Search Results (233)

Surface nanocrystallization is a critical approach for improving mechanical and functional properties of materials. Beyond conventional mechanical routes, chemical loading presents a promising pathway for nanocrystallization via interstitial-driven phase transformation. However, the characteristics and mechanisms underlying chemical load-induced nanostructuring remain insufficiently elucidated. This work investigates the surface nanocrystallization of 17-4 PH martensitic stainless steel during low-temperature plasma nitriding at 350 °C. Microstructural characterization combining XRD, EPMA, and TEM revealed a nitrogen-saturated layer with a maximum hardness of 13.5 GPa. The modified layer consists of nanoscale domains formed via a diffusionless martensite-to-austenite transformation, as evidenced by broadened FCC peaks, dark-field images, and the absence of elemental partitioning in EDX maps. This process is driven by the cyclic accumulation of chemical and elastic-strain energy at the advancing nitrogen diffusion front, triggering a self-sustaining, periodic transformation. This study introduces a chemical-driven nanocrystallization mechanism for novel design of surface-nanostructured steels via controlled thermochemical processing. Full article

This review highlights conventional forging steels and advanced medium-Mn steels containing retained austenite (RA), emphasizing their potential for industrial forging applications. Modern steels intended for forgings are required to combine strength, ductility, toughness and fatigue resistance with good hardenability and machinability at minimal cost. Medium-Mn multiphase steels fulfill these requirements by the strain-induced martensitic transformation (SIMT) of fine, lath-type RA, which can create a strength–ductility balance. Ferritic–austenitic steels provide high ductility with moderate strength, martensitic–austenitic steels show very high strength at the expense of ductility, and bainitic–austenitic steels achieve intermediate properties. Impact toughness and fatigue resistance are strongly influenced by the morphology of RA. The lath-type RA enhances energy absorption and delays crack initiation, while blocky RA may promote intergranular fracture. Low carbon (0.2–0.3 wt.%) combined with elevated manganese (3–7 wt.%) contents provides superior hardenability and machinability, enabling cost-effective air-hardening of components with various cross-sections. Advanced medium-Mn steels provide a superior mechanical performance and economically attractive solution for modern forgings, exceeding the limitations of conventional steel grades. Full article

The contact fatigue performance of carburized gear steels is critical for transmission durability, yet the mechanisms linking alloy-specific microstructure to failure modes remain complex. This study systematically compares the contact fatigue behaviors of 20MnCr5 and 20CrMoH gears using step-loading tests and multi-scale characterization. The results demonstrate a significantly higher contact fatigue limit for 20MnCr5 of 1709 ± 12 MPa compared to 1652 ± 40 MPa for 20CrMoH, despite the latter exhibiting higher initial surface hardness. This hardness–toughness paradox is mechanistically elucidated by the distinct roles of alloying elements: while Molybdenum in 20CrMoH refines the grain size for high static strength, it limits retained austenite stability, resulting in a brittle hard-shell and soft-core structure prone to interface decohesion at martensite lath boundaries. Conversely, Manganese in 20MnCr5 promotes a gentler hardness gradient via favorable diffusion kinetics and stabilizes abundant film-like retained austenite. This microstructure activates a Stress Compensation Mechanism, where strain-induced martensitic transformation generates compressive volume expansion to counteract cyclic stress relaxation. Consequently, 20MnCr5 exhibits mild plastic micropitting driven by transformation toughening, whereas 20CrMoH undergoes severe brittle spalling driven by the Eggshell Effect. These findings confirm that balancing matrix toughness with hardness is more critical than maximizing surface hardness alone for contact fatigue resistance. Full article

Cavitation erosion is a predominant failure mode of austenitic stainless steels in corrosive fluid environments, severely limiting their durability in nuclear piping and hydraulic components. In this study, five 316LN steels with 0.008–0.34 wt.% nitrogen content were fabricated, and both short-term (2 h) and long-term (24 h) cavitation tests were performed to elucidate the effect and mechanism of nitrogen. Increasing nitrogen markedly enhanced cavitation resistance: after 24 h, the cumulative mass loss decreased by 36%, 52%, 60%, and 71% for 09N, 17N, 22N, and 34N relative to 00N, accompanied by lower surface roughness, shallower pit depth, and a prolonged incubation stage. SEM revealed a progressive damage process from twin/high-angle grain boundaries to intragranular deformation bands and finally to spalling at slip intersections, whereas high-N steels exhibited only slight local detachment. TEM demonstrated that nitrogen transformed dislocations from random networks into dense slip bands and planar arrays with stacking faults, raising hardness from ~140 HV to ~260 HV. EBSD further confirmed strain-induced martensite transformation under severe deformation, providing additional strengthening. These results reveal that nitrogen improves cavitation resistance by tailoring dislocation structures and enhancing strength–plasticity compatibility, offering guidance for the design of high-performance austenitic stainless steels in cavitation environments. Full article

In this study, by regulating the heating rate, we have optimized the morphology of retained austenite in a quenching and partitioning (Q&P) steel with initial microstructure consisting of lath martensite. By optimizing the heating rate to 1 °C/s during the heating stage from 700 °C to 830 °C, the formation of blocky austenite and the coarsening of film-like austenite during the two-phase region annealing process were prohibited. Ultimately, this resulted in the production of Q&P steel containing a significant volume fraction (19.0%) of fine film-like retained austenite. This fine and film-like retained austenite exhibits higher stability than blocky austenite, exhibiting an active transformation-induced plasticity (TRIP) effect over a broad strain range. This results in excellent mechanical properties characterized by a high product of tensile strength and elongation (34.1 GPa·%). Full article

Strain-induced deformations and phase evolutions are two hidden factors that may influence cytocompatibility of Gum Metal alloys during processing for relevant implant applications. In the present research, changes in cell viability of a new Gum Metal Ti-Nb-Zr alloy in its cold-rolled state and after heat treatments (at 700, 850, and 900 °C) were investigated by a comprehensive study of microstructural phases and their role in deformation mechanisms as well as mechanical properties. In its cold-rolled state, the alloy showed a lamellar microstructure along with stress-induced α″ martensite and ω phases, as confirmed by optical microscopy (OM) and X-ray diffractometry (XRD) analysis. The instability in the β phase led to a strain-induced martensitic (SIM) transformation from β to α′/α″ phases, causing lower viability of MG-63 cells compared with commercially pure titanium. MG-63 cell viability was significantly higher (p < 0.0001) in the alloy heat-treated at 900 °C compared with those heat-treated at 700 and 850 °C. This can be directly attributed to the increased portion of the stable and dominant β phase. The stabilized β phase greatly improved the alloy’s cellular response by reducing harmful phase interactions and maintaining mechanical compatibility with bone (admissible strain of 1.3%). Importantly, heat treatment at high temperatures (between 850 and 900 °C) effectively converted the stress-induced α″ and ω phases back into a stable β phase matrix as the dominant phase. Full article

A low-carbon ferrite/martensite-laminated 0.1C5Mn3Al dual-phase steel was hot-rolled to an engineering strain of 98%, and a tensile strength of 1277 ± 44 MPa and a total elongation of 11.8 ± 0.4% was obtained in the steel. Hot-rolling induces a laminated/layered structure characterized by alternating ferrite phases and martensite phases distributed perpendicular to the rolling direction. A deformation mechanism was evaluated using nano-indentation and in situ compression of micropillars in a scanning electron microscope. The excellent mechanical properties of the steel are attributed to the refinement of ferrite/martensite layers and ultra-fine martensite laths. The synergistic deformation of the ferrite and martensite laminates provides the steel with a good combination of high strength and tensile elongation. Full article

This study investigates the evolution of strain-induced martensite (SIM) and its effect on magnetic Barkhausen noise (MBN) in AISI 321 austenitic stainless steel subjected to uniaxial tensile testing. Using X-ray diffraction and the Barkhausen noise technique, the formation and distribution of SIM were analysed as functions of plastic strain and strain rate. The results show that MBN is primarily governed by plastic deformation and strain rate rather than residual stress. The martensite fraction increases from 10% at low strains to 42.5% at high strains; however, accelerated strain rates significantly reduce martensite formation to approximately 25%. The increase in martensite density enhances the magnetic exchange interactions among neighbouring islands, resulting in stronger and more numerous MBN pulses. The anisotropy of MBN is also influenced by the initial crystallographic texture of the austenite. These findings highlight the strong correlation between MBN and SIM evolution, establishing MBN as a sensitive, non-destructive tool for assessing martensitic transformation and optimising deformation parameters in austenitic steels. Full article

This study investigates the influence of plastic deformation and temperature on the formation of mechanically induced martensite and the associated changes in hardness in AISI 304 austenitic stainless steel. Cold rolling was performed at three temperatures (20 °C, 0 °C, and −196 °C) and various degrees of deformation (10–70%). Microstructural changes, including the formation of ε and α′ martensite, were characterized using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The results confirm that martensitic transformation proceeds via the γ → ε → α′ sequence, with transformation rates and martensite fractions increasing at lower temperatures and higher strains. The stacking fault energy of 25.9 mJ/m² favors this transformation pathway. Transformation rates of α′ martensite fractions significantly increased at lower temperatures and higher strains, 91.8% α′ martensite was observed at just 30% deformation at −196 °C. Hardness measurements revealed a strong correlation with martensite content: strain hardening dominated at lower deformations, while martensite formation became the primary hardening mechanism at higher deformations, especially at cryogenic temperatures. The highest hardness (551 HV) was observed in samples deformed to 70% at −196 °C. The findings provide insights into optimizing the mechanical properties of AISI 304 stainless steel through controlled deformation and temperature conditions. Full article

Loading and clamping schemes significantly influence the behavior of shape memory alloys, specifically, the course of their solid-state transformations. This paper presents experimental and numerical findings regarding the nonlinear response of samples of the above-mentioned type of smart materials observed during tensile tests. Hysteretic properties were studied to elucidate the superelastic behavior of the tested and modeled samples. The conducted tensile tests considered two configurations of grips, i.e., the standard one, where the jaws transversely clamp a specimen, and the customized bollard grip solution, which the authors developed to reduce local stress concentration in a specimen. The characteristic impact of the boundary conditions on the solid phase transformation in shape memory alloys, present due to the specific clamping scheme, was studied using a thermal camera and extensometer. Martensitic transformation and the plateau region in the nonlinear stress–strain characteristics were observed. The results of the numerical simulation converged to the experimental outcomes. This study explains the complex nature of the phase changes in shape memory alloys under specific boundary conditions induced by a given clamping scheme. In particular, variation in the martensitic transformation course is identified as resulting from the stress distribution observed in the specimen’s clamping area. Full article

This study examines the microstructural evolution, mechanical properties, and wear behavior of medium-carbon dual-phase steel (AISI 1040) processed via Multi-Axis Compression (MAC). The DP steel was produced through inter-critical annealing at 745 °C, followed by MAC at 500 °C, resulting in a refined grain microstructure. Optical micrographs confirmed the presence of ferrite and martensite phases after annealing, with significant grain refinement observed following MAC. The average grain size decreased from 66 ± 4 μm to 18 ± 1 μm after nine MAC passes. Mechanical testing revealed substantial improvements in hardness (from 145 ± 9 HV to 298 ± 18 HV) and ultimate tensile strength (from 557 ± 33 MPa to 738 ± 44 MPa), attributed to strain hardening and the Hall–Petch effect. Fractographic analysis revealed a ductile failure mode in the annealed sample, while DP0 and DP9 exhibited a mixed fracture mode. Both DP0 and DP9 samples demonstrated superior wear resistance compared to the annealed sample. However, the DP9 sample exhibited slightly lower wear resistance than DP0, likely due to the fragmentation of martensite induced by high accumulated strain, which could act as crack initiation sites during sliding wear. Furthermore, wear resistance was significantly enhanced due to the combined effects of the DP structure and Severe Plastic Deformation (SPD). These findings highlight the potential of MAC processing for developing high-performance steels suitable for lightweight automotive applications. Full article

In this study, we have explored the thermomechanical processing on 0.1C3Mn steel to produce an ultrafine-grained (UFG) dual-phase (DP) microstructure. The composition was designed to allow a decrease in temperature for the warm deformation of austenite. It was found that the warm deformation of austenite induced a dramatic ferrite transformation, in contrast to the absence of the formation of ferrite in the well-annealed state. Compression by 60% at 650 °C resulted in the generation of a UFG-DP microstructure with a ferrite grain size of 1.4 μm and a ferrite volume fraction of 62%. The UFG-DP 0.1C3Mn steel presents a good combination of strength, ductility and fracture resistance, and the fracture strain of the UFG-DP is higher than the as-quenched low-carbon martensite. The high fracture strain of the UFG-DP could be attributed to delayed void nucleation and constrained void growth, as revealed by the quantitative X-ray tomography. Full article

This study proposes a novel non-destructive methodology for assessing structural integrity in liquefied natural gas (LNG) carrier cargo containment systems (CCSs), addressing limitations of conventional inspection techniques like visual inspection and vacuum box testing. The method leverages strain-induced martensitic transformation (SIMT) in austenitic stainless steel (SUS304L), widely used in CCS membranes, quantifying magnetic permeability increase via a Feritscope to evaluate deformation history and damage. To analyze SUS304L SIMT behavior, uniaxial tensile (UT) and equi-biaxial tensile (EBT) tests were conducted, as these stress states predominate in CCS membranes. Microstructural evolution was examined using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD), allowing a quantitative assessment of the transformed martensite volume fraction versus plastic strain. Subsequently, Feritscope measurements under the same conditions were calibrated against the XRD-measured martensite volume fraction for accuracy. Based on testing, this study introduces three complementary Feritscope approaches for evaluating CCS health: outlier detection, quantitative damaged area analysis, and time-series analysis. The methodology integrates data-driven quantitative assessment with conventional qualitative inspection, enhancing safety and maintenance efficiency. Full article

The lattice parameters of both the product phase and the matrix phase have determined using in situ X-ray and neutron diffraction measurements during forward and reverse transformations in steels. The lattice parameters are well known to be influenced by various factors, including temperature, internal stresses induced by transformation strains, partitioning of alloying elements, crystal defects, and magnetic strains. Therefore, it is crucial to accurately disentangle the contributions of these factors to the observed changes in lattice parameters. This review examines the evaluation of internal strain (stress) associated with ferrite, pearlite, bainite, martensite, and reverse austenite transformations, with a particular emphasis on the distinction between diffusional and displacive transformations. Additionally, the effects of plastic deformation of austenite on the bainite or martensite transformation are discussed. In this context, the roles of dislocations and vacancies are highlighted as key areas for further investigation. Full article

For investigating the effect of temperature and strain rate on martensitic transformation and establishing the corresponding kinetic model for newly TRIP (transformation-induced plasticity) aided duplex stainless steel (DSS), the tensile tests are conducted at temperatures of 20–150 °C and strain rates of 0.0001–150 s⁻¹. The stepped cross-section tensile specimen is proposed and designed for obtaining microstructure at specific strain during dynamic tensile testing. The results demonstrate that the deformation mechanism of austenite in TRIP-aided DSS is highly sensitive to temperature and strain rate. As the deformation temperature increases, strain-induced martensitic transformation is inhibited, and the deformation mechanism transforms from martensitic transformation to the co-occurrence of martensitic transformation and twinning, and finally, twinning is the main deformation mechanism. This leads to reduced strength with an initial increase followed by a decrease in elongation. As the strain rate increases, martensitic transformation is inhibited, resulting in a reduction in strength and plasticity during quasi-static tensile testing, while during dynamic tensile testing, strength increases due to enhanced resistance to dislocation motion, and plasticity displays no significant variation because of the combination of adiabatic softening and martensitic transformation suppression. Moreover, during tensile deformation, a plastic temperature rise model is established for newly developed DSSs. Based on this model, the Ludwigson–Berger model for martensitic transformation was modified to couple the effect of temperature and strain rate by considering the non-uniform distribution of temperature rise within the material and its variation with strain rate, as well as the suppression of dynamic strain rate on martensitic transformation. This new model could accurately describe the characteristics of martensitic transformation in newly developed DSSs at different deformation temperatures and strain rates. Full article