Dynamic recrystallization in high-strain,high-strain-rate plastic deformation of copper

When copper is deformed to high plastic strain (γ ∼ 3∼4) at high strain rates (γ ∼ 10⁴s⁻¹ a microstructure with grain sizes of ∼ 0.1 μm can be produced. It is proposed that this microstructure develops by dynamic recrystallization, which is enabled by the adiabatic temperature rise. By schock-loading the material, and thereby increasing its flow stress, the propensity for dynamic recrystallization can be enhanced. The grain size-flow stress relationship observed after cessation of plastic deformation is consistent with the general formulation proposed by Derby [Acta metall. mater.39, 955 (1991)]. The temperatures reached by the specimens during dynamic deformation are calculated from a constitutive equation and are found to be, for the shock-loaded material, in the 500–800 K range; these temperatures are consistent with static annealing experiments on shock-loaded specimens, that show the onset of static recrystallization at 523 K. A possible recrystallization mechanism is described and its effect on the mechanical response of copper is discussed.     

Cyclic strain hardening in polycrystalline copper

Polycrystals of pure copper were cyclically deformed, at room temperature under symmetric tension-compression fatigue at constant total strain amplitude control with an approximate constant plastic strain rate of 10⁻⁴. The relationship between the saturation stress amplitude and strain amplitude over a range of plastic strain from 2 × 10⁻⁷ to 10⁻² reveals three regions of cyclic hardening. A quasi-plateau, where the stresses show a slow constant increase, was observed in the intermediate region extending in the plastic strain range γ_pl, of 1.5 × 10⁻⁵ to 7.5 × 10⁻⁴. In this region, persistent slip bands (PSBs) which consist of “ladder” structures, similar to the case of single crystals, were found in the bulk of the fatigued polycrystals. The fatigue limit was found to be Δσ_s/2 = 73 MPa which corresponds to the plastic strain Δϵ_pl/2 = 1.5 × 10⁻⁵ where PSBs do not form.     

A SIMS investigation of impurity diffusion in amorphous Fe40Ni40B20

The diffusion coefficients of Be and Si in the amorphous alloy Fe₄₀Ni₄₀B₂₀ were measured in the temperature range 575–643 K by using the technique of secondary ion mass spectrometry (SIMS). The measurements were carried out on the pre-relaxed alloy specimens and the diffusion coefficients D were found to lie in the range 1.0 × 10⁻²³−2.0 × 10⁻²⁰ m² s⁻¹. The temperature dependence of the measured diffusion coefficients was found to be Arrhenius in nature yielding the values of the activation energy Q as 2.16 ± 0.19 eV atom⁻¹ and 3.39 ± 0.30 eV atom⁻¹ for the diffusion of Be and Si respectively. The corresponding values of the pre-exponential factors (D₀) were 9.4×10^{(−4.0±1.57)}m²s⁻¹ and 7.0 × 10^{5.0 ± 2.42} m² s⁻¹. An overall comparison of all the available data on diffusion in amorphous Fe₄₀Ni₄₀B₂₀ clearly shows that the previously reported correlations for the diffusion coefficient D, the activation energy Q, and the pre-exponential factor D₀ for amorphous alloys are seen in this alloy also. It is suggested that the mechanism of diffusion possibly involves a cooperative movement of a group of atoms.     

Hydrogen embrittlement in ductile Ni3Al—Effects of hydrogen content,strain rate and pre-deformation

Hydrogen embrittlement has been studied in continuous cast sheet of an Ni₃Al alloy (Ni_77.83Al_21.73Zr_0.34B_0.1). When tensile tests were performed at the initial strain rate of 5.8 × 10⁻⁵ s⁻, the elongation decreased from 32.7% for no charging to 1.9% for 330 min of cathodic charging with 50 mA/cm² current, but the yield stress did not change. The fracture mode changed partially from dimple to intergranular and cleavage modes. At a faster strain rate of 5.8 × 10⁻³ s⁻¹, hydrogen embrittlement was less pronounced, but the yield stress increased with hydrogen content and multiple cracks were generated. Plastically pre-deformed (2–26% elongations) and subsequently charged specimens failed after yielding, which occurred at the final pre-deformation stress. Our results suggest that the grain boundary or the interior of the grain was not weakened by hydrogen, rather hydrogen-enhanced localized plasticity caused the loss of ductility in B-doped Ni₃Al alloy.     

Diffusion measurements in the Fe82B18 amorphous alloy by rutherford backscattering spectrometry

Diffusion of Au, and Pb in the as-quenched metallic glass Fe₈₂B₁₈ was studied in the temperature range 575–645 K by employing the technique of Rutherford Backscattering Spectrometry (RBS). The diffusion coefficients (D) were obtained by fitting an error function type of solution of the diffusion equation to the tail region of the concentration versus depth profiles. In the case of Pb, where such a solution was not applicable, the equation was solved numerically. The D values were found to lie in the range ~ 1 × 10⁻²¹−4 × 10⁻²⁰ m² s⁻¹. Diffusion measurement of Au were also carried out on relaxed and pre-crystallized specimens of the alloy. The results indicated an insignificant effect of the relaxation treatment on diffusivity values, while significantly higher diffusion rates were observed in the case of pre-crystallized specimens. The temperature dependence of D could be fitted to an Arrhenius relation yielding the values of the frequency factor, D₀, and the activation energy, Q.     

Nd-doped structurally disordered YSr3(PO4)3 single crystal: Growth and laser performances

Disordered-structure crystals have drawn increasing attention as promising ultrashort laser material hosts owing to their broad linewidth. Herein, a novel disordered Nd:YSr₃(PO₄)₃ (Nd:YSP) crystal with good quality was successfully grown via the Czochralski pulling technique. The absorption and fluorescence spectra of the Nd:YSP single crystal were recorded at ambient temperature. The maximum absorption cross section for Nd:YSP single crystal is found to be approximately 3.89 × 10⁻²⁰ cm². The stimulated emission cross section for Nd:YSP crystal at ∼1060 nm was determined to be 7.64 × 10⁻²⁰ cm² with the full width half maximum value of 22 nm. The fluorescence lifetime of the Nd³⁺ ions in the Nd:YSP crystal is fitted to be 288 μs. Diode-pumped continuous-wave laser operation is firstly realized at approximately 1060 nm. The maximum output power value from the Nd:YSP crystal is 714 mW, corresponding to a slope efficiency of ∼12.8%. The results indicate that the Nd:YSP crystal with a disordered structure may be a promising disordered laser host.     

Superplastic-like behavior in as-extruded AlZnMg alloy matrix composites reinforced with Si3N4 whiskers

For as-extruded AlZnMg alloy without reinforcement of Si₃N_4ω, a maximum elongation is 140%, which was measured near a relatively high strain rate 2 × 10⁻²s⁻¹ at 818 K. On the other hand, a maximum elongation of 160% for α-Si₃N_4ω/AlZnMg composites, whose grain size was less than 4 μm, was obtained at 798 K and at the highest end in strain rate range of 8 × 10⁻¹s⁻¹ evaluated in this work, and for β-Si₃N_4ω/AlZnMg composites with grain size of about 4 μm, a maximum elongation of 230% was obtained at 818 K and a high strain rate of 2 × 10⁻¹s⁻¹. In these AlZnMg system alloys the composites reinforced with α-Si₃N_4ω exhibited a high superplastic elongation at higher strain rates than the composites reinforced with β-Si₃N_4ω.     

Effect of Cyclic Predeformation on the Uniaxial Tensile Deformation Behavior of [017] Cu Single Crystals Oriented for Critical Double Slip

The effect of cyclic predeformation at different plastic strain amplitudes γ _pl on the uniaxial tensile behavior of the [017] critical double-slip-oriented copper single crystal was investigated. A cyclic predeformation at a low γ _pl of 7.0 × 10^?4 was found able to enhance the tensile strength of the [017] crystal at nearly no expense of the decrease in plasticity. However, as the γ _pl for the prefatigue increases to a higher value of 3.0 × 10^?3, the tensile strength of the prefatigued [017] crystal decreases to be slightly less than that of the unfatigued crystal, and simultaneously its tensile plasticity decreases markedly. Therefore, a cyclic predeformation at an appropriate plastic strain amplitude might bring about an obvious strengthening effect for metallic single crystals.     

Influence of the crystalline microstructure on the diffusivity of hydrogen in palladium galvanostatic permeation and X-ray diffraction measurements

The influence of crystalline microstructure upon the apparent diffusion coefficient of hydrogen in Pd samples are reported. The apparent diffusion coefficient was measured by the galvanostatic permeation method (at 20°C), while the structure was characterized by X-ray diffraction measurements. Pd membrane electrodes of different thicknesses, l = 5 × 10⁻³cm and l = 10⁻²cm, were used. The structure of Pd membrane electrodes was changed by successive annealing and sequences of absorption and desorption of hydrogen accompanied by the α⇌β phase transition. Both the irreversible and reversible traps affect the mobility of hydrogen in Pd. The irreversible traps manifest in the variation of the apparent diffusion coefficient as a function of the average stationary bulk concentration of hydrogen, while the diffusion coefficient of the free hydrogen does not depend on the actual concentration of hydrogen in Pd. This is true provided that the average stationary bulk concentration of hydrogen is higher than the irreversible trap concentration. The diffusion coefficient of free hydrogen depends on the specific internal surface area according to McNabb-Foster-like equation. The value of the diffusion coefficient of hydrogen in single crystal of palladium was estimated, D₀ = (4.2 ± 0.3) × 10⁻⁷ cm²s⁻¹ (20°C).     

A critical assessment of flow and cavity formation in a superplastic yttria-stabilized zirconia

Tensile tests were performed on specimens of high purity 3Y-TZP (tetragonal ZrO₂ stabilized with ∼ 3 mol% of Y₂O₃) at temperatures from 1623 to 1803 K. Superplastic-like flow was achieved with elongations of up to >400%. The experiments show that the stress exponent, n, is ∼ 3.0–3.4 and the activation energy for flow, Q, is ∼ 602±20 kJ mol⁻¹. Internal cavities developed in all specimens during flow, and quantitative measurements were taken of the cavity shapes and sizes over a range of experimental conditions. The results demonstrate that there is an increase in the extent of cavitation with (i) decreasing temperature at constant strain rate and (ii) increasing strain rate at constant temperature. The influence of strain rate on cavitation in 3Y-TZP is the opposite of most superplastic metals, and the difference arises because of an inhibition in the diffusion growth process in 3Y-TZP.     

Structural and topological study of superplasticity in zirconia polycrystals

Polycrystalline samples of a fine-grained yttria partially stabilized zirconia have been superplastically compressed between 1220 and 1330°C up to strains near 100%, in an initial strain rate range varying from 1.2 × 10⁻⁵to 1.2 × 10⁻⁴/s. The electron microscope observations have shown that the structural evolution of the samples (grain size, grain shape) was weak during these tests. The principal difference resided in the formation of cavities, the density of which was nevertheless small. A topological study has shown that the evolution of the distribution function of grain type (grain side number in a planar section) was also very slight during deformation.     

On the creep of directionally solidified MoSi2-Mo5Si3 eutectics

The high temperature deformation behaviour of MoSi₂-Mo₅Si₃ eutectics has been investigated as a function of lamellar spacing over the temperature range 1100–1400°C and strain rates (∈) of 1 × 10⁻⁴ to 1 × 10⁻⁶ s⁻¹. Specimens with lamellar morphologies were produced by directional solidification using the Czochralski method at pull rates of 25–210 mm/h giving lamellar spacings (λ) of 2.6 to 1.09 μm. The measured flow stress was found to increase as the lamellar spacing decreased for a given strain rate. A constitutive model for creep that incorporates reinforcement spacing for creeping fibers in a creeping matrix was found to describe the creep behaviour of the eutectic, i.e. ∈αλ withm = 1. Creep deformation of the eutectic was controlled by ½〈110〉 (001) partial dislocations in the Mo₅Si₃ phase. The creep behaviour of a [314] oriented Mo₅Si₃ single crystal was also investigated.     

High-temperature deformation of MnO

Steady-state flow stresses have been measured for {100}-oriented MnO single crystals for temperatures from 900 to 1400°C and strain rates from 2 × 10⁻⁶to 2 × 10⁻³s⁻¹. The crystals were equilibrated in oxygen partial pressures ranging from 10¹¹ to 10²Pa, which induced deviations from stoichiometry of 4 × 10⁻⁴to 9 × 10⁻². Deformation rates are controlled by oxygen diffusion. Pipe diffusion along dislocation cores is predominant for T ≤ 1000°C; volume diffusion is predominant for T ≥ 1200dgC. In general, the primary diffusing oxygen species are singly charged vacancies for low Po₂ and neutral interstitials for high Po₂. At 1400°C, ionization states decrease for the oxygen point defects.     

Creep deformation of ultrafine grained Ni75B17Si8

Ultrafine grained, polycrystalline materials can be prepared by annealing of melt spun amorphous ribbons under carefully controlled conditions. A grain size of 6.5–12.10⁻⁵ mm was attained in ribbons of Ni₇₅B₁₇Si₈, annealed at 600°C for 15 min, with a heating rate of 0.01–1 K/s. The samples were then creep tested at low stresses (< 400 kg/cm²) and temperatures between 500 and 615°C. Coble creep, with an activation energy of 4 eV, was the dominant mechanism of deformation at low stress, and below 575°C. At higher stress the rate controlling mechanism became a dislocation climb mechanism. The transition was temperature and grain size dependent. Above 575°C, the dominant mechanism was found to be a nondiffusion controlled process, with an activation energy of 0.6 eV, close to dislocation glide activation energy in Ni.     

Hydrogen Decrepitation and Recycling of NdFeB-type Sintered Magnets

With the rapid growth in the use of NdFeB-type magnets and with the growing environmental need to conserve both energy and raw materials, the recycling of these magnets is becoming an ever important issue. In this paper it is demonstrated that hydrogen could play a vital role in this process. Fully dense, sintered NdFeB-type magnets have been subjected to the hydrogen decrepitation (HD) process. The resultant powder has been subsequently processed in one of two ways in order to produce permanent magnets. Firstly, the powder was subjected to a vacuum degassing treatment over a range of temperatures up to 1000°C in order to produce powder that would be suitable for the production of anisotropic bonded or hot pressed magnets. Secondly, the HD-powder has been used to produce fully dense sintered magnets; in which case optimisation of the milling time, sintering temperature and time was carried out. The optimum degassing temperature for coercive powder was found to be 700°C, giving powder with a remanence (B_r) of ∼1350mT (±10 mT) and an intrinsic coercivity (H_cj) of ∼750kAm⁻¹ (±10 kAm⁻¹). The best sintered magnet was produced by very lightly milling the powder (30 min, roller ball mill), aligning, pressing and vacuum sintering at 1080°C for 1 hour. The magnetic properties of this magnet were: (BH)_max = 290 kJm⁻³ (±5 kJm⁻³), Sr = 1240mT (±5 mT) and H_cj = 830 kAm⁻¹ (±5 kAm⁻¹); representing decreases of 15%, 10%, and 20% respectively, from the properties of the initial magnet.     

The Effect of Forging Variables on the Supersolvus Heat-Treatment Response of Powder-Metallurgy Nickel-Base Superalloys

The effect of subsolvus forging temperature and strain rate on the grain size developed during final supersolvus heat treatment (SSHT) of two powder-metallurgy, gamma–gamma prime superalloys, IN-100 and LSHR, was established. For this purpose, isothermal, hot compression tests were performed at temperatures ranging from 1144 K (871 °C) and 22 K (22 °C) below the respective gamma-prime solvus temperatures (T _γ′) and strain rates between 0.0003 and 10 s^?1. Deformed samples were then heat treated 20 K (20 °C) above the solvus for 1 h with selected additional samples exposed for shorter and longer times. For both alloys, the grain size developed during SSHT was in the range of 15 to 30 μm, except for those processing conditions consisting of pre-deformation at the highest temperature, i.e., T _γ′—22 K (T _γ′—22 °C), and strain rates in the range of ~0.001 to 0.1 s^?1. In these latter instances, the heat-treated grain size was approx. four times as large. The observations were interpreted in terms of the mechanisms of deformation during hot working and their effect on the driving forces for grain-boundary migration which controls the evolution of the gamma-grain size.     

The Observation of the Structure of M<Subscript>23</Subscript>C<Subscript>6</Subscript>/<Emphasis Type="Italic">γ</Emphasis> Coherent Interface in the 100Mn13 High Carbon High Manganese Steel

The M₂₃C₆ carbides precipitate along the austenite grain boundary in the 100Mn13 high carbon high manganese steel after 1323 K (1050 °C) solution treatment and subsequent 748 K (475 °C) aging treatment. The grain boundary M₂₃C₆ carbides not only spread along the grain boundary and into the incoherent austenite grain, but also grow slowly into the coherent austenite grain. On the basis of the research with optical microscope, a further investigation for the M₂₃C₆/γ coherent interface was carried out by transmission electron microscope (TEM). The results show that the grain boundary M₂₃C₆ carbides have orientation relationships with only one of the adjacent austenite grains in the same planes: \( (\bar{1}1\bar{1})_{{{\text{M}}_{ 2 3} {\text{C}}_{ 6} }} //(\bar{1}1\bar{1})_{\gamma } , \) \( (\bar{1}11)_{{{\text{M}}_{ 2 3} {\text{C}}_{ 6} }} //(\bar{1}11)_{\gamma } ,[ 1 10]_{{{\text{M}}_{ 2 3} {\text{C}}_{ 6} }} //[ 1 10]_{\gamma } \). The flat M₂₃C₆/γ coherent interface lies on the low indexed crystal planes {111}. Moreover, in M₂₃C₆/γ coherent interface, there are embossments which stretch into the coherent austenite grain γ. Dislocations distribute in the embossments and coherent interface frontier. According to the experimental observation, the paper suggests that the embossments can promote the M₂₃C₆/γ coherent interface move. Besides, the present work has analyzed chemical composition of experimental material and the crystal structures of austenite and M₂₃C₆, which indicates that the transformation can be completed through a little diffusion for C atoms and a simple variant for austenite unit cell.     

Diffusion along migrating and stationary grain boundaries in the CuAg system

The diffusion along a migrating random grain boundary of a Cu-3.8 at.% Ag bicrystal has been investigated by discontinuous precipitation and dissolution. The measurements have been carried out over an extensive temperature range. The reaction front velocities varied between 1.8 × 10⁻¹⁰ and 2.7 × 10⁻⁶ m/s. The values of sδD_b (s = segregation factor, δ = grain boundary width and D_b = grain boundary diffusion coefficient) have been determined from the precipitation data according to different models. All of the four models applied yield sδD_b values which are of the same order of magnitude as the sδD_b values of stationary grain boundaries. The best correspondence is given by the model of Turnbull resulting in the following Arrhenius parameters: pre-exponential factor (sδD_b)₀ = 5.80 × 10⁻¹¹ m³/s and effective activation energy Q^ef_b = 144.1 kJ/mol. Based on the numerous available data which permit a relevant comparison, we conclude that the diffusivities of migrating, sliding and stationary grain boundaries in the CuAg system are of the same order of magnitude. This conclusion is in accord with the general behavior of other binary systems.     

Study on Dy2O3-Doped Medium Temperature Sintering (Ba,Sr)TiO3 Series Capacitor Ceramics

The influence of Dy₂O₃ doping on the properties of medium temperature sintering (Ba, Sr)TiO₃ series capacitor ceramics was studied by single factor various amount method, and the law of the influence on the medium temperature sintering (Ba, Sr)TiO₃ series capacitor ceramics was obtained. The dielectric materials used for multilayer ceramic capacitor was obtained, of which the dielectric constant was 1375, the dielectric loss was 0.0060, the density was 5.92 g·cm⁻³, the sintering temperature was less than 1150 °C, the capacitance temperature changing rate (ΔC/C) was less than ± 15%, the voltage withstand strength was more than 9.3 kV·mm⁻¹, and the crystal grain size was about 1 μm. The surface morphology of the sample doped with various amount Dy₂O₃ was analyzed by scanning electron microscope (SEM). The results showed that doping Dy₂O₃ could form defect solid solution, stop grain growth, fine crystal grain, widen curie peak, obtaining high dielectric constant and low dielectric loss, capacitance temperature property was suited for X7R character, in the (Ba,Sr)TiO₃ series ceramics. At the same time, the voltage withstand strength was enhanced greatly.     

Activation parameters of high-temperature creep in polycrystalline nickel at ambient and high pressures

Polycrystalline specimens of pure nickel were deformed in uniaxial compression at temperatures of 1000–1550 K, strain rates of 1×10⁻⁵-3×10⁻³s⁻¹ and pressures of 0.1–1500 MPa, in order to determine the activation parameters of high temperature creep. Experiments at 0.1 MPa were conducted in an MTS apparatus with the specimen immersed in a molten heat-treating salt to prevent oxidation. The data show a decreasing power-law stress exponent with decreasing normalized steady-state flow stress (σ/G), approaching the “natural law” value of n=3 at normalized stresses <10⁻⁴. In contrast, the activation energy is constant over our range of temperatures (T/T_m = 0.55−0.90), and is indistinguishable from the activation energy of self-diffusion (284 kJ/mol). High pressure experiments were conducted in a modified piston-cylinder apparatus using the same molten heat-treating salt for the confining medium. The small activation volume could not be resolved; however, the trend of the high pressure data parallels that of the 0.1 MPa data with a systematic offset, and is consistent with the measured activation volume of self diffusion. Specimens deformed at 0.1 MPa exhibited significant strain-enhanced grain growth; this effect is greatly reduced under hydrostatic pressure, whereas subgrain size was less affected.