Surface Mechanical Attrition Treatment Sample Research Articles

Role of gradient nanograined surface layer on corrosion behavior of aluminum 7075 alloy

Gradient nano-grained structures have been a promising technique to evade the strength-ductility trade-off in metals and alloys. Therefore, in this work, the effect of surface mechanical attrition treatment (SMAT) on the microstructure and corrosion behavior of the high-strength aluminum alloy was investigated. SMAT was performed at room temperature and liquid-nitrogen (LN2) flow conditions to generate two distinctly different initial gradient microstructures. Potentiodynamic polarization, electrochemical impedance spectroscopy, and intergranular corrosion tests were performed. Surface film characterization of untreated and treated samples was performed using X-ray photoelectron spectroscopy and time of flight secondary ion mass spectroscopy techniques. Result reveals significant microstructural changes in SMAT processed samples such as the formation of precipitates and dissolution of inherent phases. In addition, a reduced anodic dissolution rate was observed with the SMAT processed samples. Furthermore, the surface film characterization revealed a thicker oxide film with Cu and SiO2 enrichment in SMAT samples.

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.

Investigations on the influence of surface mechanical attrition treatment on the corrosion behaviour of friction stir welded NiTi shape memory alloy

The surface mechanical attrition treatment (SMAT) was applied to friction stir welded NiTi shape memory alloy (SMA) as a post welding treatment to improve the corrosion properties of the weld. The influence of SMAT on corrosion behaviour of friction stir welded NiTi alloy was studied by employing electrochemical impedance spectroscopy and potentiodynamic polarization tests. The SMAT process has improved the corrosion properties of the welded NiTi alloy. Notably, the 1 h SMAT has shown the lowest corrosion rates and its oxide layer exhibited the highest resistance. The 1.15 h SMAT sample has exhibited a noblest breakdown potential of 1331 mV among other samples.

A superior strength-ductility combination in gradient structured Cu–Al–Zn alloys with proper stacking fault energy and processing time

This article aims to investigate the mechanical properties of the gradient structured (GS) Cu–Al–Zn alloys. To prepare the GS samples, three kinds of Cu–Al–Zn alloys with different stacking fault energies (SFEs) were processed by surface mechanical attrition treatment (SMAT) at cryogenic temperature. The results show that the Cu-5.5%Al-4.5%Zn alloy having the lowest SFE processed by SMAT for 5 min exhibits an optimized combination of strength and ductility. For both the initial annealed sample and the SMAT samples processed for the same processing time, uniform elongation (UE) and ultimate tensile strength (UTS) increase with decreasing SFE, whereas the yield strength (YS) is not sensitive to SFE. In the case of the same SFE, as the SMAT processing time increases, the YS and UTS of the GS alloy increase and the UE decrease. The optimized SMAT processing time can sufficiently refine the grains in the GS layer, which can increase the strength and maintain good ductility. Also, sufficient grain refinement promotes the better accumulation of geometrically necessary dislocations (GNDs), thus forming rather effective hetero-deformation induced (HDI) stress strengthening and HDI hardening to enhance strength while maintaining good ductility. The combination of low SFE and optimized SMAT processing time is beneficial to obtain an optimized combination of strength and ductility. Additionally, since the near-surface layer of the GS material is plastically deformed earlier than the sub-surface layer, the accumulation of GNDs at the interface between the two layers also contributes to HDI strengthening and HDI hardening to improve strength and ductility.

Tension–Compression Yield Asymmetry Influenced by the Variable Deformation Modes in Gradient Structure Mg Alloys

Surface mechanical attrition treatment (SMAT) was carried out on hot-rolled AZ31 Mg samples along two orthogonal directions; as a result, two types of gradient structures with different grain sizes and texture components in different layers were produced. The tension–compression yield asymmetry (YA) was studied using samples with different thicknesses, in order to elucidate the effect of combinations of variable deformation modes operating in different layers of the two oriented SMAT samples. The 0° oriented SMAT sample containing layers with strong basal texture displayed significant YA, because of either dislocation slip or extension twinning domination during tension or compression. By contrast, the 90° oriented SMAT sample containing layers with coexisting orthogonal texture components had an obviously weakened YA, which was attributed to the multi-deformation modes cooperating during tension or compression, i.e., extension twinning or detwinning in conjunction with dislocation slips, leading to close yield stresses compared between tension and compression.

Two‐Step Nitriding Behavior of Pure Iron with a Nanostructured Surface Layer

A two‐step gas nitriding treatment (280 °C for 2 h plus 550 °C for 6 h) is designed for pure iron with a nanostructured surface layer induced by surface mechanical attrition treatment (SMAT). The microstructure and surface properties of the SMAT sample after two‐step nitriding treatment are investigated in comparison with those of the SMAT sample after single‐step nitriding treatment (550 °C for 6 h). Experimental results show that a much thicker compound layer (up to 25 μm in thickness) is generated on the surface of the SMAT sample after two‐step nitriding treatment, in which the mean grain size of the Fe2‐3N phase is below 25 nm. The enhanced nitriding kinetic may be attributed to the formation of a large number of Fe(N) solid solutions and a small amount of dispersed Fe2‐3N particles on the SMAT sample after first low‐temperature nitriding, which enhances the thermal stability of nanostructured iron grains. Due to the enhanced nitriding kinetic and refined nitride structure, the SMAT sample after two‐step nitriding treatment displays higher surface hardness and improved wear resistance relative to its single‐step nitrided counterpart.

Strain hardening and tensile behaviors of gradient structure Mg alloys with different orientation relationships

Surface mechanical attrition treatment (SMAT) was employed to produce gradient structures in AZ31B Mg alloy samples with two different initial textures. The structure of both samples can be regarded as an integration of two main layers: the severely deformed layer exhibited dramatic grain refinement to nano- and submicron-scale with weakened and randomized textures; the less deformed layer exhibited the inherited coarse grains with increased dislocation density, possessing the similar texture with the sample prior to SMAT. All the samples containing different layer constituents cut from the SMAT alloys showed remarkable increase of strength compared to the original Mg alloy. However, the two integral SMAT samples with different initial textures exhibited marked difference in uniform elongation (UE) during tension. That was attributed to the different strain hardening behaviors influenced by the deformation coordination and strain partitioning between layers with different orientation relationships. The 0° oriented SMAT sample showed no macroscopic strain transfer or slip transmission across layers, which was hard to cause dislocation accumulation throughout the whole thickness of the layered sample. That resulted in a limited strain hardening and UE. However, the favored orientation relationship between layers of the 45° oriented SMAT sample facilitated the strain transfer and slip continuity across the layers. That generated a dynamically migrating interface during tension, which allowed the dislocations accumulating over the whole sample volume. This caused a pronounced strain hardening and sustained UE to an appreciable value.

Formation mechanism and mechanical properties of surface nanocrystallized Ti–6Al–4V alloy processed by surface mechanical attrition treatment

In the present work, surface mechanical attrition treatment (SMAT) was proposed to achieve surface nanocrystallization on Ti–6Al–4V surface via high-energy planetary ball milling method using a planetary ball mill. The characteristics of microstructure were studied using different methods. Surface nanocrystallization is achieved on Ti–6Al–4V substrate. The process of refinement could be summarized into four steps. During the refinement process, the reticular continuous beta phase performs a significant role, it cracked, broke up and moved to each side along the grain boundaries. The movement of beta grains has the capabilities of effectively optimizing the grain orientation and accelerating the further refinement of alpha grains. Twinning also plays an important role during the refinement. The grain orientation between different types of grains seems to be larger than that of same type grains. The interface will be divided into smaller nanocrystalline grains once the dislocation density breaks the threshold. Then, the balance will be achieved again and owns a higher critical value which cannot be broken, then a stable grain size can be achieved ultimately. The results of microhardness, friction coefficient and wear mass loss tests of SMAT samples indicate that the mechanical behaviors of substrate are greatly enhanced after this novel SMAT treatment.

Enhanced toughness of nitrided layers formed on Ti-6Al-4V alloy via surface mechanical attrition pre-treatment

This study focused on developing a beneficial nitriding process for use on Ti-6Al-4V alloy surfaces, involving surface mechanical attrition treatment (SMAT), which does not contaminate the existing nanocrystalline layer. The average grain size of this layer was refined to 10 nm with random crystallographic orientation. Differential scanning calorimetry confirmed that the recrystallization temperature of the Ti-6Al-4V nanocrystalline layer was 550 °C. The phase compositions, microstructure, hardness and toughness of the nitrided layer were studied by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and hardness testing. The nitrided layer of the SMAT sample was composed of nanoscale TiN, Ti2N and Ti with high nitrogen supersaturation. The nitriding kinetics was enhanced by the nanocrystalline layer at relatively low temperatures (600 °C). Compared to coarse-grained nitrided samples, the SMAT nitrided sample exhibited similar surface hardness but improved surface toughness, which may be attributed to the formation of a nanostructured nitrided layer.

Nanostructured laminar tungsten alloy with improved ductility by surface mechanical attrition treatment

A nanostructured laminar W-La2O3 alloy (WL10) with improved ductility was prepared using a surface mechanical attrition treatment (SMAT). φ1.5 mm ZrO2 WL10 balls subjected to SMAT (called φ1.5 mm ZrO2 ball SMATed WL10) samples possess the best surface profile and excellent integrated mechanical properties (the ductile-brittle transition temperature (DBTT) value decreases by approximately 200 °C, and the bending strength decreases by 100 Mpa). A highly dense group of laminates was detected near the surface of the φ1.5 mm ZrO2 ball SMATed WL10 sample. The SMATed WL10 laminates were composed of a micro-grain layer, an ultrafine-grain layer and a nanosized-grain layer. The nanostructured laminar surface layer of the φ1.5 mm ZrO2 ball SMATed WL10 sample is approximately 1–2 μm. The top surface of the WL10 plates with and without the SMAT process possesses residual compressive stress of approximately −883 MPa and −241 MPa, respectively, in the y direction and −859 MPa and −854 MPa, respectively, in the x direction. The SMAT process could be a complementary method to further improve the toughness of tungsten-based materials.

An Evaluation of a Borided Layer Formed on Ti-6Al-4V Alloy by Means of SMAT and Low-Temperature Boriding.

In this paper, a nanocrystalline surface layer without impurities was fabricated on Ti-6Al-4V alloy by means of surface mechanical attrition treatment (SMAT). The grain size in the nanocrystalline layer is about 10 nm and grain morphology displays a random crystallographic orientation distribution. Subsequently, the low-temperature boriding behaviors (at 600 °C) of the SMAT sample, including the phase composition, microstructure, micro-hardness, and brittleness, were investigated in comparison with those of coarse-grained sample borided at 1100 °C. The results showed that the boriding kinetics could be significantly enhanced by SMAT, resulting in the formation of a nano-structured boride layers on Ti-6Al-4V alloy at lower temperature. Compared to the coarse-grained boriding sample, the SMAT boriding sample exhibits a similar hardness value, but improved surface toughness. The satisfactory surface toughness may be attributed to the boriding treatment that was carried out at lower temperature.

Enhancement of siliconizing behaviors in pure iron induced by surface mechanical attrition treatment

A gradient nanostructured surface layer of about 100μm in thickness was synthesized on the pure iron induced by surface mechanical attrition treatment (SMAT). The average grain size in the top surface is about 20nm. The formation behaviors of Fe-Si compound layer were investigated in nanostructured Fe during the electroless siliconizing process. In comparison with the coarse-grained (CG) sample, the thickness of compound layer on the SMAT sample is about twice larger than the CG sample and the diffusion activation energy decreased from 231kJ/mol to 165kJ/mol for the sample after SMAT 20min. Evident inter-diffusion of elements such as Fe and Si could be found under the EDS analysis, suggested the achievement of metallurgical bonding. The porosity appeared at the interface of matrix and Fe3Si layer in the CG sample due to Kirkendall effect, however, the bonding of the compound layer and matrix was tighter due to the promotion of the elements diffusion in the SMAT sample. The enhanced siliconizing kinetics is attributed to numerous grain boundaries (GBs) in the nanostructured surface and deformed layer.

Enhanced bonding property of cold-sprayed Zn-Al coating on interstitial-free steel substrate with a nanostructured surface layer

By means of surface mechanical attrition treatment (SMAT), a gradient nanostructured surface layer was fabricated on a hot-rolled interstitial-free steel plate. A Zn-Al coating was subsequently deposited on the SMAT sample by using cold spray process. The bonding property of the coating on the SMAT substrate was compared with that on the coarse-grained (CG) sample. Stud-pull tests showed that the bonding strength in the as-sprayed SMAT sample is ∼30% higher than that in the as-sprayed CG sample. No further improvement in bonding strength was achieved in the coated SMAT sample after annealing at 400°C, mostly due to the formation of cracks and intermetallic compounds at the coating/substrate interface in an earlier stage (<30min) and in a final stage (>90min), respectively. The enhanced bonding property of the Zn-Al coating on the SMAT sample might be related with the promoted atomic diffusion and hardness in the nanostructured surface layer.

Strength and ductility of gradient structured copper obtained by surface mechanical attrition treatment

By using surface mechanical attrition treatment (SMAT) at room temperature, a gradient structure (GS) is generated in the surface layer of bulk pure copper samples, which exhibits good uniform elongation and high yield strength simultaneously. Changing SMAT processing time leads to different gradient structures with various component fractions and therefore tune their mechanical properties. The yield strength of the SMAT samples is much higher than the sum of standalone GS layer and coarse-grained (CG) matrix, indicating a synergetic strengthening. Repeated stress relaxation tests were performed to characterize the evolution of mobile dislocations. It was found that the relative mobile dislocation density of SMAT processed sample first drops and then increases with increasing tensile strain. The evolution of mobile dislocations correlates well with strain-hardening evolution. These observations provide insight for the superior combination of high strength and good ductility in SMAT samples.

An Aluminide Surface Layer Containing Lower-Al on Ferritic-Martensitic Steel Formed by Lower-Temperature Aluminization

An aluminide (AlFe and α-(FeAl)) surface layer containing lower-Al was formed on ferritic-martensitic steel P92 by means of surface mechanical attrition treatment (SMAT) combined with a duplex aluminization process at lower temperatures, i.e. a packed aluminization followed by a diffusion annealing treatment below its tempering temperature. Indentation tests indicated that the lower-Al surface layer formed on the SMAT sample is more resistant to cracking and has better adhesion to the substrate in comparison with the Al5Fe2 layer formed on the as-received sample after the duplex aluminization process. Isothermal steam oxidation measurements showed that the oxidation resistance is increased significantly by the lower-Al surface layer due to the formation of a protective (Fe,Cr)Al2O4 layer. The rate constant of oxidation was estimated to decrease from ~0.849 mg2 cm−4 h−1 of the as-received material to ~0.011 mg2 cm−4 h−1 of the AlFe layer at 700 °C.

Low Temperature Boronizing of Surface Nanostructured Ni-Cr-Mo Steel Using SMAT

Boronizing is a surface thermochemical treatment in which boron atoms are made to diffuse into a metallic surface at high temperatures. A nano-crystalline surface with larger defect density assists in enhancing the diffusion rate even at low temperatures. In the present work Ni-Cr-Mo steel is subjected to a surface mechanical attrition treatment (SMAT) to activate the surface with nanocrystalline structures and crystal defects. Subsequently the samples were boronized at low temperature regime (400°C - 600°C) for 5 hours using a pack boronizing technique. The microstructure, chemical analysis and hardness of borided layers were investigated using optical microscope, SEM – EDX and Microvicker’s Hardness Tester. The SMAT treated samples showed severe plastic deformation of the surface, nano-structured grains (10-30 nm) and larger defect density illustrating mechanically activated surface for diffusion. The boronizing had clearly demonstrated the diffusion of boron even at 400°C. The thickness of diffused layer was found to be about 20 µm at 400°C and 50 µm at 600°C for SMAT samples while the untreated samples showed practically no diffusion at 400°C and 12 µm at 600°C. The SEM-EDX results had confirmed the presence of boron at the diffused layer; however the hardness was found to be low. A maximum of 650 HV0.3 was achieved by low temperature boronizing of SMAT treated samples.

Enhanced mechanical properties in Cu–Zn alloys with a gradient structure by surface mechanical attrition treatment at cryogenic temperature

This paper describes the effects of gradient structure on mechanical properties of Cu–Zn alloys processed by surface mechanical attrition treatment (SMAT) at liquid nitrogen temperature (77K). This method leads to the formation of a gradient structure with surface fine-grained regions and a coarse-grained interior. In this study, a fine-grained surface layer with a thickness of 10μm is formed on a coarse-grained Cu–Zn alloy sheet after the cryogenic SMAT process. Microstructural observations and microhardness measurements demonstrate that a significant microstructure refinement in grain size and a gradient increase in hardness from the coarse-grained core to the top surface. Tensile tests at room temperature showed superior strength–ductility synergy for the SMAT sample.

Lower-temperature aluminizing behaviors of a ferritic–martensitic steel processed by means of surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT), a gradient nanostructured surface layer was fabricated on a ferritic–martensitic (F–M) steel plate. Its aluminizing behaviors were investigated during a packed aluminization process and a subsequent diffusion annealing treatment at lower temperatures. In comparison with the initial sample, a much thicker Al5Fe2 layer was formed on the SMAT sample after the packed aluminization, with a growth constant of ~3 times higher at 600°C. The transformation kinetics from Al5Fe2 phase into AlFe phase and α-(Fe,Al) solid solution are also enhanced in the subsequent annealing treatment at ~700°C. The enhanced aluminizing kinetics originates from the increased atomic diffusivities by numerous grain boundaries and a higher concentration of vacancies in the nanostructured surface layer. With the determined growth kinetics, a duplex aluminizing process was demonstrated for achieving a gradient surface layer with lower-Al containing aluminides below the tempering temperature of F–M steel.

The effect of work-hardening and pile-up on nanoindentation measurements

Nanoindentation is performed on the cross-section of copper samples subjected to surface mechanical attrition treatment (SMAT). The cross-section of the SMAT samples provides a unique microstructure with varying amounts of work-hardening depending on the distance from the SMAT surface. Results show that for a given indentation load the pile-up height decreases and the indentation depth increases as the distance from the SMAT surface increases, both following a power law relationship. Based on image analysis of the indented surface this increase in the pile-up height and decrease in indentation depth is attributed to the localization of plastic strain due to the increased resistance to dislocation motion in the work-hardened region. For a given amount of work-hardening (in terms of distance from SMAT surface), the indentation depth increased with the indentation load obeying a power law relationship with the exponent ranging from 0.58 to 0.68. However, the pile-up height increased linearly with the load, with the rate (slope) increasing with the amount of work-hardening. The observed linear increase in pile-up height with indentation load would naturally introduce an indentation size effect (ISE) if the hardness is corrected for the pile-up. Interestingly, this ISE associated with pile-up increased with an increase in indentation depth, in contradiction to the ISE associated with strain gradient. Deviation of the hardness values corrected for pile-up from the bulk behavior due to surface effect is highlighted and a method to obtain a bulk-equivalent hardness quantity representing the bulk behavior is proposed.

Enhanced boronizing kinetics of alloy steel assisted by surface mechanical attrition treatment

A nanostructured surface layer was fabricated on AISI H13 steel by means of surface mechanical attrition treatment (SMAT). Boronizing behaviors of the SMAT samples were systematically investigated in comparison with their coarse-grained counterparts. The boron diffusion depth of the SMAT sample with pack boriding treatment at 600°C for 2h was about 8μm, which was much deeper than that of the coarse-grained sample. A much thicker borided layer on the SMAT sample can be synthesized by a duplex boronizing treatment at 600°C followed by at a higher temperature. The borided layer was composed with monophase of Fe2B and the growth of it exhibited a (002) preferred orientation. Moreover, the activation energy of boron diffusion for the SMAT sample is 140.3kJ/mol, which is much lower than 209.4kJ/mol for the coarse-grained counterpart. The results indicate that the boronizing kinetics can be significantly enhanced in the SMAT sample with a duplex boronizing treatment. Furthermore, the thermal fatigue tests show that the borided layer with excellent oxidation resistance and mechanical strength at elevated temperatures could effectively delay the thermal fatigue cracks initiation and impede their propagation. Therefore, the thermal fatigue property of H13 steel with a duplex boronizing treatment can be improved remarkably.