Higher Microhardness Values Research Articles

Investigation of surface integrity in end milling of 55NiCrMoV7 die steel under the cryogenic environments

Cryogenic assisted machining is experiencing growing popularity and acceptance as a toxic-free, eco-friendly, hazardless process producing improved structural components. This article deals with the analysis of 3D and 2D roughness profiles, surface morphology, residual stress and microhardness of 55NiCrMoV7 die steel after end milling operation under dry, wet, cryogenic CO2 and LN2 cooling environments. Among different cooling methods, the cryogenic CO2 was seen enhancing the surface topography and morphology due to the presence of minimal wear track. The result indicates the production of an insignificant amount of residual stress in the machined surface by the cooling environments at a spindle speed of 1989 rpm and feed rate of 0.02 mm/rev. The surface microhardness values were higher under cryogenic conditions compared to dry and wet conditions. Cryogenic LN2 provided the highest microhardness value among the four cooling methods.

Microstructure, mechanical and tribological properties of NiAl matrix composites with addition of BaO/TiO2 binary oxides

BaO and TiO2 reacted and produced BaTiO3 in the process of sintering. As a result, the diffraction peaks of BaTiO3 became stronger with the increasing BaO/TiO2 contents. The diffraction peaks of NiAl shifted smaller angle to left with the addition of BaO/TiO2. Meanwhile, the lattice distortion obviously occurred in the composite structure. The composites with the addition of BaO/TiO2 presented high microhardness and yield strength as well. The composite with 20% BaO/TiO2 had the highest microhardness value and yield strength of 474.5 HV and 2786 MPa, respectively. The friction coefficient and wear rate of the composites decreased with the addition of BaO/TiO2 and the NiAl-30% BaO/TiO2 composite had the lowest friction coefficient at 800 °C test temperature. The BaTiO3 was decomposed to BaO and TiO2 during high temperature sliding.

Phase transformations in nickel- and titanium-based alloys during chemical heat treatment

The structure of surface areas and cores of Ni-40Cr-4Al and Ti-6Al-5V alloys in the process of thermal and chemical heat treatment - nitriding, oxidation and carburizing was studied by X-ray diffractometry, optical microscopy and hardness measurement. Ni-40Cr-4Al alloy after quenching contains two supersaturated solid solutions based on fcc-Ni and bcc-Cr and a small amount of compound Ni3Al; subsequent aging leads to decrease in the degree of supersaturation of fcc-Ni and increase in the amount of bcc-Cr and Ni3Al. Nitriding leads to the formation of continuous CrN layer 3-4 µm thick on the surface, decomposition of Ni3Al and formation of AlN in underlying layer 100-140 µm thick. After nitriding high microhardness value of CrN (up to 1060 HV) and a slight decrease from 740 to 650 HV in the subsurface layers due to the transformation of Ni3Al to AlN were found. The surface layer of Ti-6Al-5V alloy is oxidized by erosion cutting in water, but the oxides formed during this process do not influence on subsequent diffusion saturation. During nitriding, oxidation, carburizing of Ti-6Al-5V alloy, diffusion layers are formed. They are characterized by the increased amount of α-solid Ti solution and formation of the surface intermediate phases. The thickness of the hardened layers is 8-20 μm. The hardness of these layers is 1000 HV for oxidation, 1000 HV for nitriding, 570 HV for carburizing.

Spheroidal growth of graphite in arc plasma treatment

The present work is about the growth of spheroidal structure in graphite disc via thermal arc plasma treatment route. Arc plasma treatment of graphite discs was carried out between 5 and 10 min. Graphite with spheroidal structure was found to show unique physical properties like high specific surface and high microhardness. The properties of samples were characterized with XRD, XPS, micro Raman, FTIR, FESEM, TEM, EDS, microhardness and BET surface area study. The typical 10 min arc plasma treated graphite sample shows high values of BET surface area and microhardness of 1430 m2 g−1 and 620 VHN respectively than the untreated one.

New technique of radial-additive friction stir repairing for exceeded tolerance holes

Radial-additive friction stir repairing (R-AFSR) was proposed to repair exceeded tolerance holes. Here, AZ31 Mg alloys were selected as the primary research object. Good metallurgical bonding appeared in the repaired zones due to atom diffusion induced by thermo-mechanical behaviors. The diameter of the exceeded tolerance hole was reduced from 10 to 8 mm. The stir zone and partial thermo-mechanically affected zone were characterized by fine and equiaxed grains. The average grain size in the stir zone was less than 10 μm, attributing to high microhardness values. A high-quality repaired joint with the maximum tensile strength of 183 MPa was achieved, equivalent to 93.8% of the joint with the standard hole for AZ31 Mg alloys. Compressive shear strength and compression rate of the repaired joint reached 58.4 MPa and 26.1%, up to 75% and 74.6% of the joint with the standard hole. The fracture surface morphologies exhibited the typical ductile fracture. Therefore, R-AFSR has great prospects to repair the exceeded tolerance holes.

Microstructural and tribological characterization of molybdenum–molybdenum carbide structures produced by spark plasma sintering

Abstract Pure molybdenum is one of the most important refractory metals owing to its high melting point, low thermal expansion coefficient and good thermal conductivity. Molybdenum carbide exhibits superior wear resistance to molybdenum owing to its low friction coefficient and high hardness; therefore, in this study, to endure abrasive conditions, the surface of molybdenum was modified as molybdenum carbide. In accordance with this purpose, pure molybdenum powder was sintered under constant pressure (40 MPa), various temperatures (1 650 °C, 1 700 °C and 1 725 °C) and holding periods (180 s, 360 s and 540 s) using the spark plasma sintering technique. The effects of sintering temperature and holding period on relative density, microhardness, microstructure and wear properties of the specimens were investigated. Moreover, carbide formation on the surface due to carbon diffusion was also investigated. Relative density values decreased with increasing sintering temperature and period, obtaining the highest value of 97.55 % at 1 650 °C and 180 s. A microhardness value of 15.20 GPa was reached on the surface while the maximum value measured at the cross-section was 2.17 GPa, showing the formation of molybdenum carbide structure on the surface of all specimens. Furthermore, microstructural analysis supported the microhardness results and showed a minimum 288-μm thick molybdenum carbide layer on the surface. In addition, a typical eutectic microstructure was observed between molybdenum and molybdenum carbide layers; however, the decrease in temperature decreased the thickness of the eutectic layer and it gradually disappeared from the centre to edge of disc-shaped specimens. Wear resistance of surfaces was improved by decreasing holding period where a non-porous carbide layer and highest microhardness values were achieved. The cross-sectional analysis corresponding to molybdenum base metal proved that the formation of molybdenum carbide layer because of carbon diffusion improved the wear properties of the sample with the low friction coefficient and low wear depth.

Surface modification of pure titanium via friction stir processing: Microstructure evolution and dry sliding wear performance

In this study, the microstructural evaluation and wear behavior of multi-processed pure titanium using friction stir (FS) technique was evaluated. The microstructure characterization illustrates that the FS processing produced strong texture in titanium microstructure and induced Widmanstätten morphology formation in the following passes. Therefore, the multi-passes FS processing of pure titanium create the Widmanstätten morphology and improve wear resistance. The improvement in the wear properties of FS processed samples can also be expressed by higher microhardness values, grain size refinement, and higher amount of Widmanstätten morphology. The decrease in worn track width and debris size as a result of the increase in FS processing number implies a decrease in wear rate and a wear mechanism change from adhesive to abrasive.

Microstructure and Mechanical Properties of Titanium–Zirconium–Molybdenum and Ti2AlNb Joint Diffusion Bonded with and without a Ni Interlayer

Diffusion bonding of titanium–zirconium–molybdenum (TZM) and Ti2AlNb with and without a Ni interlayer is studied, and microstructure, residual stress, and mechanical properties of the joints are investigated. The results show that cracks appear in the TZM substrate near the joint during the direct diffusion‐bonding process, owing to detrimental residual stress from coefficient of thermal expansion (CTE) mismatch of the bonded couples. Herein, the maximum shear strength is only 240 MPa, corresponding to the joint bonded at 950 °C for 60 min at 20 MPa. When a 60 μm‐thick Ni interlayer is added, sound diffusion‐bonded joints without cracks are obtained, and the maximum shear strength reaches 340 MPa. The typical microstructure of the TZM/Ni/Ti2AlNb joint is TZM/NiMo + Ni3Mo/Ni4Mo/residual Ni/TiNi3/AlNi2Ti + Ti3Ni4 + (Ti, Ni, Nb) phase/Ti2Ni + B2(Ni)/Ti2AlNb. The residual Ni interlayer in the joint absorbs residual stress by plastic deformation, which is beneficial for the improvement of the joint shear strength. The residual stress in the joint is reduced from 347 to 78 MPa by increasing the thickness of the Ni interlayer from 0 to 120 μm. Fracturing in the joint occurs in the Ti2Ni phase because the Ti–Ni compounds have the highest microhardness values.

Realization of High Thermoelectric Figure of Merit in GeTe by Complementary Co-doping of Bi and In

Summary GeTe and its derivatives have recently attracted wide attention as promising thermoelectric materials. The principle challenge in optimizing the thermoelectric figure of merit, zT, is the low Seebeck coefficient (S) and high thermal conductivity of GeTe. Here, we report a high zT of ∼2.1 at 723 K in In and Bi co-doped GeTe along with an extremely high TE conversion efficiency of ∼12.3% in a single-leg thermoelectric generator for the temperature difference of 445 K. In and Bi play a distinct but complementary role. In doping significantly enhances the S through the formation of resonance level, which is confirmed with first-principles density functional theory calculations and Pisarenko plot considering two valance band model. However, Bi doping markedly reduces the lattice thermal conductivity due to the formation of extensive solid solution point defects and domain variants. Moreover, a high value of Vickers microhardness (∼200 Hv, Hv = kgf/mm2) reveals excellent mechanical stability.

Role of aluminum and silicon species in the formation of hard sludge on the secondary side of PWR steam generators: Potential geopolymers contribution

Despite the exhaustive chemical control, modern pressurized water reactors (PWRs) steam generators (SGs) have experienced damage associated with the formation of hard sludge on top of tubesheet. This phenomenon is detrimental because it is linked to tube degradation problems on the secondary side of SGs by corrosion processes, such as outer diameter stress corrosion cracking (ODSCC) enhanced by tube deformation or denting at the top of tubesheet. There is evidence of the contribution of aluminum and silicon species to the binding and adhesion of iron oxides particles in the formation and consolidation of hard sludge during normal operation. In fact, characterization of SG deposit samples mainly constituted by hard sludge determines high concentrations of these species (up to 6.0 ± 0.1 wt% of Al and 2.7 ± 0.1 wt% of Si), together with high microhardness values (above 800 HV) measured at particular locations within collars or hard sludge particles.Taking into account the physicochemical conditions (low temperature and high pH) in the gap between tube and tubesheet of SGs due to the thermal contraction of both materials during transient conditions (cooldown), this work proposes and discusses the possible formation of geopolymers (inorganic aluminosilicate polymers with cement-like mechanical properties formed at low temperatures and high alkaline conditions) during this period. In order to support this proposal, the role played by Si and Al compounds in the formation and consolidation of hard sludge is reviewed, considering information obtained from hard sludge characterization and from experimental studies reported in the literature.

Analysis of the strain and hardness in self-cured and light-cured self-adhesive resin based cement

The study showed an application of the 3 D Digital Image Correlation Method (3 D-DIC) for detection of von Mises strain in samples of the self-etch, self-adhesive resin based cement (RBC). The aim was to determine and compare strain in the self-cured and light-cured Maxcem Elite, furthermore to investigate the hardness of these two cement-types. The experiment was carried out using two groups of Maxcem Elite (Kerr, Orange, CA, USA) samples; each tested group includes five (ø5 × 2 mm sized) samples, for both self-curing and light-curing mode. All samples were prepared by filling teflon ring-type molds. In addition, Vickers micro-hardness was measured for all samples. Maxcem Elite showed similar maximum strain values from 10% to 12% for both groups. Besides the maximum strain value, the 3 D-DIC method also enabled monitoring the change of strain field even after the recommended polymerization time. This method has shown that the polymerization shrinkage continues even after 10 min which disagreed with manufacturer’s suggestion. Group II showed maximum strain values of 12% in the peripheral zone after 10 min, in the last Stage (Stage 60). Statistically significant difference was not found in the overall strain between self- and light-cured Maxcem Elite neither peripherally (p = 0.118) nor centrally (p = 0.879). However, statistical significance was found in strain regarding central and peripheral zone in both, self-cured (p = 0.020) and light-cured (p = 0.002) Maxcem Elite. The mean von Mises strain values in the periphery of the samples (Section 0) were significantly higher compared to strain values in the center of the samples (for Section 1 and 2). The last stage (Stage 60) of the light-cured Maxcem Elite polymerization showed significantly higher values of von Mises strain compared to initial stage (Stage 0). Higher values of micro-hardness were noticed on the surfaces directly exposed to LED lamp after performing measurements of micro-hardness on light-cured samples.

Effect of Alumina (Al2O3) Particles on The Mechanical Properties of Magnesium (Mg)

In the present study, magnesium-based composites reinforced with different volume fractions (3, 5, 10, and 15) vol.% of micro sized Al2O3 particulates were fabricated by powder metallurgy technique which involves mixed, compacted and sintered. Powders were mixed by ball milling (without balls) for 6 hours at rotation speed 60 rpm. Then powder was compacted at 550 MPa and sintered at 530˚C for 2 hours. Microstructures of sintered composites have been investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) energy dispersive. SEM image of sinter samples exhibit good bonding between the magnesium matrix and the alumina. The microhardness and wear resistance of micro composites has been improved significantly compared to that of pure magnesium. Highest value of microhardness is 97 HV at the volume fraction of 10 vol.% Al2O3.

An attempt to correlate the physical properties of fossil and subfossil resins with their age and geographic location

Abstract Testing of the correlation between physical properties of natural resins such as microhardness, density and UV-excited fluorescence emission with their age, geological conditions, botanical and geographical origin and chemical structure was performed. These physical parameters, especially microhardness, are the result of resins fossilization processes like cross-linking and polymerizations of compounds present in the fossils. In addition, hardening of the resins may be also an effect of miscellaneous chemical processes induced by various environmental, biological and geological conditions. The principal component analysis found that the correlation of microhardness, density and fluorescence intensity with the resin age is quite low. The results suggest that variability of physical properties is caused by geographic location and locally occurring geological conditions. The physical properties of natural resins are most strongly correlated with chemical structure and geographic location. The resins with higher microhardness values come from marine environment depositions. The same trend was observed for resins affected by volcanic activity. Moreover, high fluorescence intensity was also observed for resins affected by above mentioned geological conditions. However, the density values of tested resins revealed the lowest correlation with their age, botanical source and geological history.

EFFECT OF TiO2 NANOPARTICLES ON THE MECHANICAL AND ANTICORROSIVE PROPERTIES OF Zn-Ni COMPOSITE COATINGS

In this paper, we are interested in the study of electrochemical, morphological and structural characteristics of the properties of the deposits of Zn-Ni-TiO2 obtained by electrodeposition on the mild steel substrate in a bath of sulfate. The principal aim is to improve the coatings with better properties, by incorporation of titanium oxide, which is a hard compound, chemically stable and irreducible. The characterization of the coatings was carried out by scanning electron microscopy (SEM) and by X-ray diffraction. The morphology of the film surface varies with the concentration of oxide titanium and it was found higher values of microhardness. Electrochemical characterization of the composites had been carried out through potentiodynamic polarization. The results showed that better corrosion resistance with the incorporation of oxide titanium.

LOW-TEMPERATURE NITRIDING BEHAVIOR OF COMPRESSIVE DEFORMED AISI 316Ti AUSTENITIC STAINLESS STEELS

The effects of compressive cold deformation under the quasi-static loads on the nitride formation, nitride layer growth and surface hardness properties were researched in this study. Martensite structure did not form in AISI 316Ti stainless steel as a result of quasi-static deformation. Diffusion layer did not form in all nitrided samples. Both the deformed and undeformed samples have only compound layer on the surfaces at the low-temperature nitriding conditions (400∘C, 7[Formula: see text]h). According to the X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) and electron probe microanalysis (EPMA) results, S-phase and chromium nitride (CrN) were formed in the compound layers of the deformed samples. However, CrN did not form in the compound layer of the undeformed sample. The optical microscope (OM) results showed that the compressive cold deformation increased the nitrogen diffusion rate and led to thicker nitrided layer than the undeformed sample under the same plasma-nitriding conditions. All nitrided layers presented higher microhardness values ([Formula: see text][Formula: see text]HV) when compared with the untreated sample hardness. It was also verified that the deformation amount did not affect significantly the nitrided layer hardness.

Residual stresses and microstructural evolution of ECAPed AA2017

The mechanical behavior and microstructural evolution of an AA2017 aluminum alloy processed by ECAP with an equivalent simple shear deformation of ∼6 at 200 °C were studied. Samples were characterized by means of scanning electron microscopy (SEM-EDS- EBSD), image-assisted by focus ion beam (FIB), Vickers microhardness and X-ray diffraction (XRD) techniques. During the deformation process, the Al2Cu precipitates did not get fragmented or re-absorbed in the Al matrix. After the first ECAP pass, at least 50% of grains displayed an ultrafine size. The EBSD analysis showed an increment of the misorientation angle immediately after the first ECAP pass. The macrotexture evolution was explained in terms of the formation of f1: A1θ∗, Aθ, Āθ, A2θ∗, f2: Cθ, B¯θ, Bθ, Āθ, Aθ, A1θ∗ and f3: Cθ, Bθ, B¯θ, Aθ, Āθ, A2θ∗ fibers. The macro-residual stress measurements of the highly deformed samples showed linear sin2ψ profiles. The micro and macro-residual stresses were compatible with dislocation rearrangement, in which the annihilation and formation were in quasi-equilibrium. It was found that the highest microhardness value (1176 MPa) and grain refinement (at least 20% of grains showed a size smaller than 100 μm2) appeared after the first extrusion pass. The decrease in hardness, after the second pass and the residual stress stability, could be associated to a dynamic recovery phenomenon.

Alumina-based composites reinforced with single-walled carbon nanotubes

Monolithic Al2O3 ceramics and Al2O3 - 3 vol% single-walled carbon nanotubes (SWCNTs) composites were prepared by spark plasma sintering. The influence of SWCNTs and sintering temperature on sintering behavior and mechanical properties were investigated. Nanotubes were relatively homogeneously distributed in the composite powder, although some agglomerates/bundles existed. It was found that SWCNTs addition retards slightly the sinterability of alumina by nanotubes hindering of particle rearrangement. The average microhardness of the composites was lower than that of Al2O3 ceramics, but there were also high microhardness values of composites (20.41 GPa). The reduction in microhardness is explained in this paper. The average values of the fracture toughness (4.94 MPa-m1/2) from the composite sintered at 1500 °C were approximately 6% and 12% higher than those from the Al2O3 ceramics sintered at 1500 °C and 1600 °C, respectively. In addition, the analysis of the phase composition and the parameters of the crystal structure of the samples were made.

Development of spark plasma sintered TiAlSiMoW multicomponent alloy: Microstructural evolution, corrosion and oxidation resistance

Abstract TiAlMoSiW high entropy alloy was fabricated using spark plasma sintering under different sintering temperatures. The microstructural and morphological characteristics of the alloy were investigated using X-ray diffractometry (XRD) and scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The mechanical and corrosion properties of the alloy were tested using diamond base indentor and potentiodynamic polarization. Thermogravimetric analysis and density measurements were used to check the thermal stability and densification of the samples. Microstructural analysis of the samples showed that the samples exhibited a bcc matrix with secondary phase precipitate of TiSi2. The microhardness was found to be subject to sintering temperature and densification with high microhardness values obtained 1000 °C and 98% densification. The presence of elements that form stable protective film were attributed to the high corrosion performance of the samples. These elements also form stable oxide scales in elevated temperatures improving the thermal stability of the alloy. Excessive oxidation was noticed to occur at temperatures above 535 °C showing that the alloy has superior thermal stability than Ti-6V-4Al.

Влияние матрицы на свойства керметных плазменных покрытий на основе карбида титана

Comparative studies of cermet coatings with titanium carbide and matrices from high-speed steel grade 10Р6М5 and self-fluxing alloy based on nickel H77X15C3P2 were carried out. The powders were obtained by self-propagating high temperature synthesis (SHS). The coatings were formed by plasma spraying with local protection. The introduction of additional carbon into cermet TiC–10Р6М5 + 2.38 C made it possible to compensate for carbon losses during plasma spraying and to form a coating with an additional carbon content of 1.35%. The content of the titanium carbide phase in the coating is retained; however, the period of the carbide lattice is reduced, the more so for the coating with a nickel-based matrix. The content of oxygen and nitrogen in the coatings with respect to the initial powders increases mainly in the cermet containing the steel matrix. The presence of silicon and boron in the cermet grade obtained with the matrix from the alloy H77X15C3P2 did not reduce the loss of carbon in the coating. The microhardness of the obtained coatings, measured with a load on the indenter of 20 g, is 19–65% higher than the microhardness of the particles of the sprayed powders. The highest values of microhardness, 16.34 GPa, were obtained in a cermet coating sprayed from a powder with an additional carbon content of 45.23 TiC–52.39 10Р6М5–2.39 C.

Invеstigation of mechanical stability of the АMA Fe82Nb2B14RE2 (RE = Y, Gd, Tb, Dy)

The aim of study is to evaluate the mechanical properties of amorphous metallic alloys samples: Fe 84 Nb 2 B 14 , Fe 82 Nb 2 B 14 Y 2 , Fe 82 Nb 2 B 14 Gd 2 , Fe 82 Nb 2 B 14 Tb 2 , and Fe 82 Nb 2 B 14 Dy 2 after the temperature treatment. The investigated amorphous metallic ribbons were synthesized by melt spinning technique in helium atmosphere onto a copper wheel with a circumferential speed of about 30 m/s. Features of the technological process of Fe 82 Nb 2 B 14 RE 2 (where RE = Y, Gd, Tb, and Dy) amorphous ribbons producing cause the difference between their contact and external surfaces in mechanical properties. By X-ray diffraction method has been established that the Fe 82 Nb 2 B 14 RE 2 (RE = Y, Gd, Tb, and Dy) initial amorphous alloys have an amorphous structure. The microhardness of the samples in ribbon form has been measured by the use of Vickers technique. It shown the external side of the initial amorphous metallic alloys was characterized by higher the average microhardness than the contact side. It was established that base alloy of the Fe 84 Nb 2 B 14 after alloyed by the rare-earth elements (RE) increase microhardness on 0.85–1.90 GPa for contact surfaces and 3.04–3.89 GPa for external surfaces. The amorphous alloy Fe 82 Nb 2 B 14 Dy 2 has the highest value of microhardness (10.63–13.33 GPa) among alloyed initial samples. The difference between the mechanical properties of the both contact and external surfaces of the corresponding alloys decreases because of annealing at the nanocrystallization temperature. That could by explained by large number of clusters and the change of the free volume in the amorphous matrix of the alloys. The main influence on the mechanical properties causes annealing of ribbon samples in the air atmosphere because of changes of the AMА structure and formation of dense surface layers. Keywords: Amorphous metallic alloys, rare earth metals, thermal annealing, microhardness.