High-temperature deformation behavior of Ti60 titanium alloy

Isothermal compressions of near-alpha Ti60 alloy were carried out on a Gleeble-3800 simulator in the temperature range of 960-1110 °C and strain rate range of 0.001-10.0 s⁻¹. The high-temperature deformation behavior was characterized based on an analysis of the stress-strain behavior, kinetics and processing map. The flow stress behavior revealed greater flow softening in the two-phase field compared with that of single-phase field. In two-phase field, flow softening was caused by break-up and globularization of lamellar α as well as deformation heating during deformation. While in the single-phase field, flow softening was caused by dynamic recovery and recrystallization. Using hyperbolic-sine relationships for the flow stress data, the apparent activation energy was determined to be 653 kJ/mol and 183 kJ/mol for two-phase field and single-phase field, respectively. The processing map exhibited two instability fields: 960-980 °C at 0.3-10 s⁻¹ and 990-1110 °C at 0.58-10 s⁻¹. These fields should be avoided due to the flow localization during the deformation of Ti60 alloy.     

Strain hardening behavior of a TRIP/TWIP steel with 18.8% Mn

Tensile tests were carried out to study the strain hardening behavior of a TRIP/TWIP steel with 18.8% manganese. The results indicated that the true stress-strain curve can be divided into 4 stages in tension testing. Material is in an elastic region when the true strain is below 0.06. In the initial stage of the plastic deformation (? = 0.06-0.14), ?-martensite was preliminarily formed, and that austenite transformed to α-martensite through the ?-martensite formation. When the true strain was between 0.14 and 0.35, the stacking fault energies were elevated by the increase of strain energy, deformation twinning occurred instead of the ?-martensite formation. The second derivative of the stress-strain curve satisfied the condition d²σ/d?² > 0. Twinning induced plasticity dominated this stage. In the last plastic deformation stage (? = 0.35-0.45), γ → α transformation occurred at the crossing of twins, and α-martensite grew along the thickness of the twinned regions.     

Effect of hot working on flow behavior of Ti–6Al–4V alloy in single phase and two phase regions

Hot deformation behavior of the alloy Ti–6Al–4V was investigated via conducting hot compression tests at temperatures of 800–1150 °C and at strain rates, ranging from 0.001 s⁻¹ to 1 s⁻¹, at an interval of an order of magnitude. The apparent differences of flow stress curves obtained in dual phase α + β and single phase β regions were analyzed in term of different dependence of flow stress to temperature and strain rate and different microstructural evolutions. The values of strain rate sensitivity and apparent activation energy were obtained respectively as 0.20 and 530 kJ/mol for two phase microstructure. However, for single phase β microstructure they were approximated as 0.19 and 376 kJ/mol, respectively. It was found that in two phase region the values of strains corresponding to peak point, ε_p, and the highest rate of flow softening, ε^*, are almost independent to Zenner–Hollomon parameter. In single phase region, ε_p and ε^* exhibited a direct relationship to Z parameter and the corresponding empirical equations were proposed.     

High frequency characteristics of tin oxide thin films on Si

High frequency characteristics of tin oxide (SnO₂) thin films were studied. SnO₂ thin films have been successfully grown on n-type Si (111) substrates by using a spray deposition technique. The capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of the metal-oxide-semiconductor (Au/SnO₂/n-Si) Schottky diodes were investigated in the high frequency range from 300 kHz to 5 MHz. It has been shown that the interface state density, D_it, ranges from 2.44 × 10¹³ cm⁻² eV⁻¹ at 300 kHz to 0.57 × 10¹³ cm⁻² eV⁻¹ at 5 MHz and exponentially decreases with increasing frequency. The C-V and G/ω-V characteristics confirm that the interface state density and series resistance of the diode are important parameters that strongly influence the electrical parameters exhibited by the metal-oxide-semiconductor structure.     

Effect of frequency and environment on fatigue behavior of a CVI SiC/SiC ceramic matrix composite at 1200 °C

The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress-strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 10⁵ cycles at the frequency of 0.1 Hz and as 2 × 10⁵ cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Prediction of hot deformation behaviour of Fe-25Mn-3Si-3Al TWIP steel

Hot deformation behaviour of Fe-25Mn-3Si-3Al twinning-induced plasticity (TWIP) steel was investigated by hot compression testing on Gleeble 3500 thermo-mechanical simulator in the temperature range from 800 to 1100 °C and at strain rate range from 0.01 to 5 s⁻¹, and the microstructural evolution was studied by metallographic observations. The results show that the true stress-true strain curves exhibit a single peak stress at certain strain, after which the flow stresses decrease monotonically until the end of deformation, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be predicted by the Zener-Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 405.95 kJ/mol. The peak and critical strains can also be predicted by Z parameter in power-law equations, and the ratio of critical strain to peak strain is about 0.7. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and the decreasing of Z value leads to more extensive DRX.     

IR spectra and dielectric properties of Cu–Ge ferrite

The mixed ferrite Cu_1+xGe_xFe_2−2xO₄; where x = 0.0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3; was prepared from high purity oxides using the standard ceramic technique. The IR spectra were recorded on the range from 200 to 1000 cm⁻¹. The two primary bands corresponding to tetrahedral ν_A and octahedral ν_B were observed around 575 cm⁻¹ and 400 cm⁻¹, respectively. It was found the threshold frequency ν_th for the electronic transition increases with increasing the Ge content. The Debye's temperature θ_D was calculated and it was found dependent on the type of charge carriers. AC conductivity σ_ac with dielectric properties (dielectric constant ?′, dielectric loss ?″ and loss tangent tan δ) as a function of frequency (f = 10² → 10⁶ Hz) at room temperature have been measured. The dispersion of the dielectric properties was discussed in the light of Koop's phenomenological theory.     

Investigation of the electrical parameters of Ag/p-TlGaSeS/C Schottky contacts

p-type TlGaSeS single crystal was used to fabricate a Schottky device. Silver and carbon metals were used as the Ohmic and Schottky contacts, respectively. The device which displayed wide RF band at 13.200 and narrow band at 62.517 kHz with Q value of 1.4 and of 6.3 × 10⁴, respectively, is characterized by means of current (I)–voltage (V), capacitance (C)–voltage characteristics as well as capacitance–frequency (f) characteristics. The device series resistance, ideality factor and barrier height are determined from the I–V curve as 35.8 MΩ, 1.2 and 0.74 eV, respectively. The apparent acceptor density and the build in voltage of the device increased with increasing ac signal frequency. The high Q value, observed at 62.517 kHz, indicated a much lower rate of energy loss relative to the stored energy of the device. The energy loss (Q⁻¹) is much less than 0.001% of the stored value. The device was tested and found to remain at the same mode of resonance for several hours. It never switched or ceased unless it was tuned off.     

Pseudo spin-valves with different spacer thickness as sensing elements of mechanical strain

Giant magnetoresistance (GMR) effect in cobalt based pseudo spin-valves (SV) is combined with the inverse magnetostriction in sensors of mechanical strain. SV with Co/Au/Co core structure were deposited onto the flexible 125 μm thick polyimide substrates. The influence of magnetostriction on GMR was studied in bending current-in-plane configuration. Total relative strain was between −8.6 × 10⁻³ (compression) and 8.6 × 10⁻³ (tension). SV were designed with respect to the oscillating nature of exchange coupling force vs. spacer thickness. The period of oscillations is not changing under the applied stress in our experimental configuration. From the magnetoresistance ratio vs. strain dependences it follows that the output signal of the strained sensor vs. unloaded one could be improved by a proper choice of the spacer thickness t_s. If t_s = 4 nm the relative output is 36% and for t_s = 2.4 nm it is 34%, however, in this case only in the half range of the strain between 0 and ±8.6 × 10⁻³. For t_s = 2.2 nm the relative output is only 10%.     

Constitutive modeling for the dynamic recrystallization evolution of AZ80 magnesium alloy based on stress-strain data

In order to improve the understanding of the dynamic recrystallization (DRX) behaviors of as-cast AZ80 magnesium alloy, a series of isothermal upsetting experiments with height reduction 60% were performed at the temperatures of 523 K, 573 K, 623 K and 673 K, and the strain rates of 0.01 s⁻¹, 0.1 s⁻¹, 1 s⁻¹ and 10 s⁻¹ on a Gleeble 1500 thermo-mechanical simulator. Dependence of the flow stress on temperature and strain rate is described by means of the conventional hyperbolic sine equation. By regression analysis, the activation energy of DRX in the whole range of deformation temperature was determined to be Q = 215.82 kJ mol⁻¹. Based on dσ/d? versus σ curves and their processing results, the ?ow stress curves for AZ80 magnesium alloy were evaluated that they have some characteristic points including the critical strain for DRX initiation (?_c), the strain for peak stress (?_p), and the strain for maximum softening rate (?*), which means that the evolution of DRX can be expressed by the process variables. In order to characterize the evolution of DRX volume fraction, the modified Avrami type equation including ?_c and ?* as a function of the dimensionless parameter controlling the stored energy, Z/A, was evaluated and the effect of deformation conditions was described in detail. Finally, the theoretical prediction on the relationships between the DRX volume fractions and the deformation conditions were validated by the microstructure graphs.     

Hydrothermal synthesis and characterization of (Bi,K)TiO3 ferroelectrics

A lead-free ferroelectric (Bi,K)TiO₃ (BKT) was synthesized by a hydrothermal process and characterized systematically at various temperatures. Well-crystallized BKT in the tetragonal phase was identified at a hydrothermal temperature over 220 °C. Small cubic particles were observed, regardless of hydrothermal temperature. The BKT sintered at 1050 °C was observed to be a typical relaxor behavior and very stable against frequency and temperatures, respectively. The sintered-BKT ceramics exhibited a high temperature of maximum dielectric permittivity (T_max = 356 °C at 10⁶ Hz) with piezoelectric constant (d₃₃ = 65 pC/N) and electromechanical coupling factors (k_p = 0.22, k_t = 0.43). Thus, the sintered-BKT showed excellent temperature stability with a high-T_max and piezoelectric properties.     

Noise in micro-bolometers with silicon-germanium thermo-sensing layer

Low frequency noise in a-Si_xGe_y:H thermo-sensing films, on glass and in micro-bolometers of planar and sandwich structures based on the same material has been studied at different temperatures. The noise spectra had the form of the 1/f-like noise with the frequency exponent within the range of 0.8 to 1.6 depending on the sample and temperature. In the temperature range from T = 340 to 400 K the amplitude of the noise and current (at constant voltage) increased. These dependences can be described as a thermal activated process with energies of E_a^S/I = 0.63 eV and E_a^film = 0.34 eV for relative spectral noise density of the current fluctuations and DC current, respectively.     

Capacitance analyses of hydrogenated nanocrystalline silicon based thin film transistor

The capacitance-voltage (C-V) measurements within 10⁶-10^− 2 Hz frequency range were performed on the hydrogenated nanocrystalline silicon (nc-Si:H) bottom-gate thin film transistor (TFT) and metal-insulator-amorphous silicon (MIAS) structure, mechanically isolated from the same TFT. It was found that the conducting thin layer in nc-Si:H film expands the effective capacitor area beyond the electrode in the TFT structure, which complicates its C-V curves. Considering the TFT capacitance-frequency (C-F) curves, the equivalent circuit of the TFT structure was proposed and mechanism for this area expansion was discussed. On the other hand, the MIAS C-F curves were fitted by the equivalent circuit models to deduce its electrical properties. nc-Si:H neutral bulk effect was revealed by the dependence of the MIAS capacitance on frequency within 10⁶-10³ Hz at both accumulation and depletion regimes. The inversion in MIAS was detected at 10²-10^− 2 Hz for relatively low negative gate bias without any external activation source. The presence of the ac hopping conductivity in the nc-Si:H film was inferred from the fitting. In addition, the density of the interface traps and its energy distribution were determined.     

Factors influencing softening temperature and hot-strength of geopolymers

This paper presents the effects of cation type, silicate concentration, compositions of low-calcium fly ash and test load on softening temperature and hot-strength (loaded while at high temperature) of fly ash-based geopolymers. It was found that softening temperature (T_s) of sodium (Na) based-geopolymer remained the same (610 °C ± 20 °C) regardless of the silicate concentration, fly ash composition, and test load. However, when the cation was changed from sodium to potassium (K), the T_s increased to 800 °C. Further, when Na/K is mixed, the T_s dropped to 570 °C.Significant increases in hot-strengths (at 530 °C and 730 °C) were found in all geopolymers. This increase in strength was observed to be in conjunction with heat release, indicating an exothermic reaction. Since the reaction between fly ash and activators is exothermic, this reaction is believed to be the reason for the strength gain at high temperatures.     

Mössbauer effect and dielectric behavior of NixCu0.8−xZn0.2Fe2O4 compound

The ferrite system Ni_xCu_0.8−xZn_0.2Fe₂O₄ with 0.0 ≤ x ≤ 0.8 was synthesized. XRD measurement confirmed the presence of single-phase spinel structure. The area ratio of Fe³⁺ at the tetrahedral A- and octahedral B-sites was deduced from the spectral analysis of Mössbauer measurements. The results give evidence that Ni²⁺ replaces Cu at B-site in the present unit cell. The dielectric properties ?′, ?″, loss tangent tan(δ) and ac conductivity σ_ac have been studied for the prepared samples in the temperature range (300-600 K) and over the frequency range (10² to 10⁵ Hz). The electrical conductivity results revealed a semiconductor behavior with increasing nickel concentration with a change in the slope at the transition temperature T_c. The variation of the dielectric parameters (?′, ?″ and tan(δ)) with frequency and temperature displayed a strong dependence on nickel concentration. Dielectric anomaly at the transition temperature T_c was pronounced in the relations of ?′ and ?″ with temperature. The determined T_c was found to increase with increasing Ni content. The relation of tan(δ) with frequency at different temperatures showed two relaxation processes where the relaxation time and maximum frequency of the hopping conduction mechanism were determined. The results are explained in the light of cation-anion-cation and cation-cation interactions over the octahedral site in the spinel structure.     

Conduction mechanism and magnetic behavior of dysprosium strontium iron garnet (DySrIG) nanocrystals

Garnet nanoparticles Dy_2.8Sr_0.2Fe₅O₁₂ (DySrIG) were prepared by citrate auto-combustion method and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and differential thermal analysis (DTA). The suitable formation of this garnet in single phase was at 1100 °C with crystallite of size 95 nm. The Curie temperature of DySrIG is obtained at 610 K. The effective magnetic moment μ_eff was calculated experimentally and theoretically and they are compatible with each other. The dielectric constant ?′ increases from order 10² at room temperature to 10⁴ at 850 K passing by four transition temperatures. The temperature dependence of the resistivity of DySrIG at different frequencies (f) 100 kHz ≤ f ≤ 5 MHz indicates the presence of 5 transition temperatures which are slightly different from those of ?′ data. The resistivity data are frequency independent at f < 1 MHz. The transition height is decreased by increasing the temperature from ≈5 MΩ cm at 320 K and 200 kHz to ≈20 Ω cm at 700 K. Accordingly, Dy_2.8Sr_0.2Fe₅O₁₂ (DySrIG) is recommended for the use in phase shifter, circulators and microwave applications.     

Orientation of B-C-N hybrid films deposited on Ni (111) and polycrystalline Ti substrates explored by X-ray absorption spectroscopy

Orientation of sp²-bonded boron carbonitride (BCN) hybrid films has been investigated. The films were synthesized on Ni (111) and polycrystalline Ti substrates by radio frequency plasma enhanced chemical vapor deposition using tris-dimethylamino borane as a single-source molecular precursor. The deposition was performed at the radiofrequency power 400-800 W at the working pressure 2.6 Pa. Formation of sp²-BCN hybrids in the samples was confirmed by X-ray diffraction (XRD). In the XRD profile, the peak at 26.3° revealed formation of crystalline phase in the samples in which the lattice planes are separated from each other by around 3.5 Å. The D band at ~ 1350 cm^− 1 and the G band at ~ 1570 cm^− 1 in Raman spectra also suggested presence of graphite-like sp²-B-C-N hybrid bonds. The films were composed of different B-N, B-C, and C-N bonds to form sp²-BCN atomic hybrids confirmed by X-ray photoelectron spectra. Orientation and local structures of the films were studied by near-edge X-ray absorption fine structure (NEXAFS) measurements. The dominant presence of π* and σ* resonance peaks of the sp² hybrid orbitals in B K-edge NEXAFS spectra revealed preferred formation of sp²-BCN atomic hybrids around B atoms like-BN₃ configuration in respect to the plane of Ni (111) substrate. Different orientations were suggested on the basis of polarization dependence of B K-edge and N K-edge of the NEXAFS spectra.     

Effect of bead milling on heat generation ability in AC magnetic field of FeFe2O4 powder

Nano-sized FeFe₂O₄ ferrite powder having a heat generation ability in an AC magnetic field was prepared by bead milling for a thermal coagulation therapy application. A commercial powder sample (non-milled sample) of ca. 2.0 μm in particle size showed a temperature enhancement (ΔT) of 3 °C in an AC magnetic field (powder weight 1.0 g, 370 kHz, 1.77 kA m⁻¹) in ambient air. The heat generation ability in the AC magnetic field improved with the milling time, i.e., due to a decrease in the average crystallite size for all the examined ferrites. The highest heat ability (ΔT = 26 °C) in the AC magnetic field in ambient air was for the fine FeFe₂O₄ powder with a 4.7 nm crystallite size (the samples were milled for 6 h using 0.1 mm? beads). However, the heat generation ability decreased for the excessively milled FeFe₂O₄ samples having average crystallite sizes of less than ca. 4.0 nm. The heat generation of the samples showed some dependence on the hysteresis loss for the B–H magnetic property. The reasons for the high heat generation properties of the milled samples would be ascribed to an increase in the Néel relaxation of the superparamagnetic material. The hysteresis loss in the B–H magnetic curve would be generated as the magnetic moment rotates (Néel relaxation) within the crystal. The heat generation ability (W g⁻¹) can be estimated using a 1.07 × 10⁻⁴fH² frequency (f, kHz) and the magnetic field (H, kA m⁻¹) for the samples milled for 6 h using 0.1 mm? beads. Moreover, an improvement in the heating ability was obtained by calcination of the bead-milled sample at low temperature. The maximum heat generation (ΔT = 59 °C) ability in the AC magnetic field in ambient air was obtained at ca. 5.6 nm for the sample calcined at 500 °C. The heat generation ability (W g⁻¹) for this heat treated sample was 2.54 × 10⁻⁴fH².     

Effect of annealing temperature on structural, electrical and optical properties of B-N codoped ZnO thin films

The B-N codoped p-type ZnO thin films have been prepared by radio frequency magnetron sputtering using a mixture of nitrogen and oxygen as sputtering gas. The effect of annealing temperature on the structural, electrical and optical properties of B-N codoped films was investigated by using X-ray diffraction, Hall-effect, photoluminescence and optical transmission measurements. Results indicated that the electrical properties of the films were extremely sensitive to the annealing temperature and the conduction type could be changed dramatically from n-type to p-type, and finally changed to weak p-type in a range from 600 °C to 800 °C. The B-N codoped p-type ZnO film with good structural, electrical and optical properties can be obtained at an intermediate annealing temperature region (e.g., 650 °C). The codoped p-type ZnO had the lowest resistivity of 2.3 Ω cm, Hall mobility of 11 cm²/Vs and carrier concentration of 1.2 × 10¹⁷ cm^− 3.     

Effect of frequency on fatigue crack growth behavior of magnesium alloy AZ61 under immersed 3.5 mass% NaCl environment

Fatigue crack growth test of AZ61 magnesium alloy was carried out under immersed NaCl environment at frequencies of 15, 5 and 0.5 Hz under a stress ratio of 0.1. In order to investigate the effect of frequency on fatigue crack growth behavior in detail, additional tests at frequencies ranged from 15 to 0.01 Hz were conducted under a constant ΔK of 3.25 MPa m^1/2. Effect of frequency was clearly observed in low ΔK region, where fatigue crack growth rate decreased with decreasing frequency. Crack closure would be a dominant factor for the frequency effect observed under immersed NaCl environment at frequencies ranged from 15 to 0.5 Hz. However, fatigue crack growth rates at frequencies lower than 0.05 Hz were higher than those at frequencies higher than 0.5 Hz. The accelerated fatigue crack growth rates at frequencies lower than 0.05 Hz would be attributed to the corrosion attack at the crack tip.