Surface Mechanical Attrition Treatment Research Articles

Designing structures with combined gradients of grain size and precipitation in high entropy alloys for simultaneous improvement of strength and ductility

Suppressing the early strain localization at the nanostructured topmost layer is crucial for achieving better tensile ductility in the gradient structure. Thus, structures with combined gradient distributions along the depth for both grain size and volume fraction of precipitates were designed and introduced in a high entropy alloy by surface mechanical attrition treatment and aging. Yield strength and uniform elongation were observed to be simultaneously improved in the structures with combined gradients as compared to the corresponding structures with only grain size gradient. More severe strain gradients and higher density of geometrically necessary dislocations were observed to be produced at various domain boundaries in the structures with combined gradients, resulting in stronger hetero-deformation-induced (HDI) extra hardening for better tensile properties. Shearing and bowing hardening mechanisms were observed for L12 and B2 precipitates, respectively. Higher volume fractions of B2 and L12 phases at the topmost layer induce stronger precipitation hardening, which compensates the diminished strain hardening due to the reduced grain size at the topmost layer for better tensile ductility in the structures with combined gradients. The observed higher yield strength in the structures with combined gradients have been discussed based on mechanisms of dislocation strengthening, precipitation strengthening and HDI strengthening.

Combining synergetic effects of gradient nanotwins and nanoprecipitates in heterogeneous bronze alloy

Both strengthening effects of gradient nanostructure (especially the gradient nanotwins) and nanoprecipitates have been well-recognized to simultaneously improve the mechanical properties of metallic materials, e.g., the high strength–ductility synergy. In this study, a surface mechanical attrition treatment technique has been applied to fabricate a gradient nanostructured bronze alloy (Cu-12Sn-1.5Ni-0.6Fe) with pre-nanoprecipitates. Atomic-scale investigations were performed to capture the microstructural evolution governing the gradient refinement process along the depth direction of the treated alloy. On one hand, the body-centered cubic (bcc) Fe-rich pre-nanoprecipitates are found to be well-retained in the fabricated gradient nanostructured specimen. On the other hand, it is interesting to reveal that the semi-coherent interfacial zones between the bcc Fe-based nanoprecipitates and face-centered cubic (fcc) Cu matrix with the specific distribution of misfit dislocations could facilitate the nucleation and growth of gradient nanotwins along the depth direction. Specifically, the twinning partials could be associated with the dissociations of the misfit dislocations in the semi-coherent interfacial region, which is evidenced by the concomitant fcc-Cu/bcc-Fe/nanotwin triplex. As a result, the synergetic strengthening effects of gradient nanotwins and nanoprecipitates are combined to enhance the strength–ductility synergy and work hardening capacity. Moreover, in situ transmission electron microscope observations were conducted to understand the synergetic strengthening mechanisms of the gradient nanotwins and nanoprecipitates during the dynamic cracking process in the fabricated gradient nanostructured bronze alloy. The strategy of combining the nanoprecipitates and gradient nanostructure synergetic effects in our work might offer a novel pathway for designing high-performance metallic materials.

On the high cycle fatigue resistance of austenitic stainless steels with surface gradient microstructures: Effect of load ratio and associated residual stress modification

The effect of gradient microstructure generated by ultrasonic surface mechanical attrition treatment (SMAT) on the fatigue performance of an austenitic stainless steel has been evaluated in the context of localized and general plasticity by comparing two load ratios: fully reversed tension-compression (TC) RTC = -1 and tension-tension (TT) RTT = +0.1, respectively.After identical SMAT processing conditions, the fatigue limit was enhanced by +17% for RTC while it was reduced by -7% for RTT. Under RTC cyclic loading, self-heating of the specimens and sub-surface martensitic transformation occurred but not for RTT. The residual stress measurements have also revealed that while the stress gradient was smoothed after the RTC fatigue loading, it was completely reverted for the RTT one. In turns, the fatigue crack initiation sites and propagation were modified between the RTC and RTT loading conditions.Considering the stabilized surface residual stress after fatigue loading, the use of a Crossland criterion allowed to explain both the effects of load ratio and SMAT on the high cycle fatigue behavior of the stainless steel. It turns out that under the studied loading conditions the modifications of residual stress state can be considered as the primary factor governing the varying fatigue performances and the observed triggering at different initiation sites.

Effect of bimodal grain size and gradient structure on heterogeneous deformation induced (HDI) stress and mechanical properties of Cu

In this study, two different types of heterogeneous structures were prepared by controlling different surface mechanical attrition treatment (SMAT) time (5 s, 10 s, 30 s, 60 s). The effect of heterogeneous Cu with bimodal grain size structure (BGSS) and gradient structure (GS) on heterogeneous deformation induced (HDI) stress and mechanical properties was investigated systematically. By combining surface morphology with microhardness distribution, it was found that the surface grains of the SMAT-ed Cu (5 s and 10 s) were partially refined to exhibit BGSS, while the surface grains of SMAT-ed Cu (30 s and 60 s) were sufficiently refined to form GS. The load-unload-reload (LUR) tests results showed that HDI stress for SMAT 60s Cu was much higher than that of the SMAT 10s Cu. Furthermore, microstructure characterizations revealed that SMAT-ed Cu with BGSS and GS suppressed strain localization, which resulted in high strength and reasonable ductility.

Low-temperature plasma nitriding of AISI 304 stainless steel with nano-structured surface layer

Abstract A nanostructured surface layer was produced on an AISI 304 stainless steel plate by means of a surface mechanical attrition treatment (SMAT). Low-temperature plasma nitriding of the SMAT stainless steel sample was investigated in comparison with the coarse-grained sample by using structural analysis (X-ray diffraction, scanning electron microscopy, and transmission electron microscopy) as well as mechanical property measurements. It shows a much thicker nitrided layer is formed on the SMAT sample relative to that in the coarse-grained counterpart nitrided under the same condition. The nitrided layer in the SMAT sample is composed of nanostructured austenite expansion and martensite expansion with high supersaturations of nitrogen. The surface hardness and the hardened surface layer thickness, as well as the abrasive and the adhesive wear resistances of the nitrided SMAT sample are much enhanced relative to the nitrided coarse-grained sample.

Effects of thermal aging on mechanical properties and microstructures of an interstitial high entropy alloy with ultrasonic surface mechanical attrition treatment

In this work, indentation tests have been carried out on the surface of an interstitial high-entropy alloy, which is treated with ultrasonic surface mechanical attrition treatment and thermal aging treatment at 400 °C. We present and discuss these two treatments on the mechanical behavior of the material by using flat indentation method. Meanwhile, changes in its microscopic morphology, grain size, phase structure, etc. are investigated using X-ray diffraction, electron backscatter diffraction, etc. and its microstructure evolution mechanism under the long-term high temperature condition is revealed. Flat indentation tests and microstructural investigations reveal that the mechanical behavior of interstitial high entropy alloy is significantly affected by surface mechanical attrition treatment and thermal aging time. The gradient refined and strengthened surface layers are demonstrated to appreciably upgrade the mechanical performance of interstitial high entropy alloy, while thermal aging treatment will lead to a reduction of mechanical performance and a phase transformation.

Combining gradient structure and supersaturated solid solution to achieve superior mechanical properties in WE43 magnesium alloy

• Surface mechanical attrition treatment was employed to successfuly produce a gradient nanostructured layer on WE43 magnesium alloy. • High-resolution TEM was mainly performed to successfully dissect the refinement mechanisms of the gradient nanostructure. • A supersaturated solid solution is formed by re-dissolving the Mg 3 RE second phase into nanostructured α-Mg matrix phase in the gradient layer. • Gradient structure and supersaturated solid solution cooperatively contribute to the extraordinary mechanical properties. In this study, surface mechanical attrition treatment was employed to sucessfully produce a gradient nanostructured layer on WE43 magnesium alloy. X-ray diffraction, energy dispersive X-ray spectrometer, and high-resolution transmission electron microscope observations were mainly performed to uncover the microstructure evolution responsible for the refinement mechanisms. It reveals that the grain refinement process consists of three transition stages along the depth direction from the core matrix to the topmost surface layer, i.e., dislocation cells and pile-ups, ultrafine subgrains, and randomly orientated nanograins with the grain size of ~40 nm. Noticeably, the original Mg 3 RE second phase is also experienced refinement and then re-dissolved into the α-Mg matrix phase, forming a supersaturated solid solution nanostructured α-Mg phase in the gradient refined layer. Due to the cooperative effects of grain refinement hardening, dislocation hardening, and supersaturated solid-solution hardening, the gradient nanostructured WE43 alloy contributes to the ultimate tensile strength of ~435 MPa and ductility of ~11.0%, showing an extraordinary strain hardening and mechanical properties among the reported severe plastic deformation-processed Mg alloys. This work provides a new strategy for the optimization of mechanical properties of Mg alloys via combining the gradient structure and supersaturated solid solution.

Twin density gradient induces enhanced yield strength-and-ductility synergy in a S31254 super austenitic stainless steel

Gradient structure (GS), as a typical heterostructure, is arousing great interest for an improved synergy between the strength and ductility which are mutually conflicting. Recently, a novel design of GS is proposed by taking the density of twins in grains, instead of common grain size, as a gradient variable, showing the key role in strain hardening by the nano-scale twin boundaries. Following this idea, here, a deformation twin-density GS was produced by means of the technique of surface mechanical attrition treatment in a S31254 super austenitic stainless steel. To be specific, the GS consisted of a central coarse-grained (CG) core, with two sides sandwiched by the gradient-structured layer (GL), where the density of deformation twins appears gradient in grains along the depth towards the CG core. The tensile tests show that as compared to CG counterpart, yield strength in GS increases 80% to 0.5 GPa, along with comparable ductility of 36%. The interrupted tensile tests show the presence of mechanical hysteresis loops during each unload-reload cycle, indicative of the generation of hetero-deformation-induced (HDI) stress during tensile deformation. Furthermore, both the HDI stress and HDI strain hardening account for a large proportion of global flow stress and forest hardening. The deformation twins and their evolutions, with the emphasis on their interaction with the dislocations, are investigated in detail by means of EBSD and TEM observations to correlate the mechanical properties. The present results shed light on the crucial role of deformation twins in the twin-density gradient for the synergistic enhancement of both strength and ductility.

Correlation of stacking fault energy with deformation mechanism in Cu–(2, 20)Zn alloys

Copper–zinc alloys with different zinc contents contain different stacking fault energies (SFE). The influence of SFE on the deformation mechanism during surface mechanical attrition treatment was studied in this work. Research results indicate that the deformation mechanism directly correlated with the SFE in Cu–Zn alloys. Deformation twinning plays a paramount role during original deformation in Cu–20Zn (19 mJ/m2). However, for Cu–2Zn alloy with low-medium SFE (38 mJ/m2), the deformation mechanism is dominated by the dislocation slipping. Microbands are the predominant microstructural features in large strain and high strain rate regions for both Cu–Zn alloys in the present study. They are likely to be formed by the splitting of the high-density dislocation walls.

Formation mechanism for the white etching microstructure in the subsurface of the failure pearlite wheel steel

The subsurface white etching microstructure (WEM) was an important reason to cause the rolling contact fatigue (RCF) failure of wheel materials. The WEM formation mechanism in the subsurface layers of failed pearlite steel wheel was analyzed by using a scanning electron microscope, transmission electron microscope, and nanoindenter. The results showed that RCF cracks were mostly at the interface between the WEM and pearlite matrix. A large amount of the WEM was found in areas with no RCF cracks. This indicates that the WEM was formed prior to the RCF cracks. The WEM was comprised of nanostructured ferrite grains and residual cementite particles, and its hardness (approximately 920 HV) was about 3 times as high as that of the pearlite matrix. A soft area model for WEM formation is proposed based on the microstructural characteristics of two types of ferrite grains and the dissolution of cementite in pearlite resulting from continuous plastic deformation. During the wheel–rail contact, continuous plastic deformation in the subsurface under high contact stress conditions causes the successive refinement of the proeutectoid ferrite (PF) with low strength and ferrite grains in pearlite, the dissolution of cementite, and the improvement in the corrosion resistance, which cause the WEM to look white under an optical microscope. The WEM formation processes induced by alternating fatigue load were similar to that of surface nanocrystallization through surface mechanical attrition treatment. The great difference between the WEM and the pearlite matrix in the strength results in the crack propagating along their interface, therefore the nonuniform distribution of the WEM along both edges of RCF cracks was found commonly.

High-temperature oxidation behaviour of nanostructure surface layered austenitic stainless steel

The present study investigates the high-temperature oxidation behaviour of nanostructure surface layered AISI 304L stainless steel. A severely deformed layer of ∼300 μm thickness, consisting of nanoscale grains (∼40 nm size) in the topmost region, is successfully developed using the surface mechanical attrition treatment (SMAT) process. The SMATed layer is substantially stable up to 700 °C; however, the surface hardness is reduced by ∼37% at 800 °C for 25 h oxidation duration. Glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy analysis revealed the considerable difference in the chemistry and elemental distribution across the oxide scale of SMATed and non-SMATed specimens. Adherent, denser, and thinner scale, dominated by nanocrystals of Cr- and Mn-rich oxides, is formed on the SMATed steel. However, the Fe-oxide dominated scale containing micro-crystals is found on the non-SMATed specimens, which shows noticeable exfoliation. A high density of grain boundaries and lattice defects in the SMATed layer display admirable reactive diffusion properties of Cr and Mn during oxidation of steel, instigating the formation of a protective oxide scale. The SMATed specimens exhibit multiple zones in the oxide scale: (i) Cr/Mn depleted outer layer, (ii) Cr-/Mn-rich inner layer, and (iii) gradually decreasing Cr/Mn region.

Diffusion Bonding of Ti6Al4V at Low Temperature via SMAT

Titanium alloys used to be welded to gain good joint strength at 920 °C through diffusion bonding. However, due to the heat preservation at high temperatures for a long time, we obtain joints with great bond strength while the mechanical properties of the sheet are lost. In this paper, taking Ti6Al4V alloy as an example, we studied the microstructure of the surface under the different times of surface mechanical attrition treatment (SMAT). In addition, the microstructure and mechanical properties after diffusion bonding at 800 °C-5 MPa-1 h were also conducted. The results show that the shear strength of TC4 alloy welded joint after SMAT treatment is improved, and the maximum shear strength can reach 797.7 MPa, up about 32.4%

Multiple-Shot Impact Model for Vibration-Assisted SMAT Process

Surface mechanical attrition treatment enhances the mechanical properties of metallic materials by inducing high strength layer on the top surface. In this study, multiple-shot impact behavior was modeled for the 7075-T6 aluminum alloy to achieve maximum magnitudes of equivalent stress, plastic strain, residual stress depth, and residual stress. Finite element simulations have been carried out to investigate the effect of selected framework on stress and strains in constituent. The plastic deformation process during SMAT was analyzed using ANSYS/AUTODYN explicit dynamic solver according to shot velocity and diameter with a dynamic explicit finite element method (FEM). Deformation behavior was evaluated after multiple-shot impact.

Using a two-step method of surface mechanical attrition treatment and calcium ion implantation to promote the osteogenic activity of mesenchymal stem cells as well as biomineralization on a β-titanium surface.

Surface treatment is known as a very efficient measure by which to modulate the surface properties of biomaterials in terms of grain structure, topography, roughness and chemistry to determine the osseointegration of implants. In this work, a two-step method of surface modification was employed to impart high osteogenic activity and biomineralization capacity on a Ti–25Nb–3Mo–2Sn–3Zr alloy (a type of β-titanium named TLM). The preliminary surface mechanical attrition treatment (SMAT) refined the average grain size from 170 ± 19 μm to 74 ± 8 nm in the TLM surface layer and promoted the surface to be much rougher and more hydrophilic. The subsequent Ca-ion implantation did not change the surface roughness and topography obviously, but enhanced the surface wettability of the SMAT-treated TLM alloy. The in vitro evaluations of the adhesion, proliferation, osteogenic genes (RUNX2, ALP, BMP-2, OPN, OCN and COL-I) and protein (ALP, OPN, OCN and COL-I) expressions, as well as extracellular matrix (ECM) mineralization of mesenchymal stem cells (MSCs) revealed that the initial SMAT-treated sample significantly enhanced the adhesion and osteogenic functions of MSCs compared to an untreated TLM sample, and the subsequent introduction of Ca ions onto the SMAT-derived nanograined sample further promotes the MSC adhesion, proliferation, osteo-differentiation and ECM mineralization due to the adsorption of more proteins such as laminin (Ln), fibronectin (Fn) and vitronectin (Vn) on the surface, as well as the increase in extracellular Ca concentrations. In addition, the biomineralization capacity of the samples was also evaluated by soaking them in simulated bodily fluid (SBF) at 37 °C for 28 days, and the results showed that the Ca-ion implanted sample significantly boosted the deposition of Ca and P containing minerals on its surface, which was associated with the generation of more Ti–OH groups on the surface after ion implantation. The combination of the SMAT technique and Ca-ion implantation thus endowed the TLM alloy with outstanding osteogenic and biomineralization properties, providing a potential means for its future use in the orthopedic field.

Microstructure Evolution of the Carbon Steels During Surface Severe Plastic Deformation

The review is devoted to the state-of-the-art views on the microstructure evolution in structural and tool carbon steels during the surface severe plastic deformation (SPD). The main focus is on the effects of the nanocrystallization in the near-surface area of the low-carbon steel (C 0.05–0.2%), medium-carbon steel (C 0.35–0.65%), and high-carbon steel (C 1.0–1.5%). It is reviewed the following advanced surface SPD methods for the metal surfaces in recent years: an ultrasonic impact peening (UIP), high-frequency impact peening (HFIP), air blast shot peening (ABSP), surface mechanical attrition treatment (SMAT), and laser shock peening (LSP). Microstructure evolution before and after SPD is studied by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effects of the SPD parameters on the nanocrystalline modification of such main phase components of the carbon steels as ferrite, pearlite, and cementite are analysed. The atomic mechanism of the nanocrystallization is presented. The strain-hardening effect induced by SPD is demonstrated by the data of the near-surface microhardness profiles.

Aerodynamic Performance of a Nanostructure-Induced Multistable Shell

Multistable shells that have the ability to hold more than one stable configuration are promising for adaptive structures, especially for airfoil. In contrast to existing studies on bistable shells, which are well demonstrated by the Venus flytrap plant with the ability to feed itself, this work experimentally studies the aerodynamic response of various stable configurations of a nanostructure-induced multistable shell. This multistable shell is manufactured by using nanotechnology and surface mechanical attrition treatment (SMAT) to locally process nine circular zones in an original flat plate. The aerodynamic responses of eight stable configurations of the developed multistable shell, including four twisted configurations and four untwisted configurations with different cambers, are visually captured and quantitively measured in a wind tunnel. The results clearly demonstrate the feasibility of utilizing different controllable configurations to adjust the aerodynamic performance of the multistable shell.

Corrosion, Wear and In-vitro Biocompatibility Property of Surface Mechanical Attrition Treatment Processed Ti-6Al-4V Alloy

Corrosion, Wear and In-vitro Biocompatibility Property of Surface Mechanical Attrition Treatment Processed Ti-6Al-4V Alloy

Gradient Microstructure Induced by Surface Mechanical Attrition Treatment in Grade 2 Titanium Studied Using Positron Annihilation Spectroscopy and Complementary Methods.

Microstructural changes in grade 2 titanium generated by surface mechanical attrition treatment (SMAT) were studied using positron annihilation lifetime spectroscopy and complementary methods. A significant increase in the mean positron lifetime indicated many lattice defects introduced by SMAT. Two positron lifetime components were resolved in the positron lifetime spectra measured. The longer lifetime revealed the presence of vacancy clusters containing about 3 or 4 vacancies, while the shorter one was attributed to the annihilation of positrons trapped at dislocations. The changes of the positron lifetime indicated a decreasing dislocation density and the presence of a deeper layer with a higher concentration of vacancy clusters at the distance from the treated surface for which the microhardness approached the value for the strain-free matrix. Electrochemical impedance spectroscopy showed the positive effect of SMAT on the corrosion resistance of the titanium studied in a saline environment also after removal of the original oxide layer that was formed during the SMAT.

Influence of co-existing medium Mn and dual phase steel microstructures on ductility and Lüders band formation

The mechanical property spectrum of steels can be expanded to new limits through combining the characteristic microstructures of different steel grades in a single steel. Here, we demonstrate this microstructural grading concept in a single steel sheet that possesses microstructures representative of two steel grades, i.e., the duplex medium Mn steel and the dual-phase steel, in the form of a multi-layered structure. This graded steel was produced by applying the surface mechanical attrition treatment to a duplex medium Mn steel with metastable retained austenite. The resulting graded steel collects the respective advantages of the two steels: high ductility of the former, and the Lüders-band-free plastic flow of the latter. By carrying out systematic experiments and finite element simulations, we reveal that the suppression of Lüders band formation in this graded steel is due to the high early-stage strain hardening rate of the dual-phase steel microstructure, which offsets the post-yield strain softening tendency of the medium Mn steel microstructure. The ductility, on the other hand, results from the transformation-induced plasticity effect of the medium Mn steel microstructure, which enhances the overall strain hardening rate and suppresses the necking of the dual-phase steel microstructure at large strain levels.

Gradient Microstructure Design in Stainless Steel: A Strategy for Uniting Strength-Ductility Synergy and Corrosion Resistance.

Martensite transformation and grain refinement can make austenitic stainless steel stronger, but this comes at a dramatic loss of both ductility and corrosion resistance. Here we report a novel gradient structure in 301 stainless steel sheets, which enables an unprecedented combination of high strength, improved ductility and good corrosion resistance. After producing inter-layer microstructure gradient by surface mechanical attrition treatment, the sheet was annealed at high temperature for a short duration, during which partial reverse transformation occurred to form recrystallized austenitic nano-grains in the surface layer, i.e., introducing extra intra-layer heterogeneity. Such 3D microstructure heterogeneity activates inter-layer and inter-phase interactions during deformation, thereby producing back stress for high yield strength and hetero-deformation induced (HDI) hardening for high ductility. Importantly, the recrystallized austenitic nano-grains significantly ameliorates the corrosion resistance. These findings suggest an effective route for evading the strength–ductility and strength–corrosion tradeoffs in stainless steels simultaneously.