Kurdjumov-Sachs Research Articles

A novel corrosion resistant NiFeCrAlTi eutectic high entropy alloy with high strength and ductility

We present a novel corrosion resistant Ni42.6Fe24.6Cr16.4Al13.4Ti3 eutectic high entropy alloy (EHEA), which consists of FCC(L12) phase and BCC(L21) phase. The FCC and BCC phases contain numerous nano-precipitates, with sizes of ∼10 nm and 80 nm, respectively. The as-cast EHEA exhibits exceptional mechanical properties with a yield strength of ∼730 MPa, an ultimate tensile strength of 1220 MPa and elongation of 25 %, surpassing those observed in other as-cast EHEAs. The excellent mechanical properties are attributed to the extensive dislocation activities and precipitation strengthening in both BCC and FCC phases, as well as hetero-deformation induced (HDI) strengthening. Besides, the Kurdjumov-Sachs (KS) orientation relationship between adjacent phases promotes the cooperative deformation of both constituent phases, thereby enhancing the ductility of the EHEA.

Investigation on the interface microstructure and phase composition of laser cladding pure copper coating

Laser cladding is an efficient and clean method that can prepare a corrosion-resistant pure copper coating on the nuclear fuel container surface to ensure service safety. In this study, the single-layer and double-layer pure copper coatings were prepared on 20# steel tubes by laser cladding. The microstructure, elements distribution, phase composition, and interface structure, as well as strain distribution, were characterized by Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), and transmission kikuchi diffraction (TKD). The results show that the kernel average misorientation (KAM) and geometry necessary dislocation (GND) density at the coating/substrate interface in the double-layer Cu coating decrease compared to the single-layer Cu coating. Under the thermal action during laser cladding of double-layer coating, the phase composition in the substrate transforms completely from austenite, FeO, and ferrite to the entire ferrite. The Cu/Fe interface maintains certain orientation relationships, namely the Kurdjumov-Sachs (KS) and the Nishiyama-Wassermann (NW) relationships, with a mismatch of only 3.01 % for the coherent interface. Thus, the lattice distortion at the interface reduces, slightly alleviating the strain concentration.

The influence of ferrite content, ferrite-austenite morphology, and orientation relationship on bainite transformation in intercritically annealed bainitic steels

This manuscript elucidates the influence of varying ferrite/tempered martensite (TM) content, morphology and orientation relationship (OR) between ferrite/TM with reverted austenite (γ) resulting at different intercritical temperatures on the subsequent bainite transformation behaviour. A comprehensive analysis was conducted using scanning electron microscopy, electron backscattered diffraction, and dilatometry. TM/bainite and ferrite/bainite steels were produced through intercritical annealing at 735 °C, 745 °C, and 765 °C, followed by austempering at 310 °C, using initially quenched and cold-deformed martensite, respectively. In samples without cold rolling, TM and bainite are alternately distributed at all intercritical temperatures. The γ reversion process in these samples led to the formation of acicular γ at prior martensite blocks and laths while globular γ was formed at prior γ boundaries and packet boundaries. Notably, a lower TM content correlated with increased globular γ formation. The acicular γ exhibited a Kurdjumov–Sachs (K-S) OR with prior γ. The globular γ formed at prior γ grain boundaries showed the K-S OR with the prior γ where it grows. However, globular γ formed at packet boundaries maintained the K-S OR with one of its surrounding blocks in prior γ where it lies. In the subsequent austempering process, bainite formed from reversed γ followed the K-S OR with TM. The TM and bainite shared the same variant in the same block. In samples with cold rolling, polygonal ferrite and bainite were distributed at all intercritical temperatures. Decreasing the ferrite content showed a coarsening of the microstructure. All polygonal samples revealed an irrational OR between bainite/martensite and ferrite. In samples with and without cold rolling, the bainitic formation rate decreased with increasing ferrite/TM, likely due to the high C and Mn content at the α/γ interface. Comparatively, the samples without cold rolling showed faster bainitic transformation than samples with cold rolling at similar ferrite/TM percentages, particularly noticeable at higher ferrite/TM content. The alternately arranged elongated TM and reversed γ lamellae increased the α/γ interfacial density for bainitic nucleation. Moreover, K–S oriented α/γ interfaces exhibited a lower activation energy for bainite nucleation. In samples without cold rolling, both high density of α/γ interfaces and K–S oriented α/γ interfaces collectively facilitate the bainitic transformation.

Crystal plasticity modeling of prior austenite orientation effects on deformation behaviors of martensitic steels under different strain paths

The influence of prior austenite grain (PAG) orientation on the deformation behavior of a low-carbon martensitic steel is investigated using crystal plasticity (CP) modeling with hierarchical representative volume elements (RVEs). The Kurdjumov-Sachs (K-S) relationship is refined for accurate PAG reconstruction in martensitic steel. A robust calibration strategy for CP parameters based on nanoindentation tests is developed by integrating analytical calculations with inverse methods. Virtual polycrystalline aggregates are subsequently generated by manipulating initial PAG orientations. The simulations reveal that assigning cube texture to PAGs enhances strain hardening of martensite under plane strain tension. Moreover, the RVEs with brass and copper orientated PAGs exhibit similar deformation behavior, and a more homogeneous strain distribution is realized in the RVE with PAG cube orientation. The influence of PAG orientation on stress and strain fields is correlated with lattice rotation behavior during deformation. Notably, different strain paths elicit distinct lattice rotation trajectories, wherein uniaxial tension and plane strain tension favor grains reorientation toward hard γ-fiber, whilst equi-biaxial tension drives the crystal matrix to concentrate on 〈001〉 and 〈011〉 directions. These findings provide insights into the intricate relationship between microstructural orientation and deformation behavior in martensitic steels, which is crucial for performance optimization.

The effect of prior austenite grain size on crystallography and variant selection in carbide-free nano-structured bainite

Prior austenite grain size (PAGS) significantly affects the microstructure and in turn the mechanical properties of nano-structured bainite. In this study, information about crystallographic variants/blocks as well as packets has been extracted using electron backscattered diffraction (EBSD) technique for two blocks of nano-baintic steels with different PAGS obtained by austenitization at two different temperatures of 900°C and 1000°C respectively. However, the bainitic fraction has been maintained to be similar by choosing the same isothermal temperature of 250°C for transformation to bainite. The child-parent orientation relationship (OR) was determined to be close to Kurdjumov-Sachs (K-S) using EBSD for larger PAGS specimen while smaller PAGS specimen displayed closeness to K-S OR predominantly and Nishiyama-Wassermann (N-W) OR in some regions. It was observed that a higher PAGS leads to finer bainitic lath thickness, packet size and block width size. Stricter variant selection was found in smaller PAGS sample even though variant pairing was observed between variants of higher misorientation. This hints towards small number of nucleation sites for a grain and space constraint, rather than the tendency for pairing as the reason for selection.

Microstructural origins of enhanced work hardening and ductility in laser powder-bed fusion 3D-printed AlCoCrFeNi2.1 eutectic high-entropy alloys

Limited tensile ductility usually restricts the practical applications of new classes of high-strength materials in many industrial fields. Therefore, in-depth understanding of the work hardening behavior and its underlying plastic deformation mechanism are critical for the newly developed high-entropy alloys (HEAs). In this work, a geometric atomistic model of face-centered cubic (FCC)/ordered body-centered cubic (BCC (B2)) interfaces and the evolution of dislocation substructures have been investigated to explore the microstructural origins of work hardening responses for two additively manufactured AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs) with the respective lamellar and cellular microstructures. Unlike the lamellar interphase interfaces with the most classical Kurdjumov-Sachs (KS) FCC-BCC relationship of {111}FCC∥{110}B2〈011〉FCC∥〈111〉B2, the Nishiyama-Wassermann (NW) relationship, namely {111}FCC∥{110}B2〈011〉FCC∥〈001〉B2, is observed to be dominant on the cellular interphase interfaces. Furthermore, our intermittent high-resolution transmission electron microscopy (HR-TEM) results directly show that the deformation of lamellar AlCoCrFeNi2.1 alloy first proceeds with massive stacking faults (SFs) and then dislocation walls developed across phases interfaces, due to the effective dislocation transfer capability of lamellar boundaries. The large uniform elongation of the cellular AlCoCrFeNi2.1 alloy is attributed to the stable and progressive strain-hardening mechanism that is stemmed from the activated multiple slip systems, deformation-induced SF networks, and the associated Lomer-Cottrell locks in the middle and later stages of plastic deformation. Moreover, the nano-bridging of FCC cells in the 3D-printed microstructure provides unique channels for dislocation movement, which offsets the “blocking effect” of cellular boundaries and thus suppresses the pre-mature fracture.

Effect of applied stress on bainite transformation, microstructure, and properties of 15CrMo steel

The effect of external stress on the isothermal bainite transformation kinetics and microstructure of 15CrMo steel was studied using dilatometer, optical microscopes (OM), electron back-scatter diffraction (EBSD), scanning electron microscopes (SEM) and X-ray diffraction (XRD). The results show that the additional mechanical driving force provided by the external stress promotes the transformation of bainite, resulting an increase volume fraction of bainite-ferrite (BF) and causing the BF laths to become slender. The application of external stress can also induce selective orientation during the bainite transformation process, and the orientation relationship between the prior austenite and the BF under stress is closer to the Kurdjumov-Sachs (K-S) relationship. There is also an incomplete transformation of bainite transformation under stress. At the same time, the Vickers hardness of 15CrMo steel would increase after applying external stress. The mechanical driving forces calculations demonstrate that when tensile stress and compressive stress are of similar magnitudes, they provide comparable mechanical driving forces. As a result, they exert similar effects on the kinetics of bainite transformation, leading to no discernible differences in the resulting bainite microstructure.

Formation condition of lacy ferrite during solidification and subsequent transformation in austenitic stainless steels solidified with primary ferrite

The formation conditions of lacy ferrite in austenitic stainless steels solidified with primary ferrite was clarified through crystallography in order to increase the amount of lacy ferrite which contributed to the improvement of properties. Lacy ferrite was formed when the misorientation between ferrite and austenite from the Kurdjumov–Sachs (K-S) relationship was ≤ 7° on the close-packed plane and ≤ 9° in the close-packed direction during solidification. In contrast, vermicular ferrite was formed if the misorientation exceeded these values. The temperature gradient during solidification was controlled unidirectionally by two-pass laser welding. Following the performance of two-pass welding with a highly unidirectional laser, it was confirmed that the formation ratio of lacy ferrite to total ferrite significantly increased in the second-pass weld metal by remelting the first-pass weld metal, in which the heat flow direction and preferential growth direction of ferrite and austenite were controlled to be almost parallel. The growth ratio while maintaining the K-S relationship established between ferrite and austenite in the first-pass weld metal, and/or the ferrite formation ratio that satisfies the K-S relationship, increased in the second-pass weld metal because the growth of both the primary ferrite and secondary austenite was crystallographically independent of each other during solidification.

Formability of the ultra high strength automotive steel docol 1500 bor by using hot processing maps and austenite reconstruction

In this study, the hot processing map was drawn to study the hot formability of high strength automotive steel Docol 1500 Bor. The quenching microstructure of the test steel was observed under optical microscope and EBSD and the deformed PA (parent austenite) was reconstructed under different conditions according to KS (Kurdjumov–Sachs) OR (orientation relationship). The results indicate that the optimal deformation conditions in the stable region are strain rates less than 1s−1 and temperatures higher than 900 °C. Overall, the fine-grained martensite structure with a larger Schmid factor has a better deformation ability for subsequent service. The texture component {011} <100> appears in the PA at the strain rate of 5 s−1 and the temperature of 1000 °C, while the dynamic recrystallization is significantly inhibited. The quenched martensite is found coarse. In the stable region, the recrystallized austenite grains with certain deformation transforms into the martensite grains with low cumulative misorientation values and higher Schmid factors. In the unstable region, the austenite grains are smaller in size, in which severe cumulative misorientation caused by higher strain rates leads to poor plasticity.

The coupling effects of temperature and stress on the growth kinetics of Ni3Al in Ni//NiAl diffusion couples

The growth kinetics of Ni3Al were investigated in Ni//NiAl macroscopic diffusion couples annealed at 800–1000 °C under applied elastic compressive stress. By utilizing electron backscatter diffraction (EBSD) technology in conjunction with appropriate statistical methods, the mechanism underlying the impact of elastic stress on Ni3Al growth kinetics is elucidated. Based on the findings, the application of compressive elastic stress can enhance Ni3Al growth kinetics by increasing both atomic vibration frequency (ν) and activation entropy (Δs). The newly formed Ni3Al phase at the Ni/NiAl interface exhibits a specific crystallographic relationship with adjacent NiAl phase, with most Ni3Al grains maintaining a Kurdjumov-Sachs (KS) orientation relationship. Remarkably, the application of elastic compressive stress in conjunction with an increase in temperature results in a reduction of Ni3Al grains grown according to the KS orientation relationship.

Revealing the cleavage mechanism of the crack propagation process in martensitic steels

A phase field model is presented to investigate the influence of cleavage planes in martensite and austenite phases and the loading rate on the crack propagation process with considering the Kurdjumov-Sachs (K-S) orientation relationship in martensitic steels by the finite element method. The transgranular (through the martensite and austenite phases) and intergranular (along the martensite/austenite interface) fracture processes in martensitic steels can be simulated by this model. This model can simulate the crack propagation in martensitic steels with displaying the crack propagation path in austenite and martensite phases and stress distribution directly. Moreover, this model reveals the energy difference required by transgranular and intergranular fracture in martensitic steels and the hindering effect of martensite on the crack propagation process. The simulation results further indicate that the preferential cleavage planes of crack propagation in martensite and austenite phases are (100)α’ and (111)γ planes, respectively, which is consistent with the experimental results in martensitic steels. An interesting finding is that the influence of the loading rate on the maximum loading for material failure only acts on the crack propagation stage rather than the crack initiation stage. Therefore, this model can be a powerful tool to investigate the cleavage mechanism of the crack propagation process during the water quenching process and to avoid water quenching cracking in martensitic steels.

Ferrite-austenite synergistic deformation behavior in a 2205 duplex stainless steel containing equiaxed austenite domains

This study investigates the mechanical properties and deformation behavior of 2205 duplex stainless steel (DSS) through a two-stage solution (TSS) process. The TSS process involves heat treatment at 1380 °C for 20 min, followed by 1000–1200 °C for 20 min. This process achieves a favorable balance between strength and plasticity, resulting in a yield strength ranging from 585 to 607 MPa, an ultimate tensile strength ranging from 796 to 802 MPa, and a fracture elongation ranging from 28.4% to 32.8%. The activation and transmission of slip systems within the constituent phases, as well as the interaction between dislocations and different interfaces, are examined via in-situ electron backscatter diffraction, in-situ scanning electron microscopy, and transmission electron microscopy. The results indicate that slip transfer is the primary mechanism by which austenite and ferrite adapt to coordinated deformation in DSS 2205. Furthermore, the possibility of dislocation slip across boundaries is assessed using the Luster-Morris parameter (m’). When m’ exceeds 0.81, activated dislocations within the austenite grains can traverse the Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) interfaces, promoting coordinated dislocation slip in the ferrite matrix. Dislocation motion remains uninhibited at the ferrite-austenite interface, while accumulation at the ferrite grain boundaries leads to localized stress concentration.

Unveiling the transformation pathways of hierarchical γ90 – εtwin – α’ triple phase structure formation at ε-ε martensite intersection

Deformation-induced martensitic transformation (γ-austenite → ε-martensite/α’-martensite) in austenitic steels has garnered significant interest owing to its transformation-induced plasticity effect. To elucidate the orientation-dependent intricate γ/ε/α’ phase microstructure at deformation-induced ε-ε intersection, a single crystal of 316 austenitic stainless steel was compressed along the [001]γ axis at a cryogenic temperature (173 K). Electron backscattered diffraction analysis was employed to reveal the deformed microstructure on the (110)γ surface. A hierarchical triple phase structure was discovered at ε–ε intersection, where the γ rotated 90° from the matrix (γ90), {101¯2} ε-twin, and α’ phases coexist. Depending on the operative shear angle with a common intersection axis, either 90° (Type I) or 30° (Type II), three distinct atomic rearrangements of the intersection volume were observed: γ90 was present at Type I intersection, and α’-phase was developed at Type II intersection, respectively. {101¯2} ε-twin also occurred at Type I intersection, serving as an accommodation mechanism alongside the intersection γ90. EBSD results confirm the Shoji–Nishiyama (SN), Kurdjumov–Sachs (KS), and Burgers (B) orientation relationships within the complex γ – ε – α’ triple phase structures at ε–ε intersection. Transformation paths for three intersection products were visualized by the unified tetrahedron model, considering T/2 or T/3 γ-twinning shear as an intermediate state. Moreover, a novel scheme is proposed to index the α’-martensite crystallographic variants at ε-ε intersections by establishing a correlation between the Bain distortion and double shear process.

Crystallographic orientation-dependent dislocations slip during tensile testing of lath martensitic steel

Based on observations of slip traces and variant-pair boundaries obtained through the transformation of the Kurdjumov-Sachs (K-S) orientation relationship from quasi in-situ electron backscatter diffraction (EBSD), the corresponding slip systems, Schmid factors (SF), and geometric compatibility parameter (m') around the crack were calculated during tensile testing of lath martensitic steel. The occurrence of slip transfer, which typically happens at low values of m' or SF, resulted in stress concentration on crystallographic orientations of 50.5°, 60°, and twin-related orientations. However, more effective slip transfer and plastic deformation were observed at 20° due to larger m' and SF values.

Effect of orientation relationship on the morphology and strengthening effect of chromium precipitation in Cu-0.67Cr-0.3Zr alloy

High strength and high conductivity Cu-0.67Cr-0.3Zr alloy was prepared by vacuum directional solidification equipment. The face centered cubic (FCC) and body centered cubic (BCC) Cr rich precipitations in Cu matrix were characterized by high resolution transmission electron microscopy. In the as cast and solution treated state, a large amount of coherent FCC Cr precipitates is uniformly distributed in the matrix. These FCC Cr precipitates are covered by close packed {111} crystal planes and presents regular morphology. Due to their small size and cube-cube orientation with the matrix, FCC Cr precipitates follows dislocation shearing mechanism, resulting in limited strengthening effect. After aging treatment, both incoherent ordered nearly spherical and incoherent rod-shaped BCC Cr precipitates were formed in the matrix. These incoherent precipitates always follow dislocation bypassing mechanism. Due to its small size and high-volume content, the spherical precipitation plays a major role in improving the strength of the alloy. On the contrary, the influence of rod-shaped precipitates on strength is minimal. According to the minimum free energy theory and local chemical potential equilibrium requirement, the precipitation with Kurdjumov-Sachs (K-S) orientation relationship tends to be rod-shaped extending along the direction of Cu [110] and Cr [111], while the precipitation with Nishiyama-Wassemann (N-W) orientation relationship tends to be nearly spherical.

Unique Microstructure Evolution of HPT-Processed (α + γ) Two-Phase Stainless Steel

A high-pressure torsion (HPT) processed Fe-21Cr-5Ni-2Mo (mass%) two-phase stainless steel was used to study the morphology and crystallographic features of austenite (γ) precipitated from ferrite (α) during aging in the (α + γ) two-phase region. The starting material was a gas-atomized powder with a completely ferritic structure. The HPT process was carried out to produce a fully dense compact under 6 GPa for 5 revolutions. The compact was given an equivalent strain of about 130. After the HPT process, the matrix ferrite formed a pancake–like nanograined structure with a strong texture, i.e. ND (Normal Direction) // {110} α. By annealing at 1173 K for 3.6 k, an ultrafine (α + γ) microduplex structure with high-angle grain boundaries was formed. In addition, the strong texture formation of {110} α / {111} γ / ND plane was formed in the α and the γ grain duplex structure. The α and γ phases had average grain sizes of 2.1 μm and 1.6 μm, respectively. The area fraction of the γ phase was 37.2%, which exceeded that of a cold-pressed compact, 6.7%. Both ultrafine grain refinement and γ precipitation were accelerated by the HPT process. In other words, the application of the HPT process to the two-phase alloys enables the formation of the ultrafine microduplex structure. The Kurdjumov-Sachs (K-S) orientation relationship between α and γ phases is usually observed in the alloy, however, the K-S orientation relationship was not dominant except for the close packing plane parallel orientation relationship, {110} α / {111} γ, in the HPT-processed material.

Fracture Characteristics of Super Duplex Stainless Steel after Hot Tensile Testing

This study’s temperature range of hot tensile testing is 1000–1200 °C, which is consistent with ordinary industrial hot working temperatures. The fractography of super duplex stainless steel S32750 after hot tensile testing is investigated with Optical Microscopy and Scanning Electron Microscopy, and Electron Back-scatter Diffraction and Transmission Electron Microscopy study hot deformation behaviors of the necking area. The results reveal that ~1050–1150 °C is the optimal hot working temperature range regarding hot ductility and cracking sensitivity. The micro-voids nucleate at the ferrite-austenite interphase boundaries following the Kurdjumov-Sachs (K-S) or Nishiyama-Wasserman (N-W) orientation relationship at 1000 °C. Therefore, a quasi-cleavage fracture forms along the interphase boundaries. At the temperature range 1050–1150 °C, a uniform distribution of fracture surface is formed, and the highest Reduction in Area appears at 1150 °C. Dynamic phase transformation occurs with the formation of secondary austenite at ferrite grain boundaries at 1200 °C, which results in an intergranular fracture surface morphology.

Microstructure and mechanical properties of Co-free Ni2FeCrWx medium-entropy alloys reinforced by coherent precipitates

In the present work, novel Co-free Ni2FeCrWx medium-entropy alloys (MEAs) are designed by equiatomically replacing Co with Ni in the CoCrFeNi high-entropy alloys (HEAs). The effects of W addition coupled with subsequent heat treatment on microstructure evolution and mechanical properties were systematically investigated and analyzed. The W-addition is conducive to the formation of μ phase in Ni2FeCrWx MEAs, while the annealing treatment promotes the precipitation of submicron-sized BCC particles and the spheroidization of μ phase. The orientation relationship (OR) of 1¯21¯0μ phase // [011]FCC and 303¯0μ phase // 111¯FCC and the coherent interface is formed between μ phase and FCC matrix in as-cast alloys, and the orientation relationship of the interfaces between BCC precipitate and FCC matrix have been clarified as the Kurdjumov-Sachs (KS) interfaces presenting [011]FCC //1¯11BCC and 1¯1¯1FCC // 011¯BCC. The annealed Ni2FeCrW0.5 MEA exhibited outstanding strength-ductility synergy, combining a high ultimate tensile strength of ∼982 MPa and an excellent ductility of ∼29%. Strengthening mechanisms were estimated and compared with experimental measurements, revealing that the coherent BCC/μ precipitates bring out effective precipitation strengthening.

Effect of torsional deformation on crystallographic evolution of carbide-free nano-bainitic steels prepared at two different austempering temperatures

The current study investigates the nature and extent of crystallographic change after torsional deformation in nano-structured bainite. This is extremely useful as nano-structured bainite is not stable under mechanical deformation and the evolved structure under torsion is finally going to determine other mechanical responses like fracture and fatigue. Two blocks of nano-bainitic steels were prepared after austenitization at 950°C and austempering at 250°C (NB250) and 350°C (NB350) respectively. It was found that there was a decrease in the bainitic ferrite (BF) and retained austenite (RA) length scale with a reduction in austempering temperature along with reduction in the RA content. Specimens from the undeformed two blocks (NB250-UD and NB350-UD) were subjected to torsional deformation till failure (NB250-T and NB350-T) and the crystallographic characteristics of the torsion-tested and undeformed specimens were obtained using electron back-scattered diffraction (EBSD). The orientation relationship (OR) between BF and RA for NB250-UD and NB350-UD was close to Kurdjumov-Sachs (K-S) which changed to predominantly Nishiyama Wassermann (NW) after torsion deformation (NB250-T and NB350-T). Significant refinement in packet and block was observed as a result of torsional deformation. Variant selection was stricter for NB350-UD than NB250-UD but torsional deformation led to weakening of variant selection. The extent of lowest misorientation V1-V4 variant pairing was higher for NB350-UD but it did not change much with torsion in case of NB250-T whereas NB350-T displayed a reduction. Fractography revealed that the dimple size corresponded to block size range between the undeformed and torsion-tested sample for the respective austempering condition.

Influence mechanisms of underwater hyperbaric environment on nitrogen behavior, phase evolution, and mechanical properties of high nitrogen steel repaired by underwater laser direct metal deposition

Nitrogen outgassing and nitrogen pore formation are tricky problems inherent in the fabrication/repair of high nitrogen steel (HNS). In the current research, HNS powder with a nitrogen content of 0.42 wt% was used to repair damaged HNS plates by the underwater laser direct metal deposition (UDMD) technique at an ambient pressure of 0.3 MPa (sample U1). Under the same process parameters, the DMD repaired sample (sample A1) in the ground environment served as a comparison. The influence mechanisms of the underwater hyperbaric environment on the nitrogen behavior, interface crystallographic characteristics, and mechanical properties were clarified. Results showed that nitrogen outgassing and nitrogen pore formation occurred in sample A1. In sample U1, the elevated ambient pressure increased the nitrogen solubility in the molten pool from 0.628 wt% to 1.088 wt%, thus eliminating porosity defects. In addition, there was a higher content of Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) interfaces (95%) in sample A1. Conversely, the water cooling inhibited the austenite shear transformation period at the relatively low temperature of sample U1. Hence, lower KS/NW interfaces (56%) were obtained due to the changed austenite nucleation and growth mechanism. Benefitting from the denser dislocation density and finer grain size caused by the underwater environment, the comprehensive mechanical properties of defect-free sample U1 were superior to those of sample A1. The nitrogen pores in sample A1 did not weaken the tensile strength but did seriously impair the impact toughness and ductility.