Study of influence of gamma prime and eta phases on corrosion behaviour of A286 superalloy by using electrochemical potentiokinetic techniques

This work studies the influence of gamma prime and eta phases, resulting from precipitation hardening, on the corrosion behaviour of A286 superalloy by using electrochemical potentiokinetic reactivation (EPR) and double loop electrochemical potentiokinetic reactivation (DLEPR). The general trend is that I_r, Q_r, I_r/I_a and Q_r/Q_a increase with aging temperature and, for a given aging temperature, with aging time, which is consistent with the fact that gamma prime precipitate may cause discontinuities in the passive layer. I_r, Q_r, I_r/I_a and Q_r/Q_a reach a peak, which alters the general trend, for both aging temperatures (670 and 720 °C), at the aging time from which there is evidence of the presence of eta phase at grain boundary (GB). Gamma prime and eta precipitates can cause the passive layer that forms on them to become more unstable and, consequently, cause the current density through the passive layer and the susceptibility to localized corrosion attack to increase.     

Characterization of fracture behavior of a Ti alloyed supermartensitic 12%Cr stainless steel using Charpy instrumented impact tests

In this work, the toughness of a Ti-alloyed supermartensitic stainless steel with 12%Cr was evaluated by instrumented Charpy impact tests at − 46 °C. The material was heat treated by quenching and tempering at 500 °C or 650 °C. The temper embrittlement phenomena was detected in the specimen tempered at 500 °C, while the specimens as quenched and quenched and tempered at 650 °C presented a ductile fracture with high impact energy values. The predominance of cleavage fracture instead of intergranular cracks suggests that the temper embrittlement was caused by fine and disperse precipitation observed in the specimen tempered at 500 °C. The dynamic initiation fracture toughness (J_Id) was calculated from the force versus deflection curves using three different methods suggested in the literature to obtain the initiation energy.     

Evaluation of fracture toughness and impact toughness of laser rapid manufactured Inconel-625 structures and their co-relation

The present study involves evaluation of fracture toughness and Charpy impact toughness of Inconel 625 structures fabricated by laser based additive manufacturing. The results of crack tip opening displacement (CTOD) fracture toughness are close to those reported for the Inconel 625 weld metal. The nature of the load–time traces of instrumented Charpy impact tests revealed that the alloy Inconel 625 in laser fabricated condition was associated with fully ductile behavior with Charpy V-Notch impact energy in the range of 48–54 J. Stress relieving heat treatment at 950 °C for 1 h has resulted in marginal improvement in the impact toughness by about 10%, whereas no clear evidence of such improvement is seen in the CTOD fracture toughness. Fractographic examination of the Charpy specimens and the results of the instrumented impact tests imply that the mechanism of crack growth was propagation controlled under dynamic loading conditions. Dynamic fracture parameters were estimated from the instrumented impact test data and compared with the experimentally evaluated fracture toughness results.     

Microstructural changes and corrosion resistance of AISI 310S steel exposed to 600–800 °C

The microstructural changes in an AISI 310S stainless steel exposed to temperatures in the 600–800 °C range up to 210 h were investigated. Intense carbide precipitation occurs in all conditions. Sigma phase precipitation nucleates at the grain boundaries and becomes more intense with the increase of time and temperature. Intergranular corrosion resistance was evaluated by double loop electrochemical potentiodynamic reactivation tests. Sigma phase and chromium carbides cause severe sensitization, but healing is observed in specimens aged at 650 °C, 700 °C, 750 °C and 800 °C. Time to healing decreases with the increase of aging temperature. Anodic polarization tests were conducted in specimens aged for 210 h. Pitting potentials are found to decrease in all temperature tested.     

Discrete Fourier Transform as applicable technique in electrochemical detection of hydrazine using multi-walled carbon nanotube/polyacrylonitrile ceramic fiber as working electrode

Effect of “Discrete Fourier Transform” (DFT) is studied for electrochemical detection of some electroactive species using multi-walled carbon nanotube/polyacrylonitrile ceramic fiber as ultra micro electrode. Based on DFT theory, two independent phases i.e. the imaginary and real phases are evaluated during the oxidation/reduction of the quasi-reversible or irreversible electroactive species, revealing the independent components of imaginary (I_Imaginary) and real (I_Real) currents. The results show that, in different electrochemical modes such as cyclic voltammetry (CV), the contribution of DFT to the electrochemical signals significantly improves the detection limit of the electrochemical technique. More sensitive signals are obtained at high scan rates according to the combination of electrochemical techniques with the DFT theory. The reliability of DFT algorithm was evaluated for rapid determination of trace amount of hydrazine (N₂H₄) at a scan rate up to 800 V s^? 1. In this study, the amounts of phase and amplitude were estimated to 1.69 and 31.57, respectively. The detection limit of hydrazine was 4.13 × 10^? 9 M. The application of this technique was also evaluated for determination of hydrazine in different industrial wastewater samples.     

High temperature flexural strength and fracture toughness of AlN with Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic

The effects of temperature on the fast fracture behavior of aluminum nitride with 5 wt% Y₂O₃ ceramic were investigated. Four-point flexural strength and fracture toughness were measured in air at several temperatures (30–1,300 °C). The flexural strength gradually decreased with the increase of temperature up to 1,000 °C due to the change in the fracture mode from transgranular to intergranular, and then became almost constant up to 1,300 °C. Two main flaw types as fracture origin were identified: small surface flaw and large pores. The volume fraction of the large pores was only 0.01%; however, they limited the strength on about 50% of the specimens. The fracture toughness decreased slightly up to 800 °C controlled by the elastic modulus change, and then decreased significantly at 1,000 °C due to the decrease in the grain-boundary toughness. Above 1,000 °C, the fracture toughness increased significantly, and at 1,300 °C, its value was close to that measured at room temperature.     

Investigation on microstructure and properties of dissimilar joint between SA553 and SUS304 made by laser welding with filler wire

The dissimilar joints between SA553 and SUS304 were produced by CO₂ laser welding with the ERNiMo-8 and ER308L filler wire. After welding parameters were optimized, qualified weld formations were made. Investigation on the microstructure showed that there were dual phases (martensite and austenite) in the ER308L weld, but only austenite in the ERNiMo-8 weld. For both joints, not only the microstructure gradient, but also the element gradient was observed near interfaces between weld metals and base metals. The Charpy impact and tensile test at room (25 °C) and low temperature (− 196 °C) was implemented. The cryogenic impact energy of the ER308L weldment was 51 J, lower than the value (84 J) of the ERNiMo-8 weldment. The corresponding cryogenic tensile strength of the two weldments was 1070 MPa and 960 MPa, respectively. The cryogenic tensile properties of both weldments were rather higher than requirements in the relevant standards. The ERNiMo-8 weldment showed relatively better comprehensive performance when the cryogenic toughness was considered.     

A simple gel route to synthesize nano-Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> as a high-performance anode material for Li-ion batteries

Nano-Li₄Ti₅O₁₂ powders were synthesized by a simple gel route with acrylic acid, tetrabutyl titanate, and lithium nitrate as the precursors that were made into gels through thermal polymerization. The Li₄Ti₅O₁₂ powders were obtained by calcination of the gels at 700, 750, and 800 °C. They were characterized by thermal gravimetric analysis, differential thermal analysis, X-ray diffraction, and field emission scanning electron microscopy. The electrochemical performance of these nano-Li₄Ti₅O₁₂ powders was examined with galvanostatic cell cycling. The average particle size of the 700-, 750-, and 800 °C-calcined powders is about 70, 120, and 400 nm, respectively. The 750 °C-calcined powder exhibits a high capacity of over 160 mAh/g after 100 cycles and a good rate capability with a capacity of 122 mAh/g even at 10C rate.     

Mechanical and metallurgical aspects of fracture behaviour of an Al-alloy

Fracture toughness testing was carried out on an Al-⁴C% Cu alloy in the form of a fully heat treated 21/2-in thick plate. Three types of test, the three point slow bend, instrumented Charpy impact, and double cantilever beam tests, enabled values of plane strain fracture toughness (K _1c ) to be studied over a range of temperature, testing speed and specimen size. Dependence of K _1c upon temperature (?200°C to +160°C) was found to be relatively small except at very low temperatures. The alloy was found not to be appreciably strain rate sensitive over the range of testing speeds (0.002 in/min to 2000 in/min) used. Higher toughness values, obtained from plane strain fractures of the smaller size specimens, were compared with low results from large specimens. The results became variable when curved fracture surfaces developed in the smallest DCB specimens. There was an apparent increase in toughness towards the centre of the plate and the effects of specimen size were most marked in the lower yield strength metal of this layer. The results indicate a genuine increase in toughness K _1c with decreasing temperature, but also show that large specimens are required to approach the lowest and constant K _1c value in circumstances where the yield strength decreases. The DCB test is also shown to be still valid for the measurement of K _1c for this non-brittle alloy, where yield strength may be only about 40,000 lbf/in². Electron fractography showed a ductile intergranular fracture mode. The results are consistent with theories of solute precipitation in Al-alloys.     

Creep behavior of INCOLOY alloy 617

The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700 and 800 °C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)₂₃C₆ carbides and the δ-Ni₃Mo phase. M₆C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni₃(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)₂₃C₆ carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated during exposure to high temperature, also changed after 700 or 800 °C exposure. At the latter test temperature, a lower volume fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the presence of the δ-phase, detected only in specimens exposed to 700 °C, were clearly responsible for the substantially good creep response observed at 700 °C, compared with that found at 800 °C.     

Mechanical properties and oxidation behaviors of carbon/carbon composites with C–TaC–C multi-interlayer

To improve the mechanical properties and oxidation-resistance properties, a C–TaC–C multi-interlayer structure was introduced in carbon/carbon (C/C) composites by chemical vapor infiltration. Compared with conventional C/C composites, a higher fracture toughness and strength have been achieved by using the C–TaC–C multi-interlayer. In addition, the composites also exhibit a higher preliminary oxidation temperature and a lower mass loss at high temperatures. The oxidation rate of the composites increases with temperature increasing in the range of 700–1300 °C, reaching a maximum value at 1300 °C, then decreases in 1300–1400 °C. A hexagonal structure of Ta₂O₅ phase is obtained when being oxidized at 700–800 °C, and it transforms to an orthorhombic phase at temperatures above 900 °C. The structures of C–TaC–C multi-interlayer are intact without cracks or porosities after being oxidized at 700–800 °C. In 900–1300 °C, the composites are oxidized uniformly with the formation of pores. At temperatures above 1300 °C, there are oxidation and non-oxidation regions with the oxidation process being controlled by diffusion.     

Synthesis and characteristics of nature-superlattice-structured Bi3TiNbO9–Bi4Ti3O12 ferroelectric thin films by MOD

Natural-superlattice-structured ferroelectric thin films, Bi₃TiNbO₉–Bi₄Ti₃O₁₂ (BTN–BIT), have been synthesized on Pt/Ti/SiO₂/Si by metal organic decomposition (MOD) using BTN–BIT (1 mol:1 mol) solution. BTN–BIT films show natural-superlattice peaks below 2θ = 20° in X-ray diffraction patterns, which indicate that the BTN–BIT films annealed at 700–800 °C in O₂ ambient are consisted of iteration of two unit cells of Bi₃TiNbO₉ and one unit cell of Bi₄Ti₃O₁₂. As the annealing temperature increases from 600 to 750 °C, uniform and crack-free films, better crystallinity and ferroelectric properties can be obtained, but the pyrochlore phase in BTN–BIT films annealed over 800 °C would impair the ferroelectric properties. With the increase of O₂ flow rate from 0.5 to 1.5 L/min, both remanent polarization P_r and coercive electric field E_C increase, which are mainly attributed to reduction of the vacanvies of Bi and oxide ions in the films. Natural-superlattice-structured BTN–BIT thin films having 2–1 superlattice annealed at 750 °C in O₂ ambient with a flow rate of 1.5 L/min exhibit superior ferroelectric properties of P_r = 23.5 μC/cm² and E_C = 135 kV/cm.     

High-pressure synthesis of a new copper thioborate,CuBS2

A new copper thioborate, CuBS₂, was synthesized at high-pressure/temperature condition of 3 GPa and 700–900 °C. The crystal structure was refined by the Rietveld analysis of the powder X-ray diffraction data. The compound crystallizes into a tetragonal unit cell (a = 0.5044(1) nm, c = 0.8947(2) nm, space group: I-42d), isostructural with CuMS₂ chalcopyrite compounds (M: Al, Ga, and In). The compound is the first representative of the chalcopyrite-type family consisting of BS₄-tetrahedra. From the UV–Vis diffuse reflectance spectrum, the optical band gap of CuBS₂ was estimated to be E_g = 3.61 eV.     

Effects of heat treatments on microstructure and tensile properties of as-extruded TiBw/near-α Ti composites

The effects of different heat treatments on the microstructure and tensile properties of the as-extruded TiBw/Ti60 composites have been investigated. Without solution treatment, the α₂ phase precipitated from α phase and mounts of elliptical S₂ silicide are formed at the interface of α/β phases after aging at 700 °C for 5 h, which results in clear enhancement of strength at 700–800 °C. After the solution and aging treatment, more α₂ phase precipitates from α_p phase and less spherical S₂ silicide precipitated from the body of both α_p and transformed β phase. Moreover, some small silicide precipitates are formed from α matrix near the TiBw due to the stacking faults and the interface effect. The TiBw are stable without any interfacial reaction during the heat treatments. The results show that the composites performed by solution and aging treatment exhibit high strength (1377 MPa) and low elongation (2%). The annealing treatment can soften the composites and increase the elongation to 7%.     

Improvement of corrosion resistance of Cu and Nb co-added Nd–Fe–B sintered magnets

Cu and Nb powders are co-added as intergranular modifiers to improve the corrosion resistance of Nd–Fe–B sintered magnets. For the magnet co-added with 0.2 wt.% Cu and 0.8 wt.% Nb, mass loss of accelerated corrosion test in 120 °C, 2 bar and 100% relative humid atmosphere for 96 h drops from 2.47 mg/cm² to 0.49 mg/cm² in comparison with the Cu/Nb free one. The corrosion potential E_corr in 3.5% NaCl aqueous solution increases from −1.115 V to −0.799 V, which indicates the better resistance against electrochemical corrosion. The improved corrosion resistance is ascribed to the enhanced stability of the intergranular phase by forming high electrode potential Cu-containing phase and reduced Nd-rich phase at triple junctions. Besides, the distribution of (Pr, Nd)-rich phases along the grain boundaries becomes more clear and continuous through Cu/Nb co-addition, maintaining fairly good magnetic properties of B_r = 13.6 kGs, H_cj = 11.4 kOe, (BH)_max = 46.2 MGOe. Further investigation demonstrates that Nb is effective to refine the grains of hard magnetic Nd₂Fe₁₄B phase and Cu is beneficial for optimizing the distribution of the intergranular phases.     

High piezoelectric and dielectric properties of 0–3 PZT/cement composites by temperature treatment

Temperature treatment of 0–3 type PZT/cement composites before polarization yielded high dielectric and piezoelectric properties in materials with 50% PZT inclusions by volume and 50% cement matrix. Specimens were treated at seven temperatures from 23 °C to 150 °C and then applied by a 1.5 kV/mm poling field. The dielectric loss of the composites reduces at higher pretreatment temperatures, shorting the trigger time. Temperature treatment increased the piezoelectric strain factor d₃₃, the relative dielectric constant ε_r and the piezoelectric voltage factor g₃₃ of PZT/cement composites, but did not affect significantly the electromechanical coupling coefficient K_t. Piezoelectric factors reach stable values after 70 days of aging, and samples that were not temperature pretreated reached stable values earlier. Specimens pretreated at 150 °C exhibit d₃₃ = 106.3 pC/N and ε_r = 477 on the 70^th aging day, almost two times greater than the composites without temperature treatment. The resonance frequency of the composites on the 70th day decreases with increasing temperature, with the exception of 150 °C. Temperature pretreatment can also improve the phase angle of the composites. In addition, the effect of curing time for PZT/cement composites is an important factor to dominate the feasibility of polarization.     

Grazing incidence X-ray study of Ge-nanoparticle formation in (Ge:SiO2)/SiO2 multilayers

We present the study of formation of Ge-nanoparticles (Ge-NP) in germanosilicate (Ge:SiO₂) multilayer (ML) films under thermal treatment. In anticipation of controllable formation of Ge-NP, ML films were prepared by magnetron deposition at room temperature as 20 bi-layer stacks, each bi-layer comprised of a 7 nm thick layer of (Ge + SiO₂) (molar ratio: 60:40) succeeded by a 7 nm thick layer of pure SiO₂, and then annealed for 1 h, up to T_a = 900 °C. Formation and morphology of Ge-NP were analyzed by combining the information obtained from the grazing incidence small angle X-ray scattering and X-ray diffraction. It was found that precipitation of Ge-NP starts at T_a = 600 °C, while high degree of in-plane confinement and lateral ordering of rather uniform precipitated particles is achieved at T_a =  700-800 °C range. At still higher annealing temperature T_a > 800 °C, volume fraction of precipitated Ge-NP in SiO₂ matrix diminishes due to the out-diffusion of Ge atoms from the film, while Ge-NP are no more well confined to (Ge + SiO₂) layers.     

Photogeneration of charges in microcrystalline chlorophyll a

The electric-field and temperature dependence of hole photogeneration in chlorophyll a (Chla) have been analyzed in terms of electric-field assisted thermal dissociation of charge pairs based on Onsager theory. An excellent agreement between the experimental and theoretical values of the slope-to-intercept ratio, S/I, for the plot of photogeneration efficiency vs. electric field at low field strengths provides a proof for the applicability of the Onsager approach to the photogeneration of charges in Chla. A value of 19 nm has been obtained for Coulomb capture radius, r_c, from S/I. From the temperature dependence of photogeneration, the initial separation, r₀, of photogenerated electron-hole has been evaluated, and has a value of 1.24 nm. This smaller r₀ compared to r_c leads to a feeble dissociation probability of electron-hole pairs into free carriers, and may, among other factors, explain the low power conversion efficiencies of Chla photovoltaic cells.     

The effect of retrogression and re‐aging treatment on mechanical and tribological features of AA7075 aluminium alloy

In this study, T651‐applied AA7075 alloy was subjected to retrogression and re‐aging (RRA) process. Various retrogression temperatures (180 °C, 280 °C, 370 °C) and times (15 min, 30 min, 90 min) were used to determine the effects of temperature and time on the mechanical and tribological properties of the AA7075 alloy. All re‐aging stages were performed at 120 °C for 24 hours. Retrogression and re‐aging‐applied specimens were characterized by scanning electron microscope, transmission electron microscope, x‐ray diffraction, Charpy V‐notch impact and tensile tests. Brinell hardness measurements and ball‐on‐disc type tribometer measurements by using AISI 316 ball as a counterpart have also been conducted. Grain boundary precipitates in the T651‐applied specimen was transformed from continuous to the discontinuous structure after retrogression and re‐aging process. Continuous MgZn₂ precipitates at grain boundaries were disintegrated and re‐precipitated along the grain boundaries. The sizes of intragranular precipitates have become coarsened by comparison with the T651 condition. Hardness, tensile strength and wear resistance were decreased whereas impact toughness values were increased with increasing retrogression temperature and time. The best wear resistance was obtained in the sample treated at 180 °C for 15 minutes.     

Structural and ferroelectric properties of La modified Sr0.8Bi2.2Ta2O9 thin films

La modified SBT (Sr_0.8La_0.1Bi_2.1Ta₂O₉) thin films of different thickness were fabricated on Pt/Ti/SiO₂/Si substrates by the metalorganic decomposition technique. All the films were annealed layer-by-layer at 800 °C using a rapid thermal annealing furnace. X-ray diffraction analysis indicated that the relative intensity of the (2 0 0) diffraction peak [I(2 0 0)/I(1 1 5)] increased with the increase of the film thickness. Eventually, an a-axis preferentially oriented SLBT film was obtained. These results are discussed with respect to the anisotropy of the grain growth. The a-axis preferentially oriented SLBT film, whose relative intensity of the (2 0 0) peak [I(2 0 0)/I(1 1 5)] was 1.05, had a remanent polarization (2P_r) value of 21 μC/cm² and a coercive field (2E_c) value of 70 kV/cm under the electric field of 200 kV/cm.