Understanding microstructural evolution at the hot isostatically pressed (HIP) cladding AA6061/AA6061 interface is essential for advancing the Zr-laminated U10Mo monolithic fuel system for research and test reactors. In this study, AA6061 plates were diffusion bonded via HIP at two temperatures (450 and 560 °C) and two cooling rates (1 and 45 °C/min). Scanning electron microscopy and quantitative image analysis were conducted to characterize the Mg2Si precipitation, with an emphasis on precipitate linear density and size at the AA6061/AA6061 HIP-bonded interface. The results revealed a pronounced accumulation of Mg2Si precipitates at the AA6061/AA6061 interface, with the highest precipitate density and largest precipitate size occurring in samples HIP-processed at 560 °C followed by slow cooling. Post-HIP heat treatment experiments demonstrated that water quenching after solutionizing effectively suppressed Mg2Si precipitation at the interface. To evaluate the feasibility of applying such post-HIP heat treatments within the full monolithic fuel system, simple growth kinetics estimates were conducted for the Zr/AA6061 and U–Mo/Zr interfaces. Overall, these findings provide valuable guidance for optimizing HIP parameters and potential post-HIP treatments to enhance the structural integrity and performance of monolithic fuel cladding.
The part of the binary Fe-C phase diagram that deals with the solid state is an impressive manifestation of the complexity of pure iron and the consequences of carbon additions. Its origins reveal a story that inspires today, how ferromagnetism caused confusion, and yet stabilizes the most important phase in the microstructure of colossal quantities of structural steels. There are unresolved issues—if graphite is ignored, is the stable phase field at ambient temperature defined by a mixture of cementite and ferrite? Some studies favor mixtures of iron carbides (χ, η, ǫ) and ferrite to represent equilibrium at low temperatures, but it is shown that this may not be correct. Should we limit ourselves to the body-centered cubic form of ferrite when interpreting equilibrium, or does the stability of mixtures of body-centered tetragonal ferrite and austenite take precedence when the lattice change is generated by the Bain deformation? A case is made for the routine inclusion of the T0 curve on the phase diagram, on the basis that it is no different in its thermodynamic definition from the equilibrium phase boundaries of pure iron in a pressure-temperature plot.
One pivotal period in history of science is reviewed, to illustrate how the interplay of religious beliefs, technical advancements, scientific discoveries, and observations of the heavens combined within the last 500 years to introduce advancements far greater than those achieved in the prior 5000. Focus is placed on three scientists within the span of 150 years, showing how their combined endeavors overcame obstacles and set a transformational snowball in motion. An appreciation of this rapid change is important; it underscores the central role of observation in every avenue of science and can provide a young engineer with a glimpse of what he or she will experience. The next period of major change will occur in less than 50 years; probably within the working lifetime of the present generation. Three examples from a career of materials science and metallography, failure analysis, and accident reconstruction are presented. These are used to emphasize the role of critical observation, its analysis, and interpretation beyond the limits of their original environment.
Weld pool behaviour during arc welding strongly influences the microstructural evolution of copper and copper-based alloys. However, limited metallographic studies are available on the effect of activating flux on the microstructure of precipitation-hardened Cu–Cr–Zr alloys. In the present work, the influence of SiO2-assisted Activated Tungsten Inert Gas (A-TIG) welding on weld pool characteristics and microstructural evolution in pure copper and Cu–Cr–Zr alloy was investigated and compared with conventional Gas Tungsten Arc Welding (GTAW). Microstructural characterization of the base metal (BM), heat-affected zone (HAZ), and fusion zone (FZ) was carried out using scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), along with X-ray diffraction (XRD) analysis. The metallographic examination revealed dissolution of pre-existing Cr-rich precipitates during welding, followed by diffusion-controlled reprecipitation during cooling. Elemental mapping showed localized chromium enrichment within the copper matrix in the FZ. Furthermore, A-TIG welding promoted the formation of Cu–Zr intermetallic phases and fine dispersoids within the Cu–Cr–Zr alloy. In contrast, GTAW maintained a relatively homogeneous matrix in pure copper but favoured the formation of oxide particles in the alloy. The hardness variations were therefore governed by welding-induced phase transformations rather than solely by thermal effects. The study highlights the importance of metallographic characterization for understanding microstructure development in welded copper alloys.
In this study, the impact of microstructural alteration using multiaxial cryoforging on the mechanical characteristics, fracture toughness, and fracture processes of an ultrafine-grained (UFG) AZ91 alloy has been investigated. The cast alloy was homogenised at 400 °C for 4 h and underwent cryoforging for 4, 8, and 11 cycles. The 11 cycles cryoforged (CFed) sample demonstrated an improvement in yield strength (YS) exceeding 2.5 times, reaching 360 MPa, in contrast to the homogenised annealed (H-AN) sample, which measured 142 MPa. Nevertheless, the ductility was reduced to 4.6
The grain-refining efficacy of Mn in magnesium alloys is influenced by the specific alloy composition. This study systematically investigates the influence of Mn on the microstructure and mechanical properties of Mg-Zn-Sn alloys. Mn addition reduces the average grain size from 935 to 50 μm, indicating significant grain refinement. Electron backscattered diffraction analysis shows Schmid factors for prismatic and pyramidal slip remain unchanged, while basal slip decreases. With increasing Mn, tensile strength improves, but fracture elongation first increases then decreases. Fractography reveals a transition from brittle to ductile fracture features. Work-hardening rate is grain size-dependent, with Mn weakening work-hardening. Kock work-hardening model confirms the plastic deformation is primarily by dislocations. The size and number of second phase particles first increase then decrease with Mn addition. Mn promotes the precipitation of the MgZn2 phase and enhances Sn solubility, but excess Mn forms a MgZn2 layer on Mg2Sn particles, yielding coarse core–shell structures.
The precipitation behavior of an Al–Mg–Cu alloy aged at 200 °C for various times was investigated using hardness measurements, selected area electron diffraction (SAED), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The alloy reaches peak hardness after 6 h of aging. SAED analysis reveals the presence of Guinier–Preston–Bagaryatsky (GPB) zones at the early stage of aging, followed by the gradual development and intensification of diffraction reflections corresponding to the S′ phase with increasing aging time. HAADF-STEM observations confirm that GPB zones are fully coherent with the Al matrix, while S′ precipitates exhibit an ordered orthorhombic structure. Both precipitate types exhibit a similar interface trace projected parallel to ⟨021⟩Al in the ⟨100⟩Al HAADF-STEM projection, indicating a close crystallographic relationship. This crystallographic alignment suggests that GPB zones act as structural precursors for S′ phase formation during aging. Notably, GPB zones are still observed after one week of aging, demonstrating their persistence under the present aging condition. These results provide direct microstructural evidence for the crystallographic linkage and heterogeneous transformation pathway between GPB zones and the S′ phase, offering valuable insight for alloy design and precise microstructural control to achieve improved mechanical properties in Al–Mg–Cu alloys.
Living nature uses much less chemical diversity than we do in our technical systems but in turn achieves advanced functionalities through evolutionarily optimized surfaces in the scale of micrometers and sub-micrometers. Using such functional topographies as an additional degree of freedom for surface design provides promising prospects for current and future technical applications, especially facing the demands in modern human society considering energy efficiency and material circularity. This has become achievable by means of well-defined biomimetic surface patterns on technical materials that are generated via direct laser interference patterning (DLIP). The variety of optimized functional surfaces spans from fields like tribology, electrical contacts up to antimicrobial or biocompatible properties in health applications. The technology benefits from parallel topographic as well as physicochemical surface conditioning which applies a multi-modal impact on surface functionality creating a novel class of “META-surfaces.” This article discusses examples of the various functionalization with significant functional effects, as well as recent extensive experiments on antimicrobial material surfaces in space, based on DLIP to support the health of astronauts on the ISS and, in the future, also on the Moon and Mars.
Text messages, e-mail, faxes, work from home, virtual meetings, and other technical advances have reduced in-person contacts among engineers and scientists working in the same or related arenas but in different departments or organizations. A technologist might assume that rapid-fire online communication among such individuals improves technical development, while a gregarious individual might believe that a lack of in-person gatherings impedes progress. Additionally, emerging accounting practices tended to minimize any work that must be charged to overhead; thus, we do not listen to talks and symposia outside our program arena. However, my experience suggests that in-person discussions provide valuable contributions to most investigations and that knowledge broadening beyond your own area of expertise is helpful and can be essential for meaningful progress. Faced with the current trends, the engineering and science communities must determine whether the minimization of in-person gatherings and/or overhead charges provides an improvement or an impediment to progress.
The measurement of the grain size of metallic materials via metallographic investigations is intrinsically influenced by experimental uncertainties arising from the uniform determination of the grain boundaries or the presence of different microstructures. To date, the methods defined by technical standards and implemented in commercial software involve partial or total manual intervention by the technician/researcher, at the cost of considerably long processing times. The need to distinguish between ferritic and pearlitic grain sizes in mixed structures adds a further complication to manual counting. The present research concerns the development of an automated process for extrapolating grain characteristics in mixed pearlitic–ferritic structures, following the “intercept procedure” described in ASTM E112.
In this study, the microstructural evolution, mechanical properties, and corrosion behavior of tungsten inert gas (TIG)-welded dissimilar joints (DJs) between Nimonic 75 superalloy and Nitronic 50 austenitic stainless steel were investigated using three welding approaches: autogenous welding (DJ1), welding with a Ni-interlayer (DJ2), and welding with ERNiCrMo-4 filler metal (DJ3). Microstructural analysis revealed that the autogenous weld exhibited predominantly cellular and columnar dendritic structures, whereas the incorporation of a Ni-interlayer and ERNiCrMo-4 filler promoted the formation of finer equiaxed dendritic morphologies. Energy-dispersive spectroscopy of the interdendritic regions in the autogenous weld indicated Ti- and N-rich precipitates, suggesting the possible formation of Ti-rich carbides and/or nitrides. The addition of Mo and Cr through the ERNiCrMo-4 filler enhanced the tendency for carbide formation in the weld metal. Mechanical testing showed that the Ni-interlayer joint exhibited the highest ultimate tensile strength ( 714.33 ± 6.03 MPa), followed by the ERNiCrMo-4 filler joint ( 697.33 ± 7.51 MPa) and the autogenous joint ( 636.67 ± 4.16 MPa). Corrosion studies further demonstrated that the Ni-interlayer weld possessed the lowest corrosion current density (icorr ≈ 0.43 μA/cm2), indicating superior corrosion resistance compared with the ERNiCrMo-4 filler weld (icorr ≈ 3.27 μA/cm2) and the autogenous weld (icorr ≈ 4.51 μA/cm2). The improved performance of the Ni-interlayer joint is attributed to its refined microstructure, reduced segregation, and enhanced compositional homogeneity across the weld region.
Three-dimensional microstructural representations are essential for accurately predicting the mechanical response of heterogeneous materials, yet experimental 3D characterization techniques are often costly and time-consuming. This work presents a framework that integrates deep learning–based microstructure segmentation with three-dimensional reconstruction and mechanical modeling of gray and nodular cast irons from 2D metallographic images acquired at different depths. The methodology combines advanced AI-based image analysis with conventional reconstruction techniques to overcome the limitations of conventional 2D characterization. Several segmentation approaches were evaluated to distinguish the metallic matrix and graphite phases, with a modified architecture combining the Segment Anything Model and a convolutional encoder–decoder network showing the best performance, particularly for complex lamellar and spheroidal graphite morphologies. The segmented image stacks were aligned and compiled to generate consistent 3D microstructures that preserve phase distribution, size, and connectivity. Mechanical response predictions were obtained through finite element simulations of indentation loading. The numerical results, validated against experimental microhardness measurements and nanoindentations, demonstrate that the reconstructed microstructures capture the key features governing local mechanical behavior, supporting the proposed framework as an efficient alternative for 3D microstructure reconstruction and mechanical property prediction.
Artificial intelligence offers significant potential for automating microstructure analysis. However, its effectiveness is limited by the need for reliable ground truth data—accurately labeled microstructures used to train AI models. Generating this data is challenging for complex microstructures due to the time-consuming manual annotation process and the subjectivity of expert interpretation. This work explores how correlative microscopy, which combines multiple imaging techniques, can be used to create robust training datasets that overcome these limitations. Through case studies utilizing optical microscopy, scanning electron microscopy, and electron backscatter diffraction, it is demonstrated how to create reliable ground truth for AI-driven microstructure analysis. The ultimate goal is to use correlative microscopy only once to generate training data and simplify routine evaluations using the simplest microscopy technique.
Delayed hydrogen cracking represents a critical damage phenomenon in metallic materials, characterized by a temporal separation between hydrogen uptake and crack initiation. This time dependency often complicates failure analysis, as cracking may occur hours, days, or even years after manufacturing or commissioning. While hydrogen embrittlement has been extensively studied, the mechanisms governing delayed cracking are frequently discussed in isolation rather than as an integrated process. This paper presents a mechanistic framework for delayed hydrogen cracking by systematically linking hydrogen uptake, hydrogen configurations, stress-assisted hydrogen redistribution, and hydrogen-induced damage mechanisms. It is shown that the mere presence of hydrogen is insufficient to cause delayed cracking. Instead, the phenomenon arises from the ability of hydrogen to remain mobile or to be re-activated from initially benign configurations, followed by time-dependent redistribution toward regions of elevated mechanical stress and thus strain. Only when a critical local hydrogen concentration is reached do hydrogen-enhanced damage mechanisms such as hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity become operative. The resulting fracture morphologies are discussed as the fractographic manifestation of these time-dependent processes rather than as indicators of a single dominant mechanism. Differences in fracture appearance across various material classes are addressed in this context. Finally, selected technical failure cases are presented to illustrate how hydrogen introduced during manufacturing or service can lead to delayed cracking under practical operating conditions. By integrating fundamental hydrogen–material interactions with real-world failure examples, this paper provides a comprehensive understanding of why hydrogen-induced cracking may occur long after hydrogen exposure and offers a structured basis for failure analysis and risk assessment of hydrogen-exposed components.
WC–Co is valued for its high hardness and wear resistance, but its limited toughness restricts broader applications. To address this, a unique architecture was designed where steel layers are sandwiched between WC–17Co regions to improve ductility by development of a functionally gradient structure. Laser direct energy deposition (LDED) technique as a relatively new additive manufacturing technology (AM) was employed to develop this design due to its precision and suitability for developing complex geometries. Because LDED operates on a layer-by-layer basis, it allows gradual compositional transitions, enabling the creation of a functionally graded material (FGM) that minimizes thermal mismatch and stress concentration. The resulting microstructure was examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). For comparative purposes, all experiments were performed on a conventional (non-FGM) LDED-processed WC–17Co sample as well. The average porosity and grain size was smaller in FGM compared to the non-FGM sample. Substantial deviations in lattice parameter of WC for the FGM sample were observed compared to the non-FGM one and reported literature for conventionally-processed WC–17Co. XRD results revealed that the FGM sample exhibits a shift in detected peaks followed by reduced residual stress than the non-graded structure, thereby decreasing the likelihood of crack initiation. The measured crystallite sizes aligned well with previously published data. While FGM sample exhibited higher hardness than non-FGM, it should also be noted that detailed analysis of mechanical properties is not within the scope of this study. This is a preliminary study only focusing on microstructural characteristics, and results could be used to explain mechanical, thermal, and electrical properties of newly developed structures in future studies.
The present study investigates the underwater friction stir welding (UFSW) of dissimilar aluminum alloys AA6063-T6 and AA5083-H32, widely used in marine, transportation, and structural applications. The novelty lies in the systematic evaluation of key process parameters, namely tilt angle, rotational speed, and welding speed, under controlled underwater cooling conditions, which remains limited for this alloy combination. Experiments were conducted using a specially developed setup on a conventional milling machine integrated with a water tank. A Taguchi L16 orthogonal array was employed to optimize process parameters. Mechanical properties such as ultimate tensile strength (UTS), yield strength, elongation, impact toughness, and microhardness were evaluated along with detailed microstructural characterization using optical microscopy, SEM-EDS, XRD, and fractography. The results revealed that optimized welding conditions produced defect-free joints with stable material flow and uniform grain refinement in the stir zone, achieving a maximum UTS of 265 MPa compared to a minimum of 245 MPa, corresponding to an improvement of approximately 20 MPa (about 8.16
The microstructure and mechanical properties of an equiatomic FeCoCrNi high-entropy alloy were examined in as-solidified state and after annealing in current study. XRD results exhibited the existence of a single FCC phase. Microstructural observations indicated that the as-solidified alloy has a typical cast dendritic structure, where interdendritic regions were indicated the fiber texture component with ND // < 100 > orientations. The results indicated that the as-solidified alloy had ultimate strength of 451 MPa and ductility of 75
Utilizing a higher reduction ratio in the production of heavy plates not only facilitates the healing of internal shrinkage voids, porosity, and other defects in continuous casting slabs but also effectively refines the grain structure, enhancing the overall material performance. However, the higher reduction ratio imposes limitations on the product’s thickness. Applying heavy reduction at the end of solidification effectively heals internal shrinkage and porosity, while promoting solute redistribution and suppressing segregation. However, the effects on the microstructure and performance of subsequently rolled materials have not been systematically investigated. This study focuses on the continuous casting slabs, investigating the effects of heavy reduction process applied at the solidification endpoint on shrinkage, porosity, and segregation. Additionally, it explores how different heavy reduction regimes influence the microstructure and properties of subsequent rolled materials under varying reduction ratios. The findings reveal that the application of a heavy reduction process significantly mitigates shrinkage and porosity defects in the center area of slab while also reducing segregation.
