Nanocrystalline Surface Layer Research Articles

Preparation and cavitation erosion resistance of nanocrystalline surface layer on 304 stainless steels

Cavitation erosion is a common damage form of hydraulic components. Constructing a cavitation-erosion-resistant layer on the surface of hydrodynamic components can effectively prolong the service life of the equipment. In this study, 304 austenitic stainless steel was machined by a diamond blade under specific processing parameters, and a nanocrystalline layer (304-Nano) was formed by the work hardening during the machining process. The machining process also allowed strain-induced martensitic transformation, creating martensite in the nanocrystalline layer. The nanocrystalline layer had an average grain size of approximately 34.7 nm, showing a Vickers hardness of 600 ± 30 Hv, which was much higher than that of the original 304 austenitic stainless steel (230 ± 14 Hv). After the 15-h cavitation erosion test, the cavitation erosion rate of the 304-Nano sample was only 19.4 % that of the original ordinary 304 stainless steel, showing that the 304-Nano sample has excellent cavitation erosion resistance. Furthermore, the in-situ SEM results showed that the 304-Nano sample could significantly inhibit crack propagation, which was probably attributed to its high density of grain boundaries and ultrahigh hardness. The excellent cavitation erosion resistance of the nanocrystalline layer is expected to be used as a protective material for anti-cavitation erosion applications.

Improving the fatigue resistance of plasma electrolytic oxidation coated titanium alloy by ultrasonic surface rolling pretreatment

Plasma electrolytic oxidation (PEO) damages fatigue strength of titanium alloy, ultrasonic surface rolling process (USRP) pretreatment is used to mitigate this adverse effect. The microstructure, stress distribution and fatigue performance of Ti6Al4V alloy with different treatment are tested by FESEM, TEM, X-ray stress tester and rotating-bending fatigue tests, and anti-fatigue mechanism of co-strengthened sample is discussed. USRP treatment refines surface grains and introduces a compressive stress field of above 560 μm, which helps to improve the compactness and reduce surface roughness of PEO coating, and significantly increase fatigue resistance of PEO coated samples. Under covering the coating of 20 μm, the fatigue life of co-strengthened sample is about 14 times higher than that of simple PEO sample at 600 MPa, and fatigue crack source also changes from multi-source to single-source and transfers to the subsurface. PEO consumes part of surface nanocrystalline layer and releases some compressive stress for ultrasonic-rolled sample, but the effective compressive stress field with a thickness of 180 μm is still present, and the grains at coating/substrate interface also remain at the nanoscale (∼81 nm), which are the key reasons for USRP to improve fatigue resistance of titanium alloy treated by PEO.

Surface nano-crystallization and its effect on the microstructure evolution and mechanical properties of lightweight Fe–28Mn–8Al–1C steel subjected to supersonic fine particle bombardment

The surface nano-crystallization of lightweight Fe–28Mn–8Al–1C steel was achieved via the supersonic fine particle bombardment (SFPB) technique. The effects of impact time of SFPB and gas pressure on the surface morphology, microstructure, and mechanical properties were systematically investigated. The results indicated that SFPB treatment can form a gradient nanostructure on the surface of the lightweight Fe–28Mn–8Al–1C steel. With a gas pressure of 1.0 MPa and an impact time of 60 s, the surface roughness was minimized, and the corresponding strength parameters reached their highest values. With increases in the impact time and gas pressure, the grain size of the surface nanocrystalline layer gradually decreased, whereas the surface microhardness and the depth of the severe plastic deformation layer increased. However, excessively long impact time or excessively high gas pressure led to the formation of microcracks, resulting in a decrease in the strength. After SFPB treatment, lightweight Fe–28Mn–8Al–1C steel still exhibited good plasticity with elongation above 46 %, but the tensile fracture morphology changed from ductile fracture (before SFPB) to mixed ductile and brittle fracture (after SFPB).

Effect of ultrasonic vibration extrusion cutting on the corrosion resistance of ultra-fine grained strips

Ultrasonic vibration extrusion cutting (UVEC), a new processing method, is proposed to prepare ultra-fine grained materials with better corrosion resistance. The mathematical models, finite element simulations, and experiments are combined and validated against each other to investigate the mapping relationships and influence mechanisms among ultrasonic amplitude, stress field parameters, microstructure, and corrosion resistance. It is found that with the increase of ultrasonic amplitude, the equivalent strain, equivalent strain rate, grain refinement degree of strip and thickness of a nanocrystalline surface layer on the side of the strip in contact with a tool (front side) increase gradually. The immersion corrosion test shows that both sides of the strip have better corrosion resistance than original matrix, and the corrosion resistance of the strip front side is better than that of strip side in contact with the extrusion block (back side). The electrochemical corrosion test shows that with the increase of ultrasonic amplitude, the electrochemical parameters are improved. It is determined that the corrosion resistance of ultra-fine grained strips prepared by UVEC is significantly improved compared with the original matrix, and the overall corrosion resistance is best at 2 μm ultrasonic amplitude.

Mechanical properties of ultra-fine grained strips under ultrasonic vibration extrusion cutting

Ultrafine grained materials often have superior mechanical properties. In order to complement the advantages of other preparation methods of ultrafine materials, a new process of ultrasonic vibration extrusion cutting was proposed to induce severe plastic deformation of materials to prepare ultrafine grained strips with superior mechanical properties. The mathematical model, finite element simulation and cutting test were combined to validate each other. The mapping relationship and influence law among process parameters, physical field parameters, microstructures and mechanical properties were studied. After ultrasonic vibration extrusion cutting, average grain size of material was refined from tens of microns to ultra-fine grain level. With the decrease of extrusion ratio, thickness of nanocrystalline surface layer, degree of grain refinement, average microhardness and tensile strength of strip increase gradually. When extrusion ratio is 1.2, 1.5, 1.8 and 2.1, thickness of nanocrystalline surface layer is 52.9 μm, 44.1 μm, 26.5 μm and 22.8 μm, respectively. Compared with original matrix, average internal microhardness of strip is increased by about 39.9%, 37.6%, 32.8%, 24.1%, and tensile strength is increased by about 306.9%, 273.7%, 254.1%, 240.9%, respectively.

Effect of Heat Treatment on the Cavitation Erosion Behavior of Nanocrystalline Surface Layer of 304 Stainless Steel

In this study, a nanocrystalline layer composed primarily of martensite phase was prepared on the surface of 304 stainless steel. Furthermore, the martensite phase content in the nanocrystalline layer was adjusted by heat treatment at 500 °C and 550 °C, respectively, and the cavitation erosion resistance of the nanocrystalline layer before and after heat treatment was investigated. The results showed that the nanocrystalline layer before and after heat treatment exhibited excellent erosion resistance, with cumulative mass loss of approximately 1/7, 1/5, and 1/3 that of the traditional 304 stainless steel, respectively. The nanocrystalline layer could significantly inhibit the growth of cavitation pits due to the high density of grain boundaries. However, due to the decrease in hardness of the nanocrystalline layer after heat treatment, the propagation speed of cavitation cracks was accelerated, and the cavitation erosion performance of the nanocrystalline layer showed a downward trend.

Microstructure, thermal stability and corrosion behavior of ultrasonic shot peened AA7034

Microstructure, thermal stability at room temperature and corrosion behavior of ultrasonic shot peened AA7034 were investigated in this work. Microstructure modification induced by ultrasonic shot peening was obtained by using OM, SEM, EBSD and XRD, and it was found that a ∼200 μm thick gradient surface layer with nano-scale grains on the topmost layer was induced on the peened sample. XRD peaks of the pre-existing aging-induced precipitates, i.e., η′ and η phases, disappeared after peening and did not show up during 52 months-long natural aging treatment, showing good thermal stability at room temperature. After peening, a significant improvement in intergranular corrosion resistance is evident. Electrochemical characterizations like open circuit potential and electrochemical impedance spectroscopy were conducted on various depths of the peened sample and the results indicated that uniform corrosion rate was largely accelerated by the presence of foreign Fe-rich contamination, implying that modifications should be made to reduce/avoid the foreign contamination. Additionally, mechanisms of corrosion improvement and good thermal stability of the nanocrystalline surface layer of AA7304 are also discussed.

Thermal stability of nanocrystalline surface layer in an aged Al–Zn–Mg–Cu alloy induced by ultrasonic surface rolling processing

This paper investigates the thermal stability of nanocrystalline surface layer in an aged Al–Zn–Mg–Cu alloy fabricated by ultrasonic surface rolling process (USRP). The microstructure of the nanocrystalline surface layer and the microstructure changes of grains and precipitates in the nanocrystalline surface layer after annealing at various temperatures were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The microhardness after annealing was also analyzed. The results show that a nanostructured lamella with a thickness of about 40 nm was prepared in the surface layer of the aged Al–Zn–Mg–Cu alloy after USRP. The granulation of the nanostructured lamellae occurs during subsequent annealing due to recovery and becomes more significant with the increase of annealing temperature. Besides, the coarsening η phases occur at the lamellae boundaries by heterogeneous nucleation. Meanwhile, the precipitate inside lamellae gradually changes from GP zone and η' phase to η phase. The thermal stability of the nanostructured lamellae is excellent below 160 °C, but it deteriorates significantly when the temperature exceeds 200 °C. The hardness variation of the nanostructured lamellae is mainly determined by the variation of lamellae thickness and the precipitation behavior plays an auxiliary role.

Effect of ultrasonic surface rolling process on the surface properties of CuCr alloy

A nanocrystalline surface layer was fabricated on the surface of CuCr8 alloy by surface ultrasonic rolling treatment (USRT) with different rolling depth. The surface morphology, microstructure, hardness, surface strength and corrosion resistance were systematically characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the corresponding test methods. The results showed that the USRT treatment did not change the surface phase of the CuCr8 alloy, but only refined the surface grain. The rolling depth had great influence on the surface properties of the CuCr8 alloy. When the rolling depth was 0.15 mm, the surface comprehensive performance was the best. The surface grain size was about 78 nm, the surface hardness was up to 120 HV0.05, the tensile strength reached to 400 MPa, and the corrosion performance was significantly enhanced. The content of Cr, CuO and Cu(OH)2 in the passivation film of the USRT treated alloy was higher than that of the CG alloy. The USRT treatment provides a new method for fabricating CuCr alloy vacuum break tubes with high surface properties.

Effect of Surface Nano-crystalline Layer Formed by Heavy Plastic Deformation Process on Rolling Contact Fatigue

Effect of Surface Nano-crystalline Layer Formed by Heavy Plastic Deformation Process on Rolling Contact Fatigue

Machining-induced surface integrity and nanocrystalline surface layers in cryogenic finishing turning of Inconel 718

In this study, the effects of cryogenic and flood cooling on the surface integrity of Inconel 718 are investigated for face turning with four selected cutting speeds of 25, 50, 75 and 100 m/min. Surface integrity of machined samples was characterized in terms of surface morphology, sub-surface microstructure, microhardness, x-ray diffraction textures, and residual stresses. While the differences between cryogenic and flood cooling were relatively limited for the majority of surface integrity metrics, a substantially increased (+80% vs flood condition) nanolayer depth was observed at the highest cutting speed of 100 m/min with cryogenic cooling. Additionally, cryogenic cooling resulted in slightly improved surface roughness and slightly increased compressive residual stress, particularly at elevated cutting speeds. Nb-rich secondary phases were detected after machining for all conditions, however, cryogenic cooling and low cutting speed led to reduced mixing of these nanocrystalized phases in the recrystallized surface layer. Based on these observations a and qualitative model for surface generation and nanocrystallization under flood and cryogenic machining conditions was proposed. Overall, the effect of cryogenic cooling on nanolayer generation was most pronounced at elevated speeds, suggesting the potential for cryogenic cooling to allow for more aggressive, yet sustainable, processing strategies with improved surface integrity.

Nanocrystalline Surface Layer of WO3 for Enhanced Proton Transport during Fuel Cell Operation

High ionic conductivity in low-cost semiconductor oxides is essential to develop electrochemical energy devices for practical applications. These materials exhibit fast protonic or oxygen-ion transport in oxide materials by structural doping, but their application to solid oxide fuel cells (SOFCs) has remained a significant challenge. In this work, we have successfully synthesized nanostructured monoclinic WO3 through three steps: co-precipitation, hydrothermal, and dry freezing methods. The resulting WO3 exhibited good ionic conductivity of 6.12 × 10−2 S cm−1 and reached an excellent power density of 418 mW cm−2 at 550 °C using as an electrolyte in SOFC. To achieve such a high ionic conductivity and fuel cell performance without any doping contents was surprising, as there should not be any possibility of oxygen vacancies through the bulk structure for the ionic transport. Therefore, laterally we found that the surface layer of WO3 is reduced to oxygen-deficient when exposed to a reducing atmosphere and form WO3−δ/WO3 heterostructure, which reveals a unique ionic transport mechanism. Different microscopic and spectroscopic methods such as HR-TEM, SEM, EIS, Raman, UV-visible, XPS, and ESR spectroscopy were applied to investigate the structural, morphological, and electrochemical properties of WO3 electrolyte. The structural stability of the WO3 is explained by less dispersion between the valence and conduction bands of WO3−δ/WO3, which in turn could prevent current leakage in the fuel cell that is essential to reach high performance. This work provides some new insights for designing high-ion conducting electrolyte materials for energy storage and conversion devices.

Modelling the Shear Banding in Gradient Nano-Grained Metals.

Extensive experiments have shown that gradient nano-grained metals have outstanding synergy of strength and ductility. However, the deformation mechanisms of gradient metals are still not fully understood due to their complicated gradient microstructure. One of the difficulties is the accurate description of the deformation of the nanocrystalline surface layer of the gradient metals. Recent experiments with a closer inspection into the surface morphology of the gradient metals reported that shear bands (strain localization) occur at the surface of the materials even under a very small, applied strain, which is in contrast to previously suggested uniform deformation. Here, a dislocation density-based computational model is developed to investigate the shear band evolution in gradient Cu to overcome the above difficulty and to clarify the above debate. The Voronoi polygon is used to establish the irregular grain structure, which has a gradual increase in grain size from the material surface to the interior. It was found that the shear band occurs at a small applied strain in the surface region of the gradient structure, and multiple shear bands are gradually formed with increasing applied load. The early appearance of shear banding and the formation of abundant shear bands resulted from the constraint of the coarse-grained interior. The number of shear bands and the uniform elongation of the gradient material were positively related, both of which increased with decreasing grain size distribution index and gradient layer thickness or increasing surface grain size. The findings are in good agreement with recent experimental observations in terms of stress-strain responses and shear band evolution. We conclude that the enhanced ductility of gradient metals originated from the gradient deformation-induced stable shear band evolution during tension.

Effect of grain size and crystallographic structure on the corrosion and tribocorrosion behaviour of a CoCrMo biomedical grade alloy in simulated body fluid

CoCrMo alloys are used in hip and knee replacements due to their excellent long-term survival rates. However, high failure rates have recently been observed associated with adverse tissue reactions. CoCrMo alloy surfaces undergo microstructural changes during wear, including the formation of ε-martensite and, occasionally, a nanocrystalline surface layer. It is not clear whether these changes are beneficial or detrimental to the performance of the component. Thus, high-pressure torsion (HPT) was employed to produce different grain sizes and crystallographic structures in a CoCrMo alloy and the corrosion and tribocorrosion behaviour were critically investigated as a function of grain size. The results reveal a degradation of the corrosion resistance for the HTP processed samples. The contributions of mechanical and corrosion material loss in tribocorrosion is also examined.

Effect of surface nanocrystallization on fatigue properties of Ti–6Al–4V alloys with bimodal and lamellar structure

High-energy shot peening (HESP) was employed to obtain a nanocrystalline surface layer in Ti–6Al–4V alloy with bimodal and lamellar structure. The microstructure and residual stress after HESP treatment were characterized by transmission electron microscope (TEM) and X-ray diffractometer (XRD), respectively. The fracture morphology of fatigue was analyzed by scanning electron microscope (SEM). The results showed that the average grain sizes of Ti–6Al–4V alloy with bimodal and lamellar structure were 18.9 nm and 17.1 nm at the topmost surface layer, respectively. The maximum compressive residual stress of the bimodal structure was −906MPa, which was higher than that of the lamellar structure (−859 MPa). The fatigue limits of the HESP-treated Ti–6Al–4V alloy with the bimodal and lamellar structure were increased by 13.5% and 32.8%, respectively. Finally, a Goodman equation was employed to calculate the local fatigue strength and analyze the residual stress-fatigue strength correlation. Highly defected substructure, reduced effective slip length and improved yield strength were important factors for the improvement of fatigue strength. The synergy effect of surface gradient nanostructure and compressive residual stress improved the fatigue limit of the HESP-treated Ti–6Al–4V alloy with bimodal and lamellar structure.

Surface Boriding and Titanization Stainless Steel by Integrated Processes

The phase and element composition, defect sub-structure in the surface layer of stainless high-chromium steel 12Cr18Ni10Ti (0.12% C, 1.8% Cr, 10% Ni, 1% Ti) alloyed in the electric explosion have been investigated in an X-ray analysis and with the help of optical, scanning and transmission electron microscopy in the diffraction mode. Titanium and boron have been selected as alloying elements. Once a steel surface modified via the electrical explosion alloying is irradiated by an intensive pulsed electron beam, a multi-phase sub-micro nano-crystalline surface layer (up to 60 μm) with no micro-pores and micro-craters is produced. The research has demonstrated two-element compounds only form in the modified layer of steel. The microhardness of a modified layer is related to a part of titanium borides in the surface of steel, being more than 18 times higher after the electrical explosion alloying than in as delivered state. As a result of the subsequent irradiation by an electron beam, a surface layer is formed in the steel alloyed via the electrical explosion; the microhardness and wear resistance of this layer are seven and nine times, respectively, higher than these parameters in as delivered state.

Formation Mechanism of Micro- and Nanocrystalline Surface Layers in Titanium and Aluminum Alloys in Electron Beam Irradiation

The reported study discusses the formation of micro- and nanocrystalline surface layers in alloys on the example of Ti-Y and Al-Si-Y systems irradiated by electron beams. The study has established a crystallization mechanism of molten layers in the micro-and nanodimensional range, which involves a variety of hydrodynamic instabilities developing on the plasma–melt interface. As suggested, micro- and nanostructures form due to the combination of thermocapillary, concentration and capillary, evaporation and capillary and thermoelectric instabilities. This mechanism has provided the foundation for a mathematical model to describe the development of structures in focus in the electron beam irradiation. The study has pointed out that thermoelectric field strength E ≥ 106 V/m is attributed to the occurring combination of instabilities in micro- and nanodimensional ranges. A full dispersion equation of perturbations on the melt surface was analyzed.

Further refinement mechanisms of nanograins in nanocrystalline surface layer of TC17 subjected to severe plastic deformation

Further refinement mechanisms of nanograins in the surface layer of TC17 subjected to severe plastic deformation via high energy shot peening were investigated in detail by using the high-resolution transmission electron microscope. The abnormal face-centered cubic titanium (fcc-Ti) in nanoscale was observed in the surface layer of the HESP processed TC17. Specifically, the lattice constant of the fcc-Ti was of 0.356 nm, and the orientation relationship between the fcc-Ti and the hcp-Ti took as <011>fcc||<011¯1>hcp and {200}fcc||{1¯011}hcp, different from the typical orientation relationship. The two deformation mechanisms, i.e., deformation twinning and dislocation motions including statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs), were both contributed to the further refinement of the nanograins. Due to the extremely high strain rate and stress level during HESP, the stacking faults in the abnormal fcc-Ti were characterized as extrinsic type, namely extrinsic stacking fault (ESF)/E-type stacking fault, which dominated the occurrence of twins. Different from the shear-induced twinning, the deformation twinning occurred in the abnormal nano fcc-Ti was induced by overlapping ESF ribbons, resulting in the further subdivision of nanograins.

Microstructural evolution in the ultrafine-grained surface layer of Mg-Zn-Y-Ce-Zr alloy processed by sliding friction treatment

A nanocrystalline surface layer with average grain size of 98 nm of Mg-6Zn-0.2Y-0.4Ce-0.5Zr (wt%) alloy was fabricated successfully by sliding friction treatment (SFT). The abundant second-phase particles are heavily broken and homogeneously distributed in the matrix after SFT. Electron backscattered diffraction (EBSD) indicates that the intensity of basal texture shows a gradually decreasing trend with the decreasing distance from the treated surface due to the new recrystallized grains with random grain orientation. The process of grain refinement is found to involve the following stages: (i) the twins and dislocations are activated under the low strain area; (ii) the high density of dislocations accumulated near the second-phase particles promote the formation of substructures and the shear bands; (iii) the increasing strain arouses the transformation of substructures; (iv) multiple stacking faults will induce the nucleation of new strain-free grains under the high strain; (v) the nanosized grains are acquired. The abundant fine second-phase particles give large contribution for the refining process due to the pinning effect on the dislocations.

A study of the corrosion behavior of AZ31 Mg alloy in depth direction after surface nanocrystallization

The ultrasonic shot peening (USSP) treatment formed a nanocrystalline surface layer and a gradient microstructure of 200 μm depth. Corrosion test of the USSP treated and certain layers of surface materials being removed by polishing was performed. It was found exfoliated Fe particles during USSP increased the corrosion rate. The Fe particles provided more cathodic reaction sites during the corrosion process, leading to an obviously increased current density of the cathodic reaction. The nanocrystalline resulted in a fast formation of protective film and the high density of grain boundary acted as an effective corrosion barrier, which reduced the current density of the anodic reaction. The work provides an efficient way to investigate the effect of grain size and other factors on the corrosion resistance by measuring the corrosion rate of the gradient nanostructure along the depth.