Surface Mechanical Rolling Treatment Research Articles

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.

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.

Anti-corrosion and corrosive wear properties of AISI 316L stainless steel with surface nanocrystallization by surface mechanical rolling treatment

In this work, surface mechanical rolling treatment (SMRT) was adopted to obtain the gradient nanostructured (GNS) layer on AISI 316L stainless steel (SS) surface. To evaluate the effect of the GNS, anti-corrosion and corrosive wear tests of the SMRT sample were conducted in pure water and corrosive medium in comparison with a unSMRT (i.e., coarse-grained 316L SS). The corrosion behaviors, tribological properties, damage mechanism as well as relation of pitting and delamination, have been investigated to clarify their synergetic effects and failure mechanism. The results show that, the GNS of 316L SS has poor corrosion resistance due to initiation in local of stress corrosion crack (SCC). However, in comparison with the unSMRT sample, the pitting resistance of 316L SS after SMRT is significantly enhanced. For both samples, the wear loss by delamination is obviously accelerated by corrosion in the friction process. The tribo-corrosion resistance is markedly enhanced with the GNS layer relative to the unSMRT sample, the GNS layer can effectively inhibit the formation of delamination in the corrosive environment, and can also mitigate the abrasive wear in the water lubrication condition, due to the beneficial combination of superfine nanocrystalline structure and high hardness of the GNS layer.

Deformation-induced martensitic transformation and strain hardening in a nanocrystalline FeMn alloy processed by severe austenite pre-deformation

The FeMn alloy is processed by the surface mechanical rolling treatment when being heated beyond 200 °C for an austenitic state. The nanocrystalline microstructure mainly consisting of austenite phase is resulted in by the severe austenite pre-deformation, which is attributed to the significant mechanical stabilization. This is in contrast with the coarse-grained counterpart that is dominated by epsilon-martensite. The nanocrystalline microstructure in the graded specimen can undergo a large tensile deformation. Martensitic transformation occurs during mechanical loading, and strain hardening of the nanocrystalline microstructure is achieved. Microscopic observations show the growth of epsilon-martensite plates, indicating the multiplication of stacking faults in the nanograins. This study demonstrates a novel approach combining alloy design and plastic processing to produce nanocrystalline materials with strain hardening capacity.

Enhanced Fatigue Property of Welded S355J2W Steel by Forming a Gradient Nanostructured Surface Layer

Welded joints are usually characterized by microstructural and compositional inhomogeneities, which may significantly degrade their fatigue properties and result in unpredictable failures. The present work demonstrates a novel and simple method to effectively optimize the microstructure in the surface layer and promote the fatigue properties of welded specimens. By a recently developed approach—surface mechanical rolling treatment (SMRT), a gradient nanostructured surface layer is formed on welded S355J2W steel specimens. The mean grain size is refined to nanometer scale, and the hardness is significantly enhanced in the SMRT surface layer. Independent of the initially inhomogeneous microstructure and hardness distributions, the microstructure and hardness distributions in the surface layers are comparable on different zones of a welded specimen after SMRT with the same procedure. Consequently, fatigue property of the SMRT specimens is significantly enhanced relative to that of the as-welded specimens within the high cycle fatigue regime.

Cyclic deformation and fatigue behavior of 316L stainless steel processed by surface mechanical rolling treatment

Fully reversed strain-controlled tension-compression fatigue experiments were conducted to study the cyclic deformation and fatigue behavior of 316L stainless steel with a gradient nanostructured surface layer formed by surface mechanical rolling treatment (SMRT). The grain sizes of the nanostructured surface layer range from 30 nm to 300 nm. The SMRT significantly enhances the fatigue properties. The strain-life fatigue curve of the SMRT samples displays a distinguishable kink point which demarcates surface and subsurface fatigue crack initiations. Mechanisms associated with the enhanced fatigue property by SMRT are discussed in light of the mechanics of the gradient material and the microstructures.

Thermal stability of nanogradient microstructure produced by surface mechanical rolling treatment in Zircaloy-4

A nano-/ultrafine grain gradient microstructure, which is composed of high-angle grain boundaries (HAGBs) and low-angle grain boundaries or subgrains of dislocation–twin, was fabricated in Zircaloy-4 using surface mechanical rolling treatment (SMRT). Thermal stability of gradient microstructure has been investigated through characterizing the evolution of microstructure during post-SMRT annealing treatment from 400 to 600 °C using optical microscopy and transmission electron microscopy. Experimental results show that the gradient microstructure exhibits a good thermal stability at 400 °C, since the overall grain size remains similar, except a decrease in dislocation density due to recovery. In comparison, a hierarchical microstructure is formed after annealing at 600 °C. An obvious grain growth was observed at the depth of 50 μm. The activation energy for grain growth of nanograined Zircaloy-4 is estimated to be ~ 161 kJ/mol between 400 and 600 °C. Nano-/ultrafine grains predominantly consisting of HAGBs have the highest thermal stability. Both yield strength and ultimate tensile strength of Zircaloy-4 decrease due to anneal, specifically at 600 °C.

Gradient structure design to strengthen carbon interstitial Fe40Mn40Co10Cr10 high entropy alloys

In this work, gradient structure is successfully produced in the surface of FCC structured (Fe40Mn40Co10Cr10)C3.3 high entropy alloy (HEA) by using surface mechanical rolling treatment (SMRT). The depth of surface gradient layer with higher hardness increases with increasing the SMRT passes. The introducing of gradient layer not only significantly improves the strength of HEAs but also remains sufficient ductility. The ultra-high strength of the gradient structured HEA is mainly related to the synergetic strengthening mechanism, while the comparatively good ductility is ascribed to the back stress strain hardening mechanism. Gradient structure design provides a novel strategy for producing HEAs with high performance.

Microstructure and corrosion properties of FeCoNiCrMn high entropy alloy coatings prepared by high speed laser cladding and ultrasonic surface mechanical rolling treatment

Abstract The FeCoNiCrMn high-entropy alloy (HEA) coatings were prepared by high-speed laser cladding (HSLC) technology, and the surface was subjected to ultrasonic surface mechanical rolling treatment (SMRT). After SMRT, the surface of the coatings show a gradient structure. The coatings consist of planar crystal, columnar dendrites and equiaxed crystals from inside to outside. Corrosion behavior in a 3.5% NaCl solution indicates that SMRTed coatings exhibit higher corrosion resistance than the coatings without SMRT. Anti-corrosion mechanism was discussed based on grain refinement, grain boundary wetting phase transitions and compressive residual stress.

Synergetic Strengthening of Grain Refinement and Texture in Gradient Zircaloy-4 by Surface Mechanical Rolling Treatment

A Zircaloy-4 rod was subjected to surface mechanical rolling treatment (SMRT) to form a gradient structure, and the evolution of the resulting sub-grain-boundary property and texture across the gradient structure was characterized using electron backscattered diffraction. Dual-gradient structures in grain size and orientation were formed; the grain size was refined from several microns at the center to approximately 400 nm at the topmost surface. Texture analysis revealed that the c-axis gradually tilted from a random orientation toward the parallel-to-radial direction on the radial–tangential plane. The SMRT-induced formation of the dual-gradient microstructure is attributed to the formation of gradient distributions of stress and strain, which resulted in various deformation mechanisms (twinning and dislocation) being active at different depths. During the SMRT process, twinning and dislocations were activated to refine the grains. When the dual-gradient microstructure formed, twinning was mainly activated at the subsurface near the matrix, whereas dislocations were activated across the entire gradient. The geometrically necessary dislocation density increased with decreasing depth and then slightly decreased near the surface. The synergetic strengthening of the dual-gradient microstructure resulted in a gradient distribution of the microhardness near the surface. Thus, the Zircaloy-4 rod exhibited a good combination of strength and ductility.

Effects of surface gradient nanostructuring on the fatigue behavior of the friction stir welded AlZnMgCu alloy

Surface gradient nanostructuring was performed on the friction stir welded joints of high strength AlZnMgCu alloy by means of surface mechanical rolling treatment (SMRT). Uniaxial symmetric tension-tension fatigue tests (R = 0.1) showed that two orders of magnitude extension in fatigue lifetime was achieved on the FSW joint of the AlZnMgCu alloy, exhibiting the transition of fatigue crack initiation mode from surface to interior. The enhanced fatigue mechanism was discussed based on the measured in-depth distribution hardness and compress residual stress, as well as the observed microstructure modifications.

Incredible improvement in fatigue resistance of friction stir welded 7075-T651 aluminum alloy via surface mechanical rolling treatment

Friction stir welding (FSW) is able to provide high static strength for aluminum alloys, but it always leads to a remarkable loss in fatigue strength. By means of surface mechanical rolling treatment (SMRT), a near-polished, gradient nanostructured surface and high compressive residual stresses were generated on the FSW joint of the 7075-T651 aluminum alloy. Uniaxial fatigue tests showed that an enhanced fatigue property with two orders of magnitude extension in fatigue lifetime was achieved on the SMRT joints, exhibiting failure mode transitions from surface to interior. Transmission electron microscopy analyses showed that the Al18Ti2Mg3 constituent phase was responsible for the internal crack initiation. Furthermore, the enhanced fatigue mechanism and the role of SMRT were discussed. The findings offer a potential pathway to greatly compensate the fatigue resistance deterioration of materials resulting from welding.

Simultaneous enhancement of stress- and strain-controlled fatigue properties in 316L stainless steel with gradient nanostructure

A gradient nanostructured (GNS) surface layer with full austenitic phase was synthesized on AISI 316L stainless steel by surface mechanical rolling treatment at ∼280 °C. The mean grain size is ∼45 nm at the top surface and increases gradually with depth. Deformation-induced martensite (DIM) transformation was suppressed and the microstructural refinement was dominated by dislocation activities and twinning during the formation of the GNS layer. Axial tension-compression fatigue tests showed that fatigue strength and life are simultaneously enhanced in the GNS samples relative to the corresponding coarse-grained counterparts in both stress- and strain-controlled tests. This is very different from fatigue behavior of conventional nanostructured materials, i.e. an enhanced stress-controlled fatigue strength with a decreased strain-controlled fatigue life. Besides contributions from the enhanced mechanical properties and the suppressed surface defects formation, analyses of fatigue mechanisms demonstrated that the promoted formation of DIM during cyclic strain plays a crucial role in enhancing fatigue properties of the GNS samples in strain-controlled tests.

Effect of Nanostructured Surface on the Corrosion Behavior of RAFM Steels

This study investigated the effect of a preformed nanostructured surface on the corrosion behavior of 9Cr2WVTa reduced activation ferritic/martensitic (RAFM) steel and 9Cr+AlSi steel (9Cr2WVTa with the 0.12 wt.% Al and 0.68 wt.% Si addition) at 700 °C in air and at 550 °C in liquid lead–bismuth eutectic (LBE) alloys. The nanostructured surface layer was fabricated by surface mechanical rolling treatment (SMRT). The results showed that the SMRT 9Cr+AlSi sample has a lower oxidation rate than the SMRT 9Cr2WVTa steel at 700 °C in air, due to the faster diffusion rates of Al, Cr and Si in the nanostructure and a higher diffusion driving force increased by Cr. The SMRT 9Cr+AlSi sample at 550 °C in oxygen-saturated LBE alloy also had a higher oxidation rate, due to the formation of Al and Si oxides in the internal oxide layer.