Nanocrystalline Surface Layer Research Articles

Microstructure and Wear Properties of Friction Induced Nano-crystalline Surface Layers of Carbon Steels

Pin-on-disc sliding friction tests were carried out for 0.45 mass % carbon steels, and TEM microstructure and hardness of the sub-surfaces of the friction specimens were investigated. Particularly effects of friction conditions on the microstructure at the surfaces and wear properties of the friction induced microstructure were studied. It was found that ultra-fine equi-axed grains in the 30 - 50 nm size range were produced in the case of a high friction speed of 5.0 m/s in an air atmosphere. Moreover, nano-crystalline microstructure can be produced in a vacuum atmosphere even if the friction speed was low. The friction induced nano-crystalline surface layers, which exhibited significant high hardness, showed good wear resistance.

Dynamic recrystallization in nanocrystalline AZ31 Mg-alloy

A nanocrystalline surface layer of AZ31 Mg alloy was fabricated successfully by sliding friction treatment. The microstructure was characterized by a transmission electron microscopy (TEM). In the process of plastic strain-induced grain refinement, the nucleation and growth of new grains as well as grain split are all observed. These experimental phenomena indicate that discontinuous dynamic recrystallization (dDRX) and continuous dynamic recrystallization (cDRX) are responsible for grain refinement of Mg nanocrystallites. Further analysis reveals that stored strain energy (SSE) plays a critical role in the grain refinement mechanisms (dDRX or cDRX): high SSE facilitates dDRX and low SSE conduces to cDRX.

Simultaneously enhanced strength and corrosion resistance of Mg–3Al–1Zn alloy sheets with nano-grained surface layer produced by sliding friction treatment

Simultaneous improvement of the two disparate properties such as mechanical strength and corrosion resistance of Mg alloys is the object researchers have been pursuing all the time. In this study, by means of sliding friction treatment (SFT), a nanocrystalline surface layer (with an average grain size of 70 nm on the topmost surface) was successfully generated on coarse-grained (CG, mean grain size: 25 μm) Mg–3Al–1Zn (AZ31) alloy sheet. The SFTed AZ31 exhibited simultaneously enhanced mechanical strength and corrosion resistance as compared to the CG counterpart. Specifically, the yield strength (YS) of SFTed sample is 236 MPa, which is almost twice that of CG sample (123 MPa). Meanwhile, the hydrogen evolution rate during 80 h immersion in 3.5% NaCl is reduced from 0.44 ml/cm2/h to 0.17 ml/cm2/h after SFT treatment. Effective grain refinement was responsible for the notable strength increment. The improvement in corrosion resistance might be correlated with SFT-induced grain refinement, good surface quality, second-phase particles fragmentation and strong basal texture. These combined capabilities of SFT render it a commercially viable manufacturing process for comprehensive properties modification of Mg products.

Surface self-nanocrystallization of α+β titanium alloy by surface mechanical grinding treatment

In this work, the heavy deformation was performed on Ti6Al4V alloy by using surface mechanical grinding treatment (SMGT) in order to obtain surface nanocrystalline layer. The phase structure and microstructures in the deformation zones were characterized by XRD, SEM and TEM. The nanocrystallization mechanisms of α and α phase were clarified. The results show that a gradient structure including nano grain, quasi nanograin and micro-grains was achieved within the depth of 500 μm below the surface. The depth of nanocrystallines layer was at least 20 μm. From the non deformation zone in the center to the heavy deformation zone close to treated surface, dislocation densities gradually increased. The original coarse grains gradually evolved into dislocation cell structures and subgrains through dislocation movement. In the depth of about 20 μm below the treated surface, the subgrains began to split into nano-scale grains with high angle grain boundaries by lattice rotation and tilting mechanisms. During SMGT, β phase deformed and refined prior to α phase, and inhibited the generation of twinning in α grain. The coordinated deformation between a and β phase promoted the formation of nanocrystallines.

Correlation of the wear transition in CoCrMo alloys with the formation of a nanocrystalline surface layer and a proteinaceous surface film

CoCrMo alloys have been widely used in metal-on-metal (MoM) hip replacements. On the one hand, they have exhibited excellent long-term survival rates, but recently high failure rates have been observed, associated with adverse local tissue reactions. It is still a puzzle why CoCrMo alloys sometimes work very well, while at the other times the wear rate is unacceptably high. There have been several reports that the formation of a surface proteinaceous layer appears to reduce the wear rate. Equally, the formation of a surface nanocrystalline layer has been observed, but it remains unclear whether this is a good or bad phenomena. The present work directly correlates the surface deformation structure and the presence of a proteinaceous film with the wear behaviour. The lowest specific wear rates were associated with the formation of a proteinaceous layer. A wear transition was observed at higher load, associated with the loss of the proteinaceous layer and the formation of a thick surface nanocrystalline layer. The formation of the proteinaceous layer appeared to be associated with some surface deformation of the CoCrMo, which presumably removed the surface oxide. Although the formation of a thick, fine nanocrystalline layer resulted in the highest surface hardness and yield strength, it also coincided with the highest wear rate.

Strain-Induced Diffusion of Nickel in Nanocrystalline Fe Produced by High Energy Shot Peening

A nanocrystalline surface layer without oxidation, porosity or contamination was obtained by high energy shot peening (HESP) to a pure iron cylinder. The thickness of the nanocrystalline surface layer was more than 100 μm and the average grain size was about 50 nm. The pulse pressure diffusion (PPD) and constant pressure diffusion (CPD) were used to diffuse nickel into nanocrystalline iron at 850 °C on Gleeble 1500. Results show that the diffusion coefficient of Ni in the nanocrystalline Fe is 1 order of magnitude higher than that in coarse-grained Fe and 1 order of magnitude higher than that using CPD. The enhanced diffusivity of Ni may originate from the following three reasons: (1) pulse pressure can break the metallic compound formed during diffusing which may block the diffusion of Ni to inner nanocrystalline Fe; (2) a considerable amount of triple junctions and (3) a large volume fraction of non-equilibrium grain boundaries (GBs) in the present nanocrystalline Fe sample processed by the HESP technique.

Generation of nanocrystalline surface layer in short pulse laser processing of metal targets under conditions of spatial confinement by solid or liquid overlayer

The effect of spatial confinement by a solid or liquid overlayer on short pulse laser-induced surface microstructure modification is investigated in a series of large-scale atomistic simulations performed for Ag targets irradiated in the regime of melting and resolidification, below the thresholds for laser spallation and ablation. For Ag targets with free surfaces, the formation of a nanocrystalline region with random crystallographic grain orientation is observed under irradiation conditions leading to the generation of numerous sub-surface voids that slow down the solidification process. When no voids are generated, the resolidification produces grains misoriented with respect to the bulk of the target by just several degrees and separated from each other by low angle grain boundaries or dislocation walls. The presence of a liquid or solid overlayer suppresses nucleation of sub-surface voids, provides an additional pathway for cooling through the heat conduction to the overlayer, and facilitates the formation of nanocrystalline structure in a region of the metal target adjacent to the overlayer. Moreover, the stabilizing effect of the solid overlayer may result in an incomplete melting of metal in the vicinity of the interface, making it possible for grains growing from the interface to retain “memory” of the target orientation and to produce nanocrystalline interfacial region with small misorientation of grains with respect to the bulk of the target. In all simulations, the nanocrystalline layers generated by laser processing of single crystal Ag targets are characterized by a high density of stacking faults, twin boundaries, and point defects produced in the course of the rapid resolidification.

Thermal stability of surface nanostructure produced by laser shock peening in a Ni-based superalloy

A nanocrystalline layer with the grain size of about 20–200nm in a Ni-based superalloy was fabricated by the means of laser shock peening (LSP). The microstructure characterization of nanocrystalline layer was systematically investigated by transmission electron microscopy (TEM) and nanoindentation. And the thermal stability of surface nanocrystalline layer was studied by in-situ TEM annealing with different temperatures and duration times (500°C/1h, 700°C/1h and 900°C/3h). In addition, we designed a kind of nanoindentation test with the sample being post annealed at 900°C for 10h to verify the thermal stability of nanocrystalline layer. The results indicated that nanostructure has superior thermal stability after annealing at 700°C for 1h, which is higher than the dynamic crystallization temperature, 0.36 Tm (melting temperature Tm is about 1300°C). Grain growth occurred at 900°C, but some nano-grains with the size of around 50nm still existed, and the nanohardness test further strengthened the evidence of good thermal stability. All these experimental results indicated that such Ni-based superalloy with nanocrystalline layer exhibited relatively good thermal stability after annealing at 900°C. Lastly, the mechanism of thermal stability of surface nanostructure produced by LSP in a Ni-based superalloy was discussed in detail.

Neutral Molten Salt-Bath Carburizing of Ti6Al4V Alloy with Nanocrystalline Surface Layer at Low Temperature Assisted by Surface Mechanical Attrition Treatment

Nanocrystalline surface layer about 10~15μm thick was fabricated on the surface of Ti6Al4V sheet by means of the surface mechanical attrition treatment (SMAT). The average grain size was about 10nm and the grain characteristic presented equiaxed morphology. The nanocrystalline surface layer could be perfectly maintained below 550°C in the following thermal stability analysis. Neutral salt mixture was about 21% NaCl, 31% BaCl2 and 48% CaCl2 and additionally 5% Na2CO3 of total was utilized. After carburizing process, a continuous charcoal grey carburized layer was composed of TiC and carbon supersaturated solid solution, the hardening layer was about 10~15μm thick. The hardness of the outermost surface reached 1000HV, which was much higher than its coarse-grained counterpart in the same carburizing condition. The experimental result indicated that the carburizing kinetics was obviously enhanced by nanocrystalline surface layer assistance. Furthermore, the neutral molten salt-bath carburizing was verified that it could be performed in a relatively low temperature of 650°C.

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.

Improved corrosion inhibition of 3-amino-1,2,4-triazole on mild steel electrode in HCl solution using surface nanocrystallization

AbstractA nanocrystalline surface layer with average grain size of 77 nm was produced on mild steel through a wire brushing process. Its corrosion behavior was studied in the absence and presence of 3-amino-1,2,4-triazole (3-AT) as corrosion inhibitor. Although the increase in the inhibitor concentration shows a positive effect on inhibition efficiency of micro- and nano-grained surfaces, this effect is more remarkable for the sample with a nanocrystalline surface layer. In addition, in the presence of the optimum concentration of 3-AT, temperature increment has a more negative effect on the sample with a microcrystalline surface layer. The results also revealed that due to enhanced adsorption of 3-AT inhibitor and formation of a protective layer with a high polarization resistance, the inhibition effect of 3-AT increased as the grain size decreased from microcrystalline to nanocrystalline.

Microstructure and tensile behaviour of pure titanium produced after high-energy shot peening

In this study, the microstructure and tensile behaviour of pure titanium processed by means of high-energy shot peening have been studied. The results show that a nanocrystalline surface layer is prepared on the surface of pure titanium. During severe deformation, the dislocations, twins and slip bands with single orientation are formed first, then the intersections of twins and slip bands are observed, and then the subgrains appear near or in the slip bands and finally the randomly oriented nano-grains are formed due to the further breakdown. After high-energy shot peening, the ultimate tensile strength and the yield strength increase by 27 and 40%, while the elongation decreases by 64% after treatment, due to the increasing dislocation density, micro-strain and the grain refinement.

Surface nanocrystallization of Mg-3 wt.% Li-6 wt.% Al alloy by surface mechanical attrition treatment

A nanocrystalline surface layer was produced on Mg-3wt.% Li-6wt.% Al alloy by means of surface mechanical attrition treatment (SMAT). Microstructure features of various sections were systematically characterized by transmission electron microscopy. The results indicate that grain refinement induced by SMAT is dominated mainly by dislocation slip. Twinning is active at the early stage of grain refinement process when the grain size is large. The dislocation-dominated deformation mechanism is attributed to the change of c/a ratio due to the alloying of Li in Mg matrix and the suppression of twinning due to grain refinement. Nanoindentation results show that the hardness of the surface is enhanced by SMAT.

Enhancing the surface integrity and corrosion resistance of Ti-6Al-4V titanium alloy through cryogenic burnishing

In the surface layer of Ti-6Al-4V titanium alloy bar produced by cryogenic burnishing, reduced surface roughness, refined grain structure, and an obviously improved corrosion resistance are achieved. The surface roughness of cryogenic burnished sample was much lower than that of dry burnished sample. A nanocrystalline layer was fabricated on Ti-6Al-4V titanium alloy by cryogenic burnishing. There was a significant impact of the number of passes on structural refinement. The corrosion behavior of samples before and after cryogenic burnishing in a 0.9 % NaCl solution was investigated by potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), and Mott–Schottky measurements. Ti-6Al-4V alloy with nanocrystalline layer has better corrosion resistance than the unburnished coarse-grained counterpart. The positive effect can be explained by the more rapid, stable, and less defective passive film formed on the surface nanocrystalline layer of Ti-6Al-4V alloy due to its high density of grain boundaries and dislocations.

Research on Mechanism of Surface Nanocrystalline Layer in 2219 Al Alloy Induced by Supersonic Fine Particles Bombarding

A nanostructured surface layer is formed on 2219 Al alloy plate by means of supersonic fine particles bombarding (SFPB). The surface microstructure formation mechanism of morphology change is systematically characterized by using scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average grain size of 30nanostructured layer is about 30 nm when the surface of sample is induced by severe plastic deformation. Based on the experimental observations, nanoscale dislocation cells exist in strain layer about 200 from the surface, and we put forward a new formation mechanism of Al alloy surface nanocrystalline layer through theoretical analysis. The original grain is segmented fast into nanoscale dislocation cell or lamellar cell. As the Burgers vector of cell walls continue to accumulate, grain orientation difference constantly increases to form the nanoscale subboundary. Eventually, equiaxed nanocrystallites with random crystallographic orientations are formed by grain rotating or grain boundary sliding.

Effect of electropulsing-ultrasonic surface treatment on the surface properties and the corrosion behavior of 45 steel

Abstract

Effect of laser shock peening on microstructure and fatigue crack growth rate of AZ31B magnesium alloy

The fatigue crack growth tests were introduced for studying the effect of Laser Shock Peening (LSP) on fatigue crack growth rate of AZ31B Magnesium (Mg) alloy. The surface layer structures of laser treated samples were analyzed by using transmission electron microscopy (TEM) and X-ray diffraction (XRD) method. The fracture surface morphologies of samples were characterized by scanning electron microscopy (SEM). Moreover, the residual stress and surface roughness were also examined. A modified incremental polynomial method was used to obtain the fatigue crack propagation rates of six samples each group under the same crack length. The results show that the nanometer grains (with an average size of 17.5 nm) can be generated in the surface layer by using the optimized laser parameters. Surface roughness decreased from 1.177 μm to 0.713 μm after LSP. The depth of compressive residual stress induced by LSP reached to about 0.8 mm from the top surface. Comparing with the original samples, the obviously lower fatigue crack growth rates for the LSP treated samples was observed due to a combination of grain refinement, residual compressive stress and the barrier effect of surface nanocrystalline layer.

The activity of nanocrystalline Fe-based alloys as electrode materials for the hydrogen evolution reaction

In view of alkaline water electrolysis, the activities for the hydrogen evolution reaction of nanocrystalline Fe-based electrode materials were investigated and compared with the activities of polycrystalline Fe and Ni. Electrochemical methods were used to elucidate the overpotential value, the charge transfer resistance and the double layer capacity. Structural properties of the electrode surface were determined with SEM, XRD and XPS analyses. Thus, a correlation between electrochemical and structural parameters was found. In this context, we report on a cyclic voltammetric activation procedure which causes a significant increase of the surface area of Fe-based electrodes leading to a boost in effective activity of the activated electrodes. It was found that the intrinsic activity of activated Fe-based electrodes is very high due to the formation of a nanocrystalline surface layer. In contrast, the activation procedure influences only the intrinsic activity of the Ni electrodes without the formation of a porous surface layer.

Improve oxidation resistance at high temperature by nanocrystalline surface layer.

An interesting change of scale sequence occurred during oxidation of nanocrystalline surface layer by means of a surface mechanical attrition treatment. The three-layer oxide structure from the surface towards the matrix is Fe3O4, spinel FeCr2O4 and corundum (Fe,Cr)2O3, which is different from the typical two-layer scale consisted of an Fe3O4 outer layer and an FeCr2O4 inner layer in conventional P91 steel. The diffusivity of Cr, Fe and O is enhanced concurrently in the nanocrystalline surface layer, which causes the fast oxidation in the initial oxidation stage. The formation of (Fe,Cr)2O3 inner layer would inhabit fast diffusion of alloy elements in the nanocrystalline surface layer of P91 steel in the later oxidation stage, and it causes a decrease in the parabolic oxidation rate compared with conventional specimens. This study provides a novel approach to improve the oxidation resistance of heat resistant steel without changing its Cr content.

Wettability and corrosion of alumina embedded nanocomposite MAO coating on nanocrystalline AZ31B magnesium alloy

In this paper, micro- and nanocrystalline AZ31B magnesium alloy were coated by micro-arc oxidation method. In order to fabricate nanocrystalline surface layer, surface mechanical attrition treatment was performed and nano-grains with average size of 5–10 nm were formed on the surface of the samples. Coating process was carried out at different conditions including two coating times and two types of electrolyte. Alumina nanoparticles were utilized as suspension in electrolyte to form nanocomposite coatings by micro-arc oxidation method. Potentiodynamic polarization, percentage of porosity, and wettability tests were performed to study various characteristics of the coated samples. The results of scanning electron microscope imply that samples coated in silicate-based electrolyte involve much lower surface porosity (∼25%). Besides, the results of wettability test indicated that the maximum surface tension with deionized water is for nanocrystalline sample. In this regard, the sample coated in silicate-based suspension was 4 times more hydrophilic than the microcrystalline sample.