Microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites

The microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites were studied by electron microscopy observation and internal friction measurement, respectively. Both the microstructure and the internal friction behavior of the composites were strongly influenced by the thermomechanical treatment parameters. All the Al particles were still covered by the native amorphous oxide shells in those composites sintered at T < 823 K, and no indication of Al grain boundary relaxation was detected. Some Al oxide shells were cracked, resulting in the formation of a few Al–Al grain boundaries between adjacent particles in the sample sintered at 823 K, and one internal friction peak centered at ～440 K was detected. All the oxide shells were broken into small fragments in those samples sintered at T ? 843 K, and two internal friction peaks were detected, one prominent peak at ～440 K and one weak peak at ～540 K. A microstructure with a bimodal grain size distribution of Al was formed via partial recrystallization after thermomechanical treatment of the sample sintered at 893 K, and two internal friction peaks with comparable intensity were detected. The internal friction peaks were associated with the relaxation of Al grain boundary in the composites.     

Effect of hydrogen on internal friction and elastic modulus in titanium alloys

The effect of hydrogen on the variation with temperature of internal friction (Q^?I) and elastic modulus (E) of a number of Ti-based alloys has been studied in the Hz and kHz frequency ranges. A relaxation peak of internal friction with a high degree of relaxation (Q^?I_max ～ 10^?1) and with a ΔE effect is observed in all hydrogen-doped samples at T ～ 600 K at ～1 kHz, and at T ～ 500 K at ～1 Hz. Such a peak is not present in samples without hydrogen. The activation energy W and the frequency factor v₀ of the observed relaxation are determined to be W ～ 1.55 eV, v₀ ～ 10¹⁷ s^?1. It is shown that the observed effects are connected with the mechanism of grain boundary relaxation, as the introduction of hydrogen into titanium alloys leads to the formation of fine-grained structures.     

Elastic constants of amorphous and single-crystal Pd40Cu40P20

We have measured the adiabatic second-order elastic constants of amorphous Pd₄₀Cu₄₀P₂₀ (isotropic, two independent elastic constants) and single-crystal Pd₄₀Cu₄₀P₂₀ (tetragonal, six independent elastic constants) over the range 3.9 < T < 300 K. Below ∼20 K, the elastic constants of single-crystal Pd₄₀Cu₄₀P₂₀ vary as C(T) = C(0)[1 − bT² − dT⁴], which is the same temperature dependence found in metallic elements. In contrast, below ∼20 K the elastic constants of amorphous Pd₄₀Cu₄₀P₂₀ vary as C(T) = C(0¹)[1 − aT]. The bulk modulus of amorphous Pd₄₀Cu₄₀P₂₀ is approximately 2% lower than that of crystalline Pd₄₀Cu₄₀P₂₀, whereas the shear modulus of the glass is 13–36% lower than the four independent shear moduli of the tetragonal crystal. The lower moduli of the glass are best explained by additional atomic displacements, beyond those experienced by the atoms in a crystal, that occur because the atoms in glasses do not occupy centers of symmetry.     

Internal friction in Fe–Al–Si alloys at elevated temperatures

Anelastic effects in ternary Fe–Al–Si alloys at elevated temperatures (from 400 to ∼700 °C) have been studied by mechanical spectroscopy. The contributions of several damping mechanisms (high temperature background, Zener and grain boundary relaxations), which are sensitive to the structural state in the temperature range studied, are analysed. The structure of the alloys was examined by different methods. The results of computer analyses of the experimental data justify the superposition of several relaxation processes. Parameters of the Zener relaxation in two ternary Fe–Al–Si alloys are determined: the peak temperature is about 545 °C (f ≈ 2 Hz); the mean value of the activation energy of the Zener relaxation is about 2.54 eV.     

Thermal stability and strength of deformation microstructures in pure copper

The plastic flow field produced by machining is utilized to access a range of deformation parameters in pure copper: strains of 1–7, strain rates of 1–1000 s^?1 and temperatures as low as 77 K. The strength and stability of the severe plastic deformation microstructures including cellular, elongated, equiaxed and twinned types are characterized. Unique combinations of strengthening and stability are identified in the case of heavily twinned microstructures. These observations offer insights for improving the stability of both single-phase and multicomponent ultrafine-grained alloys.     

Relaxation currents from macroscopic depolarization in poly-4-vinylphenol dielectrics

Polarization and depolarization currents were obtained from a capacitor structure with poly-4-vinylphenol (PVP) dielectrics by on-off switching. A single Debye relaxation was revealed for thin films; while some dispersion appeared for thick samples. The relaxation time constant (τ) of the currents was ～1 s, and the polarization density depended on the type of top electrodes. The slow relaxation at on-off switching corresponded to a much higher susceptibility (χ ≈ 10³), compared to that (χ ≈ 10⁰) of the PVP molecular dipoles, suggesting the existence of macroscopic dipoles. Meanwhile, fast (τ < ～0.1 ms) polarization and depolarization of the molecular dipoles in PVP layers were detected by measuring a P–E hysteresis loop at a high frequency. In both cases, the electric susceptibility decreased for thin (～100–1000 nm) PVP layers, showing the dominant effects of the interfacial states.     

Mechanical properties and rolling behaviors of nano-grained copper with embedded nano-twin bundles

By means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT), bulk nano-grained copper samples with embedded nano-twin bundles were prepared. Subsequent cold rolling (CR) of the LNT-DPD Cu led to a reduction in quantity of nano-twin bundles and a slight grain coarsening, accompanied by a decrease in grain boundary (GB) energy from 0.34 to 0.22 J m⁻². An increasing CR strain leads to a saturation grain size of ∼110 nm, which is less than half of that in the severely deformed Cu from the coarse-grained form. Decreased strength and enhanced ductility were induced by CR in the LNT-DPD sample. The saturation yield strength in the LNT-DPD Cu during CR was ∼105 MPa higher than that in conventional severely deformed Cu, which originates from the finer grains as well as the nano-scale twins in the LNT-DPD sample. The enhanced ductility is primarily attributed to CR induced GB relaxation.     

Microstructural evolution in copper subjected to severe plastic deformation: Experiments and analysis

The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: B_C, A and C. The microstructural refinement was dependent on processing with route B_C being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of ∼200–500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener–Hollomon parameters (ln Z ∼ 25). Calculations show that grain boundaries with size of 200 nm can rotate by ∼30° during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.     

The resistance to sliding along Coulombic shear faults in ice

Sliding experiments were performed along Coulombic shear faults in laboratory-grown freshwater ice over a range of sliding velocities (4 × 10⁻³–8 × 10⁻⁷ m s⁻¹) and temperatures (−3, −10 and −40 °C). The Coulombic failure criterion was used to describe the observed linear relationship between the shear stress along and the normal stress across the fault. From this relationship the coefficient of friction was determined. At each temperature the coefficient of friction peaks at a transitional velocity (∼8 × 10⁻⁶ m s⁻¹). For a given velocity the coefficient of friction increases with decreasing temperature. We propose that the peaked shape of the coefficient of friction vs. sliding velocity graphs are a consequence of a change in sliding behavior from “ductile-like” at low velocities to “brittle-like” at higher velocities. The velocity-strengthening and velocity-weakening friction regimes are attributed to creep and to frictional melting, respectively.     

The effect of grain refinement on the adhesion of an alumina scale on an aluminide coating

To understand the effect of grain refinement on the thermally grown alumina scale adhesion to the metal substrate, two δ-Ni₂Al₃ coatings, one coarse-grained (∼70 μm) and the other ultrafine-grained (generally below ∼500 nm), were prepared. The cyclic oxidation in air at 1100 °C shows that the ultrafine-grained (UFG) coating is better oxidation resistant than the coarse-grained (CG) coating due to the formation of a more adherent alumina scale. The latter is intrinsically correlated with the fact that the aluminide grain refinement helps to increase the oxide/metal strength through a route to prevent the formation of large-sized voids at the interface.     

Electrical,structural and magnetic properties of polyaniline/pTSA-TiO2 nanocomposites

Polyaniline (PANI)/para-toluene sulfonic acid (pTSA) and PANI/pTSA-TiO₂ composites were prepared using chemical method and characterized by infrared spectroscopy (IR), powder X-ray diffraction (XRD), scanning electron microscopy (SEM). The electrical conductivity and magnetic properties were also measured. In corroboration with XRD, the micrographs of SEM indicated the homogeneous dispersion of TiO₂ nanoparticles in bulk PANI/pTSA matrix. Conductivity of the PANI/pTSA-TiO₂ was higher than the PANI/pTSA, and the maximum conductivity obtained was 9.48 (S/cm) at 5 wt% of TiO₂. Using SQUID magnetometer, it was found that PANI/pTSA was either paramagnetic or weakly ferromagnetic from 300 K down to 5 K with H_C ≈ 30 Oe and M_r ≈ 0.015 emu/g. On the other hand, PANI/pTSA-TiO₂ was diamagnetic from 300 K down to about 50 K and below which it was weakly ferromagnetic. Furthermore, a nearly temperature-independent magnetization was observed in both the cases down to 50 K and below which the magnetization increased rapidly (a Curie like susceptibility was observed). The Pauli susceptibility (χ_pauli) was calculated to be about 4.8 × 10^?5 and 1.6 × 10^?5 emu g^?1 Oe^?1 K for PANI/pTSA and PANI/pTSA-TiO₂, respectively. The details of these investigations are presented and discussed in this paper.     

Stick–slip behavior of serrated flow during inhomogeneous deformation of bulk metallic glasses

Accurate compression tests with a piezoelectric load cell and an acquisition rate of up to 10 kHz were performed on a Zr-based bulk metallic glass in the temperature range 210–320 K at a strain rate of 10^?3 s^?1. Information about the stress drop magnitude and the associated size of shear displacements as a function of temperature and strain provides detailed insights into the shear band characteristics, which can be described by a stick–slip process. The average shear slip displacement is on average about 1–2 μm, irrespective of temperature, whereas the associated slip time (or stress drop time) increases from ～1 ms at 320 K to ～0.4 s at 213 K, yielding values on the deformation kinetics and the shear viscosity. Scanning electron microscopy investigations on shear surfaces and in situ acoustic emission measurements provide further understanding into the complex multistep shear slip process.     

Ductile tensile failure in metals through initiation and growth of nanosized voids

We here reveal the initiation of ductile failure in metals at the nanometer scale by molecular dynamics simulations coupled with a novel analytical model. This proceeds by the emission of a special type of dislocation shear loop, which can expand as a partial or perfect dislocation, evolve into a prismatic loop through reaction, or develop into twins. Molecular dynamics (MD) simulations predict a strong dependence of the stress required for the initiation of plastic flow at the surface of the void for both Cu (a model fcc metal) and Ta (a model bcc metal). The decrease in stress with increasing void size is also analyzed in terms of a new analytical approach based on the energetics of dislocation loop emission. For both fcc (copper) and bcc (tantalum) metals initiation of plastic flow in MD simulations takes place at voids as small as a tri-vacancy (radius R ≈ 0.1 nm). Extensive calculations for tantalum combined with the analytical model, which tracks the simulations, enable extrapolation to R ≈ 300 nm, in the realm of second phase particles and inclusions. Thus we conclude that this is a general mechanism of tensile failure in pure monocrystalline metals where other initiation sites are absent.     

Internal friction in γ-TiAl at high temperatures

The internal friction in TiAl polycrystals of technical purity was studied in the temperature range of 300–1500 K using an inverted torsion pendulum. Extruded single-phase γ-TiAl with an aluminium content of 54.1 at.% shows a large, frequency-dependent relaxation maximum near 1300 K during cooling from temperatures above 1400 K, which is neither observed during heating from ambient temperature nor in two-phase α₂/γ-TiAl alloys with a lower Al content. This relaxation maximum is tentatively ascribed to the motion of grain boundaries or dislocations, which are pinned by precipitates in γ-TiAl. The precipitates dissolve at temperatures above 1350 K and form again below 1200 K. No relaxation is observed in polycrystalline TiAl with a carbon content in the range from 0.009 to 0.22 at.% at temperatures below 900 K. This behaviour may be an indication of hardening by finely dispersed precipitates, as observed in TEM and SEM micrographs.     

Deformation behavior of an ultrafine-grained Al–Mg alloy produced by equal-channel angular pressing

Presented here is the deformation behavior of Al–1.5 wt% Mg alloy severely plastically deformed to equivalent pre-strains of 8 and 13 using the equal-channel angular pressing technique. The average subgrain size after severe plastic deformation was 280 and 230 nm respectively. Strain rate change and stress relaxation tests in the range 10⁻⁴–10⁻² s⁻¹ and 298–523 K were performed. The strain rate sensitivity of ultrafine-grained (UFG) Al–1.5Mg was enhanced and the peak strain rate sensitivity shifted to lower temperatures as compared with the coarse-grained (CG) alloy. The increased strain rate sensitivity is a direct consequence of the reduced activation volume. The increase in pre-strain from 8 to 13 has a small effect on both the microstructural refinement and the subsequent deformation behavior. With increasing temperature the UFG material softens compared with the CG material. This demarcation has been clearly shown on a strain rate by temperature plot. Refinement of grain results in an enhanced solute drag regime, primarily due to the decreased activation energy of diffusion.     

First-principle study on structural,elastic and electronic properties of rare-earth intermetallic compounds: TbCu and TbZn

Structural, elastic and electronic properties of TbCu and TbZn have been studied using the full-potential augmented plane waves plus local orbital (APW + lo) within density functional theory (DFT). Results on elastic properties are obtained using both the local density approximation (LDA) and generalized gradient approximation (GGA) for exchange correlation potentials. The equilibrium lattice parameter, bulk modulus and its pressure derivative have been obtained using optimization method. Young’s modulus, shear modulus, Poisson ratio, sound velocities for longitudinal and shear waves, Debye average velocity, Debye temperature and Grüneisen parameters have been calculated. Taking elastic moduli (calculated from first-principle approach) as reference values at 0 K, temperature variation of elastic moduli has also been calculated using electrostatic and Born repulsive potentials and taking interactions up to next nearest neighbours. Calculated structural, elastic and other parameters are consistent with available data. From electronic calculations, it has been found that electronic conductivity in TbCu and TbZn is attributed to 3d-orbital electrons of Cu and Zn.     

Reinforcement architectures and thermal fatigue in diamond particle-reinforced aluminum

Aluminum reinforced by 60 vol.% diamond particles has been investigated as a potential heat sink material for high power electronics. Diamond (CD) is used as reinforcement contributing its high thermal conductivity (TC ≈ 1000 W mK^?1) and low coefficient thermal expansion (CTE ≈ 1 ppm K^?1). An Al matrix enables shaping and joining of the composite components. Interface bonding is improved by limited carbide formation induced by heat treatment and even more by SiC coating of diamond particles. An AlSi7 matrix forms an interpenetrating composite three-dimensional (3D) network of diamond particles linked by Si bridges percolated by a ductile α-Al matrix. Internal stresses are generated during temperature changes due to the CTE mismatch of the constituents. The stress evolution was determined in situ by neutron diffraction during thermal cycling between room temperature and 350 °C (soldering temperature). Tensile stresses build up in the Al/CD composites: during cooling <100 MPa in a pure Al matrix, but around 200 MPa in the Al in an AlSi7 matrix. Compressive stresses build up in Al during heating of the composite. The stress evolution causes changes in the void volume fraction and interface debonding by visco-plastic deformation of the Al matrix. Thermal fatigue damage has been revealed by high resolution synchrotron tomography. An interconnected diamond–Si 3D network formed with an AlSi7 matrix promises higher stability with respect to cycling temperature exposure.     

The yield strength anomaly of single-slip-oriented Fe–Al single crystals

Many features of the well-documented yield strength anomaly in B2-structured Fe–Al alloys have been successfully described or predicted by the vacancy-hardening model [George EP, Baker I. Philos Mag 1998;A77: 737]. Interestingly, the model does not predict any orientation dependence for the yield anomaly. Here, we examine this by measuring the yield stress of three different single-slip-oriented Fe–43Al single crystals as a function of temperature. It was found that the critical resolved shear stress of all the alloys decreased rapidly with temperature from 77 K to ∼300 K, showed a plateau from 300 K to 723 K, increased to a peak at 873 K, and then decreased again with further increase in temperature. While neither the low-temperature strength (<300 K) nor the temperature of the yield stress peak depended on the orientation (in agreement with the vacancy-hardening model), the yield stress in the plateau region around room temperature did.     

Ti50Cu23Ni20Sn7 bulk metallic glasses prepared by mechanical alloying and spark-plasma sintering

A powder metallurgy technology was developed to prepare Ti₅₀Cu₂₃Ni₂₀Sn₇ bulk metallic glasses (BMGs). Firstly, amorphous powder was prepared by mechanical alloying (MA) method successfully after being milled for 30 h. Phase transformation of the as-milled powder was characterized by X-ray diffraction (XRD). Morphology of the as-milled amorphous powder was observed by scanning electron microscopy (SEM). Onset temperature of glass transformation and onset temperature of crystallization (T_x and T_g) of the as-milled amorphous powder were evaluated by differential scanning calorimeter (DSC). Secondly, the as-milled amorphous powder was then consolidated by spark-plasma sintering (SPS) method into a specimen with the shape of cylindrical stick, with a diameter and height of about 20 and 10 mm, respectively. The SPS experiment was conducted under a pressure of 500 MPa at a heating rate of 40 K/min, sintering and holding for 1 min at the temperature of 763 K. It was confirmed that the as-milled powder is of fully amorphous however the consolidated specimen shows to be an amorphous matrix with partial crystallization. Compressing strength, Young's modulus, micro-hardness, friction and density of the consolidated specimen are about 975 MPa, 121 GPa, 13 GPa, 0.12 and 6599 kg/m³, respectively. Fractograph of the specimen appears to be shear fracture and very few defects can be seen from the picture of SEM.