High cycle fatigue and fracture behaviour of a hot isostatically pressed nickel-based superalloy

Powder of a nickel-based superalloy, RR1000, has been hot isostatically pressed (HIPped) at a supersolvus temperature and post-HIP heat treated to produce different microstructures. Microstructures were investigated using a scanning electron microscope together with an energy dispersive X-ray spectrometer and a wave-length dispersive X-ray spectrometer. High cycle four-point bending fatigue and tension–tension fatigue tests have been performed on the fabricated samples. It was found that HIPped and aged samples showed the best four-point bending fatigue limit while HIPped and solution-treated and aged samples had the lowest fatigue limit. The four-point bending fatigue crack initiations all occurred from the sample surfaces either at the sites of inclusion clusters or by cleavage through large grains on the surfaces. The tension–tension fatigue crack initiation occurred mainly due to large hafnia inclusion clusters, with lower fatigue lives for samples where inclusions were closer to the surface. Crack initiation at the compact Al₂O₃ inclusion cluster led to a much higher fatigue life than found when cracks were initiated by large hafnia inclusion clusters. The tension–tension fatigue limits were shown to decrease with increased testing temperature (from room temperature to 700?°C).     

Impact toughness of EK-181 ferritic-martensitic chromium (12%) steel under loading by concentrated bending

The low-temperature fracture of a high-temperature low-activated ferritic-martensitic EK-181 chromium (12%) steel (RUSFER-EK-181: Fe-12Cr-2W-V-Ta-B) is studied using impact and static concentrated bending tests as a function of the specimen dimensions (standard, small), the type of stress concentrator (V-shaped notch, fatigue crack), and the temperature (from −196 to +100°C). The ductile-brittle transition temperature falls in the range from −85 to +35°C. The temperature dependences of stress-intensity factor K _Ic and fracture toughness J _Ic are determined. The severest type of impact toughness tests is represented by tests of V-notched specimens with an additional fatigue crack and two lateral V-shaped notches (three-sided V-shaped notch with a central fatigue crack). The fracture energy of the steel depends on the type of stress concentrator and the specimen dimensions and is determined by the elastic energy and the plastic deformation conditions in the near-surface layers of a specimen, which are controlled by the lateral notches. At the same test temperature, the impact toughness and the fracture toughness are interrelated. Irrespective of the type of specimen (including notches and a fatigue crack), the ferritic-martensitic steel exhibits the same fracture mechanism.     

Fatigue damage rule of LY12CZ aluminium alloy under sequential biaxial loading

A series of biaxial two-level variable amplitude loading tests are conducted on smooth tubular specimens of LY12CZ aluminium alloy.The loading paths of 90°out-of-phase,45°out-of-phase and 45°in-phase are utilized.The fatigue damage cumulative rules under two-level step loading of three loading paths are analyzed.By introducing a parameter which is a function of the phase lag angle between the axial and the torsional loading,a new multiaxial nonlinear fatigue damage cumulative model is proposed.The proposed model is evaluated by the experimental data for two-level loading,multi-level loading of LY12CZ aluminium alloy,and multi-level loading of 45 steel.Fatigue lives predicted are within a factor of 2 scatter band.     

Scatter of fatigue data owing to material microscopic effects

A common phenomenon of fatigue test data reported in the open literature such as S-N curves exhibits the scatter of points for a group of same specimens under the same loading condition.The reason is well known that the microstructure is different from specimen to specimen even in the same group.Specifically,a fatigue failure process is a multi-scale problem so that a fatigue failure model should have the ability to take the microscopic effect into account.A physically-based trans-scale crack model is established and the analytical solution is obtained by coupling the micro-and macro-scale.Obtained is the trans-scale stress intensity factor as well as the trans-scale strain energy density(SED)factor.By taking this trans-scale SEDF as a key controlling parameter for the fatigue crack propagation from micro-to macro-scale,a trans-scale fatigue crack growth model is proposed in this work which can reflect the microscopic effect and scale transition in a fatigue process.The fatigue test data of aluminum alloy LY12 plate specimens is chosen to check the model.Two S-N experimental curves for cyclic stress ratio R=0.02 and R=0.6 are selected.The scattering test data points and two S-N curves for both R=0.02 and R=0.6 are exactly re-produced by application of the proposed model.It is demonstrated that the proposed model is able to reflect the multiscaling effect in a fatigue process.The result also shows that the microscopic effect has a pronounced influence on the fatigue life of specimens.     

Influence of water cavitation peening with aeration on fatigue behaviour of SAE1045 steel

Water cavitation peening (WCP) with aeration is a recent potential method in the surface enhancement techniques. In this method, a ventilation nozzle is adopted to improve the process capability of WCP by increasing the impact pressure, which is induced by the bubble collapse on the surface of components in the similar way as conventional shot peening. In this paper, fatigue tests were conducted on the both-edge-notched flat tensile specimens to assess the influences of WCP on fatigue behaviour of SAE1045 steel. The notched specimens were treated by WCP, and the compressive residual stress distributions in the superficial layer were measured by X-ray diffraction method. The tension-tension (R = S_min/S_max = 0.1, f = 10 Hz) fatigue tests and the fracture surfaces observation by scan electron microscopy (SEM) were conducted. The experimental results show that WCP can improve the fatigue life by inducing the residual compressive stress in the superficial layer of mechanical components.     

Very-High-Cycle-Fatigue of in-service air-engine blades, compressor and turbine

In-service Very-High-Cycle-Fatigue(VHCF)regime of compressor vane and turbine rotor blades of the Al-based alloy VD-17and superalloy GS6K,respectively,was considered.Surface crack origination occurred at the lifetime more than 1500 hours for vanes and after 550 hours for turbine blades.Performed fractographic investigations have shown that subsurface crack origination in vanes took place inspite of corrosion pittings on the blade surface.This material behavior reflected lifetime limit that was reached by the criterion VHCF.In superalloy GS6K subsurface fatigue cracking took place with the appearance of flat facet.This phenomenon was discussed and compared with specimens cracking of the same superalloy but prepared by the powder technology.In turbine blades VHCF regime appeared because of resonance of blades under the influenced gas stream.Both cases of compressor-vanes and turbine blades in-service cracking were discussed with crack growth period and stress equivalent estimations.Recommendations to continue aircrafts airworthiness were made for in-service blades.     

PFM characterization of (K0.5Na0.5)0.95La0.05(Nb0.9Ti0.05)O2.9 ceramics lead free

The results of ferroelectric properties studies of KNN doped with La and Ti and sintered at temperatures in the interval of 1100 °C–1190 °C are presented in this work. The doping was achieved by the substitution of La for ions in A sites and Ti for ions in the B sites. Values of 94 % of the theoretical density were accomplished. The effect of the sintering temperature and the inclusion of the La and Ti cations in the KNN structure is evident through the shift in the ferroelectric-paraelectric transition temperature of ～110 °C with respect to that of pure KNN (420 °C). Microstructure and ferroelectric analyses were carried out using Piezoresponse Force Microscopy (PFM) and hysteresis loops with interesting results, ΔP _r=9 (μC/cm²) and P _r/P _max=0.41, even when the saturation of the materials is not reached.     

Self-organization of the structure in the Cu60Fe40 composite during deformation-thermal treatment

The deformation-thermal stability of a clusterized amorphous-crystalline structure prepared from a Cu₆₀Fe₄₀ powder mixture at a logarithmic strain e = 4.6 and subjected to isochronous (40 min) annealings at T _a = 200–800°C has been investigated. Periodic changes (ΔT = 300°C) in the order and disorder with a maximum ordering at T _a = 300 and 600°C and a maximum disordering at T _a = 400 and 700°C have been observed. The periodicity of the dominant crystallographic order with a period ΔT = 400°C in the annealing temperature has been revealed for face-centered cubic copper phase planes separated by a singular point at T = 500°C characterized by the dominant body-centered cubic iron phase ordering. It has been shown that the sawtooth shape of the size distribution of strain clusters formed within the crystal structure of deformed samples slowly changes with increasing annealing temperature from exponential (T _a = 200–700°C) to linear (T _a = 800°C). This indicates a high density of internal local distortions in structural units.     

Die Temperaturabhängigkeit der magnetischen Jordan-Nachwirkung von Nickel

The magnetic after effect of pure polycristalline nickel and of a single crystal was measured between ?196 and +340 °C by a Förster second harmonic magnetometer. The viscosity constantS _v is shown to be proportional toB(T)·H_c(T) with an increasing functionB(T) for specimens of different magnetic hardness. The temperature dependence S_v(T) atH _c is rather complex in comparison to the simpleT- or √T-behaviour of former theoretical models. The latter is observed only forT??100 °C, whileS _v is nearly independent ofT between ?100 and +300 °C, and drops abruptly to zero forT?300 °C.     

Fabrication of NdCeCuO and effects of binding agents on the growth,micro-structural and electrical properties

NdCeCuO superconducting samples were fabricated using ethyl alcohol, acetone and ethylenediaminetetraacetic acid (EDTA) as binding agents. For evaporation of binding agents, the samples were heat treated at 1050 °C for 24 h and then at 950 °C for 6–48 h under argon atmosphere to obtain the superconducting phase. The best superconducting performance was found in the sample heat treated at 1050 °C for 24 h and then 950 °C for 12 h which was fabricated by using acetone as binding agent. The T_c and T₀ value was found to be ～25 K and 23.4 K, respectively. Grain size in the samples fabricated was calculated using Scherer equation and SEM data. It was found that grain size strongly depends on the binding agents and heat treatment conditions. Some cracks and voids on the surface of the samples were observed, which influences the superconducting and electrical transport properties of the samples.     

Magnetic susceptibility of solid and liquid metals

Apparatus for the measurement of the magnetic susceptibilities of solid and liquid metals at temperatures up to 800°C is described. Prior to studying the susceptibilities of various metals through the melting point, the apparatus was tested by measuring the susceptibilities of tin and lead at room temperature, and the temperature dependences of susceptibility of several pure copper specimens enclosed in quartz bulbs. For copper, in the temperature range 20°C to 700°C,? _χ /?T = 1.9₂ × 10^?11 emu/g · deg C. This result is discussed in terms of the change of electronic susceptibility with volume expansion.     

Corrosion products on steel 20 in aerated hydrazine solutions: Mössbauer study

Composition of corrosion layers on steel 20 in aerated solutions with hydrazine concentrations less than 11 ppm was studied at 50, 60, and 80°C in dynamic conditions by transmission Mössbauer spectroscopy and X-ray diffraction as supplementary technique. Corrosion rates were determined by gravimetric method. A comparison with corrosion in water at 80°C was made. The observed layers have not any protective character. For 0.1 m/s linear velocity, they are composed by nonstoichiometric magnetite, (Fe_3?x O₄,x=0.02–0.04) with lepidocrocite (γ-FeOOH) as secondary phase at 50°C. Haematite (α-Fe₂O₃) is observed at 60 and 80°C with a 19 nm particle size. It becomes smaller for higher velocity (0.7 m/s).     

Behavior of the nickel-titanium alloys with the shape memory effect under conditions of shock wave loading

The behavior of the Ti_51.1Ni_48.9 and Ti_49.4Ni_50.6 alloys with shape memory effects has been studied under submicrosecond shock wave loading in the temperature range from −80 to 160°C, which includes both the regions of the stable state of the specimens in the austenite and martensite phases and the regions of thermoelastic martensitic transformations. The grain size of the studied alloys varies from initial values 15–30 to 0.05–0.30 μm. The dependences of the dynamic elastic limit on the temperature and on the elemental composition are similar to the dependences of the yield stress of these alloys under low strain rate loading. The rarefaction shock wave formation as a consequence of the pseudoelastic behavior of the alloy during a reversible martensitic transformation has been revealed. A decrease in the grain size leads to an increase in the dynamic elastic limit and decreases the temperatures of martensitic transformations.     

Electrical conductivity of fully densified nano CaZr0.90In0.10O3 − δ ceramics prepared by a water-based gel precipitation method

Water-based gel precipitation method was first applied to synthesize high purity nano CaZr_0.90In_0.10O_3 ? δ powders suitable for fabricating dense ceramics at lower temperature. By using CaCO₃, Zr(NO₃)₄ and In(NO₃)₃ as raw materials, PEG as dispersant, CaZr_0.90In_0.10O_3 ? δ with an average particle size of about 40 nm was obtained at 850 °C, which was nearly 350 °C lower than that of traditional solid-state reaction method. Fully densified ceramics with an average grain size of 200–300 nm were obtained at 1350 °C, a temperature about 250 °C lower than that of traditional sintering techniques. Experimental results showed that the flexure stress, total, bulk and grain boundary protonic conductivities of the ceramics were more favorable than those of the ceramics fabricated at 1500 °C and 1600 °C from the powders synthesized by solid-state reaction method.     

Proton nuclear magnetic resonance spectroscopy of normal and edematous brain tissue in vitro: Changes in relaxation during tissue storage

Proton nuclear magnetic resonance relaxation times, T₁ and T₂, of water in unfixed gray and white matter from normal and edematous rabbit brain tissues were measured in vitro at 23°C and 100 MHz to evaluate the effects of the temperature (?25°C to 37°C) and duration (0 to 96 h) of tissue storage on relaxation times. T₁ and T₂ tended to decrease during storage, probably from slow dehydration of the tissue. This effect was greatest in tissues stored at 37°C and least in those stored at 4 and ?25°C; decreases in T₁ and T₂ were greater in white matter than in gray matter. Freezing brain tissue to ?25°C caused a sudden decrease in the T₂ of normal white matter. Relaxation times were constant for 5 h in tissues stored at 23°C and for 40 h at 4°C. These results correlated well with corresponding tissue water loss.     

Dielectric and ferroelectric properties of 0.8PZT–0.2PCN ceramics under sintering conditions variation

The influences of sintering conditions on electrical properties of the 0.8Pb(Zr_1/2Ti_1/2)O₃–0.2Pb(Co_1/3Nb_2/3)O₃ ceramics have been investigated with sintering temperatures of 1175, 1200, 1225, and 1250 °C and dwell times for 2, 6, and 10 h. The crystal structure of dense specimens showed coexistence between tetragonal, rhombohedral and pseudo cubic phases in all sintering temperatures, while tetragonal-rich phase appeared with increasing dwell times. A maximum dielectric constant was observed at sintering condition of 1200 °C for 2 h, while the transition temperature slightly increased with increasing dwell time. All ceramics also showed diffused phase transition behaviors with a minimum diffusivity at sintering condition of 1200 °C for 2 h. In addition, the polarization–electric field (P–E) hysteresis loops of the ceramic systems also changed significantly with sintering conditions. Interestingly, the ferroelectric parameters; remnant polarization (P_r) and loop squareness (R_sq) tended to increase with increasing sintering temperatures and dwell times.     

Optimization of calcination temperature of lead-free BiFeO3–BaTiO3 high-temperature piezoceramics

0.7BiFeO₃-0.3BaTiO₃+1mol% MnO₂(0.7BFO-0.3BTO) ceramics were synthesized by conventional solid-state powder method under different calcination temperatures (T_cal) between 770 and 830 °C. The phase structure, microstructure, and ferroelectric and piezoelectric properties changed greatly depending on the applied T_cal. Benefitting from the formation of low defect levels and large grain size and an appropriate morphotropic phase boundary (MPB) with the rhombohedral-to-pseudocubic phase ratio = 49.1 : 50.9, BFO-BTO ceramics calcined at 785 °C showed the best ferroelectric, piezoelectric, and insulating properties (P_r = 23.1 μC/cm², E_C = 25.8 kV/cm, d₃₃ = 167.8 pC/N, k_p = 0.342%). Above T_cal = 800 °C, however, the ferroelectric and piezoelectric properties deteriorated because volatilization of Bi and reduction of Fe caused a poor insulating property and high degree of chemical inhomogeneity. Moreover, the ceramics calcined at 785 °C showed a high Curie temperature (T_C) of 509.2 °C and excellent thermal aging resistance of d₃₃ up to 450 °C, demonstrating great potential for use in high-temperature applications.     

Static and cyclic mechanical behaviours and fracture mechanisms of Zr-based metallic glass at elevated temperatures

To comprehensively understand the quantitative mechanical–thermal properties and fracture mechanisms of Zr-based bulk metallic glass (BMG), by using self-made miniature tensile/compressive device and motor-piezoelectric coupling driven fatigue device, a series of static and dynamic mechanical tests at elevated temperatures of bulk Zr₅₅Cu₃₀Al₁₀Ni₅ BMG with glass transition temperature T_g of 411°C were carried out. Uniaxial tensile and compressive behaviours at temperatures with range from RT to 400°C were experimentally obtained by means of digital speckle correlation analysis, the estimation of Young’s modulus and fracture strengths as a function of the applied temperatures were investigated. The static and cyclic fracture morphologies at various temperatures were obtained to describe the mechanical–thermal fracture mechanisms in detail. The coupling effect of loading types, stress-induced temperature rise and applied temperature on the evolution of cleavage features, dimples, vein patterns and shear softening behaviours were investigated.     

Preparation and characteristics of?a?BaO–Al2O3–B2O3–SiO2–La2O3 glass ceramic via spray pyrolysis

The characteristics of a BaO–Al₂O₃–B₂O₃–SiO₂–La₂O₃ glass ceramic prepared by spray pyrolysis were studied. Glass powders with spherical shape and amorphous phase were prepared by complete melting at a preparation temperature of 1 500°C. The mean size and geometric standard deviation of the powders prepared at the temperature of 1 500°C were 0.6 μm and 1.3. The glass powders had similar composition to that of the spray solution. The glass transition temperature (T _g) of the glass powders was 600.3°C. Two crystallization exothermic peaks were observed at 769.3 and 837.8°C. Densification of the specimen started at a sintering temperature of 600°C, in which Ba₄La₆O(SiO₄)₆ as main crystal structure was observed. Complete densification of the specimen occurred at a sintering temperature of 800°C. The specimens sintered at temperatures above 800°C had main crystal structure of BaAl₂Si₂O₈.     

Ionic conduction in BaxCe0.8Er0.2O3−α

Ba_xCe_0.8Er_0.2O_3−α (x=0.98–1.03) solid electrolytes were synthesized by high temperature solid-state reaction. X-ray diffraction (XRD) patterns showed that the specimens were a single perovskite-type orthorhombic phase. The conduction properties of the specimens were studied electrochemically in the temperature range of 600–1000 °C. The influence of nonstoichiometry in the specimens with x≠1 on conduction properties was investigated and compared with that in the specimen with x=1. These three specimens showed a good protonic conduction under wet hydrogen, a mixed conduction of oxide-ion and electronic hole under dry higher oxygen partial pressure, and a mixed conduction of proton, oxide-ion and electronic hole under wet higher oxygen partial pressure. Both of the protonic and oxide-ionic conductivities increased with increasing barium content in the specimens under wet hydrogen and dry air, respectively.