The relationship between minimum creep rate and rupture time in Cr-Mo steels

The Monkman-Grant equation is widely used to relate the minimum creep rate, ε_min,and the rupture time, t_r, in creep. However, for materials that exhibit large tertiary creep and limited secondary creep, the above equation has not been found to be valid, and a correction in terms of the rupture strain ε_r and the strain at the onset of tertiary stage, ε₂₃, has to be incorporated. The relation is given in the form.     

On the origin of essential differences in the coercive force,remanence, and initial permeability of ferromagnetic steels in the loaded and unloaded states upon plastic tension

The behavior of remanence M _r, initial permeability μ, and coercive force H _c of the steels during their plastic tension in both loaded and unloaded states upon “slow” and “rapid” loading is explained from a unified point of view. It is shown that (1) a sharp (severalfold) decrease in M _r and μ and a similar abrupt increase in H _c are caused by the same effect, namely the appearance of high residual compressive stresses in most grains along the force direction; (2) in the loaded state, M _r and μ slowly decrease and H _c similarly slowly increases with increasing degree of deformation ε due to a common effect of increasing dislocation density ρ with increasing ε; (3) the values of M _r, μ, and H _c in the transition region, in which ε ≤ ε_cr, substantially differ from each other upon “slow” and “rapid” loadings; (4) at ε ≥ ε_cr, the dependences of the M _r, μ, and H _c parameters on ε both in the loaded and unloaded states are parallel to each other.     

Anisotropy and formability of AA5182-0 aluminium alloy sheets

This paper presents a new yield criterion for orthotropic sheet metals and its implementation in a theoretical model of the forming limit diagrams. The equivalent stress equation shows that the shape of the yield surface is defined by eight material parameters. The minimisation of an error-function has been used for the numerical identification of these coefficients. The parameters are established in such a way that the constitutive equation associated to the yield surface reproduces the plastic behaviour of the actual material. The uniaxial yield stresses (σ₀, σ₄₅, σ₉₀), biaxial yield stress (σ_b), uniaxial anisotropy coefficients (r₀, r₄₅, r₉₀) have been used in identification. The new yield criterion has been implemented in the Marciniak-Kuczynski theory in order to predict the limit strains. The theoretical forming limit curves have been compared with the experimental ones. The friction free tests, the hydraulic bulge test (for the positive minor strains) and the tensile test for plane strain and for uniaxial tensile test (for the negative minor strains) are used. The predicted yield surface and forming limit diagrams for AA5182-0 aluminium alloy sheets are in good agreement with the experimental ones.     

Nanocrystallinity and magnetic property enhancement in melt-spun iron-rare earth-base hard magnetic alloys

Refinement of the grain size below ~35 nm mean diameter in melt-spun FeNdB-base alloys leads to en-hancement of remanent polarization,J _r, above the level predicted by the Stoner-Wohlfarth theory for an aggregate of independent, randomly oriented, and uniaxial magnetic particles. This article summarizes the results of the recent systematic research on this phenomenon, including the influence of alloy compo-sition and processing conditions on the crystallite size, degree of enhancement ofJ _r, and maximum en-ergy product(BH) _max. It has been shown that the effect can also occur in ternary FeNdB alloys, without the addition of silicon or aluminum, which was originally thought necessary, providing the nanocrystal-lites are not magnetically decoupled by a paramagnetic second phase. Values of(BH) _max above 160 kJ m_-3 have been achieved. The relationship between grain size,J _r, intrinsic coercivity,_JHc, and(BH) _max are discussed in terms of magnetic exchange coupling, anisotropy, and other parameters. Recent exten-sion of this work to the enhancement of properties in Fe-Mischmetal-Boron-base alloys and to bonded magnets with a nanocrystalline structure is also described.     

Electronic transport in doped poly (3,4-ethylenedioxythiophene) near the metal-insulator transition

The temperature dependences of the conductivity and magnetoconductivity of poly(3,4-ethylenedioxythiophene) doped with PF₆, BF₄ and CF₃SO₃ in the metallic and critical regimes have been studied. Doped films exhibit a weak temperature dependence of the resistivity, ϱ(T), with the characteristic resistivity ratio ϱ_r = ϱ(1.4 K)/ϱ(291 K) = 1.5–20; i.e. close to the metal-insulator transition. For metallic samples (ϱ_r<2.1) prepared with each of the dopants, the sign of the temperature coefficient of resistivity (TCR) changes from negative to positive below 10 K; the temperature of the resistivity maximum, T_m, decreases with increasing ϱ_r. For samples with ϱ_r ~ 20, the power-law temperaturedependence characteristic of the critical regime of the metal-insulator transition is observed, with ϱ(T) ~ T^−0.6. High magnetic fields induce the transition from positive to negative TCR for all metallic samples with ϱ_r < 2.1 and decrease the low-temperature conductivity for samples with ϱ_r > 2.1. In both cases (negative and positive TCR), the low-temperature conductivity of metallic samples is well described by a T^1/2 dependence, both in the presence of a magnetic field and with the magnetic field equal to zero. The magnetoconductance of samples in metallic and critical regimes is negative, Isotropic, and, for metallic samples, exhibits H² and H^1/2 dependences at low and high magnetic fields, respectively. The results for metallic samples are explained as resulting from the influence of electron-electron interactions on the lowtemperature conductivity.     

Grain boundary pinning by Al6Mn precipitates in an Al–1wt%Mn alloy

The grain boundary area per unit of volume, S_V, and the interface area per unit of volume of Al₆Mn precipitates, S_VP, were measured in a high purity Al–1wt%Mn alloy for an annealing time of 3600 s at temperatures ranging from 500 to 620 °C. The experimental results show very good agreement with the expression proposed by Rios [Acta Metall. 35 (1987) 2805]: S_VL=3S_VP where S_VL is the limiting grain boundary area per unit of volume. This expression is easier to verify experimentally than its equivalent form: R_L=r/6f, where r is the particle radius, f is the volume fraction of the particles and R_L is the limiting grain radius.     

超高碳钢中枣核状马氏体形态及亚结构

超高碳(1.58%C)钢中的马氏体相变产物除了板条马氏体、片状马氏体外,还发现枣核状马氏体.HRTEM观察表明,枣核状马氏体的亚结构是高密度位错,位错密度高达1013/cm2,未观察到孪晶.基于盘片状马氏体的理论分析表明,应变能与氏体片的临界厚度(2t0*)无关,而与其临界直径(2r0*)有关;临界形核功(相变能垒)△G*与马氏体晶核临界厚径比t0*/r0*的二次方成反比.t0*/r0* <1时,马氏体核呈圆片状;t0*/r0*>1时,晶核呈枣核状;当t0*/r0*>1时,晶核呈棒状.     

Length-scale effects in the nucleation of extended dislocations in nanocrystalline Al by molecular-dynamics simulation

The nucleation of extended dislocations from the grain boundaries in nanocrystalline aluminum is studied by molecular-dynamics simulation. The length of the stacking fault connecting the two Shockley partials that form the extended dislocation, i.e., the dislocation splitting distance, r_split, depends not only on the stacking-fault energy but also on the resolved nucleation stress. Our simulations for columnar grain microstructures with a grain diameter, d, of up to 70 nm reveal that the magnitude of r_split relative to d represents a critical length scale controlling the low-temperature mechanical behavior of nanocrystalline materials. For r_split>d, the first partials nucleated from the boundaries glide across the grains and become incorporated into the boundaries on the opposite side, leaving behind a grain transected by a stacking fault. By contrast, for r_split<d two Shockley partials connected by a stacking fault are emitted consecutively from the boundary, leading to a deformation microstructure similar to that of coarse-grained aluminum. The mechanical properties of nanocrystalline materials, such as the yield stress, therefore depend critically on the grain size.     

The effect of thiourea on the corrosion kinetics of mild steel in H2SO4

The effect of thiourea (TU) on the corrosion kinetics of mild steel in I N H₂SO₄ has been studied at various temperatures and concentrations. The efficiency of inhibitor in reducing the rate of the hydrogen evolution reaction, P_io was compared to its efficiency in reducing the rate of the corrosion reaction, P_incorr. The ratio r = P_io/P_icorr was shown to be a useful diagnostic criterion; i.e. r ? 1 indicates cathodic rate control while r ? 1 indicates predominance of anodic rate control. At low concentrations ($\text{C < C}{}_{\mathrm{<mtext>crit</mtext>}}\text{)r ? 1}$, where C_crit is the critical concentration above which TU starts to lose its efficiency at C >C_crit the efficiency loss is rather sudden, thus pointing to the failure of TU to inhibit either the anodic or cathodic half reactions. Consideration of the protonation of TU and of the possible zeta potential effects suggests that the observed inhibition is due mainly to the blocking effect of the adsorbed molecular species and that the loss of efficiency at C >C_crit is not primarily due to protonation. The activation energies of both the cathodic and overall reactions have been evaluated at constant concentration and at constant degree of coverage.     

Densification, microstructural evolution, and dielectric properties of hexagonal Ba(Ti1−xMnx)O3 ceramics sintered with fluxes

Recently, doped hexagonal BaTiO₃ (6h-BaTiO₃) ceramics have been reported as potential candidates used in microwave dielectric resonators. However, similar to other common microwave ceramics, doped 6h-BaTiO₃ ceramics require a high sintering temperature, greater than 1300 °C. In this study, the effect of sintering aids, including Bi₂O₃, B₂O₃, BaSiO₃, Li₂CO₃, CuO, V₂O₅, 5ZnO·2B₂O₃, and 5ZnO·2SiO₂, on the densification, microstructural evolution, and microwave properties of the 6h-Ba(Ti_0.85Mn_0.15)O₃ ceramics was examined. Results indicate that among the fluxes studied, Bi₂O₃, B₂O₃, and Li₂CO₃ could effectively reduce the sintering temperature of 6h-Ba(Ti_0.85Mn_0.15)O₃ ceramics through liquid phase sintering, while retaining the hexagonal structure and the microwave dielectric properties. The best results were obtained for the 6h-Ba(Ti_0.85Mn_0.15)O₃ with the additions of 5 wt% Bi₂O₃ sintered at 900 °C (_r: 54.7, Qf_r: 1323, and τ_f:183.3 ppm/°C), 10 wt% B₂O₃ sintered at 1100 °C (_r: 54.4, Qf_r: 3448, and τ_f: 254.5 ppm/°C), and 5 wt% Li₂CO₃ sintered at 950 °C (_r: 43.7, Qf_r: 2501, and τ_f: −29.8 ppm/°C).     

A new approach for reverse analyses in depth-sensing indentation using numerical simulation

This paper seeks to present a new approach to reverse analysis in depth-sensing indentation which makes use of numerical simulation. This methodology allows the results of experimental hardness tests acquired with single indenter geometry to be used to determine the plastic properties of materials. Forward and reverse analyses of high deformation three-dimensional numerical simulations of Vickers indentation tests are used to determine different mechanical properties of materials: Young’s modulus, yield stress and strain-hardening exponent. The Vickers indenter used in the numerical simulations is formulated as a rigid body and takes into account the presence of the most common imperfection of the tip, so-called offset. The contact friction between the Vickers indenter and the deformable body is also considered. The forward analysis uses materials with Young’s modulus values from 50 to 600 GPa, yield stress values from 0.3 to 10 GPa and strain-hardening exponents from 0 to 0.6; the Poisson ratio did not vary from 0.3. The representative plastic strain ε_r and the correspondent stress σ_r, as previously defined by other authors [Dao M, Chollacoop N, Vliet KJ, Venkatesh TA, Suresh S. Acta Mater 2001;49:3899], were identified by an independent numerical method. The values of the representative plastic strain ε_r obtained for the Vickers indenter confirm those of the above-mentioned authors, despite showing a slight influence from the Young’s modulus values. The forward study enables the production of a unique plot of the hardness H_IT vs. representative stress σ_r, where both are normalized by the Young’s modulus E. The proposed reverse analysis provides a unique solution to the representative stress σ_r and the strain-hardening exponent, n, given that the Young’s modulus is predetermined from the experimental hardness test. Depending on the material properties, the value of n can be more or less sensitive to the scatter of the experimental results obtained using the depth-sensing equipment, particularly the stiffness of the unloading curve. The validity of the proposed reverse analysis method is checked using three real materials: stamping quality steel (DC 06), stainless AISI 304 steel and BK7 glass.     

Optimization of the Mode of Isothermal Thermomagnetic Treatment of Magnetically Hard Alloy of the Fe – Cr – Co System with 8% Co

The magnetic properties of deformable magnetically hard alloy Fe – 27.5% Cr – 7.9% Co – 0.3% V – 0.65% Si – 0.4% Ti are studied after isothermal thermomagnetic treatment (ITMT) optimized using the method of experimental design. Mean values of residual induction B _r, coercive forces H _ci (with respect to induction) and H _cm (with respect to magnetization), and maximum energy product (BH)_max after optimum ITMT are determined. A three-dimensional dependence of the coercive force H _cm on the temperature and time of the ITMT is plotted.     

Growth of non-polar ZnO films on a-GaN/r-Al2O3 templates by radio-frequency magnetron sputtering

In this work, non-polar (1,1,?2,0) ZnO thin films have been grown on a-GaN/r-Al₂O₃ templates by radio-frequency magnetron sputtering. By varying the substrate temperature, the ZnO thin films were transformed from polycrystalline structure to epitaxial one on a-GaN/r-Al₂O₃ templates. High-quality (1,1,?2,0) ZnO epitaxial thin films were grown on the a-GaN/r-Al₂O₃ template at the optimized condition of 300 °C, which was confirmed by X-ray diffraction. The full width at half maximum (FWHM) of the (1,1,?2,0) ω-rocking curve of the a-plane ZnO films grown on the a-GaN/r-Al₂O₃ template was 0.51°, indicating a small mosaicity and a low dislocation density in the ZnO film grown on the a-GaN/r-Al₂O₃ template. Its surface roughness observed by atomic force microscopy was about 2.32 nm. Furthermore, the comparative investigation results show that the ZnO films grown on a-GaN/r-Al₂O₃ templates are more likely to form in the (1,1,?2,0)-orientation than those directly grown on r-sapphire.     

Effects of pulsed and continuous Nd–YAG laser beam waves on welding of Inconel alloy

Abstract

In the present work a 2·5 kW high power Nd–YAG laser is used in the bead on plate (BOP) and butt welding of Inconel 690 plates of thickness 3 mm. Welding is performed using a rectangular laser pulse, for which the peak to base power ratio W_r is reduced from an initial value of 10 to a value of 1, maintaining an identical mean power of 1·7 kW. Therefore, the welding mode changes from a pulsed wave to a continuous wave. The BOP results indicate that the depth of the weld penetration increases at a lower travel speed and/or a higher value of W_r. In the butt welding process, as W_r is increased from 1 to 10, the cellular microstructure of the weld remains relatively unchanged, but the macroporosity formation ratio decreases from 7·1% to 0·6%. At low values of W_r, macroporosity is identified primarily in the root region. However, as W_r increases, the associated periodic high power increases the agitation of the molten pool and probably causes bubbles to float upwards. Consequently, at higher values of W_r, the regions of macroporosity are distributed randomly throughout the weld. Although microcracks are not apparent within any of the welds, each weld exhibits slight microporosity. This microporosity decreases as W_r increases. The present results confirm that a pulsed laser beam with an appropriate peak power can be used to achieve a compromise between the mechanical properties and surface roughness of the weld for Inconel 690 in Nd–YAG laser welding.     

Comparative study of one-dimensional NiCo alloy nanostructures assembled by in situ and ex situ applied magnetic fields

For the study of magnetic field-assisted assembly behavior, one-dimensional (1D) NiCo alloy nanostructures were solvothermally obtained at 180 °C under an in situ magnetic field (the magnetic field as applied during the chemical reduction) and ex situ field (after the chemical reduction was finished). Microscopic morphology and magnetic properties differences were investigated using scanning electronic microscope (SEM) and vibrating sample magnetometer (VSM) for these products. Magnetic measurement results show that 1D ordered microstructures under in situ magnetic field possess higher saturation magnetization M_s, remnant magnetization M_r, coercivity H_c and reduced magnetization M_r/M_s than 1D ordered microstructures under ex situ field, and the four magnetic parameters of the two ordered microstructures are much higher than those randomly distributed alloy particles prepared in the absence of external magnetic field.     

Relative porosity in aluminium and in aluminium alloys

Abstract

Pores are readily formed in aluminium alloy castings. Density and porosity area fraction are often used quantitatively to evaluate the quality of aluminium alloy castings. This study used the relative porosity, the ratio (d _c ? d _r)/d _c, where d _c is the density of the chilled sample and d _r is the density of the reduced-pressure test sample, to evaluate the quality of aluminium and Al–6Si and Al–13Si alloys. The bursting of degassing bubbles on the free surface of the melt brought debris of oxide particles into the melt, which were eventually trapped in the castings after pouring. These particles were accompanied with air pockets and/or engulfed by bubbles, resulting in pores shown on the sections of poured castings.     

Effects of mechanical properties, residual stress and indenter tip geometry on instrumented indentation data in thin films

In this work, an axisymmetric two-dimensional finite element model was developed to simulate instrumented indentation testing of thin ceramic films deposited onto hard steel substrates. The level of film residual stress (σ_r), the film elastic modulus (E) and the film work hardening exponent (n) were varied to analyze their effects on indentation data. These numerical results were used to analyze experimental data that were obtained with titanium nitride coated specimens, in which the substrate bias applied during deposition was modified to obtain films with different levels of σ_r. Good qualitative correlation was obtained when numerical and experimental results were compared, as long as all film properties are considered in the analyses, and not only σ_r. The numerical analyses were also used to further understand the effect of σ_r on the mechanical properties calculated based on instrumented indentation data. In this case, the hardness values obtained based on real or calculated contact areas are similar only when sink-in occurs, i.e. with high n or high ratio Y/E, where Y is the yield strength of the film. In an additional analysis, four ratios (R/h_max) between indenter tip radius and maximum penetration depth were simulated to analyze the combined effects of R and σ_r on the indentation load-displacement curves. In this case, σ_r did not significantly affect the load curve exponent, which was affected only by the indenter tip radius. On the other hand, the proportional curvature coefficient was significantly affected by σ_r and n.     

Dielectric investigation of MM(PO4)2 double orthophosphates (M = Ca, Sr, Ba, Pb; M = Ti, Zr, Hf, Ge, Sn)

M^IIM^IV(PO₄)₂ (M^II = Ca, Sr, Ba, Pb; M^IV = Ti, Zr, Hf, Ge, Sn) ceramics were prepared by solid state reaction method and consolidated by spark plasma sintering in order to study their electrical properties. The dielectric study performed at room temperature was aimed to determine the basic electrical properties of these compounds. Maxwell-Wagner polarization contributions, which are active at low frequencies cause a strong extrinsic increase of both real and imaginary part of permittivity as well of the dc-conductivity for BaZr(PO₄)₂, BaTi(PO₄)₂, BaHf(PO₄)₂, and SrGe(PO₄)₂ double orthophosphates. Moderate real (ε^′r=20-150) and imaginary low-frequency permittivity (ε^′r=7-300) and typical Debye relaxation with relaxation time in the range of ms is typical for: PbHf(PO₄)₂, PbZn(PO₄)₂, and CaGe(PO₄)₂. At high-frequency (f = 10⁹ Hz), the ceramics have permittivities of 2.29÷8.02 and tangent loss of 0.003÷0.153. The compounds SrGe(PO₄)₂, BaGe(PO₄)₂, BaZr(PO₄)₂ and BaSn(PO₄)₂ have excellent high-frequency dielectric characteristics, with losses of 3÷6% and permittivity slightly above 2 and are possible candidate as microwave ceramics.     

Statistical modeling of electrochemical reactivation conditions for detecting sensitization to IGC of austenitic stainless steel type 316L

The aim of this study was to determine the optimum conditions of the DL EPR test in order to evaluate the highest values of the degree of sensitization (DOS) of a forged austenitic stainless steel type 316L evaluated by the ratio I _r/I _a or Q _r/Q _a in %. The criteria of sensitization to the IGC corresponds toI _r/I _a > 1% and Q _r/Q _a > 1%. A model using a full factorial design has been established, and the selected factors were the sweep rate (dE/dt), sulphuric acid concentration (H₂SO₄), ammonium thiocyanate concentration (NH₄SCN), and temperature in °C. A first order model is obtained by using a 2⁴ full factorial design. An experimental test carried out using a 2⁴ factorial design indicated that all the factors and their interactions have a positive effect on the response I _r/I _a. Furthermore, the highest value detected ofI _r/I _a was found to be 65.6%. On the other hand, a microstructural study based on an optical microscope and a scanning transmission electron microscope (STEM) has been carried out for annealed and aged samples (550°C-80000 h). The profiles of the chromium, nickel, and molybdenum concentration established by X-ray microanalysis in STEM confirm a decrease of the content of these elements leading to the formation of Cr-depleted zones responsible for the sensitization to the IGC. The text was submitted by the authors in English.     

Reactive Element Effects in High-Temperature Alloys Disentangled

Reactive elements—REs—are decisive for the longevity of high-temperature alloys. This work joins several previous efforts to disentangle various RE effects in order to explain apparently contradicting experimental observations in alumina forming alloys. At 800–1000 °C, “messy” aluminum oxy-hydroxy-hydride transients initially formed due to oxidation by H₂O which in turn undergo secondary oxidation by O₂. The formation of the transient oxide becomes supported by dispersed RE oxide particles acting as water equivalents. At higher temperatures, electron conductivity in impurity states owing to oxygen vacancies in grain boundaries (GBs) becomes increasingly relevant. These channels are subsequently closed by REs pinning the said vacancies. The universality of the emerging understanding is supported by a comparative first-principles study by means of density functional theory addressing RE(III): Sc₂O₃, Y₂O₃, and La₂O₃, and RE(IV): TiO₂, ZrO₂, and HfO_2, that upon reaction with water, co-decorate a generic GB model by hydroxide and RE ions. At 100% RE coverage, the GB model becomes relevant at both temperature regimes. Based on reaction enthalpy ΔH_r considerations, “messy” aluminum oxy-hydroxy-hydride transients are accessed in both classes. Larger variations in ΔH_r are found for RE(III)-decorated alumina GBs as compared to RE(IV). For RE(III), correlation with GB width is found, increasing with increased ionic radius. Similarly, upon varying RE(IV), minor changes in stability correlate with minor structural variations. GB decorations by Ce(III) and Ce(IV) further consolidate the emerging understanding. The findings are used to discuss experimental observations that include impact of co-doping by RE(III) and RE(IV).