Search

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]

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]