Microstructure Of Surface Layer Research Articles

Eutectic resolidification and ultrafast self-quenching of the microstructure in the surface layer of high-speed steel by scanning electron beam treatment

Improving the service life and efficiency of high-speed steel (HSS) tools remains a formidable challenge. Herein, scanning electron beam (SEB) was employed to modify the surface of M2 HSS. Results revealed that SEB induced eutectic resolidification and ultrafast self-quenching of microstructure in surface layer of HSS. The melted layer exhibited a microstructure comprising martensite, residual austenite, dispersed finely rod-like MC and lamellar M2C eutectic carbides along grain boundaries. Martensite and eutectic carbide grid structure significantly enhanced the hardness and wear resistance of surface layer. Compared to substrate, the microhardness of SEB sample was elevated by more than threefold, reaching 914.7HV0.2, which exceeded the hardness (828.6HV0.2) of conventional quenching treatment. Notably, the wear volume of SEB sample had been reduced to 0.54 × 10−11 m3, resulting in a wear resistance that was 3.8 times greater than that of quenched sample.

Effect of Dual Shot Peening on Microstructure and Wear Performance of CNT/Al-Cu-Mg Composites.

This work systematically investigated the effect of dual shot peening (DSP) and conventional shot peening (CSP) on the microstructure, residual stress and wear performance of the CNT/Al-Cu-Mg composites. The results indicated that compared with CSP, DSP effectively reduced surface roughness (Rz) from 31.30 to 12.04 μm. In parallel, DSP introduced a smaller domain size (33.1 nm) and more dislocations, higher levels compressive residual stress and a stiffer deformation layer with deeper affected zones. Moreover, DSP effectively improved the uniformity of the surface layer's microstructure and residual stress distribution. The improvement is mainly due to secondary impact deformation by microshots and fine grain strengthening. In addition, the transformation of the hard second phases such as Al4C3 and CNT and its effects on improving the surface strength and deformation uniformity were discussed. Significantly, DSP improved the wear resistance by 31.8% under the load of 6 N, which is attributed to the synergistic influence of factors including hardness, compressive residual stress, surface roughness, and grain size. In summary, it can be concluded that DSP is an effective strategy to promote the surface layer characteristics for CNT/Al-Cu-Mg composites.

Laser Surface Transformation Hardening for Automotive Metals: Recent Progress

This article discusses recent advancements in the Laser Surface Transformation Hardening (LSTH) process applied to industrial metals. It focuses on examining the microstructure of the metal surface layer and explores different methods of performing LSTH to evaluate mechanical and surface properties. The study also investigates the utilization of various industrial lasers and simulation software for the LSTH process. The careful analysis of heat transfer and temperature control during LSTH aims to prevent the generation of surface defects like micro-cracks and surface melting. Finite element method (FEM) software effectively simulates the LSTH process. The research provides a comprehensive overview of recent developments in LSTH, categorized based on different metals and subsequent testing, highlighting its applications in the automotive industry. Electrochemical, wear, and microhardness tests are investigated to assess the potential applications of automotive metals.

INVESTIGATION OF THE MICROSTRUCTURE OF AISI 321 STAINLESS STEEL AFTER LASER SURFACE MELTING

The aim of the present paper is to investigate the microstructure of laser-melted surface layers of austenitic steel for biomedical applications. The surface of prismatic specimens from AISI 321 stainless steel was treated by continuous CO2 laser. Three modes of laser processing were used, ensuring surface melting. The microstructure was observed by OM and SEM, while the chemical composition was investigated by EDX analysis. It was found that the microstructure of as-delivered steel was two-phase and relatively inhomogeneous in morphology and chemical composition. It consisted of austenite with grain sizes between 20 - 150 μm, relatively large amount of striped δ-ferrite and spherical carbides along the grain boundaries. After laser melting, the microstructure remained two-phase (δ-ferrite and austenite), but became more homogeneous in morphology and composition. Different dendrites morphology in the particular regions of the molten layer was confirmed - fine equiaxed dendrites on the top surface and columnar at the bottom of the molten pool. Delta-ferrite is located in the interdendritic areas and in larger amounts in the transition zone between the molten.

Microstructure and mechanical properties of superficial surface and subsurface layers in the cutting of hardened steel under cryogenic cooling

Hardened steel parts extensively utilized in manufacturing fields, including automobiles and molds, are difficult to cut and process. Cryogenic cooling is an effective method for reducing tool wear and improving surface quality. However, knowledge about the microstructure and mechanical properties of the hardened steel cutting under a cryogenic cooling condition is severely limited. Herein, low-speed and high-speed cutting experiments with different wear tools were designed to investigate the characteristics and mechanisms of the microstructure, residual stress distribution, and hardness distribution of the superficial surface layer (SSL, including the white and transition layers) and subsurface layer (SL) of hardened steel cutting via cryogenic cooling. Results showed that microstructure changes caused by plastic deformation occurred in both the white and transition layers, whereas no changes occurred in the area below the transition layer. No phase transformation and recrystallization occurred in the white layer, the transition layer, and the area below the transition layer. The intense dislocation slip and strong plastic deformation are the keys to the formation of the white and transition layers. The hardness distribution curve included the surface hardening peak, SL softening valley, and SL hardening peak. The combined effect of fine grain strengthening and dislocation strengthening is the primary mechanism for forming the surface hardening peak. The peak residual tensile stress induces a SL softening valley in the hardness curve, whereas the peak residual compressive stress induces a SL hardening peak in the hardness curve.

Distinctive performance evolution of surface layer in TC4 alloy oxidized at high temperature: Softening or hardening?

The characteristics of metal surface layer at high-temperature significantly affect the service performance of TC4 titanium alloy components, so that the evolution mechanism of mechanical properties of metal surface layer under high-temperature oxidation conditions is an important scientific issue. In the present work, the mechanical behavior and the relevant mechanism of metal surface oxidized at the temperatures of 500 ℃, 700 ℃, and 900 ℃ for different times have been systematically investigated. The experimental results show that with the increase of temperature, the oxidation kinetics curve of the alloy changes from parabolic law to linear law, and the oxidation reaction changes from diffusion control to interfacial reaction control. Furthermore, as the temperature increases from 500 °C to 900 °C, the feature of oxide scale changes from the single-layer structure to the multi-layer alternating structure, and the thickness of oxide scales and depth of alpha case increased with increasing temperature. The surface oxide scale is mainly composed of low valent oxides such as Ti3O and Ti6O during low-temperature oxidation as well as short-term high temperature oxidation. After long-term high temperature oxidation, the oxide scale is mainly composed of high valent oxides such as TiO2, Al2O3, and TiVO4. Simultaneously, significant changes occur in the composition, microstructure and properties of metal surface layer. In general, the microstructure and mechanical properties of the surface-layer metal are strongly dependent upon the competitive influence of the outward diffusion of Al and the inward diffusion of O. In the temperature range of 500–900ºC, when the temperature is relatively low, the softening effect caused by the outward diffusion of Al is stronger than the strengthening effect caused by the inward diffusion of O, and a surface softening effect thus occurs. On the contrary, when the temperature is high, the strengthening effect caused by the inward diffusion of O is stronger than the softening effect caused by the outward diffusion of Al, and a surface strengthening effect accordingly happens.

Effect of austempering time on multiphase microstructure evolution and properties of carburized Cr-Mn-Si alloyed steel subjected to bainitization quenching & partitioning heat treatment

An innovative multistage heat treatment, that combines Bainitization and Quenching & Partitioning processes (BQ&P) can be applied to obtain multiphase microstructure in the carburized surface layer comprising nanobainite, ultra-fine martensite and retained austenite. In this study, a carburized medium-carbon Cr-Si-Mn alloyed steel was treated by BQ&P treatments with 20%, 40% and 100% of advancement of bainitic transformation in the top surface to investigate the effect of isothermal holding time on microstructure evolution and properties.Extending the bainitization time leads to an increase in the bainitic ferrite content in the microstructure and more significant fragmentation of the residual austenite blocks. It was shown that multiphase microstructure consisting of nanobainite, ultra-fine martensite and retained austenite results in the higher hardness and wear resistance of the case as compared to the fully bainitic microstructure. The enhanced properties results from the strengthening effect of additional martensite fraction and higher amount of stabilized austenite prone to transformation under friction (transformation induced plasticity, TRIP effect). Furthermore, adjusted parameters of the process lead to favorable mechanical properties in a medium-carbon core (YS ∼ 1430 MPa, UTS ∼ 1820 MPa, KV ∼ 20 Jcm−2). The ability to tailor mechanical and service properties of steel by controlling the fraction of particular phases along with a significant reduction of heat treatment time might designate the BQ&P process to industrial application, e.g. carburized gear wheels.

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

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

In-situ transmission electron microscopy investigation on surface oxides thermal stability of niobium

Niobium is commonly used for superconducting quantum systems as readout resonators, capacitors, and interconnects. Structural defects at the Nb/Si and air/Nb interface may be a major source of two-level systems (TLS), which are detrimental to the device's coherence time. Thus, identifying and understanding the microscopic origin of possible TLS in Nb-based devices and their relationship to processing is key to superconducting qubit performance improvement. This work studied the structure and thermal stability of surface oxide on physical vapor deposited Nb films on Si wafers, using aberration-corrected (scanning) transmission electron microscopy and spectroscopy. All Nb films exhibit columnar growth with strong [110] textures. After in-situ heating of the heterostructure at 360 °C inside the microscope, the initial amorphous niobium surface oxides decompose into face-centered cubic Nb nanograins in the amorphous Nb-O matrix, which may reduce microwave dissipation. Despite changes in the microstructure and chemistry of the niobium oxide surface layer due to heat treatment, the interface between the Nb and the surface oxide layer remains almost unchanged. Our comprehensive study of the Nb surface oxide decomposition mechanism may guide future superconducting qubit device optimization through interfacial scattering center and TLS minimization.

Microstructure and Mechanical Properties of Gradient Nanostructured Q345 Steel Prepared by Ultrasonic Severe Surface Rolling

In this work, ultrasonic severe surface rolling (USSR), a new surface nanocrystallization technique, is used to prepare gradient nanostructure (GNS) on the commercial Q345 structural steel. The microstructure of the GNS surface layer is characterized by employing EBSD and TEM, and the result indicates that a nanoscale substructure is formed at the topmost surface layer. The substructures are composed of subgrains and dislocation cells and have an average size of 309.4 nm. The GNS surface layer after USSR processing for one pass has a thickness of approximately 300 μm. The uniaxial tensile measurement indicates that the yield strength of the USSR sample improves by 25.1% compared to the as-received sample with slightly decreased ductility. The nanoscale substructure, refined grains, high density of dislocations, and hetero-deformation-induced strengthening are identified as responsible for the enhanced strength. This study provides a feasible approach to improving the mechanical properties of structural steel for wide applications.

Investigation of the Influence of Anti-Wear Coatings on the Surface Quality and Dimensional Accuracy during Finish Turning of the Inconel 718 Alloy.

This piece of work deals with the influence assessment of the kind of coating of the cutting inserts and their wear on the dimensional accuracy and the top layer microstructure and roughness of the surface machined with constant cutting parameters vc = 85 m/min, f = 0.14 mm/obr and ap = 0.2 mm. The tests were performed on shafts made of Inconel 718 material under the conditions of finish turning, requiring a tool life of more than 20 min. The cutting inserts of identical geometry made of fine-grained carbide covered with coatings were applied by the PVD and CVD method. The values of the obtained diameter dimensions were assessed in reference to the assumed ones, as well as the values of the surface roughness and stereometry and the microstructure of the top layer. The nature and mechanisms of edge wear and its value expressed by the VBC parameter were also assessed. It was determined in the tests that the machined surface quality defined by the Ra and Sa roughness parameters and the dimensional accuracy were influenced not only by the coating microhardness but also by the method of applying the given coating. The lowest values of the tested roughness parameters were observed for the surface machined with an edge, with the S205 coating applied by the CVD method, which was characterized by the lowest microhardness. The edge with this coating also showed the lowest wear, defined by the VBC parameter, which translated into dimensional accuracy. Furthermore, the edge with the S205 coating also provided the best results with regard to the surface layer microstructure. The least favorable results, both in terms of dimensional accuracy and surface roughness, were obtained for the surface machined with a 1115-PVD-coated edge. The highest wear value was also recorded for this edge.

Effect of thermal cyclic nitriding modes on microstructure and chemical composition of titanium alloy surface layers

A method has been developed for cyclic low-temperature ion nitriding using a plasma source with an integrally cold hollow cathode, which is superior in performance to a source with an incandescent cathode. The results of the effectiveness of the developed method are presented. It has been established that the greatest influence on the microhardness of titanium alloys is exerted by the bias voltage and the l ocation of the surface with respect to the plasma flow. Microstructural studies have shown that the α-phase is stabilized in the near-surface layer due to nitrogen diffusion, grain growth is not observed. Alloy VT8M1 retains a two-phase structure. The possibility of controlling nitriding by changing the duration and direction of surface saturation is also shown.

Corrosion properties of aluminized 16Mo3 steel

Chromium-molybdenum steel (16Mo3) is widely used in petroleum, gas, automotive, and construction industries due to its good oxidation resistance and mechanical properties at moderately elevated temperatures. The aim of the research was to evaluate the corrosion susceptibility of 16Mo3 steel in hot rolled and aluminized states. Aluminization was performed by diffusion pack aluminization process at 900?C/2h and 730?C/4h, respectively. Electrochemical corrosion testing included measuring open circuit potential (EOCP), linear polarization resistance (LPR), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in potassium phosphate buffer (KH2PO4, pH = 7). Optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used for surface layer microstructure characterization before and after corrosion tests. It was demonstrated that corrosion resistance of aluminized steel increased substantially. Corrosion properties were related to the structure and properties of intermetallic phase (FeAl, FeAl2 and Fe2Al5) that formed on the surface of 16Mo3 steel.

Influence of silicon and cobalt laser alloying on the microstructure and nanomechanical properties of the gray cast iron surface layer

Influence of silicon and cobalt laser alloying on the microstructure and nanomechanical properties of the gray cast iron surface layer

Phase transformation pre-treatment of diamond wire-sawn multi-crystalline silicon wafers for metal-assisted chemical etching of solar cells

Phase transformation by the heat treatment, prior to metal-assisted chemical etching (MACE) of solar cells, has been proposed and studied, as a pre-treatment approach for diamond wire sawn (DWS) Si wafers. The results of the study show that the phase transformation pre-treatment can modify the microstructure of amorphous Si surface layers of DWS multi-crystalline (mc) Si wafers. The modified surface layer proved to be more reactive to the chemical etching solution, which was the primary factor for the improvement in the treated solar cells. A significant improvement on the surface uniformity and morphology of the textured Si wafer was achieved, subsequent to the pre-treatment, during the MACE texturization process. These solar cells based on the treated DWS mc-Si wafers displayed a gain of about 0.18% (absolute value), in the overall photoelectric conversion efficiency (η), when compared to the solar cells of the same batch, for the untreated wafers, on the same production line. It is hence, proven that this pre-treatment method, when used as a supplementary technique to the wet-chemical texturization process, has great potential as an effective application technology for solar cells based on DWS mc-Si wafers.

Selective laser reaction synthesis of SiC, Si3N4 and HfC/SiC composites for additive manufacturing

Selective laser reaction sintering techniques (SLRS) techniques were investigated for the production of near net-shape non-oxide ceramics including SiC, Si3N4, and HfC/SiC composites that might be compatible with prevailing powder bed fusion additive manufacturing processes. Reaction bonded layers of covalent ceramics were produced using in-situ reactions that occur during selective laser processing and layer formation. During SLRS, precursor materials composed of metal and/or metal oxide powders were fashioned into powder beds for conversion to non-oxide ceramic layers. Laser-processing was used to initiate simultaneous chemical conversion and local interparticle bonding of precursor particles in 100 vol% CH4 or NH3 gases. Several factors related to the reaction synthesis process—precursor chemistry, gas-solid and gas-liquid synthesis mechanisms, precursor vapor pressures—were investigated in relation to resulting microstructures and non-oxide yields. Results indicated that the volumetric changes which occurred during in-situ conversion of single component precursors negatively impacted the surface layer microstructure. To circumvent the internal stresses and cracking that accompanied the conversion of Si or Hf (that expands upon conversion) or SiOx (that contracts during conversion), optimized ratios of the precursor constituents were used to produce near isovolumetric conversion to the product phase. Phase characterization indicated that precipitation of SiC from the Si/SiO2 melt formed continuous, crack-free, and dense layers of 93.7 wt% SiC that were approximately 35 µm thick, while sintered HfC/SiC composites (84.2 wt% yield) were produced from the laser-processing of Hf/SiO2 in CH4. By contrast, the SLRS of Si/SiOx precursor materials used to produce Si3N4 resulted in whisker formation and materials vaporization due to the high temperatures required for conversion. The results demonstrate that under appropriate processing conditions and precursor selection, the formation of near net-shape SiC and SiC composites might be achieved through single-step AM-compatible techniques.

Микроструктура и остаточные напряжения многослойных покрытий ZrN/CrN, полученных плазменно-ассистированным вакуумно-дуговым методом

Introduction. The current state of the art in the field of hard coatings application requires the formation of nanostructured compositions using different chemical elements. Modern hard coatings are able to combine different properties such as high hardness, wear resistance, corrosion resistance. At present, coatings formed by layer-by-layer deposition of zirconium and chromium nitrides are promising. When depositing combinations of chemical elements on various substrates, studies are required aimed at investigating its microstructure and, mainly, residual stresses formed during the deposition of multilayer coatings. The purpose of this work is to investigate the structural-phase state and residual stresses of ZrN/CrN system coatings formed by plasma-assisted vacuum-arc method from the gas phase. Research methods. Samples with coatings of zirconium and chromium nitrides deposited on substrates of hard alloy VК8 are investigated. Transmission electron microscopy is used to study the microstructural characteristics of multilayered coatings and X-ray diffraction analysis is used to quantify macroscopic stresses. Results and discussion. Based on the experimental results obtained it is found that changing the modes of deposition of multilayer ZrN/CrN coatings with regard to rotation speeds of table and substrate holder leads to variations in microstructure, morphology and internal stresses of surface layers of multilayer coatings. It is shown that by changing conditions for the multilayer coating deposition the possibilities of forming ZrN/CrN coatings on the substrate made of VK8 alloy with nanoscale thickness of coating layers open up. X-ray diffraction analysis indicates mainly insignificant stresses, and at high table and substrate rotation speeds – high compressive stresses in the multilayer coating. Transmission electron microscopy revealed that CrN and ZrN coatings have a common multilayer coating growth texture at low rotation speeds, and at high speeds a textural misorientation of the phases of the coating layers is observed. Based on the results obtained it is possible to recommend coatings of ZrN/CrN system as hard coatings.

Laser Surface Alloying of Sintered Stainless Steel.

A characteristic feature of sintered stainless steel (SSS) is its porosity. Porosity results in a lower density of steel, making attractive components for producing lightweight structures and materials used in industry (e.g., the automotive industry or aerospace). Scientists also observe that porosity adversely affects steel’s properties, especially its strength properties. One of the proposals for improving the discussed properties is the use of surface treatment of sintered stainless steels, e.g., with the use of concentrated energy sources such as plasma beams or laser beams. However, this proposal is an incidental subject of research, which is not justified from the point of view of the obtained research results presented by a few research groups. In this study, the surface modification (surface treatment) of sintered stainless steel was presented. The authors proposed the use of two surface treatments in order to compare them and obtain the best results. The first treatment was the deposit of Cr3C2–NiCr coatings on SSS surfaces using the atmospheric plasma spraying (APS) method. The second treatment was to create surface layers on SSSs by laser alloying the surface with a CO2 laser. Due to high precision and ease of automation, the most common methods in surface alloying treatment are laser technologies. This research’s main aim was to analyze the microstructure and strength properties of the SSS surface layer. The research confirms that applying the Cr3C2–NiCr coating and modifying the surface layer through the laser alloying method improves the mechanical properties of SSSs.

Surface layer microstructure and mechanical characteristics analysis of cast iron during milling process under supercritical CO2-MQL

Compacted graphite cast iron (CGI) has a potential application for diesel engines in the automotive industry, but the poor machinability has further impeded the application of CGI. The supercritical CO2-based MQL (ScCO2-MQL), a combination of advantages in both ScCO2 and MQL, has excellent heat dissipation and lubrication performance. The effects of ScCO2-MQL on the surface integrity of CGI at different cutting parameters in the milling process were systematically investigated by experiments and modelling analysis. The variation of microstructure, nano-mechanical properties, and residual stress distribution in machined surface layers of CGI at dry and ScCO2-MQL with different cutting parameters are obtained. To further reveal the underlying reasons for cooling and lubrication effects of ScCO2-MQL in the milling of CGI. The cooling mechanism of ScCO2-MQL was explained by the introduction of the forced heat convection and coolant boundary layers. The heat transfer model of ScCO2-MQL was established. The temperature distribution along the tool surface is presented considering similarity theory analysis. The lubrication mechanisms are also revealed by the liquid phase penetration model. The results verified that the ScCO2-MQL, by the introduction of higher compressive stress amplitude/depths and large hardened layers, can improve anti-fatigue characteristics and service performance of the CGI component in the automotive industry.

Microstructure and mechanical properties of SiC particle reinforced Zr-based metallic glass surface composite layers produced by laser alloying

Metallic glasses (MGs) are a promising candidate for advanced structural applications due to their superior mechanical properties. Improving their surface mechanical properties would be of great importance for promoting their structural and functional applications. In this study, SiC particles were used as the reinforcement to improve the mechanical properties of Zr-based MG via laser surface alloying. The influences of the average laser power and overlap ratio between neighboring laser processing lines on the microstructure and mechanical properties of the laser-alloyed surface layer were investigated. The experimental results indicated that ZrC and SiC phases were successfully introduced into the MG matrix by laser surface alloying, and the content of these two hard ceramic phases was dependent on the laser processing parameters. The formed laser-alloyed surface layers exhibited a significant improvement in overall hardness compared with the as-cast specimen. At a relatively high overlap ratio of 70%, the average hardness of the MG matrix within the laser-alloyed surface layer reached 28.91 GPa, which was three times higher than that of the as-cast specimen (6.46 GPa). Furthermore, the microstructural characteristics and mechanical properties of the cross-sections of the laser-alloyed samples were characterized. The thickness of the laser-alloyed surface layer reached several tens of microns, and the SiC particles were uniformly dispersed in the whole laser-alloyed surface layer. This study confirms the feasibility for improving the mechanical properties of MGs by laser surface alloying, which is expected to broaden the application of MGs as structural and functional components under harsh severe conditions.