Search Results (204)

The thermodynamic instability and relatively low mechanical strength of γ/γ′ phase boundaries in Ni-based single-crystal superalloys compromise the service safety of these materials. The interfacial segregation behavior of alloying elements is expected to enhance the thermodynamic stability and mechanical strength of γ/γ′ phase boundaries. In the present research, first-principles computations grounded in density functional theory were performed to examine the unclarified cosegregation characteristics of W-Re/Cr solutes at the γ-Ni/γ′-Ni₃Al phase boundary, as well as the impacts of such cosegregation on interfacial formation heat and Griffith fracture work. The results indicated that Re and Cr atoms tend to segregate preferentially at the γ-L₁-3.52-cp site within the W-alloyed phase boundary. This phenomenon can be attributed to the attractive interactions between W and Re/Cr, along with the fact that this site exhibits the most negative segregation energy. The thermodynamic stability of W-Re and W-Cr cosegregated phase boundaries is significantly enhanced, being much higher than that of clean or W-segregated phase boundaries, which is ascribed to deeper pseudogaps at the Fermi level. Notably, the preferred fracture path remains in region-1 after cosegregation, as directly evidenced by its lower Griffith fracture work compared to region-2. This disparity is rationalized by charge density analysis, which reveals a pronounced charge accumulation and consequently stronger bonding in region-2. Our results may provide atomistic insights into the solute cosegregation behaviors and their interfacial strengthening and stabilizing effects, and also the interfacial composition manipulation of Ni-based single-crystal superalloys. Full article

We investigate a non-Hermitian two-spin XY model driven by alternating real and imaginary transverse fields and derive an explicit analytic formula for the ground-state entanglement negativity. This provides a systematic analytic characterization of how ground-state entanglement behaves across PT-symmetry breaking in a non-Hermitian spin dimer. In the PT-symmetric regime, the anisotropy $\gamma$ enhances entanglement, whereas the real field h₀ suppresses it; in the PT-broken regime dominated by $\left|{\phi }_3\right.⟩$, the negativity decreases monotonically with the imaginary field ${\eta }_0$. Moreover, the first derivative of the negativity exhibits a cusp-type non-analyticity at the exceptional point (EP), consistent with the ground-state phase boundary and revealing a direct correspondence between entanglement transitions and exceptional-point physics. To facilitate implementation in integrated quantum photonics, we map $\left(h_0,{\eta }_0,\gamma \right)$ onto the device parameters $\left(\Delta \beta ,g,\kappa \right)$ of a PT-symmetric directional coupler and propose a two-qubit quantum state tomography readout based on local Pauli measurements, thereby offering a concrete entanglement-based probe of exceptional-point signatures in a realistic photonic platform. Within this model, we identify parameter regimes for observing this signature: a cusp feature is expected near $\Delta \beta \approx 0$ and $g\approx \kappa ,$ which remains observable under small detuning and moderate loss mismatch. These results offer a testable avenue for entanglement-based probing of PT-symmetry breaking and may inform device characterization and quantitative assessment in integrated quantum photonics. These combined advances provide both analytical insight into non-Hermitian entanglement structure and a feasible route toward experimentally diagnosing PT-symmetry breaking using entanglement. Full article

The distinct solidification behavior of additively manufactured (AM) Inconel 718 (IN718) produces a unique microstructure and precipitation response compared with its conventionally forged counterpart, leading to fundamentally different responses to heat treatment and intermediate-temperature deformation behaviors. In this work, the intermediate-temperature (450–750 °C) deformation mechanisms of laser powder bed fusion (LPBF)-fabricated and forged IN718 alloys were systematically compared under various heat-treatment conditions. Overall, under solution treatment state, the LPBF alloy exhibited fine columnar grains, a high dislocation density, and retained δ phases along the grain boundaries, whereas the forged alloy showed coarse equiaxed γ grains without the δ phase. Under solution + aging (STA) treatment, the δ phase in the LPBF alloy effectively pinned grain boundaries and enhanced flow stress, while in the forged alloy, strengthening was dominated by the uniform precipitation of γ″ and γ′ phases. Owing to Nb consumption by δ-phase formation, the STA-treated LPBF alloy contained fewer γ″/γ′ precipitates and exhibited slightly lower strength than the STA-treated forged alloy. This study demonstrates that the inherent δ phase retention and Nb segregation in LPBF-built IN718 critically influence its precipitation behavior and deformation resistance, distinguishing it from conventionally processed alloys and providing valuable insights for microstructure design in AM-built high-temperature superalloys. Full article

During the heat treatment of nickel-based superalloy (for instance Inconel 718 alloy), the non-isothermal heating and cooling processes significantly influenced precipitation behaviors as well as the final mechanical properties. This study compared the precipitation behaviors and the resulting hardening and softening behaviors of additively manufactured and conventionally forged Inconel 718 alloys under non-isothermal heat treatment processes. The results indicated that additively manufactured Inconel 718 alloy accelerated aging precipitation behavior due to the fine dendritic structure during both heating and cooling processes. As a result, the additively manufactured alloy reached peak hardness of ~480 HV at ~650 °C (~100 °C earlier than the forged alloy’s peak hardness of ~460 HV at ~750 °C) during heating and gained almost constant hardness during cooling. Further, the heating rate significantly affected the precipitation behaviors of γ″ and γ′ phases in both alloys. Slower heating rates provided sufficient time for phase transformation, leading to a more pronounced precipitation., e.g., the volume fraction of precipitates in the SLM alloy increased from 1.5% at 5 °C/min to 5.9% at 0.5 °C/min when heated to 850 °C. During cooling process, the twisted grain boundaries of additively manufactured alloy facilitated the precipitation of δ-phase, which in turn inhibited the formation of γ/γ′/γ″ phase. Both alloys exhibited minimum hardness of ~380–390 HV at 1000 °C due to complete dissolution of strengthening phases. This study provides a comparative understanding of non-isothermal phase evolution in AM and forged Inconel 718, which is critical for optimizing heat treatment in aerospace applications. Full article

Phase transformations significantly influence the mechanical properties of Fe-based alloys, making their understanding essential for the design of high-performance alloy materials. This study investigates microstructural evolution and martensitic transformations induced by B2 phase precipitation in an Fe-20Ni-4.5Al-1.0C (wt.%) alloy. The alloy was solution-treated at 1100 °C, followed by isothermal holding between 750 °C and 1000 °C, and water quenching. Microstructural analysis revealed that the as-quenched alloy consisted of a single-phase austenite (γ). Isothermal holding led to the precipitation of a (Ni,Al)-rich B2 phase within the grains and along grain boundaries. An α′-martensitic phase was also observed within γ-grains adjacent to the B2 precipitates in the isothermally held samples. Martensitic transformation is attributed to localized nickel depletion in the matrix surrounding B2, which reduced γ-phase stability and raised the martensite start temperature (Ms), promoting γ-to-α′ transformation during cooling. The co-existence of B2 and α′ phases significantly increased the hardness of the alloy, with a maximum observed at an 850 °C holding temperature. At higher temperatures, coarsening and partial dissolution of B2, as well reduced martensite formation, led to a decline in hardness. These findings highlight the role of B2 precipitation in promoting martensitic transformation and optimizing mechanical properties through controlled heat treatment. Full article

TiAl alloys are ideal candidates to replace nickel-based superalloys in aero-engines due to their low density and high specific strength, yet their industrial application is hindered by narrow heat treatment windows and unbalanced mechanical performance. To address this, this study investigates the microstructure and mechanical properties of Ti-44.9Al-4.1Nb-1.0Mo-0.1B-0.05Y-0.05Si (TNM-derived) alloys hot-rolled in the (α₂ + γ) two-phase region. The research employs varying heat treatment temperatures (1150–1280 °C) and cooling rates (0.1–2.5 °C/s), combined with XRD, SEM, EBSD characterization, and 800 °C high-temperature tensile tests. Key findings: Discontinuous dynamic recrystallization (DDRX) of γ grains is the primary mechanism refining lamellar colonies during deformation. Higher heat treatment temperatures reduce γ/β phases (which constrain colony growth), increasing the volume fraction of lamellar colonies but exerting minimal impact on interlamellar spacing. Faster cooling shifts γ lamella nucleation from confined to grain boundaries to multi-sites (grain boundaries, γ lamella peripheries, α grains) and changes grain boundaries from jagged and interlocking to smooth and straight, which boosts nucleation sites and refines interlamellar spacing. Fine lamellar colonies and narrow interlamellar spacing enhance tensile strength, while eliminating brittle βo phases and promoting interlocking boundaries with uniform equiaxed γ grains improve plasticity. Full article

In this study, the solidification behavior of 310S stainless steel was systematically investigated by combining high-temperature confocal laser scanning microscopy (HT-CLSM), microstructural characterization, and thermodynamic calculations. The focus was on the formation and transformation of ferrite, secondary-phase precipitation, and elemental segregation behavior, with comparisons made with 304 stainless steel. The effects of an Al addition and cooling rate were also explored. The results show that the solidification sequence of 310S stainless steel is L → L + γ → L + γ + δ → δ + γ, in which austenite nucleates early and grows rapidly, followed by the precipitation of a small amount of δ-ferrite in the later stages of solidification. In contrast, 304 stainless steel solidifies according to L → L + δ → L + δ + γ → δ + γ, with a rapid δ → γ transformation occurring after solidification. Compared with 304, 310S stainless steel exhibits a reduced ferrite fraction and a significantly increased σ phase content. The σ phase primarily precipitates directly from δ-ferrite (δ → σ), while M₂₃C₆ preferentially forms at grain boundaries and δ/γ interfaces, where δ-ferrite not only provides fast diffusion pathways for Cr but also nucleation sites. The solidification segregation sequence in 310S stainless steel is Cr > Ni > Fe, with Cr and Ni showing positive segregation and Fe showing negative segregation. The addition of Al does not alter the solidification mode of 310S stainless steel but refines austenite grains, reduces interdendritic solute enrichment, decreases segregation, lowers both the size and fraction of ferrite, and suppresses the precipitation of σ and M₂₃C₆ phases. This effect is mainly attributed to the reduction of δ/γ interfaces, which weakens the preferred nucleation sites for M₂₃C₆. Increasing the cooling rate enhances non-equilibrium solute segregation, promotes ferrite formation, inhibits the δ → γ transformation, and ultimately retains more ferrite; the intensified segregation further accelerates the δ → σ transformation. Full article

Voids have a crucial effect on the fatigue performance of materials. The general viewpoint is that voids, as possible sources of cracks, are harmful to the fatigue performance of materials. However, this study finds that microvoids enhance the low-cycle fatigue resistance of TiAl alloys, both in single crystal and polycrystal, using molecular dynamics simulations. Due to the difference between the simulation and test, the selected strain value is larger. It is found that during cyclic loading, Shockley partial dislocations preferentially nucleate around the microvoid in the single crystal, with stacking fault tetrahedra forming progressively to obstruct dislocation motion. The polycrystal model exhibits the synergistic effect of the microvoid–grain boundary, and the fatigue resistance is substantially enhanced through the combined mechanisms of Lomer–Cottrell lock formation, twin boundary migration, and phase transformation. In addition, simulation models with microvoids exhibit lower plastic strain energy density and enhance fatigue life compared to microvoid-free counterparts. The present study provides significant insights into designing γ-TiAl alloys through controlled microvoids to optimize fatigue resistance. Future work should include experimental validation to substantiate these computational findings. Full article

Additive manufacturing (AM) via selective laser melting (SLM) is increasingly deployed in aerospace, biomedical, and tooling applications where complex geometries and high performance are required. Yet, process-induced anisotropy and microstructural heterogeneity can strongly affect mechanical and tribological behavior. This study systematically evaluates the combined effects of build orientation (0°, 45°, and 90° relative to the build plate) and post-build heat treatment (as-built, 600 °C, and 860 °C) on the phase constitution, microstructure, hardness, tensile response, and dry sliding wear of SLM-fabricated 316L stainless steel. X-ray diffraction indicated a fully austenitic (γ-fcc) structure without detectable secondary phases across all conditions. Orientation-dependent substructures were observed: ~1 µm equiaxed cellular features at 0°, finer 0.3–0.5 µm cells at 45°, and 1–2 µm elongated features at 90°. Microhardness varied with orientation; relative to 0°, 45° specimens were ~15 HV higher, whereas 90° specimens were ~10 HV lower. Heat treatment at 600 °C promoted refinement and recovery of the cellular network, most pronounced in the 45° orientation, while treatment at 860 °C largely erased melt pool boundary contrast, producing a more homogeneous particle-like microstructure. Tensile fractography revealed dimpled rupture in all cases; the 90° orientation showed finer dimples and lower hardness, consistent with a ductile failure mode under reduced constraint. Dry sliding wear tests identified adhesive wear, intensified by the build-up of transferred fragments, as the dominant mechanism in both as-built and 600 °C conditions. Changes to melt pool morphology after 860 °C heat treatment correlated with altered wear track widths, with the 0° condition showing a notable narrowing relative to the 600 °C state. These results highlight processing pathways for tailoring anisotropy, strength–ductility balance, and wear resistance in SLM 316L. Full article

This study investigates the microstructure evolution and mechanical properties of DSS2209 duplex stainless steel fabricated via cold metal transfer wire arc additive manufacturing (CMT-WAAM). The as-deposited thin-wall components exhibit significant microstructural heterogeneity along the build height due to thermal history variations. Optical microscopy, SEM-EDS, and EBSD analyses reveal distinct phase distributions: the bottom region features elongated blocky austenite with Widmanstätten austenite (WA) due to rapid substrate-induced cooling; the middle region shows equiaxed blocky austenite with reduced grain boundary austenite (GBA) and WA, attributed to interlayer thermal cycling promoting recrystallization and grain refinement (average austenite grain size: 4.16 μm); and the top region displays coarse blocky austenite from slower cooling. Secondary austenite (γ₂) forms in interlayer remelted zones with Cr depletion, impacting pitting resistance. Mechanical testing demonstrates anisotropy; horizontal specimens exhibit higher strength (UTS: 610 MPa, YS: 408 MPa) due to layer-uniform microstructures, while vertical specimens show greater ductility (elongation) facilitated by columnar grains aligned with the build direction. Hardness ranges uniformly between 225–239 HV. The study correlates process-induced thermal gradients (e.g., cooling rates, interlayer cycling) with microstructural features (recrystallization fraction, grain size, phase morphology) and performance, providing insights for optimizing WAAM of large-scale duplex stainless steel components like marine propellers. Full article

To address the central cracking problem in continuous casting slabs of 38CrMoAl steel, high-temperature tensile tests were performed using a Gleeble-3800 thermal simulator to characterize the hot ductility of the steel within the temperature range of 600–1200 °C. The phase transformation behavior was computationally analyzed via the Thermo-Calc software, while the microstructure, fracture morphology, and precipitate characteristics were systematically investigated using a metallographic microscope (MM), a field-emission scanning electron microscope (FE-SEM), and transmission electron microscopy (TEM). Additionally, the effects of different holding times and cooling rates on the microstructure and precipitates of 38CrMoAl steel were also studied. The results show that the third brittle temperature region of 38CrMoAl steel is 645–1009 °C, and the fracture mechanisms can be classified into three types: (I) in the α single-phase region, the thickness of intergranular proeutectoid ferrite increases with rising temperature, leading to reduced hot ductility; (II) in the γ single-phase region, the average size of precipitates increases while the number density decreases with increasing temperature, thereby improving hot ductility; and (III) in the α + γ two-phase region, the precipitation of proeutectoid ferrite promotes crack propagation and the dense distribution of precipitates at grain boundaries causes stress concentration, further deteriorating hot ductility. Heat treatment experiments indicate that the microstructures of the specimen transformed under water cooling, air cooling, and furnace cooling conditions as follows: martensite + proeutectoid ferrite → bainite + ferrite → ferrite. The average size of precipitates first decreased, then increased, and finally decreased again with increasing holding time, while the number density exhibited the opposite trend. Therefore, when the holding time was the same, reducing the cooling rate could increase the average size of the precipitates and decrease their number density, thereby improving the hot ductility of 38CrMoAl steel. Full article

GTD 111 has been employed in first-stage blades in different gas turbines. The study of microstructural evolution is essential for the lifetime assessment and development of turbine blades. The microstructural stability of a 130 MW gas turbine first-stage blade at 800 °C was studied. The microstructure’s evolution was analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermodynamic calculation. As thermal exposure time increases, the shape of γ′ precipitates changes from square to spherical. During thermal exposure, MC particles formed and coarsened along the grain boundaries, and primary MC carbide decomposed into the η phase and M₂₃C₆. The stability of MC carbide at the grain boundaries was lower than that within the grains. MC carbide precipitated at the grain boundaries tends to grow along the boundaries and eventually forms elongated carbide. High-resolution transmission electron microscopy (HRTEM) images indicate that the orientation of the γ′ precipitate changes during the coarsening process. The GTD 111 alloy can be deformed through dislocation shearing at 800 °C. The hardness value initially increases, then decreases with further exposure, which is related to the reduced precipitation strengthening by γ′ precipitates and the reduction in the hardness of the γ matrix. Full article

This study systematically explores the synergistic effects of direct aging treatment (480 °C for 6 h) combined with cryogenic treatment (−196 °C for 8 h) on the mechanical properties and microstructural evolution of 18Ni300 maraging steel fabricated via selective laser melting (SLM). Three conditions were investigated: as-built, direct aging (AT6), and direct aging plus cryogenic treatment (AT6C8). Microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), while the mechanical properties were evaluated via microhardness and tensile testing. The results show that the AT6C8 sample achieved the highest microhardness (635 HV_0.5) and tensile strength (2180 MPa), significantly exceeding the as-built (311 HV_0.5, 1182 MPa) and AT6 (580 HV_0.5, 2012 MPa) samples. Cryogenic treatment induced a notable phase transformation from retained austenite (γ phase) to martensite (α phase), with the peak relative intensity ratio ranging from 1.42 (AT6) to 1.58 (AT6C8) in the XRD results. TEM observations revealed that cryogenic treatment refined lath martensite from 0.75 μm (AT6) to 0.24 μm (AT6C8) and transformed reversed austenite into thin linear structures at the martensite boundaries. The combination of direct aging and cryogenic treatment effectively enhances the mechanical properties of SLM-fabricated 18Ni300 maraging steel through martensite transformation, microstructural refinement, and increased dislocation density. This approach addresses the challenge of balancing strength improvement and residual stress relaxation. Full article

In this paper, the low-cycle fatigue deformation behavior of polycrystalline γ-TiAl alloys at different temperatures was investigated by molecular dynamics simulations. The results showed that the fatigue process comprises an initial cyclic softening stage followed by saturation, and the stress–strain response of the material shows significant asymmetry. With an increase in temperature, the asymmetry between tensile and compressive stresses gradually decreases, and the amplitude of saturated stress decreases significantly. The decrease in dislocation density leads to the cyclic softening of the alloy, and the evolution of dislocation density is temperature-dependent. The dislocation density first decreases and then tends to be stable, while at 900 °C and 1000 °C, it shows an abnormal trend of decreasing first and then increasing. In addition, microscopic mechanism analysis shows that grain coarsening, dislocation annihilation, and phase instability lead to the cyclic softening of the alloys. The fatigue plastic accumulation at low temperatures is mainly dominated by dislocation slip, while at high temperatures, grain boundary slip gradually replaces dislocation slip and becomes the main deformation mechanism. This work reveals new insights into the mechanical behavior of polycrystalline γ-TiAl alloys under cyclic plasticity and temperature-dependent deformation mechanisms. Full article

This work provides a comprehensive investigation into the synergistic effects of zirconium and oxygen on the microstructural evolution, high-temperature oxidation resistance, and mechanical properties of γ-phase Ti52AlxZr alloys (x = 0, 0.5, 1, and 2 at.%) under systematically controlled oxygen concentrations. Unlike prior studies that have examined these alloying elements in isolation, this study uniquely decouples the contributions of interstitial (oxygen) and substitutional (zirconium) solutes by employing low (LOx) and high (HOx) oxygen levels. Alloys were synthesized via vacuum arc melting and subsequently subjected to homogenization annealing at 1250 °C for 100 h to ensure phase and microstructural stability. Characterization techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) were employed to elucidate phase constitution and grain morphology. Zirconium addition was found to stabilize the γ-TiAl matrix, suppress α₂-phase formation, and promote grain coarsening in LOx specimens. Conversely, elevated oxygen concentrations led to α₂-phase precipitation along grain boundaries. Mechanical testing, comprising Vickers hardness and uniaxial compression at ambient and elevated temperatures (800 °C), revealed that both zirconium and oxygen significantly enhanced strength and hardness, with Ti52Al2Zr delivering optimal mechanical performance. Moreover, zirconium substantially improved oxidation resistance by promoting the formation of a thinner, adherent Al₂O₃ scale while simultaneously inhibiting TiO₂ growth. Collectively, the findings demonstrate the critical role of zirconium in engineering advanced γ-TiAl-based intermetallics with superior high-temperature structural integrity and oxidation resistance. Full article