Presence Of Oxide Particles Research Articles

Advanced characterization of bulk alloy and in-situ debris nanoparticles formed during wear of Fe–Nb–B ultrafine eutectic laser cladding coatings

The microstructure of Fe76·5Nb8·5B15 (at.%) ultrafine eutectic (UE) coatings were processed by laser cladding, then the bulk alloy coating and wear debris were evaluated by advanced electron microscopy techniques in order to understand the ultrafine matrix and nanoscale intermetallic sub-micron and nanoscale phases formed, as a result of rapid solidification. In addition, according to the laser cladding processing parameters used, the coatings presented nanoscale borides that increased mechanical properties, with the coating michohardness values reached being four times higher than those of the AISI mild steel substrate. Combined with the sliding speeds, it played an important role in the definition of different wear mechanisms. The wear performance of the coatings and substrate was evaluated by three-body wear tests using a pin-on-disc device. It can be seen at lowest sliding speed a plastic deformation and for higher slidding speed, an accumulation of debris on the laser clad coating surface. Different wear mechanisms were observed in the coatings samples: abrasive, adhesive, shearing and delamination. The presence of oxide particles, responsible for the formation of a tribofilm, was also observed for some coatings. Debris nanoparticles from 10 to 100 nm, resulting from fracture or breakage of the coatings during wear tests, were also be identified as Fe-based and oxide nanoparticles. Thus, the Fe–Nb–B ultrafine eutectics laser clad coatings showed wear resistance properties comparable to or even superior to commercial alloys and other materials processed by rapid solidification.

Toward the Understanding of CoAl2O4 Additions on the Formation of Microstructure in Alloy 718 Processed by Laser Powder Bed Fusion

This investigation rationalizes the impact of oxide particles on the melt pool and subsequent formation of microstructure in superalloy Inconel 718 (Alloy 718) processed by laser powder bed fusion (L-PBF). A mixture of Alloy 718 powder with 0.2 wt pct of CoAl2O4 was processed without scan rotation. It was found that the physical presence of oxides appeared to increase the degree of melt pool penetration by inhibiting the heat transfer. Without favorable thermal conditions to trigger heterogeneous nucleation, a more noticeable tendency to form columnar grains was characterized in samples with CoAl2O4. It is believed that the presence of oxide particles in the melt pool contributed to decreasing the cooling rates during solidification and retarded the columnar-to-equiaxed transition. A comparatively deeper degree of melt pool penetration also enhanced the relative density of samples with CoAl2O4. Despite the occurrence of the balling phenomenon, the reduced Marangoni convection also contributed to less micro-humping and better continuity of the melt tracks that decreased the surface roughness by at least 16.7 pct. The addition of CoAl2O4 to the powder feedstock significantly impacted the processing window of L-PBF processed Alloy 718 and may potentially be used as a means to engineer the microstructure.

Different Types of Particle Effects in Creep Tests of CoCrFeNiMn High-Entropy Alloy.

Compressive creep tests were performed on a CoCrFeNiMn equiatomic alloy with the dispersion of (i) aluminum nitride or (ii) boron nitride at temperatures of 973 K and 1073 K. The results are compared with previously published creep rates of the unreinforced matrix alloy and the alloy when strengthened by yttrium + titanium oxides. The comparison reveals that the creep rate is essentially unchanged by the presence of aluminum nitride particles, whereas it is reduced by the presence of oxide particles. Boron nitride particles do not influence the creep rate at low stresses but reduce it substantially at high stresses.

A study of the microstructural stability and defect evolution in an ODS Eurofer steel by means of Electron Microscopy and Positron Annihilation Spectroscopy

An approach to improve the performance of steels for fusion reactors is to reinforce them with oxide nanoparticles. These can hinder dislocation and grain boundary movement and trap radiation-induced defects, thus increasing creep and radiation damage resistance. The present work investigates the thermal stability of the microstructure and the evolution of defects in a 0.3% Y2O3 dispersed Eurofer steel. Samples were annealed for 1 h under vacuum, from 600 K to 1600 K, followed by cooling inside the furnace. Electron Microscopy techniques and Vickers Hardness were used to characterize the microstructure and evaluate its thermal stability. Positron Annihilation Spectroscopy Doppler Broadening (PASDB) was used to monitor the evolution of defects, such as dislocations and vacancies, and their interaction with Y-O based nanoparticles. Several types of events take place simultaneously in the material, due to its initial deformation caused by mechanical alloying, the presence of oxide particles and austenitic phase transformation. Annealing up to 1000 K shows that the Y-O based nanoparticles keep the microstructure refined. Upon cooling from 1200 K (above Ac3), martensite forms with an equiaxed morphology, instead of the conventional lath form, due to the pinning of prior-austenite grain boundaries by the oxide nanoparticles. Annealing at 1400 K and 1600 K results in the progressive coarsening of Y-O based nanoparticles and their loss of ability to pin grain boundaries. PASDB shows that annealing up to 1200 K leads to an overall decrease in defect concentration, mainly due to recovery of dislocations. After annealing at 1400 K and 1600 K, PASDB indicates the formation of a different type of positron trap. The hypothesis is that, at these temperatures, clusters of thermal vacancies are trapped by the oxide nanoparticles, accumulating at their interfaces with the matrix and being retained in the material upon cooling to room temperature.

Effect of Nano-oxide addition on corrosion performance of hot dip zinc coating

Corrosion performance of hot dipped zinc coating on low carbon steel was studied at the presence of different nanoand micronsize oxide particles in the liquid zinc bath. Nano-silica, nano-alumina and micro-alumina powders were loaded to the different liquid baths, in the range of 0.05–0.2 wt %. Low carbon steel specimens were immersed in the baths for a constant time of 10s. It was evident that the presence of oxide particles in the liquid bath increased the coating thickness at a constant immersing time; micro-alumina particles provided the thickest coating among the others. Salt spray and potentiodynamic polarization tests were conducted to evaluate corrosion performance of the galvanized coatings, including oxide bearing ones. The results confirmed improvement in corrosion resistance of the nano-oxide bearing zinc coatings; while incorporation of micro-alumina in the bath declined its corrosion resistance. It was shown that incorporation of nano-silica powder in the liquid bath yielded superior corrosion resistance of the zinc layer, in comparison to the other ones. The optimum corrosion performance of zinc coating was achieved via loading 0.1 wt % nanosilica to the liquid zinc bath in this work.

Microstructure and mechanical properties of Fe-C alloy produced by mechanical alloying and spark plasma sintering

Nanocrystalline Fe with carbon concentrations of 0.2 wt.% and 0.4 wt.% was prepared by the mechanical alloying technique. The grain size after milling for 100 h was 22 and 17 nm for 0.2 and 0.4 wt.% C, which shows the carbon addition is beneficial in getting smaller nano-crystalline grain size. Consolidation into bulk shape with a relative density above 98% was performed with the spark plasma sintering technique at 650, 675, 700 and 730 °C. A maximum compression strength of approximately 2000 MPa was obtained for 650 °C consolidation sample with an absolute lack of plasticity, whereas a strength to plasticity balance could be obtained by increasing the temperature. Detailed microstructural analysis has been done with scanning electron microscopy and transmission electron microscopy. The microstructure shows a bimodal grain size distribution with a fine dispersion of oxide particles having a size ranging from 20 to 200 nm and cementite particles having a size range from 100 to 500 nm depending on the sintering temperature. The presence of oxide particles along with bimodal grain distribution resulted in a high yield strength value of 1620 MPa and high plasticity of 45%.

Polypyrrole and La 1− xSr xMnO 3 (0≤ x ≤0.4) composite electrodes for electroreduction of oxygen in alkaline medium

Composite G/PPy/PPy(La 1− x Sr x MnO 3)/PPy electrodes made of the perovskite La 1− x Sr x MnO 3 embedded into a polypyrrole (PPy) layer, sandwiched between two pure PPy films, electrodeposited on a graphite support were investigated for electrocatalysis of the oxygen reduction reaction (ORR). PPy and PPy(La 1− x Sr x MnO 3) (0≤ x ≤0.4) successive layers have been obtained on polished and pretreated graphite electrodes following sequential electrodeposition technique. The electrolytes used in the electrodeposition process were Ar saturated 0.1 mol dm −3 pyrrole (Py) plus 0.05 mol dm −3 K 2SO 4 with and without containing a suspension of 8.33 g L −1 oxide powder. Films were characterized by XRD, SEM, linear sweep voltammetry, cyclic voltammetry (CV) and electrochemical impedance (EI) spectroscopy. Electrochemical investigations were carried out at pH 12 in a 0.5 mol dm −3 K 2SO 4 plus 5 mmol dm −3 KOH, under both oxygenated and deoxygenated conditions. Results indicate that the porosity of the PPy matrix is considerably enhanced in presence of oxide particles. Sr substitution is found to have little influence on the electrocatalytic activity of the composite electrode towards the ORR. However, the rate of oxygen reduction decreases with decreasing pH of the electrolyte from pH 12 to pH 6. It is noteworthy that in contrast to a non-composite electrode of the same oxide in film form, the composite electrode exhibits much better electrocatalytic activity for the ORR.

Effect of nano-scale particles on the creep behavior of 2014 Al

The creep behavior of two grades of 2014 Al was investigated over six orders of magnitude of strain rate. While these two grades were prepared by powder metallurgy (PM), they have different oxygen levels (0.3 and 1.0 wt.% oxygen). The creep data show that the creep behavior of these two grades of PM 2014 Al is similar to that of dispersion strengthened (DS) alloys with regard to the following characteristics: (a) the apparent stress exponent is high and variable, and (b) the apparent activation energy is higher than that for self-diffusion in Al. An analysis of the experimental data indicates that there exists a threshold stress for creep whose temperature dependence is strong. The values of threshold stress increase noticeably with the increase of oxygen content in the alloy. This finding together with the observation of extensive interaction between moving dislocations and nano-scale dispersion particles indicates that the presence of oxide particles is responsible for the threshold stress creep behavior in 2014 Al. By considering the effect of the threshold stress, it is demonstrated that the true creep characteristics of PM 2014 Al are consistent with those reported for Al solid-solution alloys.

Microstructural Parameter-Based Characterization of Annealing Behaviour in Metals Deformed to High Strains

The transition from discontinuous recrystallization to a continuous recovery process during annealing of aluminium deformed to high plastic strains has been investigated by following several different microstructural parameters. Samples were examined following equal channel angular extrusion (ECAE), accumulated roll bonding (ARB) and high-strain conventional rolling (HSCR). Local variations in the uniformity of coarsening have been characterized by an analysis of grain size distributions, obtained from electron backscatter pattern investigations. Except for at the lowest strains the distributions were not obviously bimodal. Methods of separating the distributions into two parts based on the mean value have therefore also been examined. The results suggest that the presence of a high fraction of high angle boundaries (>15o) in the microstructure may be a necessary but not sufficient condition for continuous recovery during annealing. The annealing behaviour can be related in part to the heterogeneity of the deformed microstructures, which can be pronounced even after strains of ≈ 10 (ECAE). The presence of oxide particles in the ARB sample has a beneficial effect in limiting the grain growth.

Structure and magnetic properties of Co +-implanted silica

Cobalt has been implanted at 30 and 160 keV in three different silica-based substrates. TEM observations and magnetization measurements performed at RT and 5 K show that, after implantation, cobalt is present both as metal particles, few nanometers in size and as smaller oxide particles. Optical absorption measurements show that the presence of oxide particles is linked to the formation of Si–Si bonds during implantation. After thermal treatment under hydrogen, cobalt is entirely in the metallic form and saturation magnetization becomes close to its theoretical value. Last, the initial amount of oxide depends on the OH − content of the substrate.

Nature of Environmentally Assisted Fractures in Polycrystals

AbstractEnvironmentally assisted fracture is in practice one of the most important fracture modes. It is especially dramatic in polycrystalline super-alloys, which are believed to be embrittled by oxygen diffusing along their grain boundaries. We used a large scale atomistic Monte-Carlo simulation to study phenomena of the environmentally assisted fracture nucleation in solids with grain boundaries exposed to an oxygen atmosphere. We find nucleation of inter-granular microcavities in a region which is well in front of the oxidized zone of the sample, apparently due to inhomogeneous stresses induced by the presence of oxide particles. This effect increases with decreasing oxidation rate since then the oxidation front becomes fuzzier and inhomogeneous stresses stronger. These findings compare well with the experimental data on super-alloys which indicate that the environmental embrittlement effects are strong for slower oxidation rates, whereas at higher oxidation rates the embrittlement may be suppressed.

Evolution of Buried Oxide “Pipe” Defects Upon Implantation Through Particles in Simox Material

ABSTRACTWe have investigated the effect of the presence of oxide particles on the surface of silicon wafers during high energy, high dose implantation of oxygen into silicon. It was found that for single implants with doses of 1.5 × 1018/cm2 or 1.8 × 1018/cm2, such particles produce a non-continuous buried oxide layer in the as-implanted condition as well as after annealing. Etching results showed that no defects, which formed etchable paths through the buried oxide, were produced for particles with diameters 0.43 um or below for the lower dose and 0.53 um for the higher dose.

Generation of heart-shaped cross section in cylindrical Cu-SiO2 alloy crystals with ?001? orientation during high temperature deformation

Over the past decade, a chain of studies concerning the high temperature deformation of oxide dispersion strengthened (ODS) copper base crystals [1-3] has helped to clarify the deformation mechanism. These crystals showed much larger deformation up to fracture than polycrystals of similar alloys prepared by internal oxidation technique. This high ductility in the crystals is helpful in discussing the deformation mechanism. In the polycrystals the presence of oxide particles at the grain boundaries leads to early failure after a small amount of deformation at elevated temperatures. It was also found that the characteristic deformation behaviour of the crystals with an initial specimen axis of(0 0 1) orientation [1,2] is different from that of crystals with other specimen axes [3]: in the crystals oriented (00 1) the specimen axis hardly rotates during deformation, and the steady-state deformation, where the flow stress remains constant, occurs at small strains from the beginning of deformation. In the present study, the deformation behaviour of these crystals also attracts attention in other respects, i.e., the morphology and cyrstallography of the deformed

The mechanical behavior of beryllium at high pressure

The pressure dependence of the flow and fracture behavior of polycrystalline beryllium has been studied at room temperature and constant strain rate. Specimens were strained in tension under constant hydrostatic pressures up to 10 kilobars (~150,000 psi) and the true stress and strain obtained from direct measurements of load and specimen cross section during the tests. The structural changes accompanying deformation and fracture were examined by both optical and transmission electron microscopy. Two types of well characterized beryllium—ingot and powder metallurgy—were studied in order to determine the influence of metallurgical variables. The yield behavior is found to be unaffected by pressure, whereas the fracture stress and strain increase with increasing pressure. At the higher pressures, plastic instability (necking) develops with 3-dimensional ductility and leads to area strains at fracture as high as 70%. Deformation mechanisms observed to operate, in addition to basal and prism slip, are non-basal slip and a new phenomenon, micro-kinking. An increase in strain hardening rate observed for the ingot Be under pressure is shown to be associated with the formation of observed sessile vacancy loops during plastic straining. The dominant fracture mode of the ingot Be is transgranular cleavage. For the pm Be, fracture is predominantly intergranular up to 6 kilobars and then becomes transgranular with a corresponding increase in the pressure dependence of fracture stress. These differences are shown to be associated with the presence of oxide particles at the grain boundaries in the pm material. The measured pressure dependence of fracture stress compared with that predicted from current theory points to a possible effect of pressure and/or strain on the effective surface energy for crack propagation. The theory is not in good agreement with the observed incidence of micro-cracks and the pressure dependence of fracture at low pressures. The observations on pm Be indicate the need to incorporate the intergranular fracture mode in any general theory of the pressure dependence of fracture.

Some creep and fatigue properties of dispersed-oxide-strengthened lead

Creep and fatigue tests have been made on dispersed-oxide-strengthened lead. The presence of dispersed oxide in the lead resulted in a marked improvement in creep and fatigue resistance. Observations showed that the presence of oxide particles affected the incidence of slip and grain-boundary migration. It is suggested that the role of the dispersed oxide, in improving the creep and fatigue properties of lead, arises from the effect of oxide particles on the amount of slip, the amount of grain-boundary sliding and migration, and, in fatigue failure, the number of vacancies available for cavity nucleation.