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In this research, ultrasonic nanocrystal surface modification (UNSM) was used to process a 7075-T651 aluminum alloy. After UNSM treatment, a work-hardened layer with compressive residual stress up to 500 MPa was generated on the near-surface region of the sample. The surface microhardness of the alloy was increased from 195 ± 4 to 225 ± 7 HV, and the surface roughness was decreased from 1.32 ± 0.04 to 0.54 ± 0.27 µm. The stress concentration factor Kt of the as-received specimen was decreased from 1.080 to 1.069 after UNSM processing. Rotating–bending fatigue tests showed that UNSM significantly improved the fatigue performance. In addition, UNSM-treated 7075-T651 samples were able to endure pitting corrosion in a 3.5 wt.% NaCl solution for 72 h without any degradation in fatigue performance. These results demonstrated that UNSM is a robust surface modification method that can improve the rotating–bending fatigue resistance of the aluminum alloy.

In this work, the nitriding efficiency in an ultrafine-grained iron was investigated. Prior to nitriding, the ultrasonic nanocrystalline surface modification (UNSM) technique was used to process the coarse-grained iron to obtain ultrafine-grained iron. Gas nitriding was conducted for the UNSM-treated iron and the coarse-grained iron at temperatures from 300 °C to 560 °C. The results indicate that the combination of UNSM and nitriding provide much higher efficiency for hardening than the sum of the efficiencies of UNSM alone and nitriding alone. Nitride nucleation was confirmed in the diffusion layer of the UNSM-treated iron following the nitriding process, enhancing the hardness after nitriding. X-ray diffraction patterns were obtained for both the UNSM-treated iron subjected to nitriding as well as iron that underwent only nitriding at different temperatures. It was also found that the minimum temperatures required to form nitrides, including γ′-Fe4N, e-Fe3N, e-Fe2N, and FeN, were reduced because of the high chemical potential at the grain boundaries. The theoretical analysis suggests that heterogeneous nitride nucleation at grain boundaries is the key factor that contributes to the enhancement in hardness in the nitriding layer in ultrafine-grained metals and nanocrystalline metals.

Nanosecond laser shock peening is an important material strengthening technique, but its application is limited by the requirement of the confining medium and protective coating. These limitations can be potentially overcome by femtosecond laser shock peening. This article presents a study on the effects of the confining medium and protective coating on femtosecond laser shock peening of 304 stainless steel. The surface hardness can be increased by 45.5% by peening directly in air without any confining medium and coating. The surface quality is also maintained at a good condition. Numerical simulation by a hydrodynamic model reveals that femtosecond laser shock peening can induce extremely strong shock waves (hundreds of GPa) directly in air, which is much stronger than those by nanosecond laser peening (~10 GPa). Surprisingly different from nanosecond laser peening, it is found that by adding the confining medium and protective layer, the peening effect is significantly weakened. It is unveiled that the super high intensity of the femtosecond laser causes strong ionization of the confining medium (water), which shields 98% of the laser energy from deposition into the sample and weakens the peening effect. The enhancement depth by femtosecond laser peening is found to be less than 100 µm, which is the reason that the peening effect is weakened when a 100 µm thick coating is used. This study shows that femtosecond laser peening works the best directly in air without any confining medium and coating, which significantly broadens its application where high flexibility and precision are required.

In this research, a surface plastic deformation layer was induced on 300M ultra-high strength steel using ultrasonic nanocrystal surface modification (UNSM). The UNSM-treated and control specimens were then nitrided at 450 °C for 6 h. After gas nitriding, the microstructure, corrosion and wear resistance of 300M steel with and without UNSM pre-treatment were investigated. After nitriding, the UNSM-treated specimens generated a dense surface nitride layer that was thicker than that of the control specimens. The high defect density and refined lath martensite induced by UNSM were found to accelerate the nitrogen diffusion and efficiency. X-ray diffraction patterns revealed that α-Fe phase occupied the largest proportion in the nitriding layer of specimens without UNSM pre-treatment. However due to the higher nitriding efficiency, the main phase of UNSM-treated specimens after nitriding was found to be e-Fe2-3N. Compared with the specimens without pre-treatment, UNSM-treated specimens exhibited higher surface hardness as well as higher corrosion and wear resistance after nitriding.

In this study, an innovative surface treatment method, electropulsing assisted laser shock peening (EP-LSP), was used to process Ti64 samples. In EP-LSP, metallic samples are subjected to simultaneous high strain rate plastic deformation and high-frequency (100–500 Hz) short-duration (100 μs) pulsed electric current. The effects of EP-LSP on surface finish, microstructure, and micro-hardness of Ti64 alloy were investigated and compared with continuous current assisted LSP (CC-LSP) having the same bulk heating effect. It was observed that EP-LSP produced higher surface hardness and deeper hardened layer, both of which indicate greater plastic deformation. Tensile tests were carried out to evaluate the plasticity of Ti64 subjected to pulsed current and continuous current with the same bulk heating effect. It was observed that pulsed current can more effectively decrease the flow stress and thus resulted in greater plasticity in Ti64 compared with continuous current, even though the bulk heating effect was the same. In addition, the higher the peak current density, the more effective the flow stress reduction. As a result, pulsed current can more effectively improve the effectiveness of LSP treatment, as manifested by higher surface hardness and deeper plastic deformation layer. It is believed that an athermal effect in addition to the thermal effect related to pulsed current exists in EP-LSP.

Surface severe plastic deformation (SSPD) can significantly improve the mechanical properties of metallic components by inducing surface nanocrystallization and beneficial compressive residual stresses. The effectiveness of the SSPD processes is significantly dependent on the plasticity of the target metals. Here, we report an innovative surface thermomechanical process called laser-assisted ultrasonic nanocrystal surface modification (LA-UNSM) that integrates localized laser heating with high strain rate plastic deformation. The laser beam locally heats the target metal and increases the local plasticity, making the SSPD treatment more effective. After LA-UNSM, a microstructure featuring a nanocrystalline layer embedded with nanoscale precipitates was achieved in Ti64, resulting in an unprecedented 75.2% increase in hardness. After LA-UNSM processing, a 25-μm severe plastic deformation layer was produced that was 2.5 times thicker than that of the room-temperature UNSM-processed material. The grains at the top surface were refined down to 37 nm, indicating a similar degree of nanocrystallization to that produced by UNSM at room temperature. Nanoscale precipitate particles with diameters in the range of 5–21 nm were non-uniformly distributed in the nanocrystalline surface layer. These precipitates were produced through laser-assisted dynamic precipitation. The extremely high surface strength obtained for the Ti64 was attributed to the composite microstructure featured by nanoscale grains embedded with nanoscale precipitates and the work-hardening.

The effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on the gas nitriding behavior of Ti6Al4V alloy have been investigated. Gas nitriding was performed at 700 and 800 °C. The microstructure after UNSM and gas nitriding was characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Microstructural investigations revealed the formation of an approximately 10 µm thick severe plastic deformation layer as well as nano-grains after UNSM treatment. The UNSM-treated Ti6Al4V alloy formed 0.26 µm and 1.35 µm thick nitride layers after nitriding at 700 °C and 800 °C, respectively, and UNSM resulted in an increased layer thickness relative to untreated samples at both temperatures. The results suggest that nitrogen adsorption and reaction capability were enhanced in the UNSM-treated Ti6Al4V alloy. This enhancement can be attributed to high-density dislocations and grain boundaries that were introduced by UNSM and served as efficient channels for nitrogen diffusion.

Mg alloys offer potential advantages over conventional biomedical implant materials because of their biodegradability and biocompatibility, but could be limited by their poor mechanical properties. In this study, laser shock peening (LSP), a surface processing technique, was applied to improve the mechanical properties of the AZ31B magnesium (Mg) alloy. It was demonstrated that LSP increased the hardness and yield strength of the Mg alloy. Due to the hardening, LSP significantly improved the wear resistance and fatigue performance of the Mg alloy. In addition, immersion tests carried out in cell culture medium revealed that LSP did not significantly increase Mg2+ release and weight loss. Furthermore, an in vitro cell culture study showed that the LSP-treated samples have cell-compatibility comparable to untreated samples. Thus, the LSP technique could, with further study, advance the clinical utility of Mg alloys in the orthopedic field.

Carbon nanotube (CNT)/graphene nanostructure has the potential to extend the superior mechanical, thermal, and electrical properties of graphene from two dimensions to three. While the theoretical investigation of CNT/graphene nanostructure based on atomistic modeling is garnering great attention, an open-source numerical tool to generate covalently bonded CNT/graphene junctions is still in lack for material scientists. In this work, a pathfinding algorithm is used to exhaust all possible configurations on graphene to seamlessly connect to a given CNT. The least squares approach method follows to sort out the configuration with minimum energy. The combined methods are able to generate CNT/graphene junction for any CNT type (m, n). Molecular dynamics simulation further reveals that the formed junctions are thermodynamically stable, and thus ready to serve as basic block for a CNT/graphene network. By providing an easy-to-use numerical tool in the form of MATLAB code, the intention is to free material scientists from the tedious preparation of atomic configuration.

Gradient nanocrystalline (GNC) metals exhibit an unprecedented combination of high strength and high ductility. In this study, GNC copper was obtained using ultrasonic nanocrystalline surface modification (UNSM), and its plasticity mechanism was investigated using tensile tests. It was found that UNSM treatment can grant copper superior yield strength and ductility which is beyond the capacity of conventional cold-working. Moreover, the UNSM-treated copper has a reduced strain-hardening capacity which does not lead to early necking and deteriorated ductility. The strain energy analysis shows that the yield strength of GNC material can be estimated using the root mean square of yield strengths of the coarse-grained (CG) layer and the gradient structure (GS) layer. The strain energy analysis also predicted a strong interaction between GS layer and CG layer during tensile deformation. Furthermore, a dislocation-based constitutive model was applied to understand the interaction between CG layer and GS layer. The migration of dislocations from the CG layer to the GS layer was proposed to explain the interaction. The migration of dislocations induces strain softening and, thus, releases internal strain energy. The proposed mechanism is supported by our experimental observations: the reduction in strain-hardening capacity is positive-related to the depression in strain localization in UNSM-treated GNC copper. The improved ductility is attributed to the depressed strain localization and the refreshed capacity for dislocation accumulation, both resulted from dislocation migration from the CG layer to the GS layer.

An ultrasonic nanocrystal surface modification (UNSM) technique has been used to treat AISI 52100 steel specimens and thrust ball bearing raceways. Tribological performances of untreated specimens were compared to UNSM-treated specimens in reciprocating sliding and rolling/sliding contact under boundary lubricated conditions. Friction and wear coefficients, micropitting, and changes in surface profiles and surface roughness of the specimens were studied. Results showed that UNSM treated specimens had higher wear and micropitting resistance than the untreated specimens. The improvements in wear and micropitting resistance may be attributed to increased surface hardness, refined grain sizes and compressive residual stress near surface region as a result of UNSM treatment. The UNSM technique has been proven to be a powerful tool to improve the durability and tribological performance of contacting surfaces of mechanical components such as bearings, gears, and seals.

Surface severe plastic deformation was induced in Inconel 718 alloy using ultrasonic nanocrystal surface modification (UNSM). Low-temperature gas nitriding was carried out on non-processed and UNSM-processed Inconel 718 samples. The microstructure and material properties were characterized using X-ray diffraction, scanning electron microscopy, the Vickers hardness test, and wear and corrosion testing. The results demonstrated that gas nitriding efficiency was significantly enhanced by the UNSM treatment. Higher nitrogen concentration was observed in the surface layer of the UNSM samples. The nitride layer in the UNSM + nitriding sample exhibited an increased hardness over that of the nitriding sample and the UNSM treated sample. Additionally, both wear resistance and corrosion resistance were significantly improved in the UNSM + nitriding sample.

Nickel-titanium (NiTi) alloys have great potential to be used as biomedical implants or devices due to their unique functional properties (i.e., shape memory properties and superelastic behavior). The machining difficulty associated with NiTi alloys is impeding their wide application. Additive manufacturing (AM), however, provides an alternative method to manufacture NiTi structures. One major concern associated with NiTi devices fabricated in this route is the potential for the release of toxic Ni ions due to the poor surface finish as well as high surface porosity. In this study, NiTi samples were produced using selective laser melting, the most common AM techniques. Then, an innovative surface processing technique, ultrasonic nano-crystal surface modification (UNSM), was used to mitigate the potential for the Ni ions release. By simultaneous ultrasonic striking and burnishing, UNSM can significantly improve surface finish and decrease surface porosity. In addition, UNSM induces plastic strain which in turn hardens the surface layer. The synergistic effect of better surface finish, lower subsurface porosity, and a hardened surface layer resulted in higher wear and corrosion resistance. It is therefore expected that UNSM can be potentially used to treat biomedical devices.

Microstructure inhomogeneity is imparted into the surface of a Vit1b Zr-based bulk metallic glass (BMG) using ultrasonic nanocrystal surface modification (UNSM). As a result, compressive residual stresses with a maximum magnitude of 1130 MPa were induced in the near-surface region. Substantial improvements in both fracture strain and strength were observed for the treated specimen in three-point bending tests. The microstructure inhomogeneity and free volume generated by severe plastic deformation resulted in increased shear band density during three-point bending tests, which is evidenced by the vein-like pattern observed on the fracture surface of the treated specimen. Moreover, the enrichment of shear bands can cause the interaction between shear bands and can thus obstruct their propagation, leading to work-hardening behavior. High magnitude compressive residual stresses are also believed to impede and slow down the propagation of the shear bands. The synergistic effect of induced inhomogeneity, increased free volume and compressive residual stresses improves the fracture stress and strain of the UNSM-treated BMG.

The effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on the thermal oxidation (TO) behavior of Ti6Al4V alloy has been investigated. The thermal oxidation was carried out at 500, 600 and 700 °C. The microstructure after UNSM and TO was characterized using scanning electron microscopy with energy dispersive spectroscopy. And phase identification was performed using X-ray diffraction. At 500 and 600 °C, the reaction capability are enhanced and the oxidation layer thickness is increased in the UNSM-treated Ti6Al4V alloy. This is attributed to nanoscale grain boundaries created by UNSM that serve as efficient diffusivity paths for interstitial gaseous atoms. When the TO temperature rises to 700 °C, due to dislocation elimination and grain coarsening induced by the high temperature, the oxidation layer thickness of the Ti6Al4V specimens show no significant difference.

The effect of ultrasonic nanocrystalline surface modification (UNSM) on the water droplet erosion (WDE) performance of Ti 6Al 4V was studied. It was observed that UNSM induces deep levels of compressive residual stresses in both the scanning and transverse directions. The treated surface revealed microdimples in a micro-tracked fashion. Mechanical deformation marks were observed within the grains due to excessive plastic deformation and variation in grain size was observed across the ultrasonically modified layer. Microhardness of the UNSM condition was enhanced significantly as compared with the untreated (As-M) condition. The WDE performance tests for the UNSM and As-M conditions were conducted in a rotating disc rig in accordance with ASTM G73 standard. Influence of impact speed on WDE was explored on two different sample geometries (T-shaped flat and airfoil). WDE results showed that the flat UNSM samples had enhanced WDE performance at speeds 250, 275 and 300 m/s as compared with the As-M condition. At 350 m/s, both UNSM and As-M conditions showed similar performance. UNSM airfoil samples showed mild enhancement in the WDE performance at 300 m/s during the advanced stage as compared with the As-M condition. At 350 m/s, the UNSM airfoils do not show enhancement in WDE performance.

We report herein the effects of Ultrasonic Nano-crystal Surface Modification (UNSM), a severe surface plastic deformation process, on the microstructure, mechanical (hardness, wear), wettability and biocompatibility properties of NiTi shape memory alloy. Complete surface amorphization of NiTi was achieved by this process, which was confirmed by X-ray diffraction and high-resolution transmission electron microscopy. The wear resistance of the samples after UNSM processing was significantly improved compared with the non-processed samples due to increased surface hardness of the alloy by this process. In addition, cell culture study demonstrated that the biocompatibility of the samples after UNSM processing has not been compromised compared to the non-processed sample. The combination of high wear resistance and good biocompatibility makes UNSM an appealing process for treating alloy-based biomedical devices.

In this study, the effects of ultrasonic nanocrystal surface modification (UNSM) treatment on the surface integrity, microstructures and wear resistance of 300M ultra-high strength steel (300M steel) were investigated. The results showed that surface roughness of 300M steels after UNSM processing was significantly decreased with a lower scanning speed even though the surface roughness values were higher than that of mechanically polished control samples. In addition, the surface hardness of 300M steel was significantly enhanced as the static load increased. It was found that using a static load of 50 N and a scanning speed of 250 mm/min in the UNSM process can significantly improve surface hardness (797 HV) while slightly increasing the surface roughness. With these parameters, the resulting microstructure of UNSM-processed samples have three layers: the layer of severe plastic deformation, the layer with gradual plastic deformation, and the unaffected layer. Due to the plastic deformation, greater and deeper compressive residual stresses were induced in the UNSM-processed samples. In addition, the wear resistance of UNSM-processed samples was significantly improved, which was attributed to the refined martensite laths, work hardening and compressive residual stress.

The tribological performance of AISI 52100 substrates subjected to several surface treatments have been evaluated in rolling, sliding, and mixed mode contact. The surface treatments include a boron-doped diamond-like carbon (B-DLC) coating deposited by plasma-assisted magnetron sputtering, an ultrasonic nanocrystal surface modification (UNSM) technique used to generate severe plastic deformation in the near surface of the steel specimens, and a B-DLC coating applied to a UNSM pretreated surface. In general, the tribological performance of the duplex surface treatment was superior to that of the untreated specimens and of the specimens with the other surface treatments in rolling, sliding, and mixed mode contact. The improved tribological performance of the duplex process was attributed to the combination of increased wear resistance provided by the B-DLC coating and the grain refinement of the martensitic structure of the AISI 52100 imparted by the UNSM process that created a beneficial substrate/coating interface.

Coarse-grained models were developed to study the evaporation and transfer of the perfluoropolyether lubricant from the disk surface under laser heating. Both moving and stationary disks were simulated with a heat flux applied to a central region under a pad. A novel concept of a reconditioning region was proposed to convert the periodic boundary conditions to more realistic conditions with the fresh material fed at the inlet. The results indicate significant influence of the disk velocity, heat flux and gap distance on the evaporation and transfer of the lubricant. With increasing heat flux, more lubricant molecules tend to reach the slider surface.

Nickel-titanium (NiTi) alloy is an attractive material for biomedical implant applications. In this study, the effects of laser shock peening (LSP) on the biocompatibility, corrosion resistance, ion release rate and hardness of NiTi were characterized. The cell culture study indicated that the LSP-treated NiTi samples had lower cytotoxicity and higher cell survival rate than the untreated samples. Specifically, the cell survival rate increased from 88 ± 1.3% to 93 ± 1.1% due to LSP treatment. LSP treatment was shown to significantly decrease the initial Ni ion release rate compared with that of the untreated samples. Electrochemical tests indicated that LSP improved the corrosion resistance of the NiTi alloy in simulated body fluid, with a decrease in the corrosion current density from 1.41 ± 0.20 μA/cm2 to 0.67 ± 0.24 μA/cm2 . Immersion tests showed that calcium deposition was significantly enhanced by LSP. In addition, the hardness of NiTi alloy increased from 226 ± 3 HV before LSP to 261 ± 3 HV after LSP. These results demonstrated that LSP is a promising surface modification method that can be used to improve the mechanical properties, corrosion resistance and biocompatibility of NiTi alloy for biomedical applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1854-1863, 2019.

Hierarchical structures on metallic implants can enhance the interaction between cells and implants and thus increase their biocompatibility. However, it is difficult to directly fabricate hierarchical structures on metallic implants. In this study, we used a simple one-step method, ultrasonic nanocrystal surface modification (UNSM), to fabricate hierarchical surface structures on a nickel-titanium (NiTi) alloy. During UNSM, a tungsten carbide ball hits metal surfaces at ultrasonic frequency. The overlapping of the ultrasonic strikes generates hierarchical structures with microscale grooves and embedded nanoscale wrinkles. Cell culture experiments showed that cells adhere better and grow more prolifically on the UNSM-treated samples. Compared with the untreated samples, the UNSM-treated samples have higher corrosion resistance. In addition, the surface hardness increased from 243 Hv to 296 Hv and the scratch hardness increased by 22%. Overall, the improved biocompatibility, higher corrosion resistance, and enhanced mechanical properties demonstrate that UNSM is a simple and effective method to process metallic implant materials.

The effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on the gas nitriding of Ti6Al4V alloy has been investigated. The gas nitriding was performed at 700 and 800 °C. The microstructure after UNSM and gas nitriding was characterized using X-ray diffraction and scanning electron microscopy. Microstructural investigations revealed the formation of an approximately 10 μm thick severe plastic deformation (SPD) layer after UNSM treatment. After nitriding at 700 °C and 800 °C, a compound layer consisting of an approximately 0.2 μm and 1.9 μm thick nitride layer was observed in UNSM-treated Ti6Al4V alloy, which exhibits a nearly two-fold increase in nitride layer thickness as compared with the un-treated sample. This suggests that the nitrogen adsorption and the reaction capability are enhanced in the UNSM-treated Ti6Al4V alloy. This enhancement can be attributed to the high density dislocations and grain boundaries introduced by UNSM that serve as efficient diffusivity channels for interstitial gaseous atoms.

Magnesium alloys have tremendous potential for biomedical applications due to their good biocompatibility, osteoconductivity, and degradability, but can be limited by their poor mechanical properties and fast corrosion in the physiological environment. In this study, ultrasonic nanocrystal surface modification (UNSM), a recently developed surface processing technique that utilizes ultrasonic impacts to induce plastic strain on metal surfaces, was applied to an AZ31B magnesium (Mg) alloy. The mechanical properties, corrosion resistance, and biocompatibility of the alloy after UNSM treatment were studied systematically. Significant improvement in hardness, yield stress and wear resistance was achieved after the UNSM treatment. In addition, the corrosion behavior of UNSM-treated AZ31B was not compromised compared with the untreated samples, as demonstrated by the weight loss and released element concentrations of Mg and Al after immersion in alpha-minimum essential medium (α-MEM) for 24h. The in vitro biocompatibility of the AZ31B Mg alloys toward adipose-derived stem cells (ADSCs) before and after UNSM processing was also evaluated using a cell culture study. Comparable cell attachments were achieved between the two groups. These studies showed that UNSM could significantly improve the mechanical properties of Mg alloys without compromising their corrosion rate and biocompatibility in vitro. These findings suggest that UNSM is a promising method to treat biodegradable Mg alloys for orthopaedic applications.

In this study, boron carbide (B4C) was reactively sputtered with acetylene to form a boron containing diamond-like carbon (DLC) coating. The flow rate of acetylene significantly affects the composition and thus the mechanical behavior of the coating. When optimal acetylene flow rate was used (10 sccm in this study), the coating has a B/C ratio of ∼1% and possesses the high hardness of B4C and the low friction of a DLC in addition to good adhesion with the 52100 steel substrate. It was found that the new coating consisted of nanocrystalline boron carbide precipitates homogeneously distributed in an amorphous hydrocarbon matrix forming a boron-doped a-C:H film. The new coating exhibited wear resistance superior to a commercial W-doped DLC coating currently used on rolling element bearings and gears. The best wear resistance and lowest friction coefficients were obtained in coatings with a B/C ratio of ∼1%.

In this study, the plasticity behavior of an AZ31B magnesium alloy subjected to short-duration (100 μs), high-frequency (120–800 Hz) pulsed current was investigated using tensile tests. The key finding is that the effect of pulsed current on plastic deformation goes beyond the Joule heating effect. In our experiments, the frequency was adjusted to maintain a constant effective current density and, thus, the same Joule heating effect. A comparison with continuous current having the same Joule heating effect was made as well. It was observed that when the peak current density is higher than a critical value, a higher peak current density will yield a more significant reduction in flow stress even though the thermal heating effect is the same. This critical current density decreases with the increase in the effective current density. Pulsed current with a higher peak current density can more effectively reduce the dislocation density through electric-induced annealing, induce more severe grain rotation, and, thus, lower the resistance for dislocations to pass through barriers like grain boundaries, resulting in a more significant flow stress reduction. X-ray diffraction characterizations were also conducted for the deformed specimen to show that a higher peak current density induces more severe grain rotation and, thus, more effectively decreases dislocation density.

An innovative surface treatment method, ultrasonic nanocrystal surface deformation (UNSM), was used to process a magnesium (Mg) AZ31B alloy in this study. In the UNSM process, ultrasonic impacts induce plastic deformation on material surface that lead to grain refinement and surface morphology changes. The hardness, tensile strength, surface morphology and weight loss from immersion testing of the materials before and after UNSM were studied systematically. Compared with the unprocessed samples, the UNSM-processed samples show significant improvements in hardness (64%) and yield stress (43%). Surface topography results reveal that UNSM generates a smooth surface with an average roughness of 93 nm. In addition, the immersion results in cell medium demonstrate that the UNSM-processed group showed lower weight loss, especially during the early immersion period. It can thus be concluded that UNSM can significantly improve the hardness and yield strength of Mg alloys and reduce the corrosion rate, indicating that UNSM is a promising new method to enhance the mechanical properties of the degradable Mg alloys.

In this study, we investigated the mechanical properties of AZ31B Mg alloy before and after laser shock peening (LSP). The hardness of the AZ31B Mg alloy increased from 57 HV to 69 HV after LSP. The yield strength increased from 128 MPa to 152 MPa. Wear resistance was significantly improved after LSP. Immersion testing showed that LSP did not significantly increase the element release and weight loss in simulated body fluid. We have demonstrated that LSP is an effective way to improve the mechanical properties of the AZ31B Mg alloy.

In this study, an innovative process called nanocrystallization-assisted nitriding was used to process 4140 steels. First, a nanocrystalline surface layer was induced in 4140 steel by ultrasonic nanocrystal surface modification (UNSM). The abundant nanoscale grain boundaries provide micro-channels for efficient nitrogen diffusion during nitriding at relatively low temperature (450 °C) and short duration (4 hours). The samples were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The hardness and corrosion resistance were examined and compared for samples after different processing conditions. It has been demonstrated that the sample processed by nanocrystallization-assisted nitriding has much higher hardness and corrosion resistance compared with the samples processed by nitriding only.

Surface amorphization of NiTi has been achieved by ultra-high strain rate dynamic loading induced by ultrasonic nano-crystal surface modification (UNSM). The amorphous microstructure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). To better understand the physical mechanism of the amorphization process, molecular dynamics (MD) simulation has been implemented to simulate the shock loading process and the results are consistent with the experiment. Central-symmetry parameter (CSP) and radial distribution function (RDF) were used to characterize the microstructure evolution. The simulation result demonstrates that the deformation is first formed as “twining” structure and then transformed into amorphization. The simulation also shows that shock speeds affect the amorphization level on the surface, while the shock amplitude mainly affects the amorphization depth.Copyright © 2015 by ASME

In this paper, the results of a study that examined the intrinsic influence of ceramic particulate reinforcements on tensile deformation and fracture behavior of aluminum alloy 2014 is presented and discussed. The candidate aluminum alloy was reinforced with two volume fractions of the alumina (Al2O3) particulates. Influence of particulate reinforcement on microstructural development is highlighted. Samples of the aluminum alloy-based metal matrix composite were deformed in uniaxial tension. The influence of nature of loading and intrinsic microstructural effects on stress versus strain response, mechanical properties, deformation and final fracture behavior are neatly elaborated and discussed in light of intrinsic microstructural effects, deformation characteristics of the microstructural constituents and nature of loading.

In this study, an innovative process, electrically assisted ultrasonic nanocrystal surface modification (EA-UNSM), is used to process Ti6Al4V alloy. As compared with traditional UNSM, EA-UNSM results in lower dislocation density and larger grains due to the thermal annealing effect caused by resistive heating. In addition, deeper plastic deformation layer is observed in the electrically assisted case. By supplying mechanical energy and thermal energy simultaneously, a strong dynamic precipitation effect is induced, which generates nanoscale precipitates in the EA-UNSM-treated Ti6Al4V alloy. These nanoscale precipitates can effectively pin dislocations during plastic deformation and thus significantly improve the surface hardness.

A Fokker–Planck code is developed to simulate the laser–plasma interaction in the femtosecond-laser shock peening and forming processes. A numerical scheme dealing with high-energy concentration and its resulting steep gradient are presented, and the source code is provided as supplementary material for further usage. The breakdown of the classical heat transport theory is observed when the laser intensity increases. The difference in heat flow between the classical theory and simulation is presented. It is found that the classical heat transport theory overestimates heat flow by orders of magnitude during femtosecond-laser shock peening or forming. As a result, the electron pressure can be underestimated using the classical hydrodynamic code.

In this study, complete solid state amorphization in nanocrystalline nickel has been achieved through cryogenic laser shock peening (CLSP). High resolution transmission electron microscopy has revealed the complete amorphous structure of the sample after CLSP processing. A molecular dynamic model has been used to investigate material behavior during the shock loading and the effects of nanoscale grain boundaries on the amorphization process. It has been found that the initial nanoscale grain boundaries increase the initial Gibbs free energy before plastic deformation and also serve as dislocation emission sources during plastic deformation to contribute to defect density increase, leading to the amorphization of pure nanocrystalline nickel.

The effect of an external magnetic field on the anodic arc root movement inside a dc plasma torch has been investigated. In this letter, a virtual instrument technology was used to measure the arc voltage of the plasma torch at atmospheric pressure, and the rotating frequency of the anodic arc root was estimated by high-speed photography. The arc voltage fluctuation which represents the degree of the arc instability was reduced. And a new peak, whose frequency is consistent with the rotating frequency of the anodic arc root, was found in the Fast Fourier Transform spectra of the arc voltage signal. From the results, the helical instability generated directly by anodic arc rotation is put forward, and only the appropriate external magnetic field could inhibit and reduce the instability of the plasma torch and prolong the life of the anodic electrode. Furthermore, the measured voltage waveforms indicated that the arc root attachment mode would be controllable by an external magnetic field.