This study employs 3D molecular dynamics simulations to investigate deformation-induced martensitic transformation (DIMT) in both chemically homogeneous and heterogeneous austenite grains, with particular emphasis on the distinctive role of chemical boundaries. Our findings reveal three fundamental differences between chemical boundaries and conventional interfaces: (i) they do not serve as nucleation sites for martensite formation, (ii) they effectively arrest propagating martensite, yet (iii) they exhibit negligible influence on stacking fault transmission. In the Fe-Ni model system, we demonstrate that DIMT behavior in compositionally graded core-shell austenite grains is predominantly governed by local Ni concentration, where increased Ni content significantly enhances phase stability. These insights demonstrate that precisely engineered chemical heterogeneities offer an effective pathway for controlling DIMT behavior, providing a novel paradigm for designing next-generation steels containing retained austenite with tunable mechanical properties.

Machine learning offers a promising approach to design high-performance alloys for laser additive manufacturing, by bypassing convoluted physical models and identifying correlations among composition, processing, microcracks/porosity and properties. However, conventional machine learning methods face limitations, e.g., overfitting or unreasonable design results, due to reliance on large, high-quality datasets. Here, we introduce a generic framework operable with smaller experimental datasets, by integrating knowledge-informed graph modeling alongside data uncertainty quantification. The generic physical-metallurgy knowledge and the stochasticity of experimental defect distributions from produced material are rationally balanced. To validate the approach, we detail the development of a new defect-free Ni superalloy possessing excellent laser printability, thermal stability, and high mechanical strength. Mechanism mining revealed a possible origin for this performance, which was confirmed by atom probe tomography. Subsequently, we developed a new laser-printable aluminum alloy through the same approach, highlighting the framework's potential to accelerate next-generation alloy design for additive manufacturing.

Accurate prediction of both the Widmanstätten start (W_s) temperature and the transformation stasis requires a quantitative description of the energy barrier governing the growth of Widmanstätten ferrite. In this study, a concise model is developed by explicitly distinguishing the energy barriers associated with lengthening and thickening. The model adopts the lengthening barrier attributed to curvature and strain energies, and further incorporates the thickening barrier caused by strain energy, together with the diffusional dissipation of substitutional solute. The W_s temperature is predicted by the condition under which the energy barriers for both lengthening and thickening can be overcome, while transformation stasis occurs when the thickening barrier becomes insurmountable due to progressive carbon enrichment of untransformed austenite. The new model enables accurate prediction of the W_s temperatures across Fe-xC and Fe-0.1C-xMn/Ni/Si/Cr/Mo systems and the carbon enrichment in austenite at stasis in Fe-C-Mn and Fe-C-Mn-Si alloys.

A novel oxide-dispersed strengthened T250 maraging steel (ODS-T250 steel) was prepared by mechanical ball milling followed by hot isostatic pressing. Martensitic transformation behavior, ultrafine-grained martensite microstructure and tensile properties of the ODS-T250 steel were compared to its oxide-free counterpart (T250 steel). The results indicated that ODS-T250 steel exhibited excellent microstructural thermal stability, maintaining an effective grain size of 0.348 μm even after quenching from 1200 °C, whereas T250 steel exhibited a much larger grain size of 5.473 μm. After quenching, the ODS-T250 steel showed significantly distinct variant selectivity, and its martensite morphology was unprecedented equiaxed structure rather than typical lath-type. Statistical analysis revealed that there was only a single variant formed when the prior austenite grain size (PAGS) was below 0.43 μm. Moreover, the martensite start (Ms) temperature of ODS-T250 steel was lower than the T250 steel due to its ultrafine PAGS and the dispersed oxide particles. The refinement of PAGS and the presence of oxide particles led to a simultaneous improvement in both strength and ductility of ODS-T250 steel.

In-situ time-resolved small-angle neutron scattering (SANS) experiments were conducted on homogenised cold-rolled ternary Fe-Au-W alloys during aging for 12 h at temperatures of 650 to 700 °C in order to study the kinetics of the nanoscale precipitation. For comparison the precipitation kinetics in the binary counterparts Fe-Au and Fe-W alloys were also studied. In the ternary Fe-Au-W alloy nanoscale Au-rich precipitates were observed by both transmission electron microscopy (TEM) and SANS, while no significant W-rich precipitation was observed. The SANS pattern of the cold-rolled Fe-Au-W alloy clearly reveals a preferred orientation for the plate-shaped nanoscale Au-rich precipitates. As these Au-rich precipitates have a fixed orientation relation with the matrix lattice this preferred orientation originates from the texture of the bcc matrix grains, as confirmed by X-ray diffraction (XRD) pole figure measurements. The effect of texture on the nuclear and the magnetic SANS signal during the precipitation kinetics was included in the data analysis. This enables us to monitor the temperature dependence of the precipitation kinetics for the Au-rich precipitates in the Fe-Au-W alloy during aging at temperatures of 650, 675 and 700 °C. It is found that an increase in aging temperature results in a faster kinetics and a lower final precipitate fraction.

In this research a machine learning model incorporating uncertainty to enhance the creep-life prediction and high-throughput design of creep-resistant steel is proposed. The framework integrates key physical metallurgical parameter linked to precipitate coarsening and applies transfer learning to correlate short-time tensile properties with the creep performance, all within a Bayesian convolutional neural network. Unlike conventional machine learning models, which often lack an assessment of prediction credibility, this uncertainty-based approach offers more accurate and stable predictions while also providing a measure of prediction credibility. By combining the model with a genetic algorithm, the framework achieves a balance between creep life optimization and uncertainty, thereby supporting robust alloy design. The validation on newly developed martensitic heat-resistant steels with tolerable prediction uncertainty showed excellent alignment between predicted and experimentally determined creep life, underscoring the effectiveness of the framework. These findings highlight the critical role of uncertainty modeling in advancing machine learning applications for alloy design.

A cyclic quenching treatment (CQT) succeeded in turning a 2.3 GPa maraging steel with a Charpy impact energy of 9 J into a new grade with the same strength but a Charpy impact energy of 20 J upon 4 cyclic treatments. The improvement of mechanical properties is attributed to the refinement and increased chemical heterogeneity of the martensitic substructure, rather than the refinement of prior austenite grain (PAG), as well as the Transformation-Induced Plasticity (TRIP) effect facilitated by small austenite grains. The role of local segregation of Ni during CQT in the formation of Ni-rich austenite grains, Ni-rich martensite laths and Ni-poor martensite laths, was investigated and verified by DICTRA simulations. This study highlights the important influence of Ni partitioning behavior during CQT, providing insights into microstructural evolution and mechanical properties.

Recycling-oriented alloy design is a crucial part of material sustainability, as it reduces the need for raw material extraction and minimises environmental impact. This requires that scraps be reused or repurposed effectively, even when the scraps are co-mingled and have higher costs for further sorting and separation. In this work, we explore an alloy design concept by creating a compositionally flexible domain that can recycle multiple alloy grades and yet maintain relatively consistent properties across chemical variations. This is demonstrated through the Fe-Cr-Ni-Mn system to identify compositionally flexible austenitic stainless steels (CF-ASS) and accommodate the recycling of mixed austenitic stainless steel scraps. Alloys within the nominal composition spaces exhibit relatively consistent mechanical properties and corrosion resistance despite significant variations in different alloy compositions. We illustrate how we can utilise the compositionally flexible austenitic stainless steels to recycle mixed 200 and 300-series stainless steel and ferronickel scraps, demonstrating its practical viability. While this demonstration focuses on the stainless steel system, the underlying principles can be extended to other systems related to mixed scrap recycling.

This work further investigated the mechanical properties of chemically heterogeneous core-shell austenite. It demonstrated the ability to enhance work hardening capacity and mitigate the formation of the Lüders band. Both experiments and molecular dynamics simulations confirmed that the Mn chemical boundary acts as a barrier to martensitic transformation during deformation.

The carbon partitioning and lengthening rate of bainitic ferrite (α_b) are excellent experimental parameters to estimate our level of understanding of the mechanism of bainitic transformation from a continuum perspective and our ability to capture it in analytical expressions. For Fe-C alloys and relatively simple steels the classical Zener-Hillert theory captures the bainitic transformation rather well but mispredicts the level of carbon in solution in the bainite and overestimates the lengthening rates for transformations at lower temperatures. To address this issue, this paper presents a new thermo-kinetic model based on the Zener-Hillert theory and the Gibbs energy balance concept to simulate the lengthening behavior of α_b in the Fe-C and low alloyed steels. The model incorporates the effect of the temperature dependent carbon diffusion within the migrating interface via a temperature dependent ferrite/austenite interfacial energy and a temperature dependent diffusion coefficient but does not impose local equilibrium across the interface. The good agreement between the model predictions and nine sets of published experiments indicates that both the carbon supersaturation in α_b and the slower lengthening rate are caused by carbon diffusion within the migrating interface. It is found that the degree of carbon supersaturation in α_b increases significantly with decreasing temperature. Consequently, the enhanced carbon solute drag effect, resulting from carbon diffusion within the interface, strongly retards the lengthening rates of α_b at lower temperatures. Transformation strain is shown to have a modest effect on the lengthening rates but to lower the degree of carbon supersaturation.

To achieve an effective design of additively manufacturable Ni superalloys with decent service performance, a hybrid computational design model has been developed, where the strategy to tailor local elemental segregations was integrated within a scheme of minimizing the cracking susceptibility. More specifically, the phase boundary of primary NbC / γ matrix was introduced into the design routine to tune the spatial distribution of critical solutes at an atomic scale, thereby inhibiting the formation of borides and segregation-induced cracking. Based on the output of the design, new grades of Ni superalloy have been developed with excellent additive manufacturability, as confirmed by the robustness of printing parameters in fabricating low-defect-density samples. The capability of the phase boundaries to evenly distribute boron atoms was validated experimentally, and the cracking induced by uncontrolled boron segregation at grain boundaries was effectively prevented. The newly designed alloys showed good tensile properties and decent oxidation resistance at different service temperatures, which are comparable to those of conventionally produced superalloys. The finding that phase boundaries can be employed to prevent undesirable clustering of boron atoms can be extended to manipulate the distributions of other critical elements, which provides a new path for designing novel Ni superalloys with balanced printability and mechanical properties.

In recent years, a new class of super saturated binary and ternary alloys have demonstrated the ability for the self-healing of creep-induced voids formed at the grain boundaries. However, a clear understanding of the parameters affecting the self-healing mechanism is still not yet complete. One of the main challenges is understanding the effect of microstructure and micromechanical stresses on the redistribution of the healing-solute and vacancies. To this end, we address this issue using a CALPHAD-informed diffusion model coupled with crystal plasticity. In principle, the approach is general and can be used for any binary Fe–X alloy, but in this work Fe–Au binary system is used since it experimentally showed the best healing efficiency. First, we present a multicomponent diffusion model considering cross and stress-driven diffusion. The effect of stress was also considered on the equilibrium vacancy concentration. To investigate the effect of the micromechanical stresses, a representative volume element (RVE) was obtained using the phase-field method. The results showed that the maximum vacancy concentration is at the grain boundaries (GBs) with the highest hydrostatic tensile stresses. These were also the regions of the highest Au enrichment. A crucial factor to achieve this is the high diffusivity of Au compared to the Fe matrix. Increasing the stresses, lead to an increase both in vacancy and Au concentration. The accompanying increased stress triaxiality is suggested to be the reason for the reduced self-healing efficiency observed in previous experimental studies.

Understanding the trapping and diffusion mechanism of hydrogen in vanadium carbide (VC) precipitates is crucial for exploring the issue of hydrogen embrittlement in steel. Although there is widespread consensus that VC can trap hydrogen, the mechanism by which hydrogen diffuses into VC is still unclear. In this study, we used first-principles calculation methods to study the influence of different spacings of carbon vacancies on the trapping and diffusion of hydrogen in VC. The increase in the number of C vacancies makes it easier for vacancies to trap hydrogen, and hydrogen tend to fill up C vacancies. The diffusion of hydrogen into VC only occurs via neighboring C vacancies at a distance of 0.295 nm (connecting vacancies), leading to a diffusion barrier of 0.63–0.78 eV. This is consistent with experimental results and validates the experimental speculation that the diffusion of hydrogen in VC requires a connecting C vacancy grid.

For decades, solid solution strengthening with up to 9 wt.% Ni has been the only successful strategy to obtain high strength and high impact toughness in bcc-structured (ferritic or tempered martensitic) cryogenic steels. Until now coherent nano-precipitates, an effective strengthening agent at room temperature, cannot be used to improve properties at cryogenic temperatures. Here, a new type of Mo-rich nano-B2 precipitates formed in a 6.5 wt.% Ni steel upon doping with 0.2 wt.% of Mo is reported. These precipitates are not only fully coherent with the matrix but are also shearable at 77 K. A high precipitate number density in excess of 2 × 10²⁴ m⁻³ has been achieved by an industrially feasible process optimization, which brings both the cryogenic strength and impact toughness of the steel to the same levels as those of 9Ni steels. The Mo-rich B2 precipitation strengthening, therefore, opens a new avenue for the design and development of low-cost high-performance cryogenic steels.

This paper elucidates the link between the near-wake development of a circular cylinder coated with a porous material in a low Mach-number flow and the related aerodynamic sound attenuation. It accomplishes this by formulating the cylinder flow-induced noise as a diffraction problem. The necessity for such an approach is driven by experimental evidence obtained through acoustic beamforming and particle-image-velocimetry measurements, which reveal that the dominant noise sources for a coated cylinder are not localised on the body surface but rather in the wake, specifically at the outbreak position of the shedding instability. The acoustic field at the vortex-shedding frequency can be hence modelled by considering a compact lateral quadrupole at this location and employing an exact Green's function tailored to a cylindrical geometry. Because of the diffraction of the sound waves radiated by the quadrupolar source on the cylinder surface, the resulting far-field directivity pattern resembles that of a dipole. The study demonstrates that the porous coating has the two-fold effect of decreasing the strength of the point quadrupole in the wake and moving its origin further downstream, reducing, in turn, the efficiency of the sound scattering. Consequently, the diffracted part of the acoustic field, which dominates the far-field noise for a bare cylinder in accordance with classical theory, provides a contribution that is comparable to the direct part. The results eventually indicate that quadrupolar sources must be considered to accurately predict the noise radiated from a porous-coated cylinder, even at low Mach numbers.

A new route towards strong yet ductile metals via architecting heterogeneities in both structure and metastability is presented. Such heterogeneities are generated and manipulated in a standard stainless steel using a heterogeneous phase transformation (het-PT) strategy, in which focused laser patterning is applied to stimulate site-specific phase transformations. The het-PT processed steel contains periodically arranged stripes of strong martensite and ductile austenite with different grain sizes and metastability. The resulting structural heterogeneity leads to a desirable strain gradient and heterogeneous deformation-induced martensitic transformation during deformation, which effectively accommodate strain localization and enhance work hardening capability in the het-PT-processed steel. These unique features result in an enhanced balance between strength and ductility, outperforming both homogeneously ultrafine-grained and coarse-grained counterparts. The het-PT strategy is expected to be applicable to tailoring structural heterogeneity and metastability in other steels and metals.

The aerodynamic noise radiated by the flow past a cylinder in the subcritical regime can be modeled by a quadrupolar sound source placed at the onset position of the vortex-shedding instability that is scattered by the surface with a dipolar directivity. When the cylinder is coated with a porous material, the intensity of the shed vortices is greatly reduced, determining a downstream shift of the instability-outbreak location. Consequently, sound diffraction is less efficient, and noise is mitigated. In this paper, an innovative design approach for a flow-permeable coating based on a further enhancement of such an effect is proposed. The results of phased-microphone-array measurements show that, once the leeward part of the cover is integrated with components that make the flow within the porous medium more streamlined, the quadrupolar source associated with the vortex-shedding onset is displaced more downstream, yielding additional noise attenuation of up to 10 dB with respect to a uniform coating. Furthermore, the same noise-control mechanism based on the weakening of the sound scattering can be exploited when these components are connected to the bare cylinder without the porous cover. In this case, the mitigation of overall sound pressure levels is comparable to that induced by the coated configurations due to the lack of noise increase produced by the inner flow interacting within the pores of the material. Remarkable sound reductions of up to 10 dB and a potential drag-force decrease are achieved with this approach, which paves the way for disruptive and more optimized noise-attenuation solutions.

The current study reports in situ TEM observations of the growth of bainitic ferrite in an Fe-0.3C-3Mn-1.5Si-0.15Mo steel held isothermally at 300 °C with a higher spatio-temporal resolution than in previous studies. Significant variations were found in the lengthening rate, with the highest being in excess of 30,000 nm.s⁻¹ while more common lengthening rates of 10–1000 nm.s⁻¹ provided the highest quality observations. Both sheaves with visible sub units and individual laths were observed during growth with the lengthening behaviour of sheaves found to be discontinuous - in the most favourably oriented sheave this could be linked to sub unit behaviour. The transformation behaviour was comparable to that of HT-LSCM observations of bainitic ferrite growth for the most comparable steel compositions and to ‘textbook’ descriptions of the formation of bainite sheaves. In addition, other relevant phenomena were recorded, including the generation and movement of dislocations in the austenite during transformation, the interaction of laths with twin boundaries and the initially slow growth of bainitic ferrite laths.

In order to capture and separate CO2 from the air or flue gas streams through nanoporous adsorbents, the influence of the humidity in these streams has to be taken into account as it hampers the capture process in two main ways: (1) water preferentially binds to CO2 adsorption sites and lowers the overall capacity, and (2) water causes hydrolytic degradation and pore collapse of the porous framework. Here, we have used a water-stable polyimide covalent organic framework (COF) in N2/CO2/H2O breakthrough studies and assessed its performance under varying levels of relative humidity (RH). We discovered that at limited relative humidity, the competitive binding of H2O over CO2 is replaced by cooperative adsorption. For some conditions, the CO2 capacity was significantly higher under humid versus dry conditions (e.g., a 25% capacity increase at 343 K and 10% RH). These results in combination with FT-IR studies on equilibrated COFs at controlled RH values allowed us to assign the effect of cooperative adsorption to CO2 being adsorbed on single-site adsorbed water. Additionally, once water cluster formation sets in, loss of CO2 capacity is inevitable. Finally, the polyimide COF used in this research retained performance after a total exposure time of >75 h and temperatures up to 403 K. This research provides insight in how cooperative CO2-H2O can be achieved and as such provides directions for the development of CO2 physisorbents that can function in humid streams.