Nanocrystalline Surface Research Articles

Mechanical property enhancement in gradient structured aluminum alloy by ultrasonic nanocrystalline surface modification

The large strength difference between hard and soft components in heterogeneous structured materials leads to the evolution of high back-stress hardening, which increases the strength and ductility of materials simultaneously. Moreover, the combination of high shear strain and elevated temperature allows an increase in the strength of low-melting temperature metallic alloys by grain refinement, solute migration, and clustering. In this study, to design a new heterogeneous microstructure in the aluminum alloy, both room temperature (RT) and high-temperature (HT) ultrasonic nanocrystalline surface modification (UNSM) were conducted, and their mechanical properties and microstructural evolutions were investigated. The large shear strain from the UNSM treatment reduces the grain size at the sample surface and creates a gradient structure. The combination of shear strain and elevated temperature during UNSM treatment induces solute migration at a certain depth of the specimens, resulting in the nano-sized Mg-rich particles at the surface region. Both grain refinement and precipitation at the surface region of the HT sample provide strong back-stress hardening in the early stages of deformation that enhances the strength and ductility of materials. Therefore, a high shear strain and control of processing temperature allow the design of a unique heterogeneous microstructure in low-melting temperature metallic alloys, which is a good strategy for enhancing the mechanical properties of sheet or thin metallic products.

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.

Ligand-engineered bandgap stability in mixed-halide perovskite LEDs.

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of commercial organic light-emitting diodes-have been demonstrated in both the infrared and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.

Resistance of surface nanostructures and ultrafine grain structures on steel 40Kh to wear and cavitation-erosive destruction

Wear resistance in oil and in conditions of dry friction, as well as resistance to cavitation-erosion destruction (CED) of samples made of steel 40Kh with a surface nanocrystalline and ultrafine grain structure formed by severe plastic deformation (SPD) by mechanical-pulse treatment (MPT) and vibration-centrifugal hardening (VCH) were studied. At the same time, for almost the same microhardness obtained by MPT, VCH forms a significantly greater thickness of the hardened layer, which makes it possible to carry out finishing operations for high-precision parts. It is shown that nanostructures and UFGS on steel 40Kh significantly reduced the friction coefficients of the test pair and its wear resistance under dry friction, as well as in oil, compared with quenching and low tempering, which is especially manifested in an oil medium by more than 2 times. It was conducted comparative studies of the stability of CED and revealed their correlation with wear resistance. At the same time, the resistance to CED depends on the processing modes, which form favorable electrochemical characteristics of the hardened surface layer.

Correction to: “Ultrasonic nanocrystalline surface modification of low strength aluminum alloy: trade-off between surface integrity and production rate aiming at desired fatigue life”

In processing of metallic material by ultrasonic nanocrystalline surface modification (UNSM), obtaining high quality sample with desired surface property needs a careful control on processing factors. The condition would be more critical when the process is designed for match production. In such condition, both of surface integrity and production rate should be taken into account. To meet both of goals, in the present study, an experimental work was made to analyze how the UNSM process factors influences the surface integrity under desired values of production rate. In order to do so, series of experiments based on small fraction central composite design were carried out including static pressure, vibration amplitude, spindle speed, feed rate, and ball size as input parameters. Also, the surface roughness, compressive residual stress, and machining time were considered performance measures. A hybrid optimization approach including gray relational analysis and desirability function was proposed to obtain desired surface integrity under minimum value of machining time. Different criteria based on importance of production rate and surface integrity were separately defined and controlled by variation of weight factors. It is found that considering the production rate as dominant criterion leads to production of samples with maximum high cycle fatigue life. This is due to selection high spindle speed that enhances both production rate and surface quality. On the other hand, considering overall value of surface integrity as dominant criterion of optimization leads to producing sample with desired values of low cycle fatigue life. This effect can be attributed to selection of low spindle speed that deteriorates surface finish but enhances the compressive residual stress.

Optimization of residual stresses generated by ultrasonic nanocrystalline surface modification through analytical modeling and data-driven prediction

In this work, we have developed a fast and robust analytical model based on the Hertzian contact theory and the Neuber's plasticity law to predict the residual stress generated by ultrasonic nanocrystal surface modification (UNSM). The measured residual stresses data for a number of different metals was used to validate the analytical model. Sensitivity studies as well as the linear regression analysis were conducted to study the effects of UNSM parameters (striking amplitude, static load, spacing, scanning speed and temperature) on the distribution of residual stresses and the surface finish. The analysis shows that a balance between residual stress and surface finish needs to be considered when choosing the UNSM parameters: the increase of ultrasonic striking amplitude and static load improve the magnitude and depth of the residual stress at the risk of deteriorating the surface finish; a higher temperature leads to a lower magnitude but deeper residual stress as well as a better surface finish; smaller tip size can significantly increase the magnitude of residual stresses but it also deteriorates the surface finish. Finally, an experimental design based on a large database (with a total of 18876 sets of UNSM parameters) was introduced to find the optimal UNSM parameters for different engineering needs.

Ultrasonic nanocrystalline surface modification of low strength aluminum alloy: trade-off between surface integrity and production rate aiming at desired fatigue life

In processing of metallic material by ultrasonic nanocrystalline surface modification (UNSM), obtaining high quality sample with desired surface property needs a careful control on processing factors. The condition would be more critical when the process is designed for match production. In such condition, both of surface integrity and production rate should be taken into account. To meet both of goals, in the present study, an experimental work was made to analyze how the UNSM process factors influences the surface integrity under desired values of production rate. In order to do so, series of experiments based on small fraction central composite design were carried out including static pressure, vibration amplitude, spindle speed, feed rate, and ball size as input parameters. Also, the surface roughness, compressive residual stress, and machining time were considered performance measures. A hybrid optimization approach including gray relational analysis and desirability function was proposed to obtain desired surface integrity under minimum value of machining time. Different criteria based on importance of production rate and surface integrity were separately defined and controlled by variation of weight factors. It is found that considering the production rate as dominant criterion leads to production of samples with maximum high cycle fatigue life. This is due to selection high spindle speed that enhances both production rate and surface quality. On the other hand, considering overall value of surface integrity as dominant criterion of optimization leads to producing sample with desired values of low cycle fatigue life. This effect can be attributed to selection of low spindle speed that deteriorates surface finish but enhances the compressive residual stress.

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.

Influence of the Surface Nanostructuring of 65G Steel on the Wear Resistance of the Disks of Furrow Openers of Seeding Machines

We study the wear resistance of the disks of furrow-openers of seeding machines. The surface nanocrystalline structure of the disks made of 65G steel is formed by the mechanical-pulse treatment under the conditions of dry abrasive friction. It is shown that, in the tests with nonfixed and fixed abrasives, the procedure of surface nanostructuring makes the wear resistance of steel 5.4 and 8.8 times higher, respectively, than in the initial state. The thermal treatment of steel prior to its surface nanostructuring considerably increases its wear resistance in the tests with fixed abrasives.

The tribological behavior of a surface-nanocrystallized magnesium alloy AZ31 sheet after ultrasonic shot peening treatment

Abstract A magnesium alloy AZ31 sheet was processed by ultrasonic shot peening treatment to fabricate a surface nanocrystalline, and a ball-on-disk dry sliding wear test was performed to evaluate the tribological behavior after treatment. The microstructure observation indicated a gradient nanocrystalline structure was formed after USSP treatment. The microhardness at the top surface was improved from 60 HV to 145 HV after treatment. The formed nanocrystalline resulted in an easy formation of MgO patches on the surface and reduced the coefficient of friction. Moreover, the formed nanocrystalline leaded to a retard of delamination with increasing the sliding speed and applied load, which was due to its stronger sub-surface. Under high sliding speed (0.5 m/s) and high applied load (50 N), it was firstly found that the formed nanocrystalline prevented the happening of thermal softening and melting. The possible reasons accounting for the prevention of thermal softening and melting were discussed accordingly.

Correlation of orientation relationships and strain-induced martensitic transformation sequences in a gradient austenitic stainless steel

Orientation relationships (ORs) play a crucial role in the phase transformation of face-centered cubic austenite (γ-fcc) to body-centered cubic martensite (α′-bcc) upon plastic deformation. So far, ORs between γ-fcc and α′-bcc are reported frequently; however, the correlation to the transformation sequences is not clear. Here, the gradient structured austenitic stainless 304 steel prepared by ultrasonic nanocrystalline surface modification was examined to clarify the ORs that were involved in the strain-induced martensitic transformation (SIMT) behaviors. The results showed changes in the ORs from Nishiyama–Wassermann (N-W) to Kurdjumov–Sachs (K-S) and Pitsch with an increase in depth from the top-treated surface. The OR of α′-bcc formed directly on γ-fcc was identified as N-W, while those via the hexagonal closed packed e-plate (e-hcp) were the K-S and Pitsch as they were nucleated at the single and two e-plates intersections, respectively. Thus, the SIMT sequence was the direct γ → α′ with the presence of the N-W OR in the high strain depth, while it was the γ → e → α′ as the existence of K-S and Pitsch ORs in the low strain depth. The findings provide essential data for the in-depth and comprehensive study of the polymorphic martensitic transformation mechanisms in various steels and alloys.

Organic photosensitizers containing fused indole-imidazole ancillary acceptor with triphenylamine donor moieties for efficient dye-sensitized solar cells

Controlling the morphology of sensitizer on a TiO2 nanocrystalline surface is beneficial to facilitating electron injection and suppressing charge recombination. Given that the N,N-dimethylaniline-substituted imidazole-fused-indole on the middle segment for preventing π aggregation can deteriorate its intrinsic photostability, we incorporate a promising building block of fused-indole-imidazole [(1,4-dihydroimidazo[4,5-b]indole) DHII)] ring as the additional acceptor to construct a novel TD2, TD3 and YD3 with D–(A)π-D, D–(DA2)π-D, D–π-D(A)-π-D architecture, which exhibits several characteristics: (i) possible chelation of imidazole ring (through N) to the titanium ions on the TiO2 surface which can assist in increasing the electron injection into the conduction band of photoanode, (ii) showing a moderate electron-withdrawing capability for an ideal push-pull balance in both promising photocurrent and photovoltage; (iii) endowing an ideal morphology control with strong capability of restraining the intermolecular aggregation and facilitating the formation of a compact sensitizer layer via N,N-dimethylaniline groups grafted onto the fused-indole-imidazole unit. The co-adsorbent-free dye-sensitized solar cell (DSSC) based on dye TD3 exhibits very promising conversion efficiency as high as 6.04 ± 0.01%, with a short-circuit current density (Jsc) of 13.57 mA cm−2, an open-circuit voltage (Voc) of 0.80 V, and a fill factor (FF) of 0.774 under AM 1.5 illumination (100 mW cm−2). TD3-based device showed better performance because of the two anchoring groups, which play a significant role for better adsorption on the TiO2 surface along with the enhanced kinetics of photoexcited electron injection.

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.

Nanograins on Ti-25Nb-3Mo-2Sn-3Zr alloy facilitate fabricating biological surface through dual-ion implantation to concurrently modulate the osteogenic functions of mesenchymal stem cells and kill bacteria

Surface mechanical attrition treatment (SMAT) method is an effective way to generate nanograined (NG) surface on Ti-25Nb-3Mo-2Sn-3Zr (wt.%) (named as TLM), a kind of β-type titanium alloy, and the achieved nanocrystalline surface was proved to promote positive functions of osteoblastic cells. In this work, to further endow the NG TLM alloy with both good osteogenic and antibacterial properties, magnesium (Mg), silver (Ag) ion or both were introduced onto the NG TLM surface by ion implantation process, as a comparison, the Mg and Ag ions were also co-implanted onto coarsegrained (CG) TLM surface. The obtained results show that subsequent ion implantation does not remarkably induce the surface roughness and topography alteration of the SMAT-treated layers, and it also has little impact on the microstructure of the SMAT-derived β-Ti nanograins. In addition, the implanted Mg and Ag ions are observed to exist as MgO and metallic Ag nanoparticles (NPs) embedding tightly in the β-Ti matrix with grain size of about 15 and 7 nm, respectively. Initial cell adhesion and functions (including proliferation, osteo-differentiation and extracellular matrix mineralization) of rabbit bone marrow mesenchymal stem cells (rBMMSCs) and the bacterial colonization of Staphylococcus aureus (S. aureus) on the different surfaces were investigated. The in-vitro experimental results reveal that the Mg and Ag single-ion implanted NG surface either significantly promotes the rBMMSCs response or inhibits the growth of S. aureus, whereas the Mg/Ag co-implanted NG surface could concurrently enhance the rBMMSCs functions as well as inhibit the bacterial growth compared to the NG surface, and this efficacy is more pronounced as compared to the Mg/Ag co-implantation in the CG surface. The SMAT-achieved nanograins in the TLM surface layer are identified to not only play a leading role in determining the fate of rBMMSCs but also facilitate fabricating dual-functional surface with both good osteogenic and antibacterial activities through co-implantation of Mg and Ag ions. Our investigation provides a new strategy to develop high-performance Ti-based implants for clinical application.

Co-TiO2 Nanoparticles as the Reinforcement for Fe Soft Magnetic Composites with Enhanced Mechanical and Magnetic Properties via Pulse Electrodeposition

This study is an attempt to produce surface nanocrystalline composite of Fe-Co-TiO2 at various current densities in the range of 20 to 50 mA/cm2 via pulse electrodeposition method. The prepared composites were characterized by field emission scanning microscope (FESEM), electron dispersive spectrum (EDS), Vickers microhardness, vibrating sample magnetometer (VSM), and x- ray diffraction techniques (XRD). The results showed that the formation of cauliflower morphology was preferred at lower current densities. Moreover, the higher current densities enhanced the Fe content and at the same time diminished the Co and TiO2 contents of prepared surface composites. XRD patterns and Rietveld analysis confirmed the formation of combinations of BCC (as dominant) and FCC phases. Higher current density enhanced the saturation magnetization and decreased lower coercivity due to the higher Fe content and the reduction of TiO2 nanoparticles in coatings. In addition, the lowest coercivity and highest saturation magnetization were gained at 50 mA/cm2, while, the maximum microhardness obtained at 30 mA/cm2.

Investigation on the Effect of Thermal and Mechanical Treatment to the Offshore Corrosion Behavior of 6351 Aluminum Alloy in Red Sea Environments.

This study investigates the effect of artificial aging treatment and mechanical attrition treatment on the corrosion behavior of 6351 Al alloy in Red Sea environment. The artificial aging of the alloy is carried out at temperature range 140°C–240°C in steps of 20°C for various time periods after the solution heat treatment at 530°C for 1 hour. Based on the hardness measurements, the aged specimens are categorized into three, namely, underaged, peak aged, and overaged. The as received alloy specimens are subjected to mechanical attrition treatment in a vacuum chamber using steel balls. Vickers hardness test reveals that there is a remarkable improvement in hardness of mechanical attrition treated specimens compared to that of aged specimens. The aged and mechanical attrition treated specimens were subjected to the corrosion test in Red Sea water using the Autolab instrument. The corrosion tests reveal that the peak-aged composite corrodes more in Red Sea water when compared to that of other groups of specimens. XRD measurements and SEM analysis are carried out to study the surface nature of attrition treated specimens. It is observed that the mechanical attrition treated specimens exhibit a nanocrystalline surface and lead to a decrease in corrosion resistance. However, the annealing of the alloy after attrition treatment optimizes the mechanical and corrosion properties of the alloy.

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.

Optimization of ultrasonic nanocrystalline surface modification of copper and its influence on fatigue life

In processing of metallic material by ultrasonic nanocrystalline surface modification (UNSM), obtaining high quality sample with desired surface property needs a careful control on processing factors. In the present investigation, an attempt was made to find optimal setting of UNSM factors viz static pressure, vibration amplitude, spindle speed, feed rate and ball size on surface integrity factors such as surface roughness and hardness. Number of 32 experiments were conducted using small fraction central composite design to correlate input-output relationship. Grey relational analysis and desirability function was also applied to experimental data to identify a setting corresponds to maximum hardness and minimum surface roughness. The optimum samples were also subjected to fatigue tests as well the microscopic observations to find how much improvement occurs as a result of optimization with respect to as turned sample. It is found from both approaches that the setting of 0.03 MPa static pressure, 10 μm vibration amplitude, 100 rpm spindle speed, 0.08mm/rev feed rate and 8 mm strike size is an optimal parameter setting that results in surface roughness of 0.356 μm and hardness of 155 HV. The sample machined by optimum condition result in fatigue life cycles improvement more than 2.5 times in different levels of applied stress. The improvement can be attributed to combined effect of smoothed surface topography, fine microstructure, hardened surface layers and exerting compressive residual stress.

Control of magnesium in vitro degradation based on ultrafine-grained surface gradient structure using ultrasonic nanocrystalline surface modification

Here, a gradient structure based on an ultrafine-grained Mg surface was prepared using ultrasonic nanocrystalline surface modification (UNSM). UNSM is expected to be commercially attractive because it enhances mechanical properties and does not lead to surface roughening, which promotes corrosion. The modified surface increases corrosion resistance by accelerating the development of compact corrosion products that provide protection. The ultrafine-grained (UFG) surface gradient structure improved corrosion resistance and maintaining strength, which increased as the grain size became smaller. In particular, the UFG structure generated by the UNSM process exhibits a dramatic decrease in corrosion rate compared to the fine-grained structure, resulting in the slope change in the plot of corrosion rate versus grain size in the UFG regime. The surface gradient structure is an optimized approach for early-stage fast bone healing and later-stage fast degradation considering the relationship between grain size, pH, and strength change.