Stability of rolling texture during heat treatment in a two-phase Ti3Al base intermetallic alloy

The stability of hot rolling texture developed in the ₂ phase of a two-phase Ti₃Al base intermetallic alloy, during subsequent heat treatment, has been systematically investigated. The basal hot rolling texture remains rather stable during recrystallization annealing at temperatures within the single phase ₂ range. Grain growth after recrystallization produces extra orientations other than the basal. For the material with a starting basal texture, heat treatment in the upper ₂, (₂+β) or β phase fields does not change the basic character of the texture, although the sharpness and especially the width of the basal component increase to some extent with the increase in the heat treatment temperature. Heat treatment of material with starting non-basal texture in the upper ₂, (₂+β) or β phase fields does not produce any basal component irrespective of the heat treatment temperature. When the starting material is reasonably strain free and possesses a weak basal texture, heat treatment in the upper ₂, (₂+β) or β phase fields helps in intensifying the basal component to a certain extent.     

The Minor Precipitation at the Final Stage of U720Li Solidification

The minor precipitations caused by B and Zr which are the normal constituents of U720 Li alloy have been studied by analyzing the solidification process and the composition evolution. The present study aims to supply the elementary information about the existing form of B and Zr in the as-cast microstructure, which is helpful for the subsequent processing, such as homogenization treatment. The M_3B_2 and Ni_5Zr phases were observed in the U720 Li alloy in as-cast state, which were usually accompanying with each other together with g-Ni_3 Ti phase at the edge of eutectic(γ+γ'). Combining the DTA analysis and heating and quenching tests, the solidification sequence was determined to be the following: c matrix, eutectic(γ+γ'), g-Ni_3Ti, M_3B_2 and Ni_5Zr. The in situ composition analysis by EDS and EPMA revealed that the precipitation and microstructure were governed by the composition evolution in the liquids. The solidification of c matrix increased the Ti concentration in the residual liquids and resulted in the eutectic(γ+γ') formation; the(γ+γ') formation increased the Ti/Al radio in the liquids and the g-Ni_3Ti was formed in front of the eutectic(γ+γ'); the g-Ni_3Ti precipitation consumed up Al and Ti and increased the concentration of B, Mo and Cr, and M_3B_2 boride is formed;the previous precipitation of the phases consumed up most of the elements other than Ni and Zr, and Ni_5Zr is formed finally. The melting points are in the ranges of 1130–1140 °C for Ni_5Zr phase, 1180–1190 °C for M_3B_2 boride and1190–1200 °C for g-Ni_3Ti phase.     

Deformation induced transformations and grain boundary thickness in nanocrystalline B2 FeAl

Precise measurement of fundamental Bragg peak shifts during milling of nanocrystalline ordered B2 Fe₆₀Al₄₀ has allowed for the first time, the deconvolution of the 110 fundamental Bragg peak intensities of the b.c.c. disordered regions and of the ordered B2 regions from the start and before full disappearance of the latter. The evolution of the lattice parameter a₀ of the b.c.c. solid solution with milling time shows two characteristics. First a jump to higher values from the initial a₀ of the B2 phase, with a Δa₀ change of the order of 1% corresponding to a volume per atom ΔV_expansion of about 3%. Subsequently, a₀ continues to increase slowly with further milling at constant grain size D and in the absence of any B2 phase. This continuing change of a₀ with further milling up to at least 180 min is attributable to a reduction of chemical short-range order (CSRO) or the number of Al–Fe heteroatomic “bonds”. The appearance of two well-defined maxima in the hyperfine field (HF) distributions derived from the Mössbauer spectra indicated the presence of two ferromagnetic environments contributing to the broadened Mössbauer resonance sextet signal. The evolution of the second component of this Mössbauer signal scales with the grain size. Using the mean grain size D derived from X-ray peak profiles and TEM pictures together with the grain boundary thickness d_gb of 1.25 nm determined by Fultz et al. (J. appl. Phys., 1996, 7, 127) for b.c.c. Fe-based alloys, the fraction of grain boundary atoms n_gb/n_total was estimated and found to be consistent with the fraction of Fe atoms contributing to the lower HF component of the Mössbauer sextet signal. The grain boundary atom count using both methods confirms that grain boundaries in materials nanocrystallized by heavy deformation are nearly as dense as in the bulk.     

Phase Evolution and Thermal Expansion Behavior of a γ′ Precipitated Ni-Based Superalloy by Synchrotron X-Ray Diffraction

The phase evolution and thermal expansion behavior in superalloy during heating play an essential role in controlling the size and distribution of precipitates, as well as optimizing thermomechanical properties. Synchrotron X-ray diffraction is able to go through the interior of sample and can be carried out with in situ environment, and thus, it can obtain more statistics information in real time comparing with traditional methods, such as electron and optical microscopies. In this study, in situ heating synchrotron X-ray diffraction was carried out to study the phase evolution in a typical γ′ phase precipitation strengthened Ni-based superalloy, Waspaloy, from 29 to 1050 °C. The γ′, γ, M₂₃C₆ and MC phases, including their lattice parameters, misfits, dissolution behavior and thermal expansion coefficients, were mainly investigated. The γ′ phase and M₂₃C₆ carbides appeared obvious dissolution during heating and re-precipitated when the temperature dropped to room temperature. Combining with the microscopy results, we can indicate that the dissolution of M₂₃C₆ leads to the growth of grain and γ′ phase cannot be completely dissolved for the short holding time above the solution temperature. Besides, the coefficients of thermal expansions of all the phases are calculated and fitted as polynomials.     

The effect of process parameters and heat treatment on the microstructure of direct laser fabricated TiAl alloy samples

Gas atomized Ti48Al2Mn2Nb powders were used as a feedstock material for direct laser fabrication (DLF) of near net shape samples. The microstructures of these laser treated samples were characterized using optical, scanning (SEM) and transmission electron microscopy (TEM), both immediately after laser fabrication and after heat treatments. These characterizations have shown that the microstructure is extremely fine but inhomogeneous in comparison to the conventionally processed material. The process parameters such as laser power and scanning speed controlled the microstructural morphology. The laser-treated microstructure remained stable up to 973 K, but rapid grain coarsening occurred at 1273 K. A fully recrystallized, uniform microstructure was obtained after annealing at 1073 K for 24 h, with an apparent compositional homogeneity. Annealing in the phase field followed by air cooling and annealing in (₂+γ) phase region resulted in uniform microstructure. However, the microstructure was much coarser than the microstructure of the DLF samples.     

FEM numerical simulation of tube axial drawing process

Tube inversion by the axial drawing is an advanced forming process for manufacturing double-walled tubes with high quality, high efficiency, and low consumption. However, to realize forming process depends on producing the tearing in deforming zone, which has a close relationship with forming load. So in this paper, the influence of forming condition parameters on the deforming force and the process is investigated by rigid-plastic FEM numerical simulation. The results show that: (1) during the whole forming process, the shape of the tube remains unchanged when the radius of the core die is larger than a certain value, so a precision forming can be easily realized; (2) for a given r/d₀ or t₀/d₀, the deforming force depends mainly on the value of r/t₀, and there is a critical parameter k, when r/t₀ < k, the steady forming force decreases with an increase of r/t₀; on the contrary, when r/t₀ > k, the steady forming force increases with an increase of r/t₀; (3) the material hardening exponent n of the tube and the friction coefficient μ have a remarkable influence on the deforming force. The smaller the value of n and the larger the value of μ the larger the forming force.     

Another unusual phenomenon for Zr7Ni10: structural change in hydrogen solid solution and its conditions

Zr₇Ni₁₀ has three hydrogen occlusion phases, , β and γ, and the following unusual features are known for the phase transitions in the Zr₇Ni₁₀–H₂ system: (1) The intermediate hydride phase (β) appears only during dehydrogenation but not during hydrogenation, and (2) The continuous hydrogen solid solution phase () exhibits a much higher hydrogen solubility during hydrogenation than during dehydrogenation. In order to clarify the mechanism about the difference in the hydrogen solubility of the phase, the relation between the pressure-composition isotherms and corresponding structural change has been examined by a conventional volumetric method and X-ray diffraction. Through the examination, we discovered that the crystal structure of the phase, which undergoes hydrogenation followed by dehydrogenation, is different from that of its pure metal phase, where the crystal structure of the dehydrogenated phase changes from an orthorhombic structure to a tetragonal structure. The conditions causing the structural change were then examined, and it has been found that the phase maintains its original orthorhombic structure as long as it is hydrogenated so as not to absorb enough hydrogen to change it to the hydride with a higher hydrogen content (γ). The phenomenon can be understood as one of the hydrogen-assisted phase transitions such as hydrogen-induced amorphization (HIA) in the sense that the phase transition requires hydrogenation under special conditions.     

Investigation of size effects on material behavior of thin sheet metals using hydraulic bulge testing at micro/meso-scales

Reliable material models are necessary for accurate analysis of micro-forming and micro-manufacturing processes. The grain-to-feature size ratio (d/D_c) in micro-forming processes is predicted to have a critical impact on the material behavior in addition to the well-known effect of the grain size (d) itself as manifested by the Hall–Petch relation. In this study, we investigated the “size effects” on the material flow curve of thin sheet metals under hydraulic bulge testing conditions. The ratio of the sheet thickness to the material grain size (N=t₀/d) was used as a parameter to characterize the interactive effects between the specimen and the grain sizes at the micro-scales, while the ratio of the bulge die diameter to the sheet thickness (M=D_c/t₀) was used to represent the effect of the feature size in the bulge test. Thin sheets of stainless steel 304 (SS304) with an initial thickness (t₀) of 51 μm and three different grain sizes (d) of 9.3, 10.6, and 17 μm were tested using five bulge diameters (D_c) of 2.5, 5, 10, 20, and 100 mm. A systematic approach for determining the flow curve of thin sheet metals in bulge testing was discussed and presented. The results of the bulge tests at different scales showed a decrease in the material flow curve with decreasing N value from 5.5 to 3.0, and with decreasing M value from 1961 to 191. However, as M value was decreased further from 191 to 49, an inversed relation between the flow curve and M value was observed; that is, the flow curve was found to increase with decreasing M value from 191 to 49, a new observed phenomenon that has never been reported in any open literature. New material models, both qualitatively and quantitatively, were developed to explain the size effects on the material flow curve by using the N and M as the characteristic parameters of relative size between the grain, the specimen (i.e., sheet thickness), and the part feature (i.e., bulge diameter). The explanation and prediction of the flow curve behavior based on these models were shown to be in good agreement with the bulge test results in this study and in the literature.     

Novel alkaline-free Er-doped fluorophosphate glasses for broadband optical fiber lasers and amplifiers

Two sets of Er³⁺-doped alkaline-free glass systems, MgF₂–BaF₂–Ba(PO₃)₂–Al(PO₃)₃ (MBBA) and Bi(PO₃)₃–Ba(PO₃)₂–BaF₂–MgF₂ (BBBM), have been prepared and investigated with the aim of using them as active media. Radiative lifetimes (τ_rad) and branching ratios (β) have been obtained for the excited states of Er³⁺. The absorption spectra were recorded to obtain the intensity parameters (Ω_t) which are found to be Ω₂ = 4.47 × 10⁻²⁰ cm², Ω₄ = 1.31 × 10⁻²⁰ cm², Ω₆ = 0.81 × 10⁻²⁰ cm² for the MBBA system and Ω₂ = 4.03 × 10⁻²⁰ cm², Ω₄ = 1.34 × 10⁻²⁰ cm², Ω₆ = 0.53 × 10⁻²⁰ for the BBBM system, respectively. The emission cross-section for the ⁴I_13/2 → ⁴I_15/2 transition is determined by the Fuchtbauer–Ladenburg method and found to be 2.35 × 10⁻²⁰ cm² and 3.54 × 10⁻²⁰ cm² for the MBBA and BBBM system, respectively. Comparison of the measured values to those of Er³⁺ transitions in other glass hosts suggests that our new glass systems are good candidates for broadband compact optical fiber and waveguide amplifier applications.     

Electron microscopy and X-ray diffraction studies of rapidly quenched Zr---Ni and Hf---Ni ribbons with about 90 at.% Ni

The structure of melt-spun ribbons of the alloys Zr₉Ni₉₁, Zr₁₀Ni₉₀ and Hf₁₁Ni₈₉ was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Ribbons of the Zr₉Ni₉₁ and Zr₁₀Ni₉₀ alloys prepared at a high cooling rate (ribbon thickness d=11 μm) were characterized by an amorphous matrix with a few per cent of quenched-in crystallites. The ribbon of the Hf₁₁Ni₈₉ alloy prepared with the same thickness (i.e. at the same cooling rate) exhibited a nanocrystalline grain structure of the HfNi₅ phase. Thicker ribbons of the Zr₉Ni₉₁ alloy (d=17–22 μm), for which the quench rate was correspondingly lower, were obtained as a b.c.c. Ni(Zr) solid solution phase with a grain size of nearly 1 μm. A high resolution (HR) TEM study of one of the Zr₉Ni₉₁ crystalline ribbons revealed a fine structure of the interior of the crystallites which can be attributed to an ordering on the Zr sublattice over distances of several nanometres within the b.c.c. grains.     

Study of extraction of the ndcl3-smcl3-hcl-h2o-di-(2-ethylhexyl)-phosphoric-acid-kerosine system

The extraction equilibrium relations of neodymium and samarium were studied in the system HCl (1.0 mol dm⁻³)-di-(2-ethylhexyl) phosphorie acid (HDEHP), for a wide range of neodymium and samarium concentration (0.02-1.20 mol dm⁻³), mole fraction (X_i=0.005-0.995) and acidity (C_H+=0.27-1.06 mol dm⁻³). A relatively simple, high precision model for the distribution ratio D_i was established by correlating the experimental data with a progressive regression program D_i=exp(a₁+a₂I+a₃C_H++a₄X_i)I^a5X_i^{a6 In C_H+ + a7 In(1-X_i})

The extraction behaviour of the neodymium-samarium binary system was studied using the above model. The study of acid equilibrium shows that the extraction proceeds according to the cation exchange reaction within this range of experimental parameters. The neodymium and samarium concentrations in the organic phase increase rapidly and reach a maximum, and then decrease as the aqueous concentration increases; the maximum concentration appears at about 0.3-0.4 mol dm⁻³ of aqueous neodymium and samarium concentration. The variation of separation factors, β_Sm/Nd, was also studied with the change in aqueous acidity, rare earth concentration and mole fraction, β_Sm/Nd vary between 6.5 and 10.2 and the average value is 8.77.     

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

The effects of milling and doping NaAlH₄ with TiCl₃, TiF₃ and Ti(OBuⁿ)₄, and of cycling doped NaAlH₄ have been investigated by infrared (IR) and Raman spectroscopy and X-ray powder diffraction. Milling and doping produce similar effects. Both decrease the crystal domain size (900 Å for milled and 700 Å for doped, as compared to 1600 Å for unmilled and undoped NaAlH₄) and increase anisotropic strain (by a factor >2.5, mainly along c). They also influence structure parameters such as the axial ratio c/a, cell volume and atomic displacement amplitudes. They show IR line shifts by 15 cm⁻¹ to higher frequencies for the Al–H asymmetric stretching mode ν₃, and by 20 cm⁻¹ to lower frequencies for one part of the H–Al–H asymmetric bending mode ν₄, thus suggesting structural changes in the local environment of the [AlH₄]⁻ units. The broad ν₃ bands become sharpened which suggests a more homogeneous local environment of the [AlH₄]⁻ units, and there appears a new vibration at 710 cm⁻¹. The Raman data show no such effects. Cycling leads to an increase in domain size (1200–1600 Å), IR line shifts similar to doped samples (except for TiF₃: downward shift by 10 cm⁻¹) and a general broadening of the ν₃ mode that depend on the nature of the dopants. These observations support the idea that some Ti diffusion and substitution into the alanate lattice does occur, in particular during cycling, and that this provides the mechanism through which Ti-doping enhances kinetics during re-crystallisation.     

Crystal fields in (La1−xGdx)OCl:Eu solid solutions

The formation of cation solid solution in the (La_1−xGd_x)OCl:Eu³⁺ (0≤x_Gd≤1; Δx_Gd=0.1) series was studied by photoluminescence spectroscopy. The luminescence from the ⁵D_0–2 to the ⁷F_0–4 levels of the Eu³⁺ ion in the (La_1−xGd_x)OCl series was recorded at 77 K by using argon ion laser excitation (457.9 nm). The interpretation of the spectra according to the C_4v site symmetry of the Eu³⁺ ion in the tetragonal PbFCl-type structure yielded nearly complete sets (18 to 19 levels) for the ⁷F_0–4 levels. Simulations of the Stark level schemes were carried out with the aid of a phenomenological c.f. theory utilizing the five non-zero c.f. B_q^k parameters (B₀², B₀⁴, B₄⁴, B₀⁶ and B₄⁶) allowed for the C_4v site symmetry. By using the calculated c.f. parameter sets a quantitative measure was obtained to monitor the formation of cation solid solutions. The strength of the c.f. effect was estimated with the c.f. strength parameters S and S^k (k=2, 4 and 6). The c.f. parameter sets reproduced the experimental ⁷F_J (J=0–4) energy level schemes with the rms deviations between 4 and 11 cm⁻¹. The individual parameters as well as the c.f. strength parameters were found to evolve in a smooth manner indicating complete solid solubility in the (La_1−xGd_x)OCl series. Some local distortions from the C_4v symmetry — probably of long range—leading to the splitting of the ⁷F₁ doubly degenerate E level were observed, however.     

Hydromechanical deep drawing of cups with stepped geometries

This paper deals with the hydromechanical deep drawing of metal cups with complex stepped geometries. Two materials, a low-carbon steel (DC04) and stainless steel (DIN 1.4301), have been researched. A die set with a maximum possible deep drawing ratio β_0,max = 3.0 for a punch diameter 100 mm has been designed and constructed. The die set is designed to withstand fluid counter pressures up to 200 MPa. Pressure control is achieved using a micro-metering pressure control valve. The process is initially simulated using the FEM solver LS-DYNA. Experiments have been conducted with two punch geometries. The punch geometries consist of cylindrical and conical wall segments. Complex positive and negative features are manufactured in the punch bottom face. The ability of transferring complex features from the punch onto the blank surface with high deep drawing ratios is investigated. Extended limiting deep drawing ratios of β_0,max = 3.0 for DC04 and β_0,max = 2.875 for DIN 1.4301 have been achieved.     

First-Principle Calculations of the MgYA_4(A=Co and Ni)Phase with AuBe_5-Type Structure

First-principles calculations have been performed to study the structural,mechanical and magnetic properties of the MgYCo_4 and MgYNi_4 phases in AuBe_5-type structure.The obtained values of cohesive energy as well as formation energy prove that the MgYCo_4 and MgYNi_4 phases have a good combination of structural stability and alloying ability,which is also supported by electronic structure.It is found that the magnetic moment of the MgYCo_4 phase is 19.06 μ_B per unit cell mainly owed to the 3d state of Co atom,and the MgYNi_4 phase exhibits no magnetism.Both the trigonal shear constant C_(44) and the shear modulus G of the Mg YNi_4 phase are larger than those of the MgYCo_4 phase.Plasticity of alloys has been estimated by the C_(11)-C_(12) and Young's modulus E,and C_(12)-C_(44),shear to bulk modulus ratio G/B and Poisson's ratio v have been studied to predict the ductility of alloys.According to the calculated results,the MgYCo_4 phase has better plasticity as well as ductility,compared with the MgYNi_4 phase.     

Tribological properties and hardness of silicon nitride ceramics after ion implantation and subsequent annealing

Silicon nitride ceramic (β-Si₃N₄) with Y₂O₃ and Al₂O₃ as sintering additives was implanted with Cr or Ti ions to a fluence of 10¹⁷ cm⁻² at energies ranging from 200 keV to 2 MeV. Changes in the phase composition in the near surface layer due to the implantation and subsequent annealing at 1000 and 1200 °C and their correlation with the behaviour of hardness and wear were investigated. Implantation results in amorphization of the near surface layer, which is buried for MeV implantations. For Ti-implanted samples, annealing leads to oxidation, resulting in the formation of γ-YSi₂O₇ and cristobalite besides the β-Si₃N₄. In the case of Cr implantation with MeV energies, the amorphous layer recrystallizes to -Si₃N₄ already at 1000 °C catalyzed by the chromium. This causes a significant suppression of the oxidation. Consequently, the hardness and the tribological behaviour of Ti- and Cr-implanted samples, respectively, are affected differently by the annealing process. For chromium, the improved wear behaviour due to the high-energy implantation remains after annealing while the reduced hardness as result of the amorphized surface is partly recovered. For titanium, the oxide formation leads to a poorer hardness and tribological behaviour.     

Mechanisms for grain size hardening and softening in Zn

An analysis of the rate-controlling mechanisms corresponding to effect of grain size d=10⁻⁹ to 10⁻³ m on the flow stress of Zn at 300K and s⁻¹ was performed. Three grain size regimes were indicated: Regime I, d≈10⁻⁶–10⁻³ m, Regime II, d≈10⁻⁶–10⁻⁸ m and Regime III, d<10⁻⁸ m. Grain size hardening occurred in Regimes I and II and grain size softening in Regime III. The intersection of pyramidal forest dislocations by basal dislocations was concluded to be the rate-controlling mechanism in both Regimes I and II, the major effect of the grain size being on the forest and gliding dislocation densities. The absence of twinning and a dislocation cell structure distinguished Regime II from I. The grain size softening observed in Regime III is in better accord with grain boundary shear than with grain boundary diffusion creep.     

Influence of A-site cation size-disorder on structural, magnetic and magnetocaloric properties of La0.7Ca0.3−xKxMnO3 compounds

The structural and magnetic properties of perovskite oxides La_0.7Ca_0.3−xK_xMnO₃ (0 ≤ x ≤ 0.15) have been investigated to explore the influence of the A-site cation size-disorder (σ²). The materials were prepared by the solid-state method and then characterized by X-ray diffraction (XRD). The XRD data have been analyzed by Rietveld refinement technique. For K doping concentration x ≤ 0.075, the samples crystallize in the orthorhombic structure, while for x ≥ 0.1, the structure becomes rhombohedral. The variation of the magnetization M as a function of the applied magnetic field μ₀H reveals the presence of a structural distortion leading to a reduction of the magnetization at low μ₀H values. When increasing μ₀H, the structural distortion decreases and for a high applied magnetic field, the M (μ₀H) curves saturate indicating the disappearance of the structural distortion. The influence of K doping concentration and the applied magnetic field on the magnetocaloric properties has been considered. A large magnetic-entropy change (|ΔS_M|  5 J/kg K) is obtained in all samples at Curie temperatures between 270 and 280 K for an applied magnetic field of 3 T. These results show that these materials can be used as candidates for magnetic refrigerants near room temperature.     

Crystal structure, magnetic behaviour and valence state of ytterbium in the Yb4Ni10+xGa21−x phase

The ternary phase Yb₄Ni_10+xGa_21−x has been synthesised from the elements by high frequency melting in argon atmosphere. The homogeneity region has been established from X-ray powder data and confirmed by EDX analysis for 0.3≤x≤1. The crystal structure of Yb₄Ni_10+xGa_21−x has been estimated from X-ray single crystal data: space group C2/m (no. 12), Z=2, a=20.6815(9) Å, b=4.0560(4) Å, c=15.3520(7) Å, β=124.800(3)°, R(F)=0.023 for 1701 symmetry independent reflections with F(hkl)>4σ(F). A special feature of the structure is the local disorder within the gallium/nickel network. Neglecting atomic disorder in the region of the Ga9 and Ga11 positions, the Yb₄Ni_10+xGa_21−x structure is an occupation variant of the Ho₄Ni₁₀Ga₂₁ type with nickel atoms partially replacing the Ga atoms in the 2d sites at the centers of distorted icosahedra. From magnetic susceptibility and from L_III-XAS spectra, the valence state of ytterbium is near 3+.