Surface Mechanical Rolling Treatment Research Articles

Interplay between deformation and fracture mechanism in gradient nanostructured austenitic stainless steel

The plastic deformation parameters are crucial for the grain size of gradient nanostructured (GNS) materials, and the multiscale grain structure within the GNS layer has a significant impact on fracture behavior. In this study, the detailed depth-dependent microstructures of GNS310S stainless steel fabricated by surface mechanical rolling treatment (SMRT) were investigated, and the relationship between the geometrically necessary dislocation (GND) density and the strain gradient was established through the local misorientation obtained by electron backscatter diffraction. The critical deformation parameters (strain gradient, shear strain, and strain rate) of SMRT-induced grain refinement at different scales were quantitatively evaluated. The tensile results show the yield strength and tensile strength of GNS310S are improved by 103 % and 23.1 % respectively compared to CG310S. Additionally, we elucidate the underlying tensile fracture mechanism of GNS310S stainless steel through detailed microstructural analysis. Due to the poor ductility of the near-equiaxed nanograins, cracking occurred on the surface of the SMRT specimen at low strain during the tensile testing. These cracks evolved into competitive shear offsets and multi-cracks with increased strain on the specimen surface, eventually leading to a hybrid fracture mode. In contrast, the subsurface nanotwinned lamellae layer displayed good ductility during tension, which can accommodate the deformation through lamellae widening and forming secondary twinning. This nanotwinned lamellae layer can suppress the inward propagation of surface cracks when the strain was not large. This work provides new insights into the fabricating parameters of the GNS stainless steel, as well as the deformation and fracture mechanisms.

Optimizing Surface Hardness and Corrosion Resistance in 316L Stainless Steel via SMRT and Plasma Nitriding.

Increasing the mechanical strength of stainless steel is fundamental to expanding its applications and extending its useful life, especially due to its impressive corrosion resistance. However, many processes that make possible increasing the hardness and wear resistance of these steels also compromise their performance when they are subjected to corrosive environments. Because of the need to increase surface hardness without jeopardizing corrosion resistance, this study explores the impact of a surface mechanical rolling treatment (SMRT) with volumetric reductions of 50% and 70%, together with high-temperature plasma nitriding (PN) (450, 500, and 550 °C), on 316L steel. X-ray diffraction analysis analyzed the composition and structure of the resulting coatings. Mechanical strength was assessed using Vickers microhardness, while corrosion resistance was tested in a 3.5%-by-weight NaCl solution. The results indicate a substantial increase in surface hardness in all of the nitrided samples. Notably, the samples were subjected to rolling with 50% and 70% volumetric reduction, and subsequent nitriding at 550 °C showed a reduced impairment of corrosion resistance. This shows that the methodology adopted in this study, consisting of SMRT and PN, can increase the useful life and expand the applicability of stainless steels commonly used in various industry sectors.

Rolling contact fatigue property of gradient rare earth addition M50 bearing steel with surface nano-grains and its inhibited martensite decay

A gradient nanostructured surface layer of ∼600 μm in thickness was prepared on M50 steel with rare earth additions by a surface mechanical rolling treatment. The initial microstructure was refined into nanosized grains (∼29 nm in diameter) in the top surface layer, causing a significant increase of surface hardness. Meanwhile, the grain size increases, and the microhardness decreases gradually to the matrix values with increasing depth. Rolling contact fatigue tests showed that the fatigue lives are significantly increased by the gradient nanostructured surface layer. Analysis of failure mechanism revealed that both softening in the near surface layer and hardening in the subsurface occur under rolling contact testing. And surface spalling is the predominant failure mode, of which the cracks initiate from primary carbides in the top surface layer owing to surface softening. In gradient nanostructured samples, the nano-grains inhibit carbon migration by impeding dislocation movement in the near surface layer, so that martensite decay is suppressed. Consequently, the crack initiation is inhibited, and the fatigue property is enhanced.

Simultaneous improvement in mechanical properties, corrosion resistance and antibacterial properties of Ti-Ag sintered alloy with gradient nanostructure

Bioinert titanium (Ti) with antibacterial properties is crucial for bone implantations. Alloying Ag can endow Ti with antibacterial properties, while the mechanical, corrosion and antibacterial properties of Ti-Ag alloys are not satisfactorily balanced. In this work, Ti-x wt%Ag alloys (x=0.5, 1 and 3) were sintered by spark plasma sintering and then subjected to a surface mechanical rolling treatment (SMRT). The Ti-Ag alloys are composed of Ti, Ti-Ag and Ti2Ag phases. After SMRT, a gradient nanostructured (GNS) layer formed on the Ti-Ag surface, the mean grain size is ∼ 6 nm at the top surface and increases gradually with depth. The GNS layer endows Ti-Ag alloys with better wettability, higher nano-roughness and compressive residual stress. Incorporating limited Ag (≤1 wt%) in Ti can effectively enhance its mechanical (surface hardness, wear resistance and compressive strength) and anti-corrosion properties, and they can be further improved by forming a GNS layer. The antibacterial efficiency of Ti-Ag is positively proportional to Ag content, and they are significantly enhanced after surface nanocrystallization. The bacteriostatic mechanism is the synergistic interaction between leach Ag+ and nano Ag-containing phases of GNS Ti-Ag alloy. Both Ti-Ag and GNS Ti-Ag alloys possess good cytocompatibility, the GNS layer has the potential to encourage early differentiation of osteoblasts. The GNS Ti-Ag alloy with 1 wt% Ag has excellent comprehensive properties, which shows larger potential in orthopedic applications.

Parameter Optimization of a Surface Mechanical Rolling Treatment Process to Improve the Surface Integrity and Fatigue Property of FV520B Steel by Machine Learning.

Surface integrity is a critical factor that affects the fatigue resistance of materials. A surface mechanical rolling treatment (SMRT) process can effectively improve the surface integrity of the material, thus enhancing the fatigue property. In this paper, an analysis of variance (ANOVA) and signal-to-noise ratio (SNR) are performed by orthogonal experimental design with SMRT parameters as variables and surface integrity indicators as optimization objectives, and the support vector machine-active learning (SVM-AL) model is proposed based on machine learning theory. The entire model includes three rounds of AL processes. In each round of the AL process, the SMRT parameters with relative average deviation and high output values from cross-validation are selected for the additional experimental supplement. The results show that the prediction accuracy and generalization ability of the SVM-AL model are significantly improved compared to the support vector machine (SVM) model. A fatigue test was also carried out, and the fatigue property of the SMRT specimens predicted by the SVM-AL model is also higher than that of the other specimens.

Surface mechanical rolling treatment-introduced gradient nanostructured Inconel 625 alloy with enhanced high-temperature wear resistance

Heterogeneous gradient nanostructures induced through surface treatment of nickel-based superalloys hold significant promise for enhancing wear resistance. In this study, a 700 µm ultra-thick gradient nanostructured (GNS) strengthening layer was fabricated on the surface of an Inconel 625 alloy plate via surface mechanical rolling treatment (SMRT). Subsequently, the tribological behaviors and wear resistance mechanisms of coarse-grained (CG) and GNS samples at temperatures ranging from 25 ℃ to 800 ℃ were comparatively investigated. The results indicated that the gradient nanostructures significantly enhanced the high-temperature wear resistance of the samples, which wear characterized by low coefficient of friction (COF), wear rate and thin friction layer. This enhancement was attributable to the stability and excellent hot hardness of the gradient nanostructures during friction.

A study of fatigue property enhancement of 1045 steel processed by surface mechanical rolling treatment with an emphasis on residual stress influence

Several factors contribute to the observed bulk mechanical properties enhanced by surface mechanical rolling treatment (SMRT), and it is important to distinguish the influences of each major factor. A commonly used engineering carbon steel, 1045 steel, was chosen for the current study because the material does not have stress-induced phase transformation. A fully reversed strain-controlled gradual decreasing amplitude loading spectrum (ENV) was employed to release the residual stresses induced by SMRT. Fatigue experiments were conducted on the 1045 steel specimens with and without ENV to assess the influences of grain refinement and the residual stress on fatigue. It was found that SMRT refined and recombined the microstructures in the surface layer through deformation and fragmentation of brittle cementite, and extrusion of ductile ferrite between adjacent cementite layers. In the high cycle fatigue regime, both the SMRT induced residual stresses and the refined/recombined microstructures contributed to the enhancement of fatigue strength and fatigue ductility but the gradient microstructure contributed significantly more than do the residual stresses. Unlike stainless steels processed by SMRT, the carbon steel does not display a kink point in the strain-life fatigue curve, and all the crack initiations were found to occur on the specimen surface.

Formation of nanocrystalline and nanolaminate structures in a twinning induced plasticity Ti-12Mo alloy: The role played by {332} 〈113〉 twinning

Grain refinement is an effective strategy to strengthen metastable β titanium alloys. {332}<113> twinning, a typical deformation mode, its role on grain refinement still remains unelucidated. Here, we produced a gradient nanostructured layer with a depth of 800 μm in a metastable twinning induced plasticity β-Ti-12Mo (wt.%) alloy by means of surface mechanical rolling treatment. We found that {332}<113> deformation twinning, mainly dominated the deformation at moderate strain and strain rates, refining coarse grains into submicro-structures. With increasing strain and strain rates, dislocation slip instead of {332}<113> twinning prevailed. These dislocations were first arranged into extended boundaries and then into laminated structures to coordinate deformation. Eventually, shear banding and fragmentation of laminates result in the formation of nanocrystalline with size of 34 ± 8 nm. The nanostructured Ti-12Mo alloy exhibited the high hardness of 4.9 GPa at subsurface. The surface hardness decreased to 4.4 GPa which was mainly caused by the inhibition of stress-induced martensitic transformation α″ and the occurrence of its reversal transformation due to adiabatic heating induced by the high strain rate. This work expands our knowledge on strengthening β titanium alloys by {332}<113> twinning induced grain refinement at different deformation conditions.

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.

Constitutive modeling and tensile behavior of a fully austenitic gradient nanostructured stainless steel

The constitutive relationship can effectively describe and predict the deformation behavior of materials, and it is critical to establish appropriate constitutive relationships to obtain the mechanical response of gradient nanostructured (GNS) metallic materials during deformation. In this work, a general method for establishing the multi-layer constitutive relationship of the GNS metallic materials was proposed. A fully austenitic 310S stainless steel specimen with a thickness of 1.4 mm was fabricated by surface mechanical rolling treatment (SMRT) technology, and its depth-dependent microstructures and macroscopic mechanical properties were experimentally investigated. Based on the experimental results of uniaxial tensile and microhardness, using the quantitative relationship between microhardness and flow stress, the reverse analysis was conducted using a separation parameter method to establish the constitutive relationship of the 28 thin shells of the GNS310S specimen. Finite element analysis (FEA) confirmed that GNS310S was in a multiaxial stress state at low strains, and the evolution of stress and strain gradients during non-uniform deformation was analyzed. The method for establishing the constitutive relationship is independent of the microstructure and synergistic strengthening factors, and the experiment performs conveniently and has great generalizability, which can provide a useful guide for the mechanical modeling of the GNS metallic materials to obtain the intrinsic mechanical response.

Assessing the tribo-corrosion resistance of surface nanostructured stainless-steel

In this work, surface mechanical rolling treatment (SMRT) was applied as a post-process nanostructuring technique to improve the tribo-corrosion degradation resistance of coarse grain (CG) 316L stainless steel (SS). The experimental findings reveal that when tribo-corrosion wear took place, the CG specimen experienced severe surface de-passivation, which increased wear-corrosion synergy and the rate of material loss. On the other hand, the SMRT specimen experienced enhanced re-passivation kinetics, which reduced wear-corrosion synergy and the rate of material loss. The wear mechanism for both the CG and SMRT specimens was abrasion-dominant, with the CG specimen having the presence of micro-pits within the wear track. In addition to these findings, the reduced surface roughness, improved hardness, formed compressive stresses, microstructural refinement, and martensite phase transformation induced by the SMRT process contributed to the enhanced tribo-corrosion resistance of the SMRT specimen. In total, SMRT was found to be a feasible technique to improve the tribo-corrosion resistance of CG 316L SS.

Unraveling the friction and wear mechanisms of surface nanostructured stainless-steel

In an effort to improve the friction and wear resistance of coarse grain (CG) 316L stainless steel (SS), the influence of surface nanostructuring formed by a novel surface mechanical rolling treatment (SMRT) technology was investigated. The tribological tests performed in this work consisted of dry reciprocating ball-on-flat tests using Al2O3 as the counter substrate. Results indicate that the SMRT process improved the abrasion resistance of the CG specimen at 15 N, 30 N, and 45 N loads. This finding was due to the combination of surface strengthening, surface smoothening, and the martensite phase transformation induced by SMRT treatment. Aside from the observed change in abrasion resistance, the frictional response of the SMRT specimen was lower than the CG specimen across all loads. Interestingly, the coefficient of friction (COF) for both the CG and SMRT specimens was observed to decrease as the load increased. The formation of surface oxidation contributed to this decrease, as a protective oxide layer was formed. As the load increased, a greater amount of frictional energy was converted into surface oxidation, which reduced the frictional response. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) investigations of the worn surfaces confirmed these findings, indicating that a combination of oxidative and abrasive wear took place. Collectively, these findings indicate that SMRT treatment is a viable technique to improve the tribological resistance of CG 316L SS.

Gradient-structured 1060 aluminum by air pressure-driven surface mechanical rolling treatment and its effects on the friction performance

Surface mechanical rolling treatment (SMRT) is well documented as a novel and promising surface self-nanocrystallization technology. An experiment platform of air pressure-driven surface mechanical rolling treatment was established innovatively, and the gradient-structured 1060 aluminum rod samples were accordingly fabricated by air pressure-driven SMRT. The air pressure, rolling pass and rotation speed of rod sample are the three parameters used for controlling the SMRT process. A three-factor and five-level orthogonal experiment table was designed to study the effects of the process parameters on the results of air pressure-driven SMRT of 1060 aluminum rod samples in terms of the surface roughness and in-depth microhardness. The experimental results show that the different combination of the three process parameters could obtain almost the same surface roughness or in-depth microhardness. Both the surface roughness and in-depth microhardness induced by air pressure-driven SMRT of 1060 aluminum rod samples are relatively more sensitive to the air pressure. Based on the microstructure observations, the grain refinement mechanism of 1060 aluminum induced by air pressure-driven SMRT was discussed in detail. The friction and wear experiments were further conducted to investigate the influences of the surface roughness and in-depth microhardness on the dry sliding wear behavior of the air pressure-driven SMRTed 1060 aluminum rod samples. In comparison to the reduction of surface roughness, the increase of the in-depth mircohardness has the more significant impact on the dry sliding wear behavior of the SMRTed 1060 aluminum rod samples.

Nano-precipitation strengthening regulated by nanotwins in CoCrNi alloy with super-high strength

The number and size of precipitates directly influence the strengthening of the high-temperature alloys. As special coherent interfaces, nano-twinned (NT) boundaries are pre-set in CoCrNi-based superalloy to regulate the distribution of nano-precipitates for further strengthening the mechanical property. High density of NTs with twin spacings of 2–200 nm is produced in CoCrNi alloy by surface mechanical rolling treatment (SMRT), and L12 (Ni3Ti)-type precipitates are induced by the low-energy NT boundaries, as well as grain boundaries, after aging at 700 °C for 4 h. The CoCrNi alloy with L12 nano-precipitates exhibits a super-high mechanical property, characterized by 2019 MPa of a yield strength (σy) and 2300 MPa of an ultimate tensile strength (σu), much higher than the CoCrNi alloy counterpart of 840 (σy) and 1100 (σu) MPa, respectively. Meanwhile, the treated CoCrNi alloy also delivers a reasonable elongation of 7.0%. The super-high strength is mainly ascribed to the high density L12 nanoprecipitates grown on the NT boundaries, while the elongation is sustained by the NTs boundaries.

Characterization of surface coarse-grained structure and improved corrosion resistance for Zr-4 alloy at high temperature

A surface coarse-grained structure (SCGS) layer is introduced in Zr-4 alloy after surface mechanical rolling treatment and then vacuum annealed at 800 ℃. The microstructures and textures of SCGS is characterized by Electron Backscattered Diffraction, and grains orientation are analyzed in detail by Inverse Pole Figure and Pole Figure. The corrosion tests of the Zr-4 alloy with SCGS layer and as-received were carried out at 360 ℃/18.7 MPa in water and in steam at 700–800 ℃, respectively. The coarse grain in SCGS layer is 10 times larger than that of as-received samples and has <101¯0> preferred orientation in cross section of cylindrical samples. The weight gain of the SCGS specimens is lower than that of the as-received specimens at high temperatures. As for corrosion in steam at 700 ℃ for 1 hour, the weight gain of SCGS sample is 40% less than that of the as-received sample. The high temperature oxidation process is dominated by the diffusion of O2− along the grain boundaries. The grain boundary density of the SCGS layer is six times less than that of the as-received specimens, which reduces the grain boundary diffusion channels for oxygen ion, and improves corrosion resistance of Zr-4 alloy at high temperature.

Twin-induced transformation strengthening of CoCrNi-based medium-entropy alloy with a gradient nanostructure

Single-phase face-centered cubic (fcc) medium-entropy alloys (MEAs) generally exhibit good ductility but insufficient room temperature strength that limits their potential application. Here, a deformation mechanism of twin-induced transformation was achieved to strength the CoCrNi-base alloy with gradient nanostructure obtained by surface mechanical rolling treatment (SMRT) technique. Significantly, the SMRT processed CoCrNi-based alloy exhibits a substantial improvement in the mechanical property comparing with the wrought counterpart by increasing the yield strength from 964 to 1664 MPa and the ultimate tensile strength from 1210 to 1883 MPa, while maintaining a uniform elongation of 11.3%. The enhanced combination of strength and ductility attributes to the extra hardening from the synergistic effect of nanostructure and phase transformation.

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.

Enhanced bonding property of ion-plated TiN coating on stainless steel by mechanically pre-forming a gradient nanostructure

An enhanced coating/substrate bonding property is crucial for the application of hard coatings (e.g., TiN) on soft substrates (e.g., stainless steel). In the present work, a gradient nanostructure, in which the grain size is ~50 nm in the top surface layer and increases gradually with depth, was pre-formed in 304 stainless steel by means of plate surface mechanical rolling treatment (PSMRT). Subsequently, a TiN coating was deposited on it by using a commercial arc ion plating approach. Scratch tests demonstrated that the critical load of coating detachment was significantly enhanced in the coated PSMRT sample, ~47 % higher than that in the coated coarse-grained counterpart sample. Analyses of the underlying bonding mechanisms revealed that a higher interfacial strength was achieved in the coated PSMRT sample, mostly due to the depressed formation of compounds and the accelerated diffusion of Ti in the interfacial region. In addition, such factors as a higher substrate hardness, a lower friction coefficient between the scratch tip and the coating, and a lower internal stress in the coating might also contribute to the enhanced bonding property in the coated PSMRT sample. This work provided insights on applying a simple mechanical approach to enhance the bonding properties of hard coatings on soft substrates.

Superior mechanical properties and deformation mechanisms of a 304 stainless steel plate with gradient nanostructure

Spatially gradient microstructures have shown a promising application in enhancing strength-ductility synergy of engineering metals such as austenitic stainless steels. However, existing approaches are limiting in producing a thick gradient nanostructured (GNS) layer with a high strengthening capability, and the underlying deformation mechanisms are still not clear in GNS austenitic stainless steels. In this work, we developed a new approach, i.e., plate surface mechanical rolling treatment, to produce a bulk gradient nanostructure in a 304 stainless steel plate of ∼1.90 mm in thickness. Uniaxial tensile tests revealed that an ultra-high yield strength of ∼1073 MPa with a considerable uniform elongation of ∼21% was achieved in the GNS sample. Subsequently, the evolutions of microstructure, phase, microhardness, and local strain distribution were systematically studied in the GNS plate during tensile tests. The results demonstrated that the mechanical incompatibilities, relating with the gradient microstructure and martensite-enclosing-austenite domains, contribute to an extra strain-hardening capability, leading to the outstanding strength-ductility synergy in the GNS 304 stainless steel. Furthermore, analyses based on experimental observations and theoretical calculations revealed that dislocation activities, instead of deformation-induced martensite transformation, microstructure refinement, and twinning, play a dominant role in the strain-hardening mechanisms of the GNS plate during tension.

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.