Search

In this research, a surface plastic deformation layer was induced on 300M ultra-high strength steel using ultrasonic nanocrystal surface modification (UNSM). The UNSM-treated and control specimens were then nitrided at 450 °C for 6 h. After gas nitriding, the microstructure, corrosion and wear resistance of 300M steel with and without UNSM pre-treatment were investigated. After nitriding, the UNSM-treated specimens generated a dense surface nitride layer that was thicker than that of the control specimens. The high defect density and refined lath martensite induced by UNSM were found to accelerate the nitrogen diffusion and efficiency. X-ray diffraction patterns revealed that α-Fe phase occupied the largest proportion in the nitriding layer of specimens without UNSM pre-treatment. However due to the higher nitriding efficiency, the main phase of UNSM-treated specimens after nitriding was found to be ε-Fe 2-3 N. Compared with the specimens without pre-treatment, UNSM-treated specimens exhibited higher surface hardness as well as higher corrosion and wear resistance after nitriding. Unlabelled Image • Refined and distorted lath martensite generated in UNSM-treated 300M steels resulted in much higher nitriding efficiency. • UNSM + nitriding generated a denser and deeper surface nitride layer that contributed to higher corrosion resistance. • UNSM + nitriding induced high surface hardness that contributed to higher wear resistance of 300M steel. [ABSTRACT FROM AUTHOR]

In this study, an innovative surface treatment method, electropulsing assisted laser shock peening (EP-LSP), was used to process Ti64 samples. In EP-LSP, metallic samples are subjected to simultaneous high strain rate plastic deformation and high-frequency (100–500 Hz) short-duration (100 μs) pulsed electric current. The effects of EP-LSP on surface finish, microstructure, and micro-hardness of Ti64 alloy were investigated and compared with continuous current assisted LSP (CC-LSP) having the same bulk heating effect. It was observed that EP-LSP produced higher surface hardness and deeper hardened layer, both of which indicate greater plastic deformation. Tensile tests were carried out to evaluate the plasticity of Ti64 subjected to pulsed current and continuous current with the same bulk heating effect. It was observed that pulsed current can more effectively decrease the flow stress and thus resulted in greater plasticity in Ti64 compared with continuous current, even though the bulk heating effect was the same. In addition, the higher the peak current density, the more effective the flow stress reduction. As a result, pulsed current can more effectively improve the effectiveness of LSP treatment, as manifested by higher surface hardness and deeper plastic deformation layer. It is believed that an athermal effect in addition to the thermal effect related to pulsed current exists in EP-LSP. • EP-LSP results in higher surface hardness and deeper hardened layer compared with LSP. • Higher peak current density leads to more effective EP-LSP treatment. • Pulsed current is more effective in improving the plasticity of Ti64 compared with continuous current. • An athermal effect that cannot be explained by resistive heating exists in EP-LSP. [ABSTRACT FROM AUTHOR]

[Display omitted] • Severe plastic deformation behavior of AM repaired Ti-6Al-4V alloy samples subjected to LA-UNSM treatment was thoroughly investigated. • Sufficient grain refinement, with a mix of elongated and ultra-fine grains, was produced on the surface of the LA-UNSM sample, eliminating larger columnar grains. • Improved roughness, hardness, compressive stress, and fatigue life were achieved after LA-UNSM treatment. • Fatigue life was enhanced by LA-UNSM's work hardening and dynamic precipitation effects. Ultrasonic nanocrystal surface modification (UNSM) combined with localized laser heating (LA) were implemented to enhance the surface fatigue life of additively repaired Ti-6Al-4V alloy parts in this study. The effect of induced surface severe plastic deformation on microstructure development and grain refinement is analyzed. The results showed that surface metallurgical defects, hardness, residual stress, and roughness were significantly improved after LA-UNSM treatment. As a result of the improved surface textures and higher grain refinement, the fatigue life of the LA-UNSM treated samples was an order of magnitude longer. Surface free of pores and displaying a healthy topography provides evidence. [ABSTRACT FROM AUTHOR]

Abstract Surface severe plastic deformation (SSPD) can significantly improve the mechanical properties of metallic components by inducing surface nanocrystallization and beneficial compressive residual stresses. The effectiveness of the SSPD processes is significantly dependent on the plasticity of the target metals. Here, we report an innovative surface thermomechanical process called laser-assisted ultrasonic nanocrystal surface modification (LA-UNSM) that integrates localized laser heating with high strain rate plastic deformation. The laser beam locally heats the target metal and increases the local plasticity, making the SSPD treatment more effective. After LA-UNSM, a microstructure featuring a nanocrystalline layer embedded with nanoscale precipitates was achieved in Ti64, resulting in an unprecedented 75.2% increase in hardness. After LA-UNSM processing, a 25-μm severe plastic deformation layer was produced that was 2.5 times thicker than that of the room-temperature UNSM-processed material. The grains at the top surface were refined down to 37 nm, indicating a similar degree of nanocrystallization to that produced by UNSM at room temperature. Nanoscale precipitate particles with diameters in the range of 5–21 nm were non-uniformly distributed in the nanocrystalline surface layer. These precipitates were produced through laser-assisted dynamic precipitation. The extremely high surface strength obtained for the Ti64 was attributed to the composite microstructure featured by nanoscale grains embedded with nanoscale precipitates and the work-hardening. Highlights • Laser assisted ultrasonic nanocrystal surface modification (LA-UNSM) integrates laser heating with UNSM. • In additional to nanocrystallization, LA-UNSM produce nanoscale precipitates through dynamic precipitation. • By fabricating a unique nanocomposite microstructure, LA-UNSM significantly increase the surface strength of Ti64. • LA-UNSM can significantly increase the plastic affected depth due to material softening by laser heating. • Local heating from laser effectively suppress grain coarsening as compared with traditional furnace heating. [ABSTRACT FROM AUTHOR]

Surface severe plastic deformation was induced in Inconel 718 alloy using ultrasonic nanocrystal surface modification (UNSM). Low-temperature gas nitriding was carried out on non-processed and UNSM-processed Inconel 718 samples. The microstructure and material properties were characterized using X-ray diffraction, scanning electron microscopy, the Vickers hardness test, and wear and corrosion testing. The results demonstrated that gas nitriding efficiency was significantly enhanced by the UNSM treatment. Higher nitrogen concentration was observed in the surface layer of the UNSM samples. The nitride layer in the UNSM + nitriding sample exhibited an increased hardness over that of the nitriding sample and the UNSM treated sample. Additionally, both wear resistance and corrosion resistance were significantly improved in the UNSM + nitriding sample. [ABSTRACT FROM AUTHOR]

Microstructure inhomogeneity is imparted into the surface of a Vit1b Zr-based bulk metallic glass (BMG) using ultrasonic nanocrystal surface modification (UNSM). As a result, compressive residual stresses with a maximum magnitude of 1130 MPa were induced in the near-surface region. Substantial improvements in both fracture strain and strength were observed for the treated specimen in three-point bending tests. The microstructure inhomogeneity and free volume generated by severe plastic deformation resulted in increased shear band density during three-point bending tests, which is evidenced by the vein-like pattern observed on the fracture surface of the treated specimen. Moreover, the enrichment of shear bands can cause the interaction between shear bands and can thus obstruct their propagation, leading to work-hardening behavior. High magnitude compressive residual stresses are also believed to impede and slow down the propagation of the shear bands. The synergistic effect of induced inhomogeneity, increased free volume and compressive residual stresses improves the fracture stress and strain of the UNSM-treated BMG. [ABSTRACT FROM AUTHOR]

The efficacy of peening treatment depends on the plasticity of the target metal. In this study, the effectiveness of high-amplitude short duration pulsed current in improving the peening efficacy was examined in a process called electropulsing-assisted ultrasonic nanocrystal surface modification (EP-UNSM). During the EP-UNSM process, the target metal, i.e., Ti64, is subjected to simultaneous ultrasonic peening and electropulsing. The high energy pulsed current can generate a critical magnetic field that can induce the transition of the radical pairs formed by dislocations and the pinning obstacles from the singlet state to the triplet state. This leads to higher dislocation mobility and thus higher plasticity for more effective peening treatment. The results show that the sample treated with EP-UNSM had a deeper plastically deformed layer than that for samples subjected to UNSM and continuous current–assisted UNSM (CC-UNSM), and the maximum depth of plastic deformation was obtained when using the highest peak current density. Due to microstructure refinement, work hardening, and dynamic strain aging, the EP-UNSM sample had a 50% higher surface hardness compared with the control sample. Moreover, the compressive residual stresses generated by EP-UNSM were higher in magnitude and greater in depth compared to those generated by traditional UNSM. These results demonstrate that pulsed current can effectively improve the peening efficacy and EP-UNSM is an effective method for strengthening Ti64. • Pulsed current leads to higher dislocation mobility and thus higher plasticity for more effective peening treatment. • EP-UNSM generates a gradient nanocrystalline layer in Ti64. • EP-USM results in high surface hardness and higher residual stress magnitude and depth. [ABSTRACT FROM AUTHOR]

Severe plastic deformation has made it possible to alter the grain size of metal surface to nanoscale. With refined nanograins, the grain boundary effect on diffusion and phase transformation cannot be neglected. Consequently, the widely used conventional 1D nitriding model is not applicable. In this study, a 2D model considering grain boundary diffusion has been developed to investigate nanocrystalline nitriding. As a multi-physical process, both phase transition and diffusion are modeled. Cellular automata method was used to integrate the two models, and more importantly to deal with the moving 2D interface induced by grain boundaries. The phase transition model and diffusion model were validated with experimental data and the Maxwell–Garnett effective diffusion model, respectively. After validation, nitriding of nanocrystalline iron at low temperature (300 °C) was simulated and compared with nitriding of coarse-grained (μm level) iron. In addition, the growth kinetic, composition and spatial distribution of the nitride layer in nanocrystalline nitriding, with different temperatures, surface nitrogen concentrations and different grain sizes, were studied. It has been found that these parameters could significantly affect the growth rate as well as the composition of the nitrided layers. The results also demonstrated that the presence of nanoscale grain can not only decrease nitriding temperature and nitriding duration making low temperature nitriding possible, but also increase the volume fraction of ∊ and γ′ phases in the nitride layer and therefore a better nitriding quality. [ABSTRACT FROM AUTHOR]

[Display omitted] • UNSM refined the martensite laths and formed a plastically deformed layer on A100 steel. • Higher surface hardness and higher fretting wear resistance were observed. • High magnitude compressive residual stresses were generated in the near surface region. • Both the plain fatigue and fretting fatigue resistance of A100 steel were improved. In this study, ultrasonic nanocrystal surface modification (UNSM) was used to improve plain and fretting fatigue properties of A100 steel. Results showed that an obvious plastic deformation layer was induced in A100 steel, and the martensite microstructure was remarkably refined. In addition, surface compressive residual stress after UNSM treatment was enhanced to 1700 MPa due to the severe lattice distortion. The increased surface hardness and the presence of beneficial compressive residual stresses after UNSM treatment significantly enhanced the plain/fretting fatigue resistance of A100 steel by retarding fatigue crack nucleation and propagation. [ABSTRACT FROM AUTHOR]

• An UNSM system is used to fabricate hierarchical surface structures on metals. • The formation of nano-scale structure originates from pile-up produced by dynamic indentation. • Dynamic indentation produces higher pile-up as compared with quasi-static indentation. • Dynamic indentation induces high resistance to dislocation motion, forcing material to pile up. Ultrasonic-assisted surface texturing can produce surface textures with hierarchical micro/nano-scale structures on metal components. In this work, we prove that the formation of nano-scale structure originates from pile-up produced by ultrasonic dynamic indentation. Both quasi-static and dynamic indentations are performed on pure copper, 316 stainless steel, and pure titanium using an ultrasonic nanocrystal surface modification system. For the first time, it is revealed that ultrasonic dynamic indentation produces significant higher pile-up as compared with that of quasi-static indentation, making nanoscale structure possible. Finite element simulations demonstrate that dynamic indentation produces larger equivalent plastic strain, an indicator for work hardening, in the region close to the edge of the contact interface. It is conjectured that higher degrees of work hardening and dislocation density result in a higher dislocation resistance as compared with quasi-static indentation. The plastic deformation is confined at the contact interface and is then forced to move upward, leading to a significant pile-up. [ABSTRACT FROM AUTHOR]

• A refined and distorted lath martensite structure was generated on the surface of 300M steels through UNSM. • UNSM significantly induced high surface hardness and compressive residual stresses. • UNSM treated 300M steels exhibited higher wear resistance by forming a surface plastic deformation layer. In this study, the effects of ultrasonic nanocrystal surface modification (UNSM) treatment on the surface integrity, microstructures and wear resistance of 300M ultra-high strength steel (300M steel) were investigated. The results showed that surface roughness of 300M steels after UNSM processing was significantly decreased with a lower scanning speed even though the surface roughness values were higher than that of mechanically polished control samples. In addition, the surface hardness of 300M steel was significantly enhanced as the static load increased. It was found that using a static load of 50 N and a scanning speed of 250 mm/min in the UNSM process can significantly improve surface hardness (797 HV) while slightly increasing the surface roughness. With these parameters, the resulting microstructure of UNSM-processed samples have three layers: the layer of severe plastic deformation, the layer with gradual plastic deformation, and the unaffected layer. Due to the plastic deformation, greater and deeper compressive residual stresses were induced in the UNSM-processed samples. In addition, the wear resistance of UNSM-processed samples was significantly improved, which was attributed to the refined martensite laths, work hardening and compressive residual stress. [ABSTRACT FROM AUTHOR]

Abstract Using either electropulsing (EP) or surface severe plastic deformation (SSPD) to process metallic glasses can improve their plasticity, however, the moderate improvement in plasticity does not warrant a commercial usage. This work, for the first time, demonstrates the integration of electropulsing and surface severe plastic deformation is much more effective in improving the plasticity of metallic glasses than EP or SSPD treatment alone, opening a new avenue towards an unprecedented combination of strength and plasticity in metallic glasses. It is found that SSPD can generate microstructure heterogeneity featured by a mixture of matrix and plastically displaced regions with increased atomic volume. When applying EP and SSPD simultaneously, a synergistic effect occurs to produce a hybrid network with excess free volume and nanocrystals uniformly embedded in amorphous matrix. Molecular dynamics simulation and fracture surface analysis further reveal that the hybrid network is able to effectively reduce shear band localization, and therefore delay the fracture of metallic glasses. Graphical abstract Unlabelled Image Highlights • Electropulsing-assisted surface severe plastic deformation (EP-SSPD) treatment increased plasticity from 0% to 2.03% ± 0.29%. • EP-SSPD generated a hybrid structure of 4.1% uniformly-distributed nanocrystals and 0.009% averaged extra free volume. • Free volume alleviates localized strain at nanocrystal/matrix interface and nanocrystals retard shear band propagation. [ABSTRACT FROM AUTHOR]