Nitriding iron at lower temperatures

Comparison of conventional and severe shot peening effects on the microstructure, texture, roughness, hardness, and electrochemical behavior of austenitic stainless steel

In the present research, microstructure, texture, roughness, hardness, and electrochemical behavior of AISI 316L austenitic stainless steel before and after shot peening were studied to elucidate the effect of conventional and severe shot peening (CSP and SSP) processes. After the shot peening, the fraction of strain-induced martensite (SIM) and mechanical twins (MTs) in the sub-surface layer was increased. The fraction of SIM and MTs in the SSP sample was higher than in the CSP sample. The XRD patterns indicated that the SSP sample had a higher peak broadening compared to the CSP sample. In the CSP and SSP samples, a gradient microstructure was formed along the depth direction. The microstructure of the topmost layer of the CSP and SSP samples exhibited numerous ultrafine grains. The grain refining during severe shot peening was faster because of the accumulation of more strain. The CSP and SSP samples revealed a gradient distribution of elements. After the SSP, the intensity of ⟨110⟩‖ED fiber texture decreased from 12.7 to 11.6 × R and the average intensity of ⟨100⟩‖ED fiber texture increased from 1.7 to 2.0 × R, respectively, compared to the CSP sample. The surface roughness of the SSP sample (Rq = 73.6 nm and Ra = 45.2 nm) was lower than that of the CSP sample which represented the roughness decreased with surface coverage increasing from 100 % to 1500 %. Also, the wettability increased after the conventional and severe shot peening processes. In addition, the microhardness of the CSP and SSP samples showed a gradient distribution. The CSP sample had the lowest corrosion current density (0.13 μA/cm2) whereas the NP (non-peened) sample exhibited the highest current density (0.65 μA/cm2). The presence of ⟨100⟩-oriented grains in both CSP and SSP samples led to the higher corrosion resistance of shot-peened steels compared to the NP sample. The presence of favorable texture with higher intensity in the CSP sample was responsible for the higher corrosion resistance of the CSP sample compared to the SSP sample. Finally, the gradient distribution of elements along the depth direction in the CSP and SSP steels improved the corrosion resistance of the surface.

Abrading bulk metal into single atoms

Single-atom catalysts have recently attracted considerable attention because of their highly efficient metal utilization and unique properties. Finding a green, facile method to synthesize them is key to their widespread commercialization. Here we show that single-atom catalysts (including iron, cobalt, nickel and copper) can be prepared via a top-down abrasion method, in which the bulk metal is directly atomized onto different supports, such as carbon frameworks, oxides and nitrides. The level of metal loading can be easily tuned by changing the abrasion rate. No synthetic chemicals, solvents or even water were used in the process and no by-products or waste were generated. The underlying reaction mechanism involves the mechanochemical force in situ generating defects on the supports, then trapping and stably sequestering atomized metals.

Gradient Microstructure Induced by Surface Mechanical Attrition Treatment in Grade 2 Titanium Studied Using Positron Annihilation Spectroscopy and Complementary Methods

Microstructural changes in grade 2 titanium generated by surface mechanical attrition treatment (SMAT) were studied using positron annihilation lifetime spectroscopy and complementary methods. A significant increase in the mean positron lifetime indicated many lattice defects introduced by SMAT. Two positron lifetime components were resolved in the positron lifetime spectra measured. The longer lifetime revealed the presence of vacancy clusters containing about 3 or 4 vacancies, while the shorter one was attributed to the annihilation of positrons trapped at dislocations. The changes of the positron lifetime indicated a decreasing dislocation density and the presence of a deeper layer with a higher concentration of vacancy clusters at the distance from the treated surface for which the microhardness approached the value for the strain-free matrix. Electrochemical impedance spectroscopy showed the positive effect of SMAT on the corrosion resistance of the titanium studied in a saline environment also after removal of the original oxide layer that was formed during the SMAT.

Additive Manufacturing of Tungsten Carbide Surfaces with Extreme Wear Resistivity

Steel surfaces have been coated with Co-based tungsten carbide (WC) in an additive printing process. This process leads to compact and extremely mechanically stable surfaces. We performed tribological measurements using WC counter bodies under dry conditions and severe mechanical load. Low coefficients of friction, even for rough surfaces, were found and the resulting wear rates were extraordinarily small, even when compared to high-quality PVD film with a similar composition. These findings suggest a wide field of application for this novel preparation process for wear-resistive surfaces.

A Facile Route for the Preparation of Monodisperse Iron nitride at Silica Core/shell Nanostructures

Uniform-sized iron oxide nanoparticles obtained from the solution phase thermal decomposition of the iron-oleate complex were encapsulated inside the silica shell by the reverse microemulsion technique, and then thermal treatment under NH₃ to transfer the iron oxide to iron nitride. The transmission electron microscopy images distinctly demonstrated that the as-prepared iron nitride at silica core/shell nanostructures were highly uniform in particle-size distribution. By using iron oxide nanoparticles of 6.1, 10.3, 16.2, and 21.8 nm as starting materials, iron nitride nanoparticles with average diameters of 5.6, 9.3, 11.6, and 16.7 nm were produced, respectively. The acid-resistant properties of the iron nitride at silica core/shell nanostructures were found to be much higher than the starting iron oxide at silica. A superconducting quantum interference device was used for the magnetic characterization of the nanostructure. Besides, magnetic resonance imaging (MRI) studies using iron nitride at silica nanocomposites as contrast agents demonstrated T ₂ enhanced effects that were dependent on the concentration. These core/shell nanostructures have enormous potential in magnetic nanodevice and biomedical applications. The current process is expected to be easy for large-scale and transfer other metal oxide nanoparticles.

Two-Dimensional Fe8N Nanosheets: Ferromagnets and Nitrogen Diffusion

We perform a first-principles study and identify two intriguing ferromagnets, hollow-Fe₈N (H-Fe₈N) and bridge-Fe₈N (B-Fe₈N) monolayers, by extensive structural searches. Both H-Fe₈N and B-Fe₈N nanosheets are buckled triangular lattices with a similar motif, but they are distinguishable by the positions of N atoms. The magnetic and electronic properties show that H-Fe₈N is a low-spin ferromagnet; in contrast, B-Fe₈N is a high-spin ferromagnet, which originates from the 3d orbital splitting of the Fe atom due to the low/high symmetric crystal field. Surprisingly, two stable Fe₈N monolayers can be transferred to each other by N atom diffusion from the bridge position to the hollow position with the migration energy barrier of 1.5 eV. The energy barrier is affected by introduced Fe defects and rare earth metal dopants. These findings introduce a new tactic to regulate the 2D Fe-nitride monolayers at the atomic scale.

Effect of ultrasonic surface deep rolling combined with oxygen boost diffusion treatment on fatigue properties of pure titanium

Ultrasonic surface deep rolling (USDR), oxygen boost diffusion (OBD), and their combination (USDR-OBD) were all used to improve the surface hardening of pure titanium. The microstructure, microhardness, and fatigue life of pure titanium treated by USDR, OBD, and USDR-OBD methods were analyzed. USDR treatment induced a severe deformation area, while OBD treatment produced a brittle oxygen diffusion zone. The USDR-OBD treated samples approached the highest hardness in comparison with other treated samples. The fatigue lives of USDR treated samples were improved, which was due to the high compressive residual stress and refined grains. However, the fatigue lives of both OBD treated samples and USDR-OBD treated samples were decreased due to premature crack initiation and rapid propagation in the oxygen diffusion zone. Finally, the fatigue fracture mechanisms of different samples were proposed.

Rapid Nitriding of Titanium Alloy with Fine Grains at Room Temperature

Multifunctional surfaces are required to design safe engineering products for human lives. Heating in a nitrogen atmosphere (nitriding) improves the tribological properties but reduces the strength of titanium (Ti) alloys owing to grain coarsening. A rapid nitriding method for Ti alloys forms the nitrided layer on the surface of a Ti alloy by bombarding with commercially pure Ti fine particles with a nitrided phase at room temperature within a short period. Furthermore, fine grains of Ti alloy are formed in the nitrided layer because of the impact of the Ti particles. These results reveal that this room-temperature method resolves the trade-off between the rapid formation of a nitrided layer and the suppression of grain coarsening for Ti alloys.

Enhanced Physicochemical and Biological Properties of a Low-Temperature Copperized Layer on Gradient Nanograined Pure Titanium

The application of titanium as medical implants is limited to a certain extent due to its insufficient corrosion resistance, biological activity, and antibacterial ability. In this work, a gradient nanograined (GNG) layer was fabricated on the titanium surface by surface ultrasonic rolling treatment (SURT). The subsequent copperizing kinetics was greatly enhanced so that a thick copperized layer could be obtained on the surface of GNG Ti at a relatively low diffusion temperature (450 °C). Meanwhile, the GNG structure accelerated the release rate of Cu²⁺, which endows GNG Cu/Ti with strong antibacterial activity. Moreover, the corrosion resistance and cytocompatibility of GNG Cu/Ti were also evidently improved compared with coarse-grained Ti, indicating a good biomedical application prospect.

Mechanochemistry for ammonia synthesis under mild conditions

Ammonia, one of the most important synthetic feedstocks, is mainly produced by the Haber-Bosch process at 400-500 °C and above 100 bar. The process cannot be performed under ambient conditions for kinetic reasons. Here, we demonstrate that ammonia can be synthesized at 45 °C and 1 bar via a mechanochemical method using an iron-based catalyst. With this process the ammonia final concentration reached 82.5 vol%, which is higher than state-of-the-art ammonia synthesis under high temperature and pressure (25 vol%, 450 °C, 200 bar). The mechanochemically induced high defect density and violent impact on the iron catalyst were responsible for the mild synthesis conditions.

The Controlled Compound Layer of Ni-Coated Nitrided Pure Iron

In order not to sacrifice nitrided layer thickness and reduce brittle compound layer thickness, Ni-coated pretreatment was carried out with electrodeposition on a pure iron surface, followed by gas nitriding. The brittle compound layer thickness of duplex surface treated samples was reduced, and the nitrided layer thickness increased to 320 μm. The microhardness was 4 times harder, and the wear loss was reduced by 68% compared with the original material. The results indicate that Ni-coated pretreatment could effectively improve microhardness and wear resistance and realize the controlled microstructure of a brittle compound layer of pure iron without compromising nitrided layer thickness. Ni coating plays an important role in ammonia adsorption and decomposition, and in the transfer of active nitrogen atoms during nitriding.

Bioinformatics analysis and identification of circular RNAs promoting the osteogenic differentiation of human bone marrow mesenchymal stem cells on titanium treated by surface mechanical attrition

BACKGROUND

To analyze and identify the circular RNAs (circRNAs) involved in promoting the osteogenic differentiation of human bone mesenchymal stem cells (hBMSCs) on titanium by surface mechanical attrition treatment (SMAT).

METHODS

The experimental material was SMAT titanium and the control material was annealed titanium. Cell Counting Kits-8 (CCK-8) was used to detect the proliferation of hBMSCs, and alkaline phosphatase (ALP) activity and alizarin red staining were used to detect the osteogenic differentiation of hBMSCs on the sample surfaces. The bioinformatics prediction software miwalk3.0 was used to construct competing endogenous RNA (ceRNA) networks by predicting circRNAs with osteogenesis-related messenger RNAs (mRNAs) and microRNAs (miRNAs). The circRNAs located at the key positions in the networks were selected and analyzed by quantitative real-time PCR (QRT-PCR).

RESULTS

Compared with annealed titanium, SMAT titanium could promote the proliferation and osteogenic differentiation of hBMSCs. The total number of predicted circRNAs was 51. Among these, 30 circRNAs and 8 miRNAs constituted 6 ceRNA networks. Circ-LTBP2 was selected for verification. QRT-PCR results showed that the expression levels of hsa_circ_0032599, hsa_circ_0032600 and hsa_circ_0032601 were upregulated in the experimental group compared with those in the control group; the differential expression of hsa_circ_0032600 was the most obvious and statistically significant, with a fold change (FC) = 4.25 ± 1.60, p-values (p) < 0.05.

Plasma-Nitriding Properties of CoCrFeMnNi High-Entropy Alloys Produced by Spark Plasma Sintering

High-entropy alloys (HEAs) were fabricated by powder metallurgy using gas-atomized powder and spark plasma sintering (SPS) followed by surface modification (plasma nitriding) of the sintered sample. Plasma nitriding forms nitride and induces solid-soluting of N; it enables the diffusion of N atoms by removing the passive film formed on the surface of alloys such as stainless steel, Al alloys, and Ti alloys, via the sputtering of cations during glow discharge. Therefore, plasma nitriding has the potential to process HEAs that contain strong oxidizing elements such as Cr, Al, and Ti. In this work, a sintered CoCrFeMnNi HEA was plasma-nitrided and its properties were subsequently evaluated. A uniform microstructure without segregation was obtained in the SPS sample, and its hardness and wear resistance were found to have improved. Analysis of the sample surface after nitriding revealed that an expanded face-centered cubic phase formed on the surface plasma-nitrided at 673 K and that a CrN phase formed on the surface plasma-nitrided at temperatures greater than 723 K. The surface hardness of the plasma-nitrided sample was 1200 HV or greater, and the wear resistance and pitting corrosion resistance were improved compared with those of the untreated sample.

New Approach to Produce a Nanocrystalline Layer on Surface of a Large Size Pure Titanium Plate

It was demonstrated that the mechanical shot peening (MSP) technique was a viable way to obtain a nanocrystalline layer on a large size pure titanium plate due to the MSP provided for severe plastic deformation (SPD) of surface high velocity balls impacting. The MSP effects of various durations in producing the surface nanocrystalline layer was characterized by optical microscope (OM), X-ray diffraction (XRD), transmission electron microscope (TEM), and Vickers micro-hardness tester. The results showed that the thickness of the SPD layer gradually increased with the MSP processing time increase, but saturated at 230 μm after 30 min. The average grain size was refined to about 18.48 nm in the nanocrystalline layer. There was equiaxed grain morphology with random crystallographic orientation in the topmost surface. By comparing with the nanocrystalline layer, acquired by surface mechanical attrition treatment (SMAT), the microstructure and properties of the nanocrystalline layer acquired by MSP was evidently superior to that of the SMAT, but the production time was cut to about a quarter of the time used for the SMAT method.

Dissociating stable nitrogen molecules under mild conditions by cyclic strain engineering

All quiet on the nitrogen front. The dissociation of stable diatomic nitrogen molecules (N₂) is one of the most challenging tasks in the scientific community and currently requires both high pressure and high temperature. Here, we demonstrate that N₂ can be dissociated under mild conditions by cyclic strain engineering. The method can be performed at a critical reaction pressure of less than 1 bar, and the temperature of the reaction container is only 40°C. When graphite was used as a dissociated N* receptor, the normalized loading of N to C reached as high as 16.3 at/at %. Such efficient nitrogen dissociation is induced by the cyclic loading and unloading mechanical strain, which has the effect of altering the binding energy of N, facilitating adsorption in the strain-free stage and desorption in the compressive strain stage. Our finding may lead to opportunities for the direct synthesis of N-containing compounds from N₂.

Abrasive Wear Resistance of Plasma-Nitrided Ti Enhanced by Ultrasonic Surface Rolling Processing Pre-Treatment

The objective of the given work was to investigate abrasive wear behaviours of titanium (Ti) treated by ultrasonic surface rolling processing (USRP) pre-treatment and plasma nitriding (PN). Simulated lunar regolith particles (SLRPs) were employed as abrasive materials during characterization of tribological performances. The experimental results showed that SLRPs cause severe abrasive wear on Ti plasma-nitrided at 750 °C via the mechanism of micro-cutting. Due to the formation of a harder and thicker nitriding layer, the abrasive wear resistance of the Ti plasma-nitrided at 850 °C was enhanced, and its wear mechanism was mainly fatigue. USRP pre-treatment was effective at enhancing the abrasive wear resistance of plasma-nitrided Ti, due to the enhancement of the hardness and thickness of the nitride layer. Nevertheless, SLRPs significantly decreased the friction coefficient of Ti treated by USRP pre-treatment and PN, because the rolling of small granular abrasives impeded the adhesion of the worn surface. Furthermore, USRP pre-treatment also caused the formation of a dimpled surface with a large number of micropores which can hold wear debris during tribo-tests, and finally, polishing and rolling the wear debris resulted in a low friction coefficient (about 0.5).

A Review on Heterogeneous Nanostructures: A Strategy for Superior Mechanical Properties in Metals

Generally, strength and ductility are mutually exclusive in homogeneous metals. Nanostructured metals can have much higher strength when compared to their coarse-grained counterparts, while simple microstructure refinement to nanoscale generally results in poor strain hardening and limited ductility. In recent years, heterogeneous nanostructures in metals have been proven to be a new strategy to achieve unprecedented mechanical properties that are not accessible to their homogeneous counterparts. Here, we review recent advances in overcoming this strength–ductility trade-off by the designs of several heterogeneous nanostructures in metals: heterogeneous grain/lamellar/phase structures, gradient structure, nanotwinned structure and structure with nanoprecipitates. These structural heterogeneities can induce stress/strain partitioning between domains with dramatically different strengths, strain gradients and geometrically necessary dislocations near domain interfaces, and back-stress strengthening/hardening for high strength and large ductility. This review also provides the guideline for optimizing the mechanical properties in heterogeneous nanostructures by highlighting future challenges and opportunities.

Synthesis of α''-Fe16N2 ribbons with a porous structure

The microstructure of FeCuB ribbons (∼20 μm thick) was modified to fabricate α''-Fe₁₆N₂ at a temperature as low as 160 °C. The ribbon samples were heat treated first at a temperature reaching 930 °C and then quenched down to room temperature. During the heat treatment, ribbon samples were oxidized, and hydrogen reduction was then conducted to remove the oxygen from the ribbon samples. The reduced ribbon samples had a porous structure, which improved the nitrogen diffusion efficiency and decreased the fabrication temperature of α''-Fe₁₆N₂ down to 160 °C. It was demonstrated that the techniques for microstructure control in this method including oxidation and reduction helped obtain the α''-Fe₁₆N₂ phase with high coercivity, thus manifesting this could be a promising technique for low-temperature nitridation on ribbons in general.

High-temperature stability of nanozirconate-toughed IMF material lanthanum synthesized by anin situreaction

Herein, powders composed of La₂Zr₂O₇ (LZ) and ZrO₂ phases were synthesized by an in situ reaction using a sol-spray pyrolysis method; moreover, 24 mol% LaO_1.5–ZrO₂ (volume ratio = 1 : 1) powders were characterized by XRD, Raman spectroscopy, SEM, and TEM. XRD and Raman results showed that the samples maintained a tetragonal ZrO₂ and a pyrochlore LZ phase from 900 to 1100 °C. The addition of LZ could be helpful in the stabilization of t-ZrO₂ and decreasing the grain size of ZrO₂. The SEM results revealed that the LZ and ZrO₂ phases were homogeneously distributed in the sintered bulk. The HRTEM results suggested that the crystal orientations of the nano-LZ and nano-ZrO₂ phases were accordant; this was in agreement with the characteristics of the coherent boundaries. The fracture toughness of LZ–ZrO₂ was markedly improved by the transformation toughening of the ZrO₂ phase, and a value that was 2.2-fold that of the LZ prepared by a similar technique was achieved.

Herein, powders composed of La₂Zr₂O₇ (LZ) and ZrO₂ phases were synthesized by an in situ reaction using a sol-spray pyrolysis method; moreover, 24 mol% LaO_1.5–ZrO₂ powders were characterized by XRD, Raman, SEM, and TEM.

The effect of duplex Surface mechanical attrition and nitriding treatment on corrosion resistance of stainless steel 316L

Surface mechanical attrition treatment (SMAT) fabricates a nanostructured layer on the surface of 316L that possesses excellent mechanical properties. In this paper a duplex process combining SMAT with nitriding was performed, then the microstructure, hardness and corrosion properties of the treated materials were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), hardness tests, potentiodynamic polarization measurements, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The results showed that this duplex surface treatment can effectively increase the hardness and corrosion resistance of the treated surface both at room temperature and in the simulated steam generator (SG) condition at high temperature and high pressure.

An Energetic Approach to Predict the Effect of Shot Peening-Based Surface Treatments

Almen intensity and surface coverage are well-known to be the defining parameters of shot peening-based surface treatments. These parameters are directly affected by material properties, the extension of the contact zone, the geometry of the impact pair, as well as the impact rate and velocity. Such intricate relationships have resulted in often dissimilar predictions of shot peening effects even while using an identical combination of Almen intensity and surface coverage. With the fast pace introduction of new generation impact-based surface treatments, there is a need to find a more widespread parameter that would facilitate the direct comparison of all different treatments and relate the main process parameters to the resultant mechanical characteristics. Herein, we propose to use an energy-based parameter to describe the peening process in a more widespread approach, which collectively incorporates the effects of the Almen intensity and surface coverage, as well as the diameter, material, and velocity of the impact media. A set of finite element analyses was developed to demonstrate the correlation of the peening process effects with this energetic approach. Comparisons with the experimental data served as proof of concept, confirming that the proposed method could provide a quite good estimation of the effect of peening parameters on the treated material.

Structural evolution behavior of manganese monophosphide under high pressure: experimental and theoretical study

The influence of external pressure on the structural properties of manganese monophosphides (MnP) at room temperature has been studied using in situ angle dispersive synchrotron x-ray powder diffraction (AD-XRD) with a diamond anvil cell. The crystal structure of MnP is stable between 0 to 15 GPa. However, the compressibility of b-axis is much larger than those of a- and c-axes. From this result we suggested that the occurrence of superconductivity in MnP was induced by suppression of the long-range antiferromagnetically ordered state rather than a structural phase transition. Furthermore, the present experimental results show that the Pnma phase of MnP undergoes a pressure-induced structural phase transition at ~15.0 GPa. This finding lighted up-to-date understanding of the common prototype B31 structure (Strukturbericht Designation: B31) in transition metal monophosphides. No additional structural phase transition was observed up to 35.1 GPa (Run 1) and 40.2 GPa (Run 2) from the present AD-XRD results. With an extensive crystal structure searching and ab initio calculations, we predict that MnP underwent two pressure-induced structural phase transitions of Pnma  →  P2₁3 and P2₁3  →  Pm-3m (CsCl-type) at 55.0 and 92.0 GPa, respectively. The structural stability and the electronic structures of manganese monophosphides under high pressure are also briefly discussed.

The Nitrocarburising Response of Low Temperature Bainite Steel

The nitrocarburising response of low transformation temperature ultrafine and nanoscale bainitic steel was investigated and compared with martensite and pearlite from the same steel composition. It was found that the retained austenite content of the bainitic steel dictated the core hardness after nitrocarburising. The refined bainitic structure showed improvements in the nitriding depth and hardness of the nitrocarburised layer, compared to coarser grained martensitic and pearlitic structures, possibly due to the fine structure and the distribution of nitride forming elements.

Diffusion and surface alloying of gradient nanostructured metals

Gradient nanostructures (GNSs) have been optimized in recent years for desired performance. The diffusion behavior in GNS metals is crucial for understanding the diffusion mechanism and relative characteristics of different interfaces that provide fundamental understanding for advancing the traditional surface alloying processes. In this paper, atomic diffusion, reactive diffusion, and surface alloying processes are reviewed for various metals with a preformed GNS surface layer. We emphasize the promoted atomic diffusion and reactive diffusion in the GNS surface layer that are related to a higher interfacial energy state with respect to those in relaxed coarse-grained samples. Accordingly, different surface alloying processes, such as nitriding and chromizing, have been modified significantly, and some diffusion-related properties have been enhanced. Finally, the perspectives on current research in this field are discussed.

A general strategy for the ultrafast surface modification of metals

Surface modification is an essential step in engineering materials that can withstand the increasingly aggressive environments encountered in various modern energy-conversion systems and chemical processing industries. However, most traditional technologies exhibit disadvantages such as slow diffusion kinetics, processing difficulties or compatibility issues. Here, we present a general strategy for the ultrafast surface modification of metals inspired by electromigration, using aluminizing austenitic stainless steel as an example. Our strategy facilitates the rapid formation of a favourable ductile surface layer composed of FeCrAl or β-FeAl within only 10 min compared with several hours in conventional processes. This result indicates that electromigration can be used to achieve the ultrafast surface modification of metals and can overcome the limitations of traditional technologies. This strategy could be used to aluminize ultra-supercritical steam tubing to withstand aggressive oxidizing environments.

Surface modification techniques can allow metals to be used in aggressive environments for emerging applications. Here authors demonstrate an ultrafast process for aluminizing austenitic stainless steels and propose an electromigration-assisted mechanism suggesting generalizability of the method.

Design of magnesium alloys with controllable degradation for biomedical implants: From bulk to surface

The combination of high strength, light weight, and natural biodegradability renders magnesium (Mg)-based alloys promising in orthopedic implants and cardiovascular stents. Being metallic materials, Mg and Mg alloys made for scaffolds provide the necessary mechanical support for tissue healing and cell growth in the early stage, while natural degradation and reabsorption by surrounding tissues in the later stage make an unnecessarily follow-up removal surgery. However, uncontrolled degradation may collapse the scaffolds resulting in premature implant failure, and there has been much research in controlling the degradation rates of Mg alloys. This paper reviews recent progress in the design of novel Mg alloys, surface modification and corrosion mechanisms under different conditions, and describes the effects of the structure, composition, and surface conditions on the degradation behavior in vitro and in vivo.

STATEMENT OF SIGNIFICANCE

Owing to their unique mechanical properties, biodegradability, biocompatibility, Mg based biomaterials are becoming the most promising substitutes for tissue regeneration for impaired bone, vascular and other tissues because these scaffolds can provide not only ideal space for the growth and differentiation of seeded cells but also enough strength before the formation of normal tissues. The most important is that these scaffolds can be fully degraded after tissue regeneration, which can satisfy the increasing demand for better biomedical devices and functional tissue engineering biomaterials in the world. However, the rapid degradation rate of these scaffolds restricts the wide application in clinic. This paper reviews recent progress on how to control the degrdation rate based on the relevant corrosion mechanisms through the design of porous structure, phase structure, grains, and amorphous structure as well as surface modification, which will be beneficial to the better understanding and functional design of Mg-based scaffolds for wide clinical applications in tissue reconstruction in near futures.

2D Metals by Repeated Size Reduction

A general and convenient strategy for manufacturing freestanding metal nanolayers is developed on large scale. By the simple process of repeatedly folding and calendering stacked metal sheets followed by chemical etching, free-standing 2D metal (e.g., Ag, Au, Fe, Cu, and Ni) nanosheets are obtained with thicknesses as small as 1 nm and with sizes of the order of several micrometers.

Synthesis of Fe16N2 compound Free-Standing Foils with 20 MGOe Magnetic Energy Product by Nitrogen Ion-Implantation

Rare-earth-free magnets are highly demanded by clean and renewable energy industries because of the supply constraints and environmental issues. A promising permanent magnet should possess high remanent magnetic flux density (B_r), large coercivity (H_c) and hence large maximum magnetic energy product ((BH)_max). Fe₁₆N₂ has been emerging as one of promising candidates because of the redundancy of Fe and N on the earth, its large magnetocrystalline anisotropy (Ku > 1.0 × 10⁷ erg/cc), and large saturation magnetization (4πMs > 2.4 T). However, there is no report on the formation of Fe₁₆N₂ magnet with high B_r and large H_c in bulk format before. In this paper, we successfully synthesize free-standing Fe₁₆N₂ foils with a coercivity of up to 1910 Oe and a magnetic energy product of up to 20 MGOe at room temperature. Nitrogen ion implantation is used as an alternative nitriding approach with the benefit of tunable implantation energy and fluence. An integrated synthesis technique is developed, including a direct foil-substrate bonding step, an ion implantation step and a two-step post-annealing process. With the tunable capability of the ion implantation fluence and energy, a microstructure with grain size 25–30 nm is constructed on the FeN foil sample with the implantation fluence of 5 × 10¹⁷/cm².

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.

Formation of a protective nitride layer by electrochemical nitridation on 316L SS bipolar plates for a proton exchange membrane fuel cell (PEMFC)

In the present study, an attempt has been made to increase the corrosion resistance of 316L stainless steel (SS) bipolar plates (Bp) through electrochemical nitridation using a nitrate bearing electrolyte solution of 0.1 M HNO₃ and 0.5 M KNO_3.

Theoretical study of the ammonia nitridation rate on an Fe (100) surface: a combined density functional theory and kinetic Monte Carlo study

Ammonia (NH3) nitridation on an Fe surface was studied by combining density functional theory (DFT) and kinetic Monte Carlo (kMC) calculations. A DFT calculation was performed to obtain the energy barriers (Eb) of the relevant elementary processes. The full mechanism of the exact reaction path was divided into five steps (adsorption, dissociation, surface migration, penetration, and diffusion) on an Fe (100) surface pre-covered with nitrogen. The energy barrier (Eb) depended on the N surface coverage. The DFT results were subsequently employed as a database for the kMC simulations. We then evaluated the NH3 nitridation rate on the N pre-covered Fe surface. To determine the conditions necessary for a rapid NH3 nitridation rate, the eight reaction events were considered in the kMC simulations: adsorption, desorption, dissociation, reverse dissociation, surface migration, penetration, reverse penetration, and diffusion. This study provides a real-time-scale simulation of NH3 nitridation influenced by nitrogen surface coverage that allowed us to theoretically determine a nitrogen coverage (0.56 ML) suitable for rapid NH3 nitridation. In this way, we were able to reveal the coverage dependence of the nitridation reaction using the combined DFT and kMC simulations.

The effects of the size of nanocrystalline materials on their thermodynamic and mechanical properties

This work has considered the intrinsic influence of bond energy on the macroscopic, thermodynamic, and mechanical properties of crystalline materials. A general criterion is proposed to evaluate the properties of nanocrystalline materials. The interrelation between the thermodynamic and mechanical properties of nanomaterials is presented and the relationship between the variation of these properties and the size of the nanomaterials is explained. The results of our work agree well with thermodynamics, molecular dynamics simulations, and experimental results. This method is of significance in investigating the size effects of nanomaterials and provides a new approach for studying their thermodynamic and mechanical properties.

Cell response to nanocrystallized metallic substrates obtained through severe plastic deformation

Cell-substrate interface is known to control the cell response and subsequent cell functions. Among the various biophysical signals, grain structure, which indicates the repeating arrangement of atoms in the material, has also proved to play a role of significant importance in mediating the cell activities. Moreover, refining the grain size through severe plastic deformation is known to provide the processed material with novel mechanical properties. The potential application of such advanced materials as biomedical implants has recently been evaluated by investigating the effect of different substrate grain sizes on a wide variety of cell activities. In this review, recent advances in biomedical applications of severe plastic deformation techniques are highlighted with special attention to the effect of the obtained nano/ultra-fine-grain size on cell-substrate interactions. Various severe plastic deformation techniques used for this purpose are discussed presenting a brief description of the mechanism for each process. The results obtained for each treatment on cell morphology, adhesion, proliferation, and differentiation, as well as the in vivo studies, are discussed. Finally, the advantages and challenges regarding the application of these techniques to produce multifunctional bio-implant materials are addressed.

Lanthanum-containing hydroxyapatite coating on ultrafine-grained titanium by micro-arc oxidation: a promising strategy to enhance overall performance of titanium

Titanium is widely used in biomedical materials, particularly in dental implants, because of its excellent biocompatibility and mechanical characteristics. However, titanium implant failures still remain in some cases, varying with implantation sites and patients. Improving its overall performance is a major focus of dental implant research. Equal-channel angular pressing (ECAP) can result in ultrafine-grained titanium with superior mechanical properties and better biocompatibility, which significantly benefits dental implants, and without any harmful alloying elements. Lanthanum (La) can inhibit the acidogenicity of dental plaque and La-containing hydroxyapatite (La-HA) possesses a series of attractive properties, in contrast to La-free HA. Micro-arc oxidation (MAO) is a promising technology that can produce porous and firmly adherent hydroxyapatite (HA) coatings on titanium substrates. Therefore, we hypothesize that porous La-containing hydroxyapatite coatings with different La content (0.89%, 1.3% and 1.79%) can be prepared on ultrafine-grained (~200–400 nm) titanium by ECAP and MAO in electrolytic solution containing 0.2 mol/L calcium acetate, 0.02 mol/L β-glycerol phosphate disodium salt pentahydrate (β-GP), and lanthanum nitrate with different concentrations to further improve the overall performance of titanium, which are expected to have great potential in medical applications as a dental implant.

Improvement of the biomedical properties of titanium using SMAT and thermal oxidation

Titanium and its alloys are excellent candidates for biomedical implant. However, they exhibit relatively poor tribological properties. In this study, a two-step treatment including surface mechanical attrition treatment (SMAT) combined with thermal oxidation process has been developed to improve the tribological properties and biocompatibility of Ti. Ti after two-step treatment shows excellent wear-resistance and biocompatibility among all Ti samples, which can be ascribed to the highest surface energy, well crystallinity of rutile layer on its surface. Overall, the two-step treatment is a prospective method to produce excellent biomedical Ti materials.

Adsorption, dissociation, penetration, and diffusion of N2 on and in bcc Fe: first-principles calculations

We report first-principles calculations of adsorption, dissociation, penetration, and diffusion for the complete nitridation mechanism of nitrogen molecules on a pure Fe surface (bcc, ferrite phase). The mechanism of the definite reaction path was calculated by dividing the process into four steps. We investigated various reaction paths for each step including the energy barrier based on the climb image nudged elastic band (CI-NEB) method, and the complete reaction pathway was computed as the minimum energy path (MEP). The adsorption characteristics of nitrogen (N) and molecular nitrogen (N2) indicate that nitrogen atoms and molecules are energetically favorable at the hollow sites on pure Fe(100) and (110). The dissociation of the nitrogen molecule (N2) was theoretically supported by electronic structure calculations. The penetration of nitrogen from the surface to the sub-surface has a large energy barrier compared with the other steps. The activation energy calculated for nitrogen diffusion in pure bcc Fe was in good agreement with the experimental results. Finally, we confirmed the rate-determining step for the full nitridation reaction pathway. This study provides fundamental insight into the nitridation mechanism for nitrogen molecules in pure bcc Fe.