Gradient Nanostructured Surface Layer Research Articles

Strengthening mechanism of abnormally enhanced fatigue ductility of gradient nanostructured 316L stainless steel

Structural materials with a good combination of high fatigue strength and high fatigue ductility are highly desirable in engineering applications. However, it remains a challenge to “break” the exclusive relationship between fatigue strength and fatigue ductility. The cylindrical 316L stainless steel specimens with a gradient nanostructured surface layer were prepared using surface mechanical rolling treatment (SMRT). Fully reversed strain-controlled tension–compression fatigue experiments indicated that the SMRT processed 316L specimens achieved simultaneous improvement of fatigue strength and fatigue ductility. The abnormal increase in fatigue ductility is attributed to the significant secondary cyclic hardening caused by the martensitic phase transformation and the transition in fatigue crack initiation mode. The martensitic phase transformation enhances the fatigue strength while reducing the proportion of plastic deformation in the controlled strain amplitude. The competition between the driving force and resistance of fatigue nucleation in different structural units results in the migration of fatigue crack initiation sites from the surface to subsurface, thereby prolonging the fatigue crack initiation life. This work reveals the synergistic strengthening mechanism of fatigue strength and fatigue ductility of a gradient nanostructured 316L stainless steel.

Understanding the high-temperature oxidation resistance of heat-resistant austenitic stainless steel with gradient nanostructure

The underlying high-temperature oxidation mechanisms of the heat-resistant austenitic stainless steel with gradient nanostructured surface layer is revealed through systematic analysis of the microstructure and composition. Nanoscale oxide grains and high-density grain boundaries promoted the formation of pronounced spinel oxides, which suppressed elemental diffusion and CrO3 volatilization. High level of residual stress in the GNS layer facilitated the formation of high-density precipitates at the oxide/matrix interface and grain boundaries, which hindered the growth of oxide scale and reactive elements diffusion. The enhanced high-temperature oxidation resistance resulted from the synergistic combination of spinel oxides, precipitates, and high-density grain boundaries.

Ultrastrong gradient M50 bearing steel with lath-shape nano-martensite by ultrasonic shot peening and its enhanced wear resistance at elevated temperature

This investigation delves into the intricacies of the process mechanism for generating a gradient nanostructured surface (GNS) layer on M50 bearing steel, with the aim of augmenting its surface hardness and wear resistance under elevated temperature. The electron backscatter diffraction and transmission electron microscopy substantiate the successful formation of a GNS layer, surpassing 200 µm in thickness, on the M50 bearing steel treated by Ultrasonic shot peening (USP) technology. The severe plastic deformation (SPD) induced by the USP process resulted in a significant refinement of the martensite in M50 bearing steel, leading to a remarkable 32.25 % increase in surface hardness compared to its untreated coarse-grained (CG) counterpart. Pin-on-Disk wear tests were conducted at an elevated temperature of 300 °C on both the as-received CG-M50 and USP-treated M50 bearing steel samples to evaluate the impact of USP treatment on the wear resistance of M50 bearing steel. Experimental results reveal that the volume loss of the USP-treated M50 bearing steel is 94.9 % less than that of the untreated CG-M50 bearing steel after 4 h of wear tests. The improved wear resistance of the USP-treated M50 bearing steel can be attributed to the creation of an ultrastrong and stable GNS layer facilitated by USP.

Research on multi-scale failure mechanism of gradient nanostructured 316L steel under strain-controlled fatigue at 650 °C

Gradient nanostructured materials have excellent fatigue resistance. At present, there are few research on the fatigue properties of gradient nanostructured (GNS) materials at high temperature. Herein, the multi-scale failure mechanism of GNS 316L steel at 650 °C is studied, and the high temperature fatigue tests under different strain amplitudes are conducted. Then, post-test microstructure observations were carried out to reveal the damage mechanisms, and the fatigue behavior was numerically simulated by crystal plasticity model. The experimental results show that the strength of the GNS 316L steel is still higher than that of the coarse-grained (CG) 316L steel at 650 °C, and the fatigue life of the GNS 316L steel is also higher than that of the CG one under strain fatigue loadings. The fatigue fracture results show that the fatigue crack presents a mixed propagation mode, and the crack source of GNS 316L transfers from surface to subsurface under low strain amplitude. The numerical results directly reflect the influence of the GNS surface layer on the plastic slip behavior. The fatigue failure mechanism of GNS 316L steel under strain-controlled cyclic loading at 650 °C was explained from a multi-scale perspective. This study provides data and method support for the application of GNS 316L steel at high temperature.

Ultrastrong gradient nanostructured CSS-42L bearing steel and its enhanced wear resistance at elevated temperature

A gradient nanostructured surface layer was successfully fabricated on the CSS-42L(15Cr14Co12Mo5Ni) aerospace bearing steel by ultrasonic shot peening (USP) technology at room temperature. The original austenite in the as-received (AR) CSS-42L bearing steel was fully transferred into lath martensite by quenching and two cryogenic tempering cycles. The lath martensitic structure at the surface of the bearing steel was refined into nanoscale martensite phases with an average size of 33.9 nm after USP treatment. The microhardness of the gradient heterogeneous nanostructured CSS-42L bearing steel increased by approximately 39 % compared to that of the tempered bearing steel. The mechanisms of martensite refinement during the USP process were analyzed and discussed using advanced materials characterization methods. Dry sliding wear tests at elevated temperatures (500 °C) were conducted on the tempered and USPed CSS-42L bearing steel samples, respectively. The wear resistance of the USPed sample was significantly enhanced compared to that of its untreated coarse-grained counterpart. The wear rate coefficient of the USPed CSS-42L was reduced by 80.42 % compared to the AR sample. The superior wear resistance of the USPed CSS-42L bearing steel was attributed to the formation of an ultrastrong surface layer with a gradient heterogeneous nanostructure fabricated by USP technology.

Enhanced mechanical, bio-corrosion, and antibacterial properties of Ti-Cu alloy by forming a gradient nanostructured surface layer

To improve the comprehensive performance of Ti-Cu alloy, a gradient nanostructured surface (GNS) layer was formed on Ti-Cu alloy by surface ultrasonic rolling treatment (SURT). The surface features, mechanical properties, corrosion resistance, antibacterial properties, and cell compatibility of Ti-Cu alloy before and after SURT were systematically investigated. The surface of GNS Ti-Cu is mainly composed of Cu2O and TiO2, as well as a small amount of Ti, and shows improved wettability. The thickness of the GNS layer is ~200 μm and the average grain size in the topmost layer is ~4 nm. The mechanical properties (microhardness, strength-ductility synergy, and wear resistance) and corrosion resistance are enhanced due to the existence of GNS layer on Ti-Cu alloy. The antibacterial tests demonstrated that GNS Ti-Cu obtains excellent antibacterial ability against Staphylococcus aureus and Escherichia coli, the antibacterial mechanism can be attributed to the accelerated Cu2+ leaching and the contact sterilization of Cu2O. Finally, biocompatibility experiments via MC3T3-E1 cells indicated that GNS Ti-Cu possesses improved cell spread ability and osteoblastic differentiation, showing satisfied biocompatibility. This work demonstrated that Ti-Cu alloy with superior comprehensive performance can be achieved by forming a GNS layer, which has great application potential in the field of metallic biomaterial.

Rotary bending fatigue behavior of a rare earth addition bearing steel: The effects of a gradient nanostructured surface layer formed by surface mechanical rolling treatment

A 700 μm-thick gradient nanostructured surface layer with enhanced hardness and compressive residual stress was produced on a rare earth addition bearing steel by surface mechanical rolling treatment under fine lubrication and cooling. Rotary bending fatigue tests showed that fatigue properties are significantly enhanced in the surface modified samples under the internal-induced failure mode, with the fatigue strength increasing from 1050 MPa in the as-received counterparts to 1200 MPa in them at a fatigue life of 107 cycles. Analyses revealed that the enhanced fatigue properties are related with the transfer of crack initiation site to a deeper region in the surface layer. And the formation of a thick gradient nanostructured surface layer with enhanced hardness and compressive residual stress contributes to the enhancement of fatigue properties of bearing steel.

Corrosion Behavior in Hydrochloric Acid of Pure Titanium after Ultrasonic Severe Surface Rolling

Designing a gradient nanostructure is regarded as an effective strategy for strengthening commercial pure Ti without seriously sacrificing ductility. However, the corrosion behavior of the gradient nanostructured (GNS) pure Ti is far from clear, especially in reducing acid in which pure Ti shows poor corrosion resistance. The present paper aims at investigating the corrosion behavior of GNS pure Ti in hydrochloric acid by electrochemical method. The GNS surface layer is produced by a recently developed method called ultrasonic severe surface rolling. The GNS pure Ti exhibits spontaneous passivation behavior as well as the coarse-grained one in 1 M HCl. Due to the GNS surface layer, the corrosion current density and passive current density decrease by 70% and 54%, respectively, giving rise to significantly enhanced corrosion resistance and passivation ability. The better corrosion resistance is believed to be ascribed to the high-density grain boundaries and dislocations induced by the surface nano-grained structure as well as the smooth surface with few surface defects. The USSR processing also enlarges the static water contact angle of the pure Ti to 61.0 ± 0.3°.

Formation mechanism and wear behavior of gradient nanostructured Inconel 625 alloy

The formation mechanism and wear behavior of a gradient nanostructured (GNS) Inconel 625 alloy were investigated using SEM, TEM and ball-on-disc sliding wear tester. The results show that surface mechanical grinding treatment (SMGT) induced an approximately 800 μm-deep gradient microstructure, consisting of surface nano-grained, nano-laminated, nano-twined, and severely deformed layers, which resulted in a reduced gradient in micro-hardness from 6.95 GPa (topmost surface) to 2.77 GPa (coarse-grained matrix). The nano-grained layer resulted from the formation of high-density nano-twins and subsequent interaction between nano-twins and dislocations. The width and depth of the wear scar, wear loss volume, and wear rate of the SMGT-treated sample were smaller than those of untreated coarse-grained sample. Moreover, the wear mechanisms for both samples were mainly abrasive wear and adhesive wear, accompanied with mild oxidation wear. The notable wear resistance enhancement of the GNS Inconel 625 alloy was attributed to the high micro-hardness, high residual compressive stress, and high strain capacity of the GNS surface layer.

Enhanced fatigue property by fabricating a gradient nanostructured surface layer in a reduced-activation steel

Reduced activation ferritic/martensitic (RAFM) steels have been selected as candidate structural materials for future advanced nuclear power systems. In the present work, the influence of a gradient nanograined surface layer on the fatigue properties of RAFM steels was studied. A gradient nanostructured (GNS) surface layer with a thickness of ∼85 ​μm was prepared on RAFM steel utilizing surface mechanical rolling treatment (SMRT). The mean grain size was approximate 43 ​nm at the topmost surface and increased gradually with depth. The results of the stress-controlled tension-compression fatigue experiments showed that the fatigue life enhanced approximately 6 times in the SMRT samples compared to the corresponding base metal counterparts. The relationship between the applied stress amplitude and the fatigue lifetime, and the fracture morphology showed that the surface strengthening and strain delocalization were caused by GNS, which suppressed surface crack initiation process, and hence the fatigue properties of RAFM steels improved. In addition, the deformation compatibility in GNS and coarse-grained boundaries leading to more dislocation interactions and accumulation during the cyclic process, also plays a crucial role in enhancing the fatigue properties of RAFM steel.

Enhanced initial biodegradation resistance of the biomedical Mg-Cu alloy by surface nanomodification

Mg-Cu alloys are promising antibacterial implant materials. However, their clinical applications have been impeded by their high initial biodegradation rate, which can be alleviated using nanotechnology by for example surface nanomodification to obtain a gradient nanostructured surface layer. The present work (i) produced a gradient nanostructured surface layer with a ∼500 µm thickness on a Mg-0.2 Cu alloy by a surface mechanical grinding treatment (SMGT), and (ii) studied the biodegradation behavior in Hank's solution. The initial biodegradation rate of the SMGTed samples was significantly lower than that of the unSMGTed original counterparts, which was attributed to the surface nanocrystallization, and the fragmentation and re-dissolution of Mg2Cu particles in the surface of the SMGTed Mg-0.2 Cu alloy. Furthermore, the SMGTed Mg-0.2 Cu alloy had good antibacterial efficacy. This work creatively used SMGT technology to produce a high-performance Mg alloy implant material.

Contribution to Improvement of Fatigue Properties of Zr-4 Alloy: Gradient Nanostructured Surface Layer versus Compressive Residual Stress.

The gradient nanostructured (GNS) layer forms beneath the surface of Zr-4 samples by the surface mechanical grinding treatment (SMGT) process, which increases the fatigue strength apparently due to the synergistic effect of the gradient nanostructured layer and compressive residual stress. The SMGTed Zr-4 samples are subjected to annealing to remove residual stress (A-SMGT) and the individual effect of the GNS layer and compressive residual stress can be clarified. The results show that the gradient nanostructure in the surface is stable after annealing at 400 °C for 2 h but residual stress is apparently removed. Both SMGTed and A-SMGTed Zr-4 samples exhibit higher fatigue strength than that of coarse-grained (CG) Zr-4 alloy. The fatigue fracture of Zr-4 alloy indicates that the hard GNS surface layer hinders fatigue cracks from approaching the surface and leads to a lower fatigue striation space than that of CG Zr-4 samples. The offset fatigue strength of 106 cycles is taken for SMRT-ed, A-SMRT-ed, and CG Zr-4 samples and the results indicate clearly that the GNS surface layer is a key factor for the improvement of fatigue strength of the Zr-4 alloy with surface mechanical grinding treatment.

Unprecedented age-hardening and its structural requirement in a severely deformed Al-Cu-Mg alloy

A gradient nanostructured surface layer was produced in an Al-Cu-Mg alloy (Al 2024) by means of a surface sliding friction treatment carried out at liquid nitrogen temperature. After aging treatment, unprecedented age-hardening was achieved at the surface layer, where microhardness values of >320 HV, more than twice that achievable by conventional precipitation hardening (150 HV), and well above the previously reported maximum value (280 HV) for this alloy, were achieved. Transmission electron microscopy and atom probe tomography analysis revealed that the formation of a nanograin structure at the deformed surface layer resulted in enhanced segregation of Cu and Mg during aging to narrowly spaced grain boundaries, with a corresponding suppression of precipitation. The enhanced grain boundary segregation instead of precipitation is considered to be responsible for the substantial age-hardening, although the underlying hardening mechanisms resulting from grain boundary segregation require further investigation.

Antibacterial property and bioadaptability of Ti6Al4V alloy with a silvered gradient nanostructured surface layer

Antibacterial property and bioadaptability of Ti6Al4V alloy with a silvered gradient nanostructured surface layer

Unprecedented Age-Hardening and its Structural Requirement in a Severely Deformed Al-Cu-Mg Alloy

A gradient nanostructured surface layer was produced in an Al-Cu-Mg alloy (Al 2024) by means of surface sliding friction treatment carried out at liquid nitrogen temperature. After aging treatment, unprecedented age-hardening was achieved at the surface layer, where microhardness values of >320 HV, more than twice that achievable by conventional precipitation hardening (150 HV), and well above the previously reported maximum value (280 HV) for this alloy, were achieved. Transmission electron microscopy and atom probe tomography analysis revealed that the formation of a nanograin structure at the deformed surface layer resulted in enhanced segregation of Cu and Mg during aging to narrowly spaced grain boundaries, with a corresponding suppression of precipitation. The enhanced grain boundary segregation instead of precipitation is considered to be responsible for the substantial age-hardening, although the underlying hardening mechanisms resulting from grain boundary segregation require further investigation.

Distributions of grains and precipitates in gradient lamellae Al–Zn–Mg–Cu alloy by ultrasonic surface rolling processing

Improving surface properties can effectively expand the industrial application of aluminum alloys. In this study, a gradient nanostructured surface layer was successfully fabricated in an aged Al–Zn–Mg–Cu alloy by ultrasonic surface rolling processing (USRP). The microstructure and microhardness of the gradient nanostructured surface layer were investigated. The results show that a perfect gradient lamellae structure was fabricated in the surface layer. The average structural size in the topmost surface layer is approximately 41 nm, and the depth of nanostructure lamellae layer is up to 50 μm. After the USRP and two years of natural aging, the gradient distribution of precipitates was formed in gradient lamellae structure. A large number of discontinuous coarse η phase are formed on the lamellae boundary throughout the lamellae structure. However, the precipitates inside the lamellae have gradient distribution. The volume fractions of precipitates increase gradually from nanostructure to ultrafine-grained structure, and the main precipitates are GPII zones. The microhardness in the topmost surface is increased by 40% and gradually decreases from the surface to the matrix. The increase in microhardness is attributed to the fact that the enhancement of the hardness by the grain refinement is greater than the decrease due to the reduction of the precipitates inside the gradient lamellae structure from the surface to the matrix. This study provides technical method and theoretical basis for improving the surface hardness of Al–Zn–Mg–Cu alloy.

Strong effect of carbides and alloy elements in gradient nanostructured 32Cr2Mo1VE steel on nitrogen atom diffusion

Metal surface nanocrystallization can improve the surface activity, reduce the diffusion activation energy, and then reduce the diffusion temperature or time. However, for gradient nanostructured surface layer on 32Cr2Mo1VE prepared by surface ultrasonic rolling process (SURP), the surface nanocrystallization seriously hindered the diffusion of N atoms during gas nitriding (430 °C, 480 °C and 530 °C). The effect of the distribution of alloying elements on the diffusion of active N atoms was studied in detail. For multi-element medium‑carbon alloy steels, it should be that carbides or nitrides formed by alloying elements usually precipitate easily at grain boundaries. So, precipitates along grain boundaries will block the rapid diffusion channel of nanocrystalline grain boundaries and also reduces the concentration gradient drive. • After surface ultrasonic rolling process (SURP), the surface grains were refined. • After SURP, the carbides were refined and precipitated on the grain boundaries. • The Cr 2 N precipitated on the grain boundaries of the SURP sample after gas nitriding. • The carbides and Cr 2 N formed on the grain boundaries hinder the diffusion of N atoms.

Controllable Martensite Transformation and Strain-Controlled Fatigue Behavior of a Gradient Nanostructured Austenite Stainless Steel.

Gradient nanostructured (GNS) surface layer with a controllable martensite fraction has been synthesized on 316L austenitic stainless steel by means of surface mechanical rolling treatment (SMRT) with temperature being controlled. The mean grain size is in the nanometer scale in the near-surface layer and increases gradually with depth. In addition, the volume fraction of martensite decreases from ~85% to 0 in the near-surface layer while the SMRT temperature increases from room temperature to 175 °C. Fatigue experiments showed that the strain-controlled fatigue properties of the GNS samples are significantly enhanced at total strain amplitudes ≥0.5%, especially in those with a dual-phase surface layer of austenite and pre-formed martensite. Analyses on fatigue mechanisms illustrated that the GNS surface layer enhances the strength-ductility synergy and suppresses the formation of surface fatigue defects during fatigue. In addition, the dual-phase structure promotes the formation of martensite and stacking faults, further enhancing fatigue properties at high strain amplitudes.

Enhanced mechanical properties and corrosion resistance of 316L stainless steel by pre-forming a gradient nanostructured surface layer and annealing

Stainless steels with high mechanical properties and corrosion resistance are promising structural materials for the next generation of aerospace and some niche industries. In this work, we pre-formed a gradient nanostructured (GNS) surface layer on 316L stainless steel by surface mechanical rolling treatment (SMRT) and subsequently annealed it at 700 °C. Tensile tests showed that the strength-ductility synergy was enhanced in the annealed-SMRT sample, while the grain size and hardness in the GNS layer retained rather stable. In addition, a remarkable Cr-enrichment was found in the GNS surface layer after annealing, resulting in a significantly enhanced corrosion resistance. The underlying mechanisms on the microstructure, composition and phases evolutions, as well as their effects on deformation and corrosion behavior, were analyzed in the annealed-SMRT sample. This work provides insights on developing a simple thermomechanical approach to produce stainless steels with enhanced mechanical properties and corrosion resistance.

Enhanced Mechanical Properties and Corrosion Resistance of 316l Stainless Steel by Pre-Forming a Gradient Nanostructured Surface Layer and Annealing

Stainless steels with high mechanical properties and corrosion resistance are promising structural materials for the next generation of aerospace and some niche industries. In this work, we pre-formed a gradient nanostructured (GNS) surface layer on 316L stainless steel by surface mechanical rolling treatment (SMRT) and subsequently annealed it at 700 °C. Tensile tests showed that the strength-ductility synergy was enhanced in the annealed-SMRT sample, while the grain size and hardness in the GNS layer retained rather stable. In addition, a remarkable Cr-enrichment was found in the GNS surface layer after annealing, resulting in a significantly enhanced corrosion resistance. The underlying mechanisms on the microstructure, composition and phases evolutions, as well as their effects on deformation and corrosion behavior, were analyzed in the annealed-SMRT sample. This work provides insights on developing a simple thermomechanical approach to produce stainless steels with enhanced mechanical properties and corrosion resistance.