Microhardness Gradient Research Articles

Nonuniformity of Fatigue Properties of Two‐Sided Electron Beam Welded Joint of Superthick Titanium Alloy

ABSTRACTThe TC4 titanium (Ti) alloy has been widely utilized in various industries such as aerospace and shipbuilding, owing to its numerous advantages. Its exceptional properties have made it a material of choice for applications requiring high performance and reliability. The total weld thickness was 140 mm, and microstructures and high‐cycle fatigue properties were investigated at three different layers within the 140‐mm section. Experimental results show that the overlap area at two weld roots has the highest microhardness and largest nonuniformity of the overall joints, so the area is found to have the poorest high‐cycle fatigue properties. The microstructures in every layer of the weld metal of the joints were analyzed to determine the causes of the poor fatigue properties in the overlapping area at the weld roots. Significant amounts of α′ acicular microstructure are present in the weld metal of joints, while the overlap area at weld roots contains more α′ acicular microstructure with a finer size. Compared with other layers, the weld metal and base metal in Layer 2 (overlap area) have the largest microhardness gradient, where the stress concentration is more serious in the fatigue experimental process. Heat dissipation conditions was a critical factor for the inhomogeneity of microstructure and mechanical properties along the thickness direction of 140‐mm double‐sided electron beam welding joint. Fatigue damage (micropore) is formed at β phases in priority, and fatigue cracks propagate via series connection of micropores, indicative of transgranular ductile cracks.

Effects of ultrasonic shot peening followed by surface mechanical rolling on mechanical properties and fatigue performance of 2024 aluminum alloy

Aiming to improve the mechanical properties and fatigue performance of 2024 aluminum alloy, the experimental investigations on composite treatment consisting of ultrasonic shot peening (USP) followed by surface mechanical rolling (SMR) were carried out. The experimental results were compared with the specimens only treated by USP or SMR in terms of surface roughness, microhardness gradient and microstructure observation. The uniaxial tension test and low cycle fatigue test were conducted on the as-received specimen and the ones treated by USP, SMR and USP/SMR composite treatment, respectively. The tensile mechanical properties were effectively improved by USP. The introduction of SMR following USP can significantly increase the number of cycles to failure. Combining fracture morphology analysis and DEM-FEM coupling simulation of USP/SMR composite treatment, it was concluded that the improvement of tensile mechanical properties is mainly attributed to the synergistic effect of the compressive residual stresses and gradient-structured layer produced by USP, and the decrease in surface roughness resulting from SMR is the main reason for the significant improvement of fatigue performance. This work could provide an insight into the surface strengthening mechanism for USP/SMR composite treatment of materials with respect to the improvement of mechanical properties and fatigue performance.

Microstructure evolution and local mechanical properties of friction stir additively manufactured (FSAM) AA5083/AA6061/AA7075 gradient composite

The current study is focused on the successful fabrication and characterization of microstructure and properties gradient composite made of 3 mm thick sheets of AA5083-O, AA6061-T6, and AA7075-T6 alloys using friction stir additive manufacturing (FSAM) technique. Two combinations of four layered FSAM builds (6756 and 7567) were fabricated successfully without any major defects at the optimized process parameters of 850 rpm (tool rotational speed) and 55 mm/min (tool transverse speed). The material mixing, interfacial bonding features, microstructure evolution, and mechanical performance within the stir zones (SZ) were thoroughly examined using advanced characterization techniques. Electron backscatter diffraction (EBSD) analysis confirmed a gradient microstructure along the build depth in both FSAM builds, with average grain sizes decreasing from 52 μm (AA5083), 46 μm (AA6061), and 40 μm (AA7075) in base metals to ∼ 4–7 μm within the SZ. The remarkable grain refinement within the SZ was mainly attributed to the dominant presence of the continuous dynamic recrystallization (CDRX) mechanism. Texture analysis corresponding to pole figures (PFs) and orientation distribution function (ODF) plots reveals the crystallographic texture evolution along the build depth. The prevalent existence of B/ B‾ and C textures throughout all regions of the FSAM builds affirms the presence of ample shear strain during the FSAM procedure. Through thickness miniature tensile and microhardness tests depict the gradient in mechanical performance for both the FSAM builds along the build depth. Microhardness gradients in the range from ∼ 80 HV0.1 to ∼ 145 HV0.1 were observed along the build depth. The axial sample in the build direction shows appreciable strength (265 MPa and 224 MPa) and uniform elongation (28 % and 24 %) for 6756 and 7567 FSAM build, respectively. Furthermore, remarkable strain hardening behaviour corresponding to hardening capacity (Hc) and hardening exponent (n) was noticed for both the axial build samples compared to AA6061-T6 and AA7075-T6 base metals. These findings underscore the potential of FSAM in fabricating functionally gradient composite materials tailored to specific functional requirements.

Electrofriction treatment of plow shares

This paper presents the results of research aimed at developing the technology of plow share hardening by means of electrofriction hardening. It is shown that in electrofriction hardening of plow shares a structure with microhardness gradient is formed along the depth of the hardened zone. After electrofriction hardening the microhardness of plow share increases in 3–3.5 times in comparison with the initial state. The reason for the gradient character of microhardness distribution along the thickness of the modified layer is the ultra-high cooling rate, which causes a high temperature gradient near the surface. On the basis of the results of scanning electron microscopy it is established that at electrofriction hardening of steel 40Kh the hardened surface layer is formed, consisting of two zones: the surface hardened zone with the structure of fine–needle martensite and austenite; the zone of thermal influence (transition layer) with martensite-perlite structure, smoothly passing into the initial ferrite-perlite structure. It is established that the phase composition of steel 40Kh in the initial state consists of α-Fe phase with BCC lattice, and after electrofriction hardening the hardening phases of residual austenite (γ-Fe) and martensite (α'-Fe) are formed. The obtained data allow us to conclude that electrofriction treatment is an effective method of plow share hardening from structural steel 40Kh.

Study of changes in the structure of zirconium alloy E125 after deformation by radial shear rolling

The rheological properties of the alloy E125 (Zr‑2.5 %Nb) were studied in the range of deformation rates 0.5–15 s‑1 and the temperature range 20–770 °C. A database for computer FEM modeling was created in the specified temperature and speed ranges. In the DEFORM‑3D software package, numerical simulation of a complex radial shear rolling process was carried out using the obtained database. The conditions conducive to the grinding of the alloy structure into an ultrafine‑grained state were determined. Based on the simulation results, a full‑scale experiment of rolling a bar made of alloy E125 on a radial shear rolling mill RSP‑14/40 in 7 passes from a diameter of 37 mm to 20 mm with a total compression in diameter of ε = 85 % was carried out. At the same time, according to the modeling, the total accumulated deformation in the most developed peripheral zone was 27,5 mm/mm. Due to the complex vortex flow of the metal, the distribution of accumulated deformation over the cross section was uneven with a gradient to the axial zone. This should also affect the change in the structure. The changes and the gradient of the structure were studied by the method of EBSD mapping of the sample section with a resolution of 2 mm. The microhardness gradient of the cross section was also investigated by the HV method. The axial and central zones of the sample were studied on the. The structure has a pronounced gradient from the formed equiaxed ultrafine‑grained (UMZ) structure on several outer millimeters of the peripheral section to an elongated rolling texture in the center of the rod. The work shows the possibility of processing with the formation of a gradient structure with enhanced properties for the E125 alloy, and also presents a database for FEM calculations.

Модифицирование поверхности быстрорежущего инструмента совместным насыщением вольфрамом и азотом

Research significance is contingent on the requirement strengthening for high-speed cutting tools performance and wide-spread introduction of automatic lines and NC-machines. An increase of the tool service life is also necessary for the reduc-tion of putting expensive alloying elements to use, primarily, tungsten. The solution of these problems requires the use of technologies for hardening cutting surfaces. Combined processes of thermochemical treatment processes, uniting diffusive surface alloying with nitrogen saturation have shown their effectiveness in the surface hardening of various steels. Now the aim is to study the process of combined surface tungsten and nitrogen saturation of high-speed steel for increasing small-sized tool durability. Experimental studies were carried out on samples and small-diameter drills made of P6M5 steel. For laboratory experiments connected with combined process of thermochemical treatment an installation for nitriding in multi-component media was used. Metallization with tungsten was carried out by the slip method with parallel nitriding of the tool in a glow discharge. To determine the regime that provides the necessary temperatures for oxygen and nitrogen saturation, the temperatures of the control steel samples were measured on the surface and in the core at different durations of the cur-rent pulse in the heating phase. Metallographic analysis proved that thermochemical treatment resulted in a modified sur-face layer with a thickness of 10…15 microns, formed in P6M5 steel. The structure of the layer is an internal nitriding zone, which consists of a solid tungsten and nitrogen solution in iron and dispersed inclusions of tungsten nitrides. Dispersion and solid solution hardening provide a two-fold increase in the microhardness of the modified W-N layer compared to the alloy base. A transitional diffusion zone of nitrogenous martensite has been revealed under the hardened layer, creating a smooth microhardness gradient from the layer to the core, protecting it from embrittlement, peeling and staining. Using a metallophysical simulated test in predeveloped methodology, the calculation of the hardening index of the modified layer (yield point increase) was made. It showed that with increase in the concentration of tungsten in the layer, the proportion of the component of the dispersion hardening by W2N particles also increases. Full-scale tests in production conditions showed that the tool with a hardened layer had increased resistance. The durability of drills, determined by the number of drilled holes made before its dropping-out, increases by 2,2 times when drilling on 30XGSA steel and by more than 7,0 times when drilling a titanium alloy VT-23.

The Influence of Two-Jet Gas Shielding Parameters on the Structure and Microhardness of Steel 45 Joints during Consumable Electrode Welding

The paper presents the study results of the parameters influence of arc welding with a consumable electrode with two-jet gas shielding in CO2 on the structure and microhardness of high-strength steel 45 welded joints with slotted edges. Controlling the dynamic impact of the internal shielding gas jet on the processes in the welding zone changes the heat and mass transfer processes in the welding zone and results in the intensive mixing of the molten electrode metal with the base metal in the weld pool. The results of the studies determined the dynamic effect of the active shielding gas jet on the structure and microhardness of multilayered steel 45 welded joints with slotted edges using the method of full factorial experiment, developed dependences of chemical elements (carbon, silicon, manganese) content in the weld metal of multilayered steel 45 welded joints on the controlled parameters of the welding mode (Q, Iw, U). Due to uneven heat introduction into each of the welded plates during edge slotting, the asymmetric distribution of microhardness in the cross sections of welded joints relative to the weld axis indicates some differences in the structure and properties of the heat-affected zone (HAZ) and the weld. According to the results of the studies, consumable electrode arc welding with two-jet gas shielding provides faster distribution and equalization of heat on the product surface and reduces its instantaneous overheating, which improves the structural phase state of the welded joint made of steel 45 and reduces the microhardness gradient in the HAZ. An increase in the heat input of welding (a simultaneous increase in the welding current and voltage of the welding arc) leads to a decrease and smoothing of the microhardness peak in the HAZ.

Effect of a gradient structure on the mechanical performance of Inconel 718 Ni-based superalloy at elevated temperatures

The effect of a gradient structure (GS) with present precipitates on the tensile properties of Inconel 718 (IN718) alloy at elevated temperatures was studied. A high-density δ-Ni3Nb phase and fine recrystallized γ grains were obtained in the surface gradient layer using an ultrasonic surface-rolling process (USRP) and subsequent heat treatment. The amount of the δ phase in the gradient layer decreased with the layer depth, while the size of the recrystallized γ grains exhibited an opposite trend. Remarkably, the GS did not show a microhardness gradient at room temperature but exhibited excellent strength and plasticity during high-temperature tensile tests. Compared with a uniform sample, the yield strength (YS) and ultimate tensile strength (UTS) of the specimen with a GS increased by 55 and 105 MPa, respectively, and the plasticity increased by 25% in the tensile tests at 650 °C. The δ phase pinned the RANS-scale grain hierarchy in the gradient layer and improved the thermal stability at 650 ℃. This GS promoted the accumulation of high-density geometrically necessary dislocations (GNDs) in the gradient layer during plastic deformation, resulting in additional back-stress strengthening. Therefore, forming a gradient structure with precipitates provides a new approach toward a good combination of strength and ductility at elevated temperatures.

Effect of process parameters of Plain Water Jet on the cleaning quality, surface and material integrity of Inconel 718 milled by Abrasive Water Jet

Abrasive Water Jet (AWJ) is considered as a promising milling method for difficult-to-machine aeronautical materials as Inconel alloy 718 (IN718). However, grit embedment during AWJ is known as a detrimental effect on certain applications as aircraft repair, where surface condition may play an important role on additive material technologies. To overcome this problem, Plain Water Jet (PWJ) has been used in the present study as cleaning process and demonstrated to be an effective method to remove grit particles from the surface with marginal alterations of surface state. In this paper, firstly the influence of AWJ process parameters on abrasive embedment and surface texture on IN718 specimens milled by AWJ were addressed. Then these surfaces were subjected to PWJ cleaning process and were extensively characterized in terms of grit embedment, surface texture and roughness, erosion depth, microhardness and residual stresses. Before cleaning, the milled surfaces presented a grit embedment level varying between 7 % and 14 %. One setting condition was selected for performing PWJ cleaning which reduced the grit level up to a quarter of the initial total surface area (less than 4 % in all cleaned surfaces) without relatively modifying neither the surface texture nor the erosion depth. Comparable microhardness gradients were observed before and after PWJ cleaning which corresponded to ∼ 30 % higher than the bulk values at surface and then decreased beneath the surface up to 200 µm before to reach bulk value (∼ 245 HV). Compressive residual stress state at surface initially induced by AWJ milling in some surfaces remained unchanged after PWJ process but in other ones was slightly relieved (∼ 50 MPa). Residual stresses after PWJ process resulted from − 630 MPa to – 315 MPa depending on the milling process parameters.

Investigation of wet shot peening on microstructural evolution and tensile-tensile fatigue properties of Ti-6Al-4V alloy

Ti–6Al–4V (TC4) alloy was treated by wet shot peening (WP) using ceramic beads compared with dry shot peening (SP). The effects of shot peening on surface integrity, microstructural evolution, fatigue crack initiation behavior and tensile-tensile fatigue performance were investigated applying multiple characterization techniques. The result exhibited that the surface plastic deformation caused by WP and SP treatment resulted in microstructure changes near the surface. Surface morphology was distinctly changed by shot peening treatment. The roughness value increased from 0.596 μm (Ra) to 1.594 μm (Ra). Microhardness gradients and compressive residual stress (CRS) distributions along the depth were induced by shot peening. The maximum hardness reached 373.5 HV, 407.8 HV and 445.3 HV. The maximum value of CRS reached 884 MPa, 1038 MPa and 1207 MPa with the corresponding depth at 21 μm, 25 μm and 40 μm. The fatigue life was significantly improved by shot peening treatment. Cracks tended to initiate at the subsurface layer instead of the treated surface after shot peening. The reason for crack initiation position altering was tentatively explained by the CRS stable zone. The competition mechanism between the surface stress concentration and local strengthening factors were discussed based on the analysis of stress concentration factor, Kt.

A study of micro hardness in the machining of Inconel 625 using TiAlSiN coated tools under dry cutting conditions

ABSTRACT Inconel 625, a nickel-based super alloy, has a high tendency to form a work-hardening layer during the milling operation, resulting in further difficulty to process it. However, Inconel 625 is extensively used in the manufacturing of aero-engine exhaust and in many other industries due to its admirable mechanical properties. Further, tighter environmental regulations and higher coolant and disposal costs associated with the nickel-based super alloys are forcing researchers to think about dry machining. PVD TiAlSiN coating is the most suitable in dry machining of difficult-to-cut materials. In this context, this study analysed the micro-hardness of machined surface and sub-surfaces up to 200 µm during the dry milling with coated tool. It was observed from the in-depth analysis of process parameters as well as their interaction effect with TiAlSiN-coated tool in dry conditions that it has a low influence on the micro-hardness gradient in the top layer of machined surface. A blend of the lowest level of cutting speed with the central level of feed per tooth, radial rake angle and depth of cut resulted in minimum micro-hardness due to the balancing of the effects of mechanical and thermal deformation.

Microstructure and element distribution characteristics of Y2O3 modulated WC reinforced coating on Invar alloys by laser cladding

As an essential material of aviation composite forming mold, Invar alloys will produce defects such as wear and crack during long-term service. In this paper, the rare earth oxide Y2O3 was added into the WC reinforced composite coating to improve the surface properties of Invar alloy. The microstructural evolution, element distribution and microhardness of composite coating were investigated by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and Vickers hardness tests. The mean area and proportion of regular cellular crystals were quantitatively measured. The effect of Y2O3 on the distribution of the W element in the coating is revealed. Results indicate that the addition of Y2O3 significantly increases the thickness of the coating and improves the melting efficiency. Furthermore, cellular dendritic crystals tend to transform into regular cellular crystals with higher Y2O3 amounts. The proportion of regular cellular crystals reaches up to 30.0% when the mass fraction of Y2O3 is 5%. The W and Y elements tend to converge at crystals boundaries. Meanwhile, the microhardness gradient of the cladding layer (CL) decreases with Y2O3.

Effect of Cryogenic Treatment on the Microstructure and Wear Resistance of 17Cr2Ni2MoVNb Carburizing Gear Steel

Cryogenic treatment as a process that can effectively improve the performance of steel materials is widely used because of its simplicity and speed. This paper investigates the effects of different low temperature treatments on the microstructure and properties of 17Cr2Ni2MoVNb steel. The low temperature treatment range is divided into cryogenic treatment (CT-80), shallow cryogenic treatment (SCT-150) and deep cryogenic treatment (DCT-196), all with a duration of 1 h. The retained austenite content and the change in carbide volume fraction at 0.2 mm in the carburised layer are studied. The microhardness gradient of the carburised layer, as well as the friction coefficient and wear scar morphology at 0.2 mm, was investigated. The results show that the low temperature treatment is effective in reducing the retained austenite content and increasing the volume fraction of carbide. The lowest retained austenite content and highest carbide volume fraction were obtained for DCT-196 specimens at the same holding time. Due to the further transformation of martensite and the diffuse distribution of carbides, the microhardness and frictional wear properties of DCT-196 are optimal. Therefore, low temperature treatment can change the microstructure of the case layer of 17Cr2Ni2MoVNb steel and effectively improve the mechanical properties of materials.

Comparison of fatigue performance of TC4 titanium alloy welded by electron beam welding and laser welding with filler wire

AbstractThis research compared high‐cycle fatigue (HCF) performances of joints of Ti–6Al–4V titanium (TC4) alloy with the thickness of 30 mm welded by vacuum electron beam welding (EBW) and laser welding with filler wire (LWFW) (hereinafter referred to as EBW and LWFW joints). Under test conditions, the fatigue strength of the LWFW joint is only 65% that of the EBW joint. Based on analysis, the main reason is that a larger microhardness gradient is present in the LWFW joint. The average microhardness of the weld metal (WM) of the LWFW joint is 41 HV lower than that of base metal (BM). A lot of punctate β phases in the WM of the LWFW joint may be an important reason for its softening. The research results provide data supports for the application of the EBW and LWFW of Ti alloy in the field of aviation manufacturing.

Microstructural and Mechanical Properties of B-Cr Coatings Formed on 145Cr6 Tool Steel by Laser Remelting of Diffusion Borochromized Layer Using Diode Laser

The paper presents study results focused on the microstructural, mechanical, and physicochemical properties of B-Cr coatings obtained by means of modification of diffusion borochromized layers by diode laser beam. The studies were conducted on 145Cr6 tool steel. Diffusion borochromized layers were produced at 950 °C in powder mixture containing boron carbides as a source of boron and ferrochrome as a source of chromium. In the next step these layers were remelted using laser beam. Powers: 600, 900, and 1200 W were used during these processes. The microstructure, microhardness, chemical composition, as well as wear and corrosion resistance of newly-formed B-Cr coatings were determined. As a result of laser beam interaction, the diffusion borochromized layer was mixed with the steel substrate. The study showed that too low laser beam power causes cracks in the newly formed B-Cr coating, and on the other hand, too higher laser beam power causes deep remelting resulting in the loss of microhardness. The reduced corrosion resistance in comparison with diffusion borochromized layers was caused by occurrence cracks or deep remelting. For B-Cr coatings produced using laser beam power 600 W, a small decrease in wear resistance was observed, but note that this coating was much thicker than diffusion borochromized layers. On the other hand, laser beam power of 1200 W caused a significant decrease in wear resistance. Newly formed B-Cr coatings had an advantageous microhardness gradient between the layer and the substrate.

Effect of shot peening on notched fatigue performance of powder metallurgy Udimet 720Li superalloy

The tensile and stress-controlled LCF tests of a powder metallurgy (PM) superalloy, Udimet 720Li have been conducted at 650 °C in order to investigate the effect of shot peening (SP) on notched low cycle fatigue (LCF) performance. The measurements of surface tomography, Micro-Vickers hardness, residual stress and failure mechanism analysis were conducted in detail. The results show that there are many regularly shaped machining traces on as-received (AS) surface while lots of highly disorganized dimples are mainly distributed on SP surface. A larger microhardness gradient is produced along the path from SP surface to the bulk of material compared with AS specimen. The maximum compressive residual stress of SP specimen is not more located at surface like AS one but at a depth of approximately 50–90 μm from surface. Compared with the applied stress, the effective stress curve of SP specimen becomes “flat” and the effective stress gradient decreases accordingly. The LCF lifetime of the notched specimen is significantly improved after SP process and the LCF lifetime increment is between 2.05 and 7.66 times. Finally, there are obvious differences on the failure mechanisms, such as fatigue crack initiation, propagation and final fracture between AS and SP specimens.

ВЫБОР МАТЕРИАЛА НАНОРАЗМЕРНОЙ ФАЗЫ И УСТАНОВЛЕНИЕ РЕЖИМОВ НАНЕСЕНИЯ НАНОКОМПОЗИЦИОННЫХ ГАЛЬВАНИЧЕСКИХ ПОКРЫТИЙ НА ОСНОВЕ ХРОМА

The article presents the results of research on the establishment of an effective nanoscale phase and modes of applying nanocomposite electroplating coatings based on chromium. As a result of the conducted studies, it was found that it is advisable to use a nanodispersed aluminum oxide powder as a nanoscale phase. The method of mathematical planning of the experiment was used to determine the optimal modes of coating and the concentration of nanoscale particles in the electrolyte. The microhardness of the obtained coatings was chosen as an optimization parameter, since it significantly affects their wear resistance. The highest microhardness of a nanocomposite electrolytic coating based on chromium is achieved when the electrolyte is heated to a temperature of 50 °C, a current density of 59 A/dm2 and a concentration of nanoscale phase particles in the electrolyte of 3.2 g/l, which ultimately corresponds to an increase in microhardness to 14.32 GPa. It is also established that nanocomposition coatings have a positive microhardness gradient in thickness, which allows leveling the difference in the values of the microhardness of the coatings and the base metal and will help to increase the adhesion strength of the coatings to the base on the one hand and their wear resistance on the other. Based on the microhardness measurements of chromium-based nanocomposition coatings, statistical series were formed. 30 samples were subjected to measurements. According to the results of microhardness measurements, the average square deviation of the values of nanocomposition coatings based on chromium was 0.05 (coefficient of variation 0.283). To equalize the obtained experimental microhardness information, the law of normal distribution is chosen, since the coefficient of variation, according to which the greatest probability of microhardness values of nanocomposite coatings based on chromium is observed in the range of 14.32–14.37 GPa.

Effect of Complex Magnetic-Abrasive and Chemical-Thermal Treatment on the Fatigue Resistance of VT8 Alloy Compressor Blades

The results of studies of the effect of complex magnetic-abrasive (MAT) and chemical-thermal (CTT) treatment on the fatigue resistance of compressor blades made of VT8 alloy are presented. The CTT method of the titanium alloy surface is a method of nitrocementation, which consists in the simultaneous saturation of the blade surface with carbon and nitrogen in a gas environment. Prior to that, MAT of blades were carried out to form favorable residual stresses in the surface layer, which stimulate the CTT process, provide its higher efficiency, productivity and the ability to create a diffusion layer of the required depth and quality. The results of measurements of microhardness of the surface layer of the blades in the initial state and after complex treatment are presented. The analysis of measurements showed that the microhardness of the initial blade tends to decrease steadily with small fluctuations to a depth of 0.25 mm, while on the blade after complex treatment by MAT+CTT+MAT we have three hardness peaks with higher values than the initial blades to a depth of 0.15 mm. The effect of complex treatment by technological cycle of MAT+CTT+MAT was estimated according to the results of comparative fatigue tests of VT8 alloy blades in the initial state and after treatment. It is shown that due to the formed reinforced layer on the surface of the blades after treatment, which has greater microhardness and microhardness gradient along the blade depth, the scattered damage accumulation under cyclic loading slows down, whereas the endurance limit of these blades based on N = 107 cycles is 14% higher than the endurance limit of the blades in the initial state.

Time-dependent Microhardness Gradients of Self-adhesive Resin Cements Under Dual- and Self-curing Modes.

Acid-functional monomers in self-adhesive resin cements may decrease their self-curing polymerization ability. Light irradiation optimizes polymerization performance. Purpose: The aim of this study was to investigate Knoop microhardness of self-adhesive resin cements under dual- and self-curing modes in simulated canals for describing the polymerization behavior.Methods and Materials: Slots in lightproof silicone cylinders with one open end were filled with the following eight materials: a traditional resin cement (Duolink), a core build-up resin material (MultiCore Flow), and six self-adhesive resin cements (RelyX Unicem 2, G-Cem Automix, Maxcem, Biscem, Multilink Speed, and PermaCem 2.0). The resins were exposed to light through the open end and then stored in a lightproof box. The Knoop hardness gradient for each resin was measured after 1 hour and 120 hours. Surface readings were obtained at 1-mm intervals from 1 mm to 10 mm away from the open ends. The data were analyzed by two-way analysis of variance and the Student-Newman-Keuls test (α=0.05).Results: All the resin materials had stable Knoop hardness numbers (KHNs) at a certain depth; their KHNs in the self-curing mode did not change (p>0.05). The region above this certain depth was regarded as having undergone the dual-curing mode, and the KHN decreased gradually with depth (p<0.05). Between 1 and 120 hours postexposure, the ratio of the KHN at a 5-mm depth (self-cured) to that at a 1-mm depth (dual-cured) increased in Duolink and MultiCore Flow. However, the ratios of the six adhesive resin cements varied.Conclusion: Without light, most self-adhesive resin cements differed from traditional dual-cured resin materials in terms of Knoop micro-hardness, and they had a lesser capacity for chemical-induced curing.

Microstructural Modification and Tensile Behavior of IF Steel Processed through Surface Mechanical Attrition Treatment

The present investigation deals with the impact of surface mechanical attrition treatment (SMAT) on the surface roughness, microstructure, phase stability, hardness, and tensile properties of interstitial-free (IF) steel. SMATed IF steel was characterized by visible-light microscopy, x-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy, and microindentation and tensile testing. The grain refinement and strengthening of the IF steel were found to be dependent on the duration of SMAT. The optimal combination of strength and elongation was observed in the IF steel SMATed for 200 s, showing a microhardness gradient up to a depth of ~ 900 µm and peak surface hardness of 2.6 GPa. The 0.2% yield strength of the IF steel was increased by ~ 125% with an appreciable elongation (~ 44%) almost comparable to that of the untreated material. Furthermore, IF steel SMATed for 200 s followed by stress relief at 300°C and 500°C was found to exhibit thermally stable gradient microstructures with good strength and ductility.