Mechanical properties of as-fabricated and 2300 °C annealed tungsten wire tested up to 600 °C

Recent efforts dedicated to the mitigation of tungsten brittleness have demonstrated that tungsten fiber-reinforced composites acquire pseudo ductility even at room temperature. Crack extension and fracture process is basically defined by the strength of tungsten fibers. Here, we move forward and report the results of mechanical and microstructural investigation of different grades of W wire with a diameter of 150 μm at elevated temperature up to 600 °C. The results demonstrated that potassium doping to the wire in the as-fabricated state does not principally change the mechanical response, and the fracture occurs by grain elongation and delamination. Both fracture stress and fracture strain decrease with increasing test temperature. Contrary to the as-fabricated wire, the potassium-doped wire annealed at 2300 °C exhibits much lower fracture stress. The fracture mechanism also differs, namely: cleavage below 300 °C and ductile necking above. The change in the fracture mechanism is accompanied with a significant increase of the elongation to fracture being ~ 5% around 300 °C.     

Plasticity of indium antimonide between −176 °C and 400 °C under hydrostatic pressure. Part II: Microscopic aspects of the deformation

Indium antimonide (InSb) has been plastically deformed over a wide temperature range, from 400 down to ?176 °C (see the companion paper: Kedjar B, Thilly L, Demenet JL, Rabier J. Acta Mater 2009) and transmission electron microscopy was used to characterize the deformation microstructures. In the ductile regime, i.e. T > T_tr1 ≈ 150 °C, the crystal deforms via the nucleation and motion of perfect dislocations belonging to the glide set. In the brittle domain, i.e. for T < T_tr1 ≈ 150 °C, two regimes are observed: for T_tr2 ≈ 20 °C < T < T_tr1 ≈ 150 °C, the crystal deformation takes place via the nucleation and glide of dissociated perfect dislocations or only leading partials, while for T < T_tr2 ≈ 20 °C, the crystal deformation proceeds via the nucleation and motion of perfect dislocations belonging to the shuffle set. In view of these observations, the brittle-to-ductile transition (at T_tr1) is confirmed to correspond to a change in the dislocation nature in the glide set, from partial-mediated plasticity at low temperature to perfect-mediated plasticity at high temperature. Another transition is observed at T_tr2 and corresponds to the glide-to-shuffle transition which is observed experimentally for the first time in a III–V compound semiconductor.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

Air-oxidation of a Co-based amorphous ribbon at 400–600 °C

The oxidation behavior of a commercial Co₆₉B₁₂Si₁₂Fe₄Mo₂Ni₁ amorphous ribbon (Co6-AR) was studied over the temperature range of 400–600 °C in dry air. The results showed that virtually no oxidation occurred at 400 °C. On the other hand, the oxidation kinetics of the Co6-AR alloy at 450–600 °C generally followed a multi-stage parabolic-rate law, and the parabolic-rate constants (k_p values) tend to increase with increasing temperature. It was found that the oxidation rates of the glassy alloy are slower than those of pure Co, indicative of a better oxidation resistance. An exclusive scale of CoO was observed after the oxidation of the glassy alloy in the temperature range of interest, and several crystalline phases formed on the substrate beneath the scale, consisting of pure Co (both FCC and HCP structures), Co₃B, Co₂Si, CoFe, and Co₂B (absent at 450 °C), which indicated the occurrence of crystallization.     

The ϵ → η → θ transition in 100Cr6 and its effect on mechanical properties

Low-temperature precipitation reactions in 100Cr6 are characterized using transmission electron microscopy and X-ray diffraction, and modelled using thermokinetic methods. Martensitically transformed 100Cr6 is shown to display a complex microstructure composed of plate martensite, primary carbides, retained austenite and one or more of the ?-, η- and θ-phases. It is demonstrated that the maximum tensile strength (in excess of 2 GPa) and ductility is achieved by the θ-phase and the maximum yield strength is found during the α′ + η → α′ + θ transition. The interplay between the amount of carbon in solid solution, the martensite tetragonality and its morphology are related to the precipitate/matrix strain energy, the precipitate species present and their morphology. The progress in precipitate volume fraction, average radius, particle number and matrix composition can be quantitatively described by performing multicomponent precipitation kinetics calculations in paraequilibrium incorporating: (i) the effects of precipitate/matrix lattice misfit strain and particle aspect ratio, (ii) nucleation at plate boundaries and dislocations and (iii) an appropriate value for the precipitate/matrix interfacial energy, which is the only parameter fitted in the calculation.     

Improvement of “Pest” Resistance of MoSi2 Intermetallic Compound at 500 °C and 600 °C via Addition of B Fabricated by Spark Plasma Sintering

Oxidation of Metals - Mo–Si–B alloys were obtained by spark plasma sintering (SPS) using fine Mo–Si–B alloy powders with a particle size of 2.5&nbsp;μm. The SPS...     

Effects of Ti,Zr, and Hf on the phase stability of Mo_ss + Mo3Si + Mo5SiB2 alloys at 1600 °C

Understanding the stability of the three-phase Mo_ss + Mo₃Si + Mo₅SiB₂ region is important for alloy design of Mo–Si–B-based refractory metal intermetallic composites. In this work, thermodynamic modeling is coupled with guided experiments to study phase stability in this three-phase region of the Mo–Si–B–X (X = Ti, Zr, Hf) system. Both the calculated and experimental results show that additions of Zr and Hf limit significantly the stability of the three-phase region because of the formation of the ternary phases MoSiZr and MoSiHf, while Ti addition leads to a much larger region of stability for the three-phase equilibrium.     

Microband evolution during large plastic strains of stable {1 1 0}〈1 1 2〉 Al and Al–Mn crystals

The deformation microstructures of Al and Al–Mn {1 1 0}〈1 1 2〉 single crystals have been characterized after room temperature channel-die compression up to true strains of 2.1. The evolution of local misorientations and microband structures were quantified by high-resolution electron backscatter diffraction in a field emission gun scanning electron microscope and their alignments compared with the traces of active slip planes and macroscopic shear stress planes. During plane-strain compression these “Brass” oriented crystals remain stable in terms of the final, average, orientation, with a small orientation spread. However, the microband alignment varies with strain and also with solute content. There is a general tendency for the microbands to be both crystallographic and non-crystallographic at low strains, then crystallographic, and finally mixed again at high strains (with some lamellar banding).     

Adaptive VN/Ag nanocomposite coatings with lubricious behavior from 25 to 1000 °C

A two-phase nanocomposite coating that consists of inclusions of silver in a vanadium nitride matrix (VN/Ag) was investigated as a potential adaptive coating with a reduced friction coefficient from 25 to 1000 °C. This nanocomposite structure was selected based on the premise that silver and silver vanadate phases would form on the surface of these coatings, reducing their friction coefficient in the (i) room to mid-range and (ii) mid-range to high temperatures, respectively. Silver and vanadium were expected to react with oxygen at high temperatures and create a lubricious silver vanadate film on the coating. The VN/Ag coatings were deposited using unbalanced magnetron sputtering and their elemental composition was evaluated using X-ray photoelectron spectroscopy. The tribological properties of the materials against Si₃N₄ balls were investigated at different temperatures. The lowest friction coefficients recorded for samples with identical compositions were 0.35, 0.30, 0.10 and 0.20 at 25, 350, 700 and 1000 °C, respectively. Post-wear testing Raman spectroscopy and X-ray diffraction (XRD) measurements revealed the formation of silver vanadate compounds on the surface of these coatings. In addition, real time Raman spectroscopy and high temperature XRD revealed that silver vanadate, vanadium oxide and elemental silver formed on the surface of these coatings upon heating to 1000 °C. Upon cooling, silver and vanadium oxide were found to combine at about 400 °C, leading predominantly to the formation of silver vanadate phases on the surface of these materials.     

Atomistic structure of the Cu(1 1 1)/α-Al2O3(0 0 0 1) interface in terms of interatomic potentials fitted to ab initio results

We propose an atomistic model to describe the copper/sapphire interface by means of simple interatomic potentials involving only a few fitting parameters. Successful results are achieved when the copper atoms in the monatomic layer closest to the interface have properties different from the bulk. This layer is to accommodate the ionic/covalent bonding in the ceramics to the metallic bonding in copper. For an oxygen terminated interface, we fit the parameters of the potentials to the results of a rigid tensile test (explained in the text) simulated from first principles. The results of atomic relaxation near the interface are shown to be consistent with ab initio and experimental results available in the literature. Calculations reveal highly interesting relaxation dynamics near the interface. In the early stage of relaxation, a periodic network of partial misfit dislocations is formed, which later transforms into an irregular network due to the instability of the layer of copper atoms atop the oxygen atoms. This explains the interface incoherency observed in high-resolution electron microscopy. Calculations based on the FK model reproduce this effect.     

Interaction between martensitic structure and defects in β ↔ β′ + γ′ cycling in CuAlNi single crystals. Model for the inhibition of γ′ martensite

The authors have previously reported an estimate of the energy associated with the inhibition effect of γ′ martensite after β ↔ β′ + γ′ cycling in CuAlNi single crystals. In this paper, a microscopic model is proposed to explain the γ′ inhibition, related to the localized interaction between a dislocation array and the twinned γ′ structure. Dislocations with Burgers vector [1 0 0]_β and line direction [1 1 1]_β in an isotropic β matrix are considered. The model takes into account the interaction between the martensitic stress-free transformation strains and the stress field created by the dislocation arrays. It is shown that the interaction is different for each twin-related variant in the γ′ martensite. The energy necessary to maintain the right volume relationship of the twinned γ′ variants to produce an undistorted β/γ′ habit plane is defined as the inhibition energy. A value of around 12 J mol⁻¹ was obtained, which is in reasonable agreement with experimental results.     

The isothermal section at 400 °C of the Pr–Ag–Sn ternary system

The phases and equilibria involved in the isothermal section at 400 °C of the Pr–Ag–Sn ternary system have been here investigated after different annealing times by X-ray diffraction, optical and scanning electron microscopy and electron probe microanalysis. Three ternary compounds have been confirmed: PrAgSn hP6 LiGaGe type, Pr₃Ag₄Sn₄ oI22 Gd₃Cu₄Ge₄ type and Pr₅AgSn₃ hP18 Hf₅CuSn₃ type. The solid solubility ranges of the binary compounds have been considered and trends of their lattice parameters studied. The tie-triangles of the ternary system have been defined. The general features of the section are discussed and compared to those of the other R–Ag–Sn ternary systems.     

Glass-like thermal conductivities in (La1-x1Yx1)2(Zr1-x2Yx2)2O7-x2 (x = x1 + x2, 0 ⩽ x ⩽ 1.0) solid solutions

The evolution of structure and thermal conductivity (k) has been studied for a range of Y–La₂Zr₂O₇ solid solutions. Within the pyrochlore range (x < 0.40) Y³⁺ solely substitutes for La³⁺ below a critical composition factor (x = 0.15), above which it substitutes for both La³⁺ and the Zr⁴⁺. A glass-like k, approaching the amorphous limit, is observed within a certain composition range (0.20 ? x < 0.40). The glass-like k behaviour is attributed to a phonon localization effect that arises from small and weakly bound Y³⁺ cations (rattlers) oscillating locally and independently in oversized anionic cages [(La/Y)O₈]. The ultralow and glassy k makes Y³⁺-doped La₂Zr₂O₇ pyrochlores promising candidate materials for high temperature thermal barrier coating topcoats.