The investigation of internal friction and elastic modulus in surface nanostructured materials

In this paper, 304 stainless steel and pure Fe specimens, which were processed by high-energy shot peening (HESP) and ultrasonic shot peening (USP) respectively, were studied by the internal friction method. Measurements were carried out on a vibrating reed apparatus. The change of internal friction and elastic modulus shows that the treatment duration of specimen is not accompanied by the corresponding persistent increase of internal friction and elastic modulus. There is a transition layer from the top surface to the inside of the materials. Young’s modulus of surface shows obviously a fluctuation along the depth profile. The phenomena have never been shown by other measurement methods. The microstructure change should be related to some basic mechanism of surface layer formation. It may also explain why the improvement of mechanical properties in surface nanocrystallized materials does not simply correspond to the duration time of severe deformation.     

Evaluation of the internal friction and elastic modulus of the superhard films

In this paper, the vibrating-reed technique was used to measure the elastic modulus and internal fiction of the superhard thin films on the stainless steel substrate. The general formulas for evaluation of the internal fiction and the Young’s modulus of the film were derived for the sample with a rectangular section. The Young’s modulus of the nanocomposite superhard TiN/Si₃N₄ films on stainless steel is about 420–460 GPa at room temperature and decreases with increasing temperature. The internal fiction of the two-side deposited films is small and increases with temperature, while the internal friction of one-side deposited film is large and decreases with temperature due to the larger internal stress in the film.     

Internal friction and elastic modulus of bulk metallic glasses

The mechanical relaxation in binary, ternary, quaternary, and quitary bulk metallic glasses with widely different glass-forming ability, or the critical cooling rate, has been studied. A single-roller melt-spinning apparatus was used for preparing thin specimens. The internal friction Q⁻¹ and the oscillation frequency f of the specimens were measured using an inverted torsion pendulum with the free decay method. The measurements were performed from room temperature, through the glass transition temperature T_g, up to the crystallization temperature T_x. As the temperature is increased, the background Q⁻¹ increases, and peaks can usually be seen near T_g and T_x. The shear modulus, which is proportional to f², is changed near the Q⁻¹ peak. The experimental data are presented and overall features of the results are discussed.     

Experimental evidence of Snoek-like relaxation in annealed metallic glass

The frequency dependence of the internal friction (IF) of annealed Co-based metallic glass (MG) has been measured at elevated temperatures in the frequency range of 10⁻³≤f≤10³ Hz. It has been found that the IF increases with frequency decrease at a constant temperature. To explain this observation, an analytical model of IF due to anelastic relaxation in a symmetrical two-well potential has been proposed. The model shows that the IF rise with decreasing frequency observed in the experiment can be interpreted as a high frequency tail of a Snoek-like relaxation with distributed activation energies in deep relaxation centers of annealed glassy structure.     

Application of the dynamical flexural resonance technique to industrial materials characterisation

The determination of the Young’s modulus and damping coefficient Q⁻¹ by means of non-destructive vibrating techniques has been applied to bulk and coated industrial materials. Extensions of a previous analytic model of composite beam allow to determine accurately the macroscopic modulus of each component of multilayered structural materials as coated superalloys or nitride-hardened steels. Furthermore, the study of glasses and polymers has been investigated. An attempt of normalisation of the modulus versus temperature curves allows to establish master curves depending on the specific structure, from metallic glasses to polymeric glasses. Finally a comparison of dynamical modulus and Q⁻¹ values measured between resonant (>1 kHz) and subresonant techniques (10⁻³ to 10 Hz) in relation to the loading frequencies applied in real conditions has been under folder. For metallic materials such as forged or rolled titanium alloys, the brittle-to-fragile transition occurs abruptly or smoothly with a shift of 300 K following the range of excitation frequencies.     

Internal friction of hydrogen-doped metallic glasses of high glass forming ability

Seven multi-component metallic glasses of high glass forming ability (four Zr-based, a Mg-based, a Pd-based and a La-based glasses) have been hydrogenized electrolytically and internal friction has been measured at temperatures between 80 and 400 K. Hydrogen damping has been observed in every alloy; the internal friction peak is quite broad, where the peak value increases and then decreases and the peak temperature decreases with increasing hydrogen content. Compared with the results of the hydrogen damping in binary metallic glasses so far reported, the peak height versus peak temperature relation is generally shifted to higher temperature side in multi-component glasses, the origin of which has been discussed.     

Mechanical relaxation in glasses

The basic properties of glasses and the characteristics of mechanical relaxation in glasses were briefly reviewed, and then our studies concerned were presented. Experimental methods adopted were viscosity, internal friction, ultrasonic attenuation, and Brillouin scattering measurements. The specimens used were several kinds of inorganic, organic, and metallic glasses. The measurements were mainly carried out from the room temperature up to the glass transition temperature, and the relaxation time was determined as a function of temperature. The “double relaxation” composed of two Arrhenius-type relaxations was observed in many materials. In both relaxations, the “compensation effect” showing a correlation of the pre-exponential factor and the activation energy was observed. These results were explained by considering the “complex relaxation” due to cooperative motions of atoms or group of atoms. Values of activation energy near the glass transition determined by the various experimental methods were compared with each other.     

Quantitative determination of the volume fraction of intergranular amorphous phase in sintered silicon nitride

In this paper, a commercial liquid phase sintered silicon nitride with Y₂O₃ and Al₂O₃ additives is investigated. The material is characterised by a large, and stable, internal friction peak near 1150 °C. The peak is linked with the glass transition in intergranular amorphous volumes, whose presence is confirmed with transmission electron microscopy. An estimate of the volume fraction of the amorphous phase is calculated from the difference in stiffness below and above the glass transition temperature. The procedure relies on accurate Young’s modulus data, which were obtained with the impulse excitation technique (IET). The amount of amorphous pocket phase was estimated at 12.4 vol.%. For the first time, microstructural evidence supporting this estimate is obtained, using analysis of scanning electron microscopy (SEM) images. The rather large amount of amorphous matter explains the limited high temperature potential of the material, which was primarily and successfully developed for wear applications.     

Ultrasonic relaxations in borate glasses

The attenuation and velocity of ultrasonic waves of frequencies in the range from 10 to 70 MHz have been measured in M₂O–B₂O₃ borate glasses (M: Li or Ag) as a function of temperature between 15 and 350 K. The velocity of sound waves decreases with increasing temperature in all the glasses, the decrease as the temperature is increased is larger in glasses containing silver than in those with lithium. A broad relaxation peak characterises the attenuation behaviour of the lithium and silver borate glasses at temperatures below 100 K and is paralleled by a corresponding dispersive behaviour of the sound velocity. Above 100 K, the ultrasonic velocity shows a nearly linear behaviour regulated by the vibrational anharmonicity, which decreases with increasing content of modifier oxide and is smaller in lithium than in silver borates. These results suggest that the relaxation of structural defects and the anharmonicity of borate glasses are strongly affected by two parameters: the number of bridging bonds per network forming ion and the polarising power of network modifier ions which occupy sites in the existing interstices.     

Superconducting state parameters of binary metallic glasses

Ashcroft’s empty core (EMC) model potential is used to study the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ^*, transition temperature T_C, isotope effect exponent and effective interaction strength N₀V of some binary metallic glasses based on the superconducting (S), conditional superconducting (S′) and non-superconducting (NS) elements of the periodic table. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used for the first time with EMC potential in the present investigation to study the screening influence on the aforesaid properties. The T_C obtained from IU-local field correction function are found an excellent agreement with available theoretical or experimental data. In the present computation, the use of pseudo-alloy-atom model (PAA) is proposed and found successful. The present work yield results in qualitative agreement with such earlier reported values either theoretical or experimental which confirm the superconducting phase in all metallic glasses. A strong dependency of the SSP of the metallic glasses on the valence Z is found.     

Effect of interstitial impurities on internal friction measurements in niobium

Measurements of internal friction as a function of temperature were carried out in samples of niobium containing different amounts of interstitial solutes (oxygen and nitrogen) and one sample of niobium containing initially only nitrogen as interstitial solute. The experimental spectra of internal friction as a function of temperature were obtained with a torsion pendulum of the inverted Kê-type and resolved, using the method of successive subtraction, into a series of constituent Debye peaks corresponding to different interactions. For each relaxation process it was possible to obtain the height (Q_max⁻¹) and temperature (T_p) of the peak, the activation energy (E) and the relaxation time (t₀). The height, shape and temperature of these peaks depend on the concentration of interstitial elements. The observed peaks were associated with matrix-interstitial (Nb---O, Nb---N) and interstitial–interstitial (O---N) interaction processes.     

Influence of steel heat treatment on ultrasonic absorption measured by laser ultrasonics

Laser ultrasonics is combined with the reverberation technique to measure ultrasound absorption during thermal cycles applied to medium-carbon steel samples. During heating to the austenitic phase, a small Snoek peak is observed around 360 °C at 10 MHz along with a signature of the ferromagnetic to paramagnetic transition near 768 °C. Upon cooling, the behavior of the internal friction is much different from that of heating: reduced high-temperature absorption, delayed phase transition, smaller magnetoelastic contribution and stronger Snoek peak are observed. The austenitizing temperature is found to have primary importance on the internal friction behavior during cooling. These measurements show that ultrasound absorption measurements using laser ultrasonics could serve as a basis for a technique to characterize the microstructure evolution of steel in various temperature ranges.     

Effect of multi-wall carbon nanotubes on the mechanical properties of natural rubber

Multi-walled carbon nanotubes (MWNTs) were used to prepare natural rubber (NR) nanocomposites. Our first effort to achieve nanostructures in MWNTs/NR nanocomposites were formed by incorporating carbonnanotubes in a polymer solution and subsequently evaporating the solvent. Using this technique, nanotubess can be dispersed homogeneously in the NR matrix in an attempt to increase the mechanical properties of these nanocomposites. The properties of the nanocomposites such as tensile strength, tensile modulus, tear strength, elongation at break and hardness were studied. Mechanical test results show an increase in the initial modulus for up to 12 times in relation to pure NR. In addition to mechanical testing, the dispersion state of the MWNTs into NR was studied by transmission electron microscopy (TEM) in order to understand the morphology of the resulting system. According to the present study, application of the physical and mechanical properties of carbon nanotubes to NR can result in rubber products which have improved mechanical, physical and chemical properties, compared with existing rubber products reinforced with carbon black or silicone.     

Anelastic relaxation in high purity molybdenum single crystals at medium temperatures

The internal friction of deformed molybdenum single crystals with two different orientations has been measured in the 300–1300 K temperature range. After annealing to 950 K, a relaxation peak is seen in the 880–840 K range, with a hysteresis between the warming and cooling runs. For higher annealing temperatures, the peak position change to 970 K for the 1 1 0 and 1040 K for the 1 4 9 sample. The influence of a bias stress on the sample relaxation was studied. Possible mechanisms for this relaxation have been considered, and an interaction of dislocations with vacancy type point defects has been proposed.     

Hydriding effects on negative cell electrode

Internal friction (IF) and modulus measurements have been carried out on a composite material taken from the negative electrode of a commercial Ni–metal-hydride (Ni–MH) battery, both in the as-received condition and after in situ gaseous hydrogen exposure, using a sub-resonant torsion pendulum at fixed frequencies between 0.1 and 10 Hz in the temperature range 90–450 K. In both cases the IF spectrum is composed by two peaks: P1 at 190 K and P2 at 290 K, with corresponding modulus variations. The P1 data are detected to be due to a thermally activated process which frequency shift activation energy is (0.175±0.004) eV and fitted to a relaxation peak with a larger wide respect to a Debye peak, being β equal to 2.2. It is interpreted to be due to the movement of misfit dislocations within the AB₅ intermetallic-type alloy particles in the presence of nearby H atoms, acting as a Cottrell atmosphere. Peak P2 is ascribed to the decomposition and formation of hydride phase in the alloy particles upon heating and cooling, respectively. The peak heights are shown to increase in the hydrided samples, supporting the above interpretation. These results are also discussed in connection with previous measurements on an electrode material prepared at the laboratory.     

Structural homology of the strength for metallic glasses

The structure-property relationship,one of the central themes in materials science,is far from being well understood for metallic glasses (MGs) due to the great complexity of their amorphous structures.Based on the analysis of published experimental data for 165 MGs from more than 15 different alloy systems,the present study reveals a universal dependence of mechanical properties (Young's moduli,shear moduli and yield strength) on simple structural parameters (the inter-atomic distance and/or valence electron density) originating from the interatomic potential and Fermi sphere-Brillouin zone interaction.This work establishes a structure-property relationship for metallic glasses and provides insights into the fundamentals of the mechanical properties of disordered systems.     

Effect of concentration on the structure of isothermally-annealed CuZr metallic glasses

To clarify the influence of isothermal annealing on Cu_xZr_100-x (x?=?36, 44, 50, 56, and 64) metallic glasses, the annealing process was conducted using molecular dynamics (MD) simulation. The local structure is analysed by means of the radial distribution functions, Voronoi tessellation and the nearest-neighbour correlation index, C_ij. The results show that Cu-centered < 0 0 12 0 > and Zr-centered < 0 0 12 4 > dramatically increases after an isothermal annealing treatment and presents a strong tendency of being the nearest neighbour with each other, as the Cu concentration is over 50%. These two kinds of clusters are formed the Cu₂Zr Laves phase which can be directly observed in the Cu-rich glasses.     

Young’s modulus measurements of nanohoneycomb structures by flexural testing in atomic force microscopy

This paper determines the Young’s modulus of nanohoneycomb structures by flexural testing in an atomic force microscope (AFM). Since the cross-sectional area of nanohoneycomb structures varies along the structure, the area moment of inertia is not a constant. The area moment of inertia is also influenced by the porosity of the nanohoneycomb structure. An anodic aluminum oxide (AAO) film is fabricated as a nanohoneycomb structure. Young’s modulus of the AAO film, measured from the results of flexural testing in AFM, is in good agreement with the results of tensile tests in a Nano-UTM (universal testing machine).     

Thermal and mechanical properties of simultaneous and sequential full-interpenetrating polymer networks

An investigation of the thermal and mechanical characteristics of new full-interpenetrating polymer networks (full-IPNs), prepared by simultaneous and sequential synthesis paths, has been performed in the temperature regions above and below the glass transition. It has been observed that simultaneous full-IPNs exhibit higher values of density than sequential full-IPNs, as a consequence of an enhanced intermolecular packing. This peculiarity leads to an increase of the glass transition temperature and to a larger “γ-suppression” effect for the γ₂-relaxation characterizing the polycyanurate phase of simultaneous full-IPNs, which has been ascribed to reduction of the free-volume available for the molecular group rearrangements.