ZrC Ultra-High Temperature Ceramic is a promising material for future extreme environment applications. However, its susceptibility to oxidation at elevated temperatures poses a significant challenge. There remains unresolved controversy in literature regarding its oxidation kinetics and activation energies. The temperature, oxygen pressure and time effects on the oxidation and passivation of ZrC are still not fully understood. To address these questions, we fabricated near-stoichiometric ZrC ceramic via spark plasma sintering (SPS) and for the first time investigated the temperature-oxygen pressure-time (T-P-t) dependent oxidation kinetics of SPS-sintered ZrC. A three-stage oxidation mechanism including a passivation stage was reported. The study also revealed the complexity of activation energy dependence on temperature and pressure within the 3D T-P-t space. Additionally, it uncovered the conditions necessary to maintain the passivation of ZrC. These findings provide valuable insights for future design of oxidation-resistant ZrC and carbides, paving the way for advancements in materials for extremes.

Liquid metal embrittlement (LME) during resistance spot welding (RSW) of twinning induced plasticity (TWIP) steel is primarily driven by stress-assisted grain boundary (GB) diffusion of zinc (Zn). Although GB diffusion is widely recognized as the dominant LME mechanism, experimental quantification is challenging due to resolution limitations. This study characterizes Zn diffusion in TWIP steel during RSW by conducting energy dispersive X-ray spectroscopy (EDS) line scans ahead of LME cracks in both the rolling direction (RD) and normal direction (ND) over weld times from 700 to 1700 ms. Results reveal that Zn diffusion distance increases with weld time, with consistently higher diffusion in the ND. To compare experimental measurements with diffusion theory, an FEA simulation based on Fick’s law was employed to approximate bulk Zn diffusion under varying temperatures. The model predicts Zn diffusion trends consistent with experimental observations. Although the diffusion distance predicted in the simulation exceeds measured values, directional trends are accurately captured. A theoretical framework to compare GB and bulk diffusion was proposed. GB diffusion distance of Zn is estimated to be approximately 30 times greater than bulk diffusion, establishing a quantitative link between weld time and Zn diffusion during RSW of TWIP steel.

Zirconium carbide (ZrC) is a candidate material for extreme environments due to its exceptional thermal and mechanical properties. However, its oxidation behavior, particularly the formation of the Zr–C–O layer, requires further clarification. In this study, we investigated the oxidation of spark plasma sintered ZrC under varying temperatures and oxygen partial pressures, revealing a double-layer oxide scale. At the interface between ZrC and the Zr–C–O layer, we identified previously unreported oxidation front stripes composed of cubic zirconia, along which elliptical submicropores formed, suggesting preferential CO₂ release pathways. The Zr–C–O layer itself was significantly enriched with amorphous free carbon. Based on these findings, we developed a phenomenological model that incorporated the formation of the compact Zr–C–O layer to predict oxide scale growth. This multiscale approach provides new insights into ZrC oxidation mechanisms and supports the design of oxidation-resistant ceramics for aerospace and nuclear applications.

Zirconium diboride (ZrB₂)–silicon carbide (SiC) composites are promising candidates for ultra-high temperature applications, yet optimizing their densification and mechanical performance without sintering additives remains a challenge. This study systematically investigates the independent and combined effects of three critical spark plasma sintering (SPS) parameters, that is, temperature, applied pressure, and dwell time, on the densification behavior, microstructure, and mechanical properties of ZrB₂–20 vol% SiC composites. Building upon prior work on powder preparation effects (e.g., Tungsten Carbide (WC) vs. ZrO₂ milling), this research uniquely focuses on how precise control of sintering conditions alone can tailor final material characteristics. The results demonstrate that optimizing sintering parameters yields significant property enhancement, achieving a maximum relative density of 99.2% (at 2100°C, 65 MPa, 15 min) and peak flexural strength of 516 MPa (at 2000°C, 65 MPa, 60 min). Hardness and fracture toughness reached 17.08 GPa and 3.85 MPa m^1/2, respectively, under optimized conditions. Through detailed microstructural and performance analysis, this work explains the fundamental role of individual sintering parameters in governing densification kinetics and mechanical outcomes. The findings offer practical guidance for additive-free, energy-efficient processing of ZrB₂–SiC ceramics for advanced aerospace and thermal protection systems.

Liquid metal embrittlement (LME) during resistance spot welding (RSW) of Zn-coated twinning-induced plasticity (TWIP) steel results in intergranular cracking driven by the interaction of liquid Zn, tensile stress, and the grain boundary (GB) network. This study investigates how GB misorientation and orientation relative to the electrode force loading axis influence the LME crack path across six weld times from 700 ms to 1700 ms. A comparative framework was developed in which, at each triple junction along the LME crack, the misorientation and angle relative to the loading axis of the chosen LME grain boundary are evaluated against those of the unchosen LME-free grain boundary. Σ3 coherent twin boundaries were LME-free at all weld times regardless of their orientation to the stress axis. Non-twin coincident site lattice (CSL) boundaries (Σ5 to Σ41) were predominantly LME-free at low weld times but progressively became LME GBs at high weld times, when their stress normalisation factor exceeded that of the competing boundary. The fraction of triple junctions where the LME grain boundary had the higher stress normalisation factor increased from 58% at 700 ms to 91% at 1700 ms, while the preference for the higher-misorientation-angle boundary declined from 73% to 56% over the same range. An LME susceptibility index incorporating grain boundary energy, temperature-dependent Zn diffusivity, and stress alignment is proposed as a framework for predicting crack path selection and guiding grain boundary engineering strategies (GBE) to reduce LME in resistance spot welded TWIP steel.

Liquid metal embrittlement (LME) presents a major barrier to the widespread adoption of advanced high-strength steels in automotive applications. Despite extensive research, decoupling its early-stage cracking and propagation micromechanisms remains challenging and is a key research gap. Distinguishing these stages is crucial to understanding the conditions and factors that are favourable for LME and to developing mitigation strategies. Moreover, it can improve the accuracy of predictive models through detailed knowledge from initiation to propagation. In this study, this challenge is addressed by performing interrupted Gleeble hot tensile tests on a Zn-galvanised twinning-induced plasticity steel, simulating resistance spot welding conditions. This approach enables tracking LME progression under applied stress and identifying fracture micromechanisms at early and advanced stages of cracking. Additionally, existing theories on LME micromechanisms are often contradictory, highlighting the need for fundamental research in this area. The findings reveal that LME begins with the contact between liquid Zn and the substrate, leading to Zn diffusion into the substrate by diffusion-induced grain boundary migration and dissolution of the substrate by erosion-corrosion. This dissolution generates defects on the substrate and facilitates Fe diffusion into liquid Zn. Subsequently, defects are filled with liquid and the Zn-rich defect tips, connected to grain boundaries, enhance Zn grain boundary diffusion and weaken intergranular cohesion. Under tensile stress, these weakened boundaries decohere and lead to crack nucleation. Newly formed crack surfaces allow fresh Fe-rich liquid Zn to penetrate, continuing the process until fracture. Future work will focus on the influence of microstructure on LME crack growth.

This study investigates the impact of different powder milling methods on the densification and mechanical properties of ZrB₂-SiC ceramic composites processed via spark plasma sintering (SPS). Powders were prepared using two ball milling techniques: tungsten carbide (WC) and conventional ZrO₂. The densification behavior during SPS was monitored, and the sintered samples were evaluated for their relative density, hardness, fracture toughness, and flexural strength. Results show that WC milling significantly enhances densification, achieving 99.2 % relative density at 2100 °C/65 MPa/15 min, compared to 96.5 % for ZrO₂-milled samples. This improvement is due to WC's sintering aid effect, which promotes grain boundary diffusion and particle packing. However, ZrO₂-milled composites exhibit superior hardness (17.38 GPa) and fracture toughness (3.97 MPa m¹/²), attributed to their refined grain structure and the absence of softer ZrO₂ phases. Conversely, WC-milled samples show slightly higher flexural strength (384–516 MPa), likely due to the transformation toughening effect of the secondary ZrO₂ phase. Overall, WC milling improves densification and flexural strength, while ZrO₂ milling yields finer-grained composites with higher hardness and toughness, making it better suited for wear-resistant and mechanically demanding applications.

Wire arc additive manufacturing (WAAM) offers a novel approach to fabricate functionally graded components. By changing the wire consumable between layers, chemical grading can be used to obtain specific properties across a part's volume. This is an interesting approach to design large metal components that achieve unconventional performance in demanding engineering applications, such as sulphide-resistant pressure vessels or sea ballast piping with extended lifetime. However, challenges derived from dissimilar material combinations draw the need to study the effect of compositional grading on the mechanical properties. This study focuses on the deformation and fracture toughness behaviour of WAAM-fabricated high-strength low-alloy (HSLA) and austenitic stainless (AS) steel bi-material specimens, particularly examining the diluted interface layer obtained during deposition. Tensile testing results indicate that the elastic modulus at the interface matches that of un-diluted AS steel (157 ±17 GPa) along the build direction. Fracture toughness showed a lower J_IC (180 kJ/m²) when compared to the undiluted AS steel (459 ±69 kJ/m²) and HSLA steel (408 ±25 kJ/m²). Scanning electron microscopy and electron backscatter diffraction are used to establish a connection between the microstructure at the interface and the observed mechanical properties. It is concluded that deformation at the interface is in large controlled by the deformation-induced martensitic transformation of metastable austenite. These results underline the influence of chemical dilution on the deformation mechanisms and fracture behaviour of HSLA and AS steel bi-material parts, which needs to be accounted for in the design of parts composed by this bi-metal couple.

In this study, three-dimensional functionally graded NiTi bulk materials were fabricated using laser powder bed fusion (LPBF) by in-situ adding Ni powder into equiatomic NiTi powder. The gradient zone exhibited a Ni composition ranging from approximately 49.6 to 52.4 at.% over a distance of about 2.75 mm. The functionalities along the compositional gradient were examined through differential scanning calorimetry analysis and spherical indentation. This unique gradient resulted in location-specific functionalities, including superelasticity characterized by wide and narrow hysteresis loops, shape memory effect, and various phase transformation temperatures. The rapid cooling rate during fabrication led to the presence of excess Ni in the solid-solute state within NiTi. This unique solid-solute compositional gradient in NiTi resulted in varying lattice parameters, influencing the compatibility between martensite and austenite and allowing for tailored hysteresis. This discovery presents new avenues for designing multifunctional materials through in-situ additive manufacturing.

Copper-steel functionally graded materials combine the thermal conductivity of copper with the mechanical strength of steel. This study examines the microstructural, mechanical, and thermophysical properties of the constitutive layers of copper-4130 steel functionally graded material fabricated via laser directed energy deposition, considering four intermediate compositions: 100% 4130, 75% 4130 – 25% Cu, 50% 4130 – 50% Cu, and 25% 4130 – 75% Cu. It was observed that the amount of Cu-rich terminal liquid governs crack formation and backfilling during solidification, while Cu-Fe liquid phase separation and Marangoni convection within the melt pool generate macrostructures composed of alternating Cu- and Fe-rich phases. Increasing Cu content progressively enhances thermal diffusivity due to the formation of interconnected copper regions. The application of quenching and tempering treatments induced softening of Cu-containing samples due to Cu recrystallization and diffusion from supersaturated Fe-rich phases. Although solidification cracking was only observed in 75% 4130–25% individual samples, the analysis of a complete multilayer structure revealed that interlayer mixing causes local compositional variations, extending cracking susceptibility beyond this region. These findings provide insights into the key factors governing laser directed energy deposition of copper-steel functionally graded materials, supporting process optimization and predictive model development to enhance manufacturability.

The sensitivity of the single-phase, low thermal expansion (LTE) alloy 36 (Fe-36Ni) to intergranular cracking hinders its processability during additive manufacturing. This study investigates the effect of accelerated cooling via a CO₂ jet and the addition of TiC particles on the cracking susceptibility of the LTE36 alloy during wire-arc additive manufacturing (WAAM). Results show that accelerated cooling reduces inter-pass deposition times and the susceptibility to cracking due to increased heat dissipation. A crack-free microstructure was achieved only with the addition of TiC particles, which pinned the high-angle grain boundaries and induced tortuosity, thereby limiting grain growth and mitigating intergranular cracking. Mechanical performance was restored compared to the cracked condition, and the critical LTE property of the as-deposited LTE36 alloy was improved due to the enhanced ferromagnetic character of the alloy. Therefore, the combined approach effectively mitigated intergranular cracking while retaining the LTE behaviour during WAAM of the LTE36 alloy.

This study concentrates on the fatigue crack propagation behaviour of a high-strength low-alloy (HSLA) steel and austenitic stainless (AS) steel bi-material part, as obtained by wire arc additive manufacturing (WAAM). Due to partial mixing in the weld pool, the first layer of AS steel laid onto the previously deposited HSLA steel results in a diluted interface layer of distinct chemical and microstructural characteristics. Average Paris parameters are obtained for the interface layer along transverse and longitudinal planes to the deposition direction (BD-LD plane: m = 2.79, log₁₀(C) = –7.83 log₁₀(da/dN)) (BD-TD plane: m = 3.47, log₁₀(C) = –8.39 log₁₀(da/dN)). However, it is observed that this interface layer manifests an intriguing crack propagation behaviour. FCGR consistently drop as the crack front transitions from undiluted AS steel to the interface. At ΔK = 20 MPa⋅m^0.5, the greatest Δ is −0.77 log₁₀ steps (R = 0.1). As cracks near the HSLA fusion line, rates re-accelerate up to + 0.75 log₁₀ steps (R = 0.5). The phenomenon is attributed to the interplay between deformation-induced martensitic transformation and pre-existing allotropic martensite. Our findings, derived from a series of fatigue tests in correlation with multiscale microstructural and fracture characterization, offer insights into the damage-tolerant behaviour of these bi-material structures.

Superelastic metamaterials have attracted significant attention recently, but achieving such functionality remains challenging due to partial superelasticity and premature fracture in additively manufactured components. To address these issues, this study investigates the premature fracture in Ni-rich NiTi metamaterials fabricated by laser powder bed fusion. A comparative analysis of two structures (Gyroid network and Diamond shell) reveals that the structural stability of bending- and stretching-dominated structures is reversed compared to typical elastic-plastic response, due to the tension-compression asymmetry of base NiTi. The premature fracture and partial superelasticity of these as-fabricated samples are attributed to low deformation ability for accommodating tensile stress. Based on these findings, a heat treatment introducing Ni₄Ti₃ precipitates was employed, successfully achieving macroscopic superelasticity in the NiTi metamaterials, with consistency between model prediction and experiments.