Crack-tip constraint under elevated-temperature conditions cannot be adequately characterized by a single crack-tip constraint descriptor. The applicability of the modified Q-parameter Qm, stress triaxiality h, and out-ofplane constraint factor Tz therefore requires clarification. In this study, three-dimensional elastic-plastic finite element analyses were performed on C(T), SEN(B), and SEN(T) specimens of 304, 316, and P92 steels. Specimen thickness and crack depth were varied independently over a wide range of J-integral levels to distinguish out-ofplane and in-plane constraint effects. The results show that Qm has the widest variation range and the highest sensitivity, capturing both thickness- and crack-depth-induced constraint variations. h responds consistently to both types of constraint but remains markedly load dependent during plastic deformation. Tz is numerically stable and weakly load dependent, making it suitable for thickness-controlled out-of-plane constraint; however, its narrow variation range limits its ability to distinguish crack-depth-induced in-plane constraint. Based on these findings, a unified Qm-h-Tz transformation framework is established for thickness-dominated constraint evolution, with h serving as the bridging variable between stress-field deviation and through-thickness confinement. For crack-depth-dominated constraint, a reduced Qm-h description is more appropriate. These results define the applicability limits of the three parameters and support elevated-temperature fracture assessment.

To overcome the adverse effects of the uneven composition of the molten pool and the poor growth of columnar crystals caused by rapid cooling underwater during underwater welding on the corrosion performance of underwater welds, this study innovatively proposed a double pulsed current (DPC) waveform modulation technology. The influence of different DPC waveforms on the microstructure evolution and corrosion resistance of local dry underwater welded 304 stainless steel (SUS304) was systematically studied. The results show that DPC induced the periodic oscillation of arc energy distribution, which achieved the stirring effect on the underwater molten pool, thereby refining the grain structure by maximum 54.6 %, increasing the ferrite content by 43.2 % and making it uniformly distributed on the austenite matrix. It promoted the transformation of ferrite morphology from skeleton to lath, and enhanced the uniformity of the molten pool composition. With the decrease of the weak pulse peak current, the stirring effect exhibited a trend of first enhancing and then weakening. The corrosion failure mode of the SUS304 underwater weldment was pitting corrosion. The fine grain structure and higher ferrite content brought by DPC facilitated to form a stable passivation film structure, increasing its thickness by 173.1 %, thereby improving the corrosion resistance of underwater weldment. This work provides a flexible solution and solid foundation for underwater welding quality optimizing of austenitic stainless steel, which promote the application of underwater welding technology in the construction and repair of large underwater structures.

In this study, a high-power laser-arc hybrid additive manufacturing process with synchronous powder feeding (LAHAM-SPF) was developed to fabricate TiC/Al-Cu composites. By optimizing the processing parameters, efficient introduction, stable deposition, and effective incorporation of TiC particles were achieved. On this basis, the evolution of TiC particles in the hybrid molten pool, the associated solidification behavior, and the resulting microstructure-property relationship were systematically investigated. The results showed that, under the high-power laser-arc hybrid thermal field, the initially added coarse TiC particles were evidenced to undergo partial dissolution and subsequent reprecipitation, leading to the formation of a multilevel strengthening architecture composed of retained coarse TiC particles, in-situ re-precipitated fine TiCp/TiCw and Al3(Ti,Zr) phases. This architecture significantly modified the solidification behavior of the hybrid molten pool, reduced the heterogeneous nucleation barrier of α-Al, and suppressed Cu segregation and the formation of a continuous grain-boundary θ-Al2Cu eutectic network. Owing to the synergistic effects of dislocation multiplication, grain refinement, Orowan strengthening, and load transfer, the ultimate tensile strength increased from 267.1 MPa for the Al-Cu matrix to 310.8 MPa for the composite. In addition, the wear resistance of the composites was markedly improved at both room and elevated temperatures, with the average wear rate reduced by approximately 88

To investigate the effects of different oscillation paths (no oscillation, 8-shaped, and triangular types) on the interface microstructure, intermetallic compound (IMC) layer thickness, tensile strength, and fracture behavior of Al/Ti dissimilar metal joints during the laser welding-brazing process, Al/Ti composite joints were prepared by laser welding-brazing technology. By using the oscillation path of the laser spot as the key process variable, the weld morphology and interface microstructure were characterized by optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy. Moreover, the mechanical properties and fracture mechanism of the joints were analyzed in combination with tensile tests. The results indicate that, compared with no oscillation, the 8-shaped and triangular oscillation paths can effectively increase the spreading length of the brazing filler metal on the surface of the Ti alloy and significantly inhibit the growth of interfacial IMC. Under the 8-shaped oscillation, the interfacial IMC thickness is the thinnest, ranging from 0.9 to 3.19 μm. element analysis confirms that Si aggregates at the Ti/Al interface, forming a Ti(Al, Si)3 ternary phase. In terms of mechanical properties, the tensile strength of the joint without oscillation is the lowest (119.19 MPa). The fracture occurs at the thicker Ti/Ti(Al, Si)3 interface or the Ti(Al, Si)3 layer, presenting typical brittle fracture characteristics. The oscillation paths effectively improve the joint strength by refining the IMC layer.

Ceramic-particle-assisted laser-arc hybrid welding (LAHW) provides a potential route for improving the mechanical performance of 2219 AlCu alloy welds. However, the effective and stable introduction of ceramic particles into the hybrid molten pool remains challenging. In this study, two wire-powder coordinated LAHW routes, namely synchronous powder feeding (SPF) and laser pre-cladding (LPC), were developed to introduce TiC particles into 2219 AlCu alloy welded joints. The effects of particle introduction route on weld formation, TiC distribution, microstructure evolution, and mechanical properties were investigated. TiC addition improved laser energy absorption and stabilized root penetration in both TiC-containing samples. During SPF, however, the incident momentum of TiC particles and their interaction with the advancing solidification front promoted particle enrichment near the fusion line and in the laser zone. The resulting particle agglomeration, together with carrier-gas disturbance and weakened melt spreading at the weld edge, increased the tendency for pore formation and undercut defects. In contrast, LPC enabled more uniform TiC redistribution in the hybrid molten pool and reduced large pores and particle agglomeration. TiC particles promoted heterogeneous nucleation of α-Al and refined the weld grains. The LAHW-LPC sample showed more uniform fine equiaxed grains and a network-like θ-Al₂Cu distribution. The yield strengths of the LAHW, LAHW-SPF, and LAHW-LPC samples were 151.2, 177.9, and 180.1 MPa, respectively. The LAHW-LPC sample achieved the best overall tensile performance, with an ultimate tensile strength of 288.9 MPa and an elongation of 3.47%. The improved strength-ductility balance was attributed to uniform TiC distribution, reduced defects, grain refinement, and the network-like θ-Al₂Cu phase that hindered crack propagation.

Pipelines are the most economical and efficient means for large-scale, long-distance transport of hydrogen gas, helping accelerate the realization of a hydrogen economy. Today, the development of hydrogen pipeline projects, including repurposing existing pipelines for hydrogen service, has become a global focus, especially in major energy producing and consuming countries. However, steel pipelines are prone to hydrogen embrittlement (HE) in high-pressure gaseous hydrogen environments, which can lead to pipeline failure. Drawing on published work, we assemble a curated knowledge base that distills how gaseous hydrogen embrittlement in pipelines is understood, characterized, and evaluated.

In laser-arc hybrid additive manufacturing (LAHAM), the high-power laser required for precise forming often leads to unstable keyhole behavior and severe keyhole-induced porosity, which compromise mechanical properties. In this study, laser beam oscillation was employed to address this contradiction. It was found that laser beam oscillation could distribute the laser energy over a larger area, reducing the average power density below the critical threshold for keyhole formation. Under high-power laser processing (laser power of 3 kW), optimizing the oscillation parameters decreased the keyhole-induced porosity from 16.3 % under the non-oscillating condition to 0.3 %, and restored the elongation from 2.2 % to 17.4 %. Moreover, the high-power laser with beam oscillation preserved the precision-forming benefits. The highly ionized plasma generated by the laser provided a low-resistance channel for the arc current, which attracted and compressed the arc. The resulting electromagnetic forces guided the droplets along a more controlled transfer path, and together with the stabilized molten pool behavior, enabled superior forming accuracy. Compared with the low-power (laser power of 1 kW) samples, the surface roughness was reduced by 68.2 %, and the material utilization increased by 29.4 %. In addition, the high-power oscillating laser promoted molten flow to disrupt dendrites, facilitated rapid crystallization, and reduced local temperature gradients, thereby breaking the directional growth of columnar grains. The resulting refined, random-oriented multimodal grain structure significantly reduced mechanical anisotropy. This work provides a feasible technical pathway and a valuable reference for achieving high-quality additive manufacturing of aluminum and other challenging materials.

To ensure safe and reliable operation of high-temperature power plant components, it is crucial to evaluate the failure risk of P92 steel welded joints after long-term service. This study systematically investigated microstructural evolution and creep damage mechanisms using a multi technique characterization approach to analyze weld metal (WM), coarse grained heat-affected zone (CGHAZ), fine grained heat-affected zone (FGHAZ), and base metal (BM). Results revealed pronounced softening in the FGHAZ, with microhardness declining to 175 HV10, while repair welding partially restored microstructure and hardness in WM and CGHAZ. Degradation in FGHAZ, including precipitate coarsening and lath structure loss, remained largely irreversible. Creep cavities preferentially nucleated at delta-ferrite, grain boundary triple junctions, and coarse M23C6 and Laves phases. TEM observations showed extensive dislocation entanglement and slip around coarse precipitates, facilitating microcrack initiation, whereas MX carbonitrides remained stable. EBSD analysis indicated severe microstructural degradation in FGHAZ, with reduced lath boundaries and kernel average misorientation, and increased fractions of recrystallized grains and subgrains, contributing to localized softening and elevated creep susceptibility. Based on these findings, a creep damage model governed by microstructural degradation and abnormal delta-ferrite distribution was proposed. Overall, the study identified delta-ferrite and coarse precipitates as primary damage nucleation sites, providing quantitative microstructural metrics to guide failure risk assessment and life prediction of P92 welded joints after long-term service.

The inherent unpredictability of the wear resistance of NiTi alloys, a stress-induced phase transformation material, stems from their dynamic phase transformation under frictional loads, which seriously hinders practical applications. This study conducted friction experiments on LPBF-NiTi alloys with different energy densities (185–93 J/mm3) and found that the wear resistance is closely related to the energy density. The wear mechanism is mainly caused by initial adhesive wear and subsequent abrasive wear. Abrasive wear is caused by the hardened austenite/martensite agglomerated deformed blocks formed by extrusion of austenite blocks during the friction process. During the wear process, LPBF-NiTi alloy with an energy density of 139 J/mm3 forms a martensitic hardening layer in-situ. This in-situ structural modification alleviates abrasive wear caused by agglomerated deformation blocks and suppresses wear caused by plastic deformation, exhibiting excellent wear resistance. This study provides theoretical and data support for the application of stress-induced phase transformation materials such as LPBF-NiTi alloy in the field of wear resistance. Friction property and corresponding wear mechanism diagram of LPBF NiTi.

The corrosion resistance of base metal, laser-arc hybrid welded AZ31B magnesium alloys with and without addition of carbon nanotubes (CNTs) was compared. The corrosion behaviors and the underlying improvement mechanism of CNTs were systematically investigated. The introduction of CNTs effectively refined the grains, weakened the texture and enhanced the microstructure homogeneity of the weld, which contributed to the enhancement of corrosion resistance. Specifically, the corrosion rates of hydrogen evolution and weight loss of weld decreased by >30% after the addition of CNTs, and the corrosion products were denser due to the formation of Al2O3 passive film. The corrosion current density and polarization resistance of weld with addition of CNTs were 1.220 µA/cm2 and 7155 Ω·cm2, respectively, in contrast to 2.480 µA/cm2 and approximately 269.5 Ω·cm2 for the weld without CNTs. Besides, the content of precipitates in the weld increased from 0.60% to 1.76% after the addition of CNTs, which can release Al3+ ions, promoting the formation of a dense Al2O3 film that serves to protect the metal matrix from further degradation.

The coarse-grained heat-affected zone (CGHAZ) was widely recognized as the most brittle region in single-pass welding with high heat input. Previous studies mainly focused on either impact toughness or fracture toughness of the CGHAZ, often assuming a linear relationship between the two. This study revealed that there was a decoupling phenomenon between fracture toughness and impact toughness in CGHAZ of HSLA steel, resulting from the combined influence of loading conditions and temperature. The fracture toughness of the simulated CGHAZ specimens first increased and then decreased with the increasing heat input, resulting in brittle fracture only when the t8/5 was reached 335 s. In contrast, the impact toughness exhibited a continuous decline, and brittle fracture was observed as early as when t8/5 was 89 s. In the impact test, Fe3C/DP phases were found to initiate microcracks even at relatively low heat input (t8/5 = 89 s), resulting in the embrittlement of acicular ferrite and granular bainite. TEM revealed that coarsened Fe3C/DP failed to generate dislocation pile-up/tangles prior to cracking under impact loading and lower temperature, acting instead as direct crack sources-a behavior not observed under CTOD test. Therefore, the impact test showed a higher sensitivity to coarsened and aggregated of Fe3C compared with the CTOD test. This work provides a mechanistic framework for understanding decoupling between fracture and impact toughness and informs the design of HSLA alloys for complex service conditions.

In this study, four Fe-18Cr-B alloys with distinct hardness levels were fabricated via laser directed energy deposition, and their microstructure, wear, corrosion, and tribo-corrosion behaviors were systematically evaluated. The alloys exhibited microstructural heterogeneity, with typical zones including fine-grained, coarsegrained, and tempered regions. Dual-phase strengthening via boride precipitation and martensitic transformation governed their hardness, which strongly correlated with wear resistance. Corrosion resistance, in contrast, was dominated by the selective dissolution of Cr-depleted matrix regions, particularly within the tempered zones, where galvanic coupling and occluded cell effects accelerated localized attack. Tribo-corrosion analysis further revealed that material loss is governed by the competition between corrosion-enhanced wear (Delta Wc) and wearenhanced corrosion (Delta Cw), with their relative contributions controlling the dominant degradation mechanism. Under 3% FeCl3, Delta Wc prevailed and hardness effectively reduced loss; under 6% FeCl3, Delta Wc continued to dominate material loss, while localized pitting substantially intensified degradation. A synergistic mechanism was proposed linking microstructure to failure depth, emphasizing that optimizing Cr retention and matrix continuity is critical to enhancing tribo-corrosion resistance in Fe-Cr-B systems.

Nickel-based superalloy 718 is widely used in gas turbines but is vulnerable to hydrogen embrittlement. This study applies first-principles density functional theory to examine hydrogen behavior in the gamma (Ni), gamma '' (Ni3Nb), and delta (Ni3Nb) phases, as well as at gamma ''/gamma and delta/gamma interfaces. Hydrogen preferentially occupies octahedral interstitial sites, while tetrahedral sites are energetically unfavorable. At interfaces, segregation occurs mainly at sixfold Ni-coordinated sites on the gamma side, with negative segregation energies confirming strong trapping. The interfacial separation energy decreases by similar to 19% at the gamma ''/gamma interface but remains nearly unchanged at the delta/gamma interface. Vacancies significantly enhance trapping, with gamma-Ni vacancies accommodating up to six hydrogen atoms. Charge transfer from Ni to H stabilizes these complexes. These results clarify the distinct roles of gamma '' and delta precipitates in hydrogen embrittlement and provide atomic-scale guidance for designing Ni-based alloys with improved resistance.

A new out-of-plane constraint parameter, Qm*, is proposed to capture fracture toughness evolution of 316 stainless steel at 550 degrees C under large-scale yielding (LSY). Unlike conventional measures, Qm* is load-independent and remains stable across specimen geometries and thicknesses, as confirmed by finite element analyses of C(T), SEN(B), and SEN(T) configurations. Crack propagation was modeled using a Rice-Tracey damage framework, producing J-R curves consistent with experiments. By correlating resistance parameters with Qm*, a generalized transformation method was established to predict J-R curves across different geometries and sizes from limited test data. This approach decouples geometry effects from intrinsic material behavior, enabling constraintcorrected fracture toughness evaluation with reduced experimental cost. The proposed framework demonstrates broad applicability for constraint-based modeling of ductile fracture and offers a transferable methodology for structural integrity assessments under service conditions.

The problems of coarse grains and significant deterioration in joint performance are common in aluminum alloy welding. Adding ceramic particles to address these issues is a novel approach. In this study, laser-arc hybrid welding of 2219 aluminum alloy was conducted with the addition of varying contents of TiC particles, which were introduced into the weld via laser pre-cladding. The particle distribution, microstructure evolution, and strengthening mechanisms of the welded joints were investigated. With increasing TiC particle content, the growth of columnar grains near the fusion line was inhibited, and these grains gradually transformed into equiaxed grains-achieving a 90 % grain refinement and homogenization of grain orientation. Furthermore, the morphological evolution of precipitates was promoted, along with improvements in mechanical properties. At a particle feeding rate of 1.2 r/min, the tensile strength and elongation reached 297 MPa and 4.86 %, representing increases of 15.6 % and 36.5 %, respectively, compared to the joint without particle addition.

This study challenges the common premise of a linear correlation between impact and fracture toughness in LBZ of High-strength low-alloy steels, revealing a fundamental mismatch between them. In this study, fracture/impact toughness tests were conducted on simulated LBZ specimens with different heat inputs, and their fracture mechanisms under quasi-static loading and impact loading were investigated respectively. The results showed the crack tip opening displacement (CTOD) value of coarse-grained HAZ (CGHAZ) exhibited a three-fold increase with increased t8/5 (cooling time from 800℃ to 500℃), while the impact absorbed energy remained stable at about 200 J without significant fluctuations. Under low heat input (t8/5 = 4  s and 7  s), a lath bainite (LB)/lath martensite (LM) mixed microstructure in the CGHAZ provided high impact toughness via martensite plasticity deformation, but quasi‑static cracks propagated along brittle bainite, lowering fracture toughness. At higher heat inputs (t8/5 = 11  s and 17  s), a “softened” acicular ferrite (AF)/granular bainite (GB) mixed microstructure effectively resisted crack propagation under both loading conditions, ensuring excellent comprehensive toughness. In the intercritically-reheated CGHAZ (ICCGHAZ), numerous martensite–austenite (M−A) constituents induced microcracks and embrittled all microstructures, causing a drastic toughness deterioration. This work provides a mechanistic framework for designing welding processes to ensure structural integrity under complex loading scenarios.

The design of Ti2AlNb-based alloys with optimized mechanical properties requires understanding the effects of dopant species and concentrations on phase stability and deformation behavior. In this work, first-principles calculations are used to investigate the influence of Cu, Zr, and Mo doping on the structural stability and mechanical properties of the O phase and B2 phase over a concentration range of 1.5625-12.5 at%. The results show that all dopants can be stably incorporated into both phases, enabling systematic evaluation of their mechanical responses. Distinct dopant- and phase-dependent behaviors are observed: Cu enhances O-phase ductility, while B2 shows a non-monotonic response; Zr induces a transition from reduced to increased deformation resistance in O phase with concentration; Mo enhances O-phase ductility but increases B2-phase deformation resistance. The optimal dopant concentrations are identified as 1.5625 at% Cu, 6.25 at% Zr, and 12.5 at% Mo. These trends originate from a synergistic interplay between dopant-induced lattice distortion and electronic structure modification, with the latter playing a dominant role in governing the mechanical response.

The mechanical performance of Ti2AlNb alloys is critically influenced by interstitial elements, yet their atomic-scale effects remain insufficiently understood. In this study, first-principles calculations were employed to systematically investigate the influence of four representative interstitial atoms (B, C, N, and O) on the structural and mechanical properties of O-phase and B2-phase in Ti2AlNb alloys, as well as on the O(001)/B2(110) interface. The results reveal that interstitial doping induces lattice expansion. Although O-phase and B2-phase each contain two distinct interstitial sites, the thermodynamic incorporation preference is consistent across both, following the sequence N > C > O > B in O-phase and O > B > C > N in B2-phase. Mechanically, all interstitials enhance the stiffness of the B2-phase but reduce its ductility, whereas nitrogen uniquely strengthens the O-phase, while interstitial doping generally improves its ductility. Furthermore, nitrogen exhibits a pronounced site preference near the O/B2 interface and enhances interfacial cohesion by significantly increasing the work of separation through covalent bonding with neighboring Ti, Al, and Nb atoms. Experimental results demonstrate that the incorporation of nitrogen into Ti2AlNb alloys leads to a significant increase in tensile strength from 945 to 1092 MPa, which can be primarily attributed to the combined effects of solid solution strengthening and grain refinement, and although a slight reduction in elongation is observed, the alloys still exhibit ductile fracture behavior. These findings elucidate the atomistic mechanisms by which interstitial species modulate mechanical behavior and provide a theoretical foundation for the targeted design of high-performance Ti2AlNb-based alloys with improved interfacial integrity.

The fracture toughness behavior of X65 welded joints was investigated under a total pressure of 10 MPa in hydrogen-mixed natural gas environments with hydrogen volume fractions ranging from 0 to 20 vol.