Surface Mechanical Rolling Treatment Research Articles

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.

Comparison of SP, SMAT, SMRT, LSP, and UNSM Based on Treatment Effects on the Fatigue Properties of Metals in the HCF and VHCF Regimes

This paper aims to provide a better understanding regarding the effects of shot peening (SP), surface mechanical attrition treatment (SMAT), laser shock peening (LSP), surface mechanical rolling treatment (SMRT), and ultrasonic nanocrystal surface modification (UNSM) on the fatigue properties of metals in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes. The work in this paper finds that SMRT and UNSM generally improve the high-cycle and very-high-cycle fatigue properties of metals, while SP, SMAT, and LSP can have mixed effects. The differences are discussed and analyzed with respect to the aspects of surface finish, microstructure and microhardness, and residual stress. SMRT and UNSM generally produce a smooth surface finish, while SP and SMAT tend to worsen the surface finish on metals, which is harmful to their fatigue properties. In addition to inducing a plastic deformation zone and increasing microhardness, surface treatments can also generate a nanograin layer and gradient microstructure to enhance the fatigue properties of metals. The distribution of treatment-induced residual stress and residual stress relaxation can cause mixed effects on the fatigue properties of metals. Furthermore, increasing residual stress through SP and SMAT can cause further deterioration of the surface finish, which is detrimental to the fatigue properties of metals.

Gradient Enhanced Strain Hardening and Tensile Deformability in a Gradient-Nanostructured Ni Alloy.

Gradient-nanostructured material is an emerging category of material with spatial gradients in microstructural features. The incompatibility between gradient nanostructures (GNS) in the surface layer and coarse-grained (CG) core and their roles in extra strengthening and strain hardening have been well elucidated. Nevertheless, whether similar mechanisms exist within the GNS is not clear yet. Here, interactions between nanostructured layers constituting the GNS in a Ni alloy processed by surface mechanical rolling treatment were investigated by performing unique microtension tests on the whole GNS and three subdivided nanostructured layers at specific depths, respectively. The isolated nanograined layer at the topmost surface shows the highest strength but a brittle nature. With increasing depths, isolated layers exhibit lower strength but enhanced tensile plasticity. The GNS sample’s behavior complied more with the soft isolated layer at the inner side of GNS. Furthermore, an extra strain hardening was found in the GNS sample, leading to a greater uniform elongation (>3%) as compared to all of three constituent nanostructured layers. This extra strain hardening could be ascribed to the effects of the strain gradients arising from the incompatibility associated with the depth-dependent mechanical performance of various nanostructured layers.

Enhanced fatigue performance of aluminum alloy through surface strengthening treatment

Surface gradient distribution of the grain size was achieved on the 7075-T651 aluminum alloy by means of surface mechanical rolling treatment (SMRT). The tensile properties and fatigue resistance have been improved due to the action of SMRT. After SMRT, fatigue crack initiation mode exhibited the transition from surface to interior as the stress decreases. The mechanism of fatigue strengthening was discussed based on the gradient grain layer generated after SMRT combined with the cyclic deformation behavior and the transition of crack initiation mode.

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.

Cyclic deformation and fatigue behavior of 7075-T651 Al alloy with a gradient structure

Fully reversed stress-controlled tension–compression fatigue experiments (R = −1) were conducted to study the cyclic deformation and fatigue behavior of 7075-T651 aluminum alloys with and without a surface mechanical rolling treatment (SMRT). The strain amplitude, ratcheting effect, and asymmetric behavior of base metal (BM) and SMRT specimens were analyzed. SMRT specimens showed more remarkable ratcheting effects and tension–compression asymmetry under high stresses. Fatigue tests showed that the fatigue strengths of 7075 Al alloys were increased significantly by the SMRT. Both BM and SMRT specimens presented a failure-mode transition from the intrusions and extrusions failure modes to the Al18Ti2Mg3 constituent phase. The hysteresis loop was utilized to obtain the plastic-strain amplitude. Furthermore, according to the cyclic-deformation behavior combined with the Manson–Coffin law, strain–life evaluation of BM and SMRT specimens under the stress-control is given.

Modeling for Cyclic Plasticity of Gradient Nanostructured Metals and Fatigue Life Prediction

This paper proposes a theoretical model for the description of tension-compression cyclic plasticity of gradient nanostructured (GNS) metals. The gradient grain size effect is considered by introducing the Hall–Petch relation for local yield stress and strain hardening. With the experimentally measured grain size distribution profile, the average axial stress can be calculated for cylindrical GNS metal specimens. The model was verified using experimental data obtained from 316L stainless steel treated by surface mechanical rolling treatment (SMRT). Moreover, the corresponding strain energy for cyclic plasticity can be calculated from the constitutive equations, providing an energy-based approach to explain the fatigue life of gradient 316L stainless steel.

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.