformation performance of maraging steel samples fabricated using the laser powder bed fusion technique was evaluated using the split Hopkinson torsion bar (SHTB) test. Thin-walled tubular maraging steel samples were deformed under dynamic torsional loading at strain rates of 260 s⁻¹ to 720 s⁻¹ using twist angles varying from 3 to 12°. Microstructural and textural investigations were carried out on deformed samples using the electron backscatter diffraction technique and scanning electron microscopy. Results showed that maraging steel samples fractured when deformed using an angle of twist of 12° and strain rate of 650 s⁻¹. As a result of deformation localization at high strain rates, adiabatic shear bands are developed in some thin-walled tubular torsion specimens deformed using the 12-degree angle of twist, leading to fracture. Textural studies showed that texture weakening occurred with an increment in strain rate ascribable to grain fragmentation. In this study, two models (empirically and semi-empirically) were employed for describing maraging steel performance during high strain-rate torsional loading. Simulation results based on Kobayashi-Odd and Nemat-Nasser models agreed well with the experimental data.

This study aims at assessing the effect of solution heat treatment (at a temperature just below the eutectic temperature) followed by various cooling rates on the microstructure and mechanical properties of additively manufactured AlSi10Mg and the cast counterpart. The mechanical properties were evaluated using a depth-sensing nanoindentation platform. The cast and additively manufactured parts were solutionized at 540 ​°C for 2 ​h followed by water quenching, air cooling, and furnace cooling. Results show extensive microstructural changes (e.g. size and morphology of eutectic-silicon phase) and evolutions in the mechanical properties of the heat-treated materials relative to the as-printed and as-cast ones. Besides, the microstructure and micromechanical properties of the materials broadly alter the cast and additive manufacturing conditions. Depending on the cooling condition, the mentioned cooling cycles directly affect the morphology of eutectic-silicon in both cast and additive manufactured materials starting with silicon fragmentation, then followed by silicon spheroidization, and silicon coarsening. The microstructural evolution affects the local micromechanical properties of the studied materials. The results of this study provide insights into the control of microstructure and hence mechanical properties of AlSi10Mg alloy by addressing suitable heat treatment cycles. This study, for the first time, assesses and compares the effect of various post-fabrication cooling rates in the cast and additive manufacturing conditions in an AlSi10Mg alloy.

Inconel 718 superalloy cylindrical rods were fabricated using the laser powder bed fusion (L-PBF) technology in the vertical orientation. The rods were stress-relieved at 980 °C for 15 min before cutting them from the build plate. The microstructure in this condition exhibited a significant amount of undesirable needle-like δ-phase precipitates and a small amount of interdendritic Laves phase that is finer in size. Differential Scanning Calorimetry (DSC) was used to determine the temperatures for solid-state phase transformations and appropriate temperature for solution-treatment. Solution-treatment was performed at 1065 °C for 1 h, followed by air cooling. The microstructures were characterized with specific reference to δ-phase and Nb segregation. Solution-treatment at 1065 °C was found to result in a significant elimination of micro-segregation (mainly Nb), complete dissolution of δ phase, considerable Laves dissolution, and partly undissolved carbide particles (few nm in size). Solution-treatment did not produce a significant change in the grain morphology (columnar dendritic) on a plane parallel to the build direction but more recrystallized and equiaxed grains were formed on a plane perpendicular to the build direction. The hardness of the solution-treated sample is comparable with wrought 718 alloys but lesser (115 HV) than in the stress-relieved condition attributing to the annihilation of dislocation tangles.

In this research, the validity of the deformation texture simulation based on the full-constraint (FC) Taylor model is assessed by a semi in-situ observation of crystal rotation during rolling. In order to study the deformation behavior of individual grains, successive rolling reductions (up to 24%) were applied to a split-sample of a commercially pure aluminum specimen. The electron backscattering diffraction (EBSD) technique was used to collect the crystallographic orientations and to investigate the microstructural evolution. The results indicate that as the deformation proceeds, the orientation spread inside the grains increases. Local variation of lattice rotation inside the grains results in the formation of a fragment boundary. As the deformation proceeds, the fragment boundary gradually moves through the original grain while it becomes narrower and carries a larger misorientation. However, up to the maximum rolling strain of 24%, the microstructural evolution did not give rise to massive generation of new high angle grain boundaries (HAGB). In general, the results of the texture predictions by the FC-Taylor model were found to be in reasonable agreement with the experimentally measured texture at low deformation. However, more plastic deformation resulted in a larger deviation. Study of individual grains rotation revealed large discrepancies in the simulated results of near cube orientations, which was attributed to the high symmetry of these orientations with respect to the deformation axis.

Characterization of the austenite phase at high temperatures is important for understanding the microstructural evolution during steel processing. The austenite phase structure can be reconstructed from the room-temperature microstructure employing the crystallographic orientation relationship between the parent and product phases. The actual orientation relationships in steels are often calculated on the basis of well known relations (e.g. Kurdjumov-Sachs), which may differ from the experimentally observed orientation relationships. This work introduces a new approach to improve the current state of the art in prior phase reconstruction. The proposed approach consists of two new algorithms that are sequentially applied on an electron backscatter diffraction (EBSD) measured data set of the product phase microstructure: (i) an automated identification of the optimum orientation relationship using the observed misorientation distribution of the entire EBSD scan and (ii) reconstruction of the parent phase microstructure using a random walk clustering technique. The latter identifies groups of closely related grains according to their angular deviation from the proposed orientation relationship. The results were validated by near in situ experimental observations of phase transformation in an Fe-Ni alloy whereby the experimentally measured parent phase structure could be compared point by point with the reconstructed counterpart.

Due to high local cooling rates and non-equilibrium directional solidification conditions, selective laser melting (SLM) processed metals exhibit microstructural and textural features significantly different from the conventionally processed ones. The evolution of crystallographic orientations in a maraging stainless steel (commercially known as stainless steel CX) sample fabricated by the SLM process was studied through experimental and modelling approaches Electron backscattering diffraction analysis showed that the dominant texture components in martensite and austenite phases are <111>|| building direction and <011>|| building direction, respectively. Texture simulation indicated that the formation of crystallographic orientations in the studied sample is the result of two consecutive phase transformations, from initially solidified delta ferrite phase with dominant cube fiber texture to austenite and austenite to martensite.

Three-dimensional Electron Back Scattering Diffraction (3D-EBSD)is a technique for microstructure characterization that works by sequential sectioning via mechanical polishing, Focused Ion Beam (FIB)milling or layer ablation by laser. In this technique, consecutive steps of sample preparation and EBSD measurement are employed to extract 2D-EBSD sections and to reconstruct the 3D microstructure. The 3D data collected by the serial sectioning technique, suffer from misalignment between the adjacent sections. In the present work, serial sectioning by mechanical polishing was employed to collect 3D EBSD data of a dual-phase (DP)steel sample. A previously developed algorithm based on the minimization of misorientation between the neighboring sections was utilized to accurately align the consecutive sections. The reconstructed microstructure was then used to validate the accuracy of the alignment algorithm. The results show that the alignment procedure has successfully improved the reconstructed 3D microstructure and produced reconstructed sections, which in terms of microstructural parameters (i.e. grain size and morphology), image quality and kernel average misorientation distribution resemble the results of a directly measured EBSD experiment and can be used to differentiate the 3D morphology and crystallography of martensite from ferrite in DP steels.