Mechanical Loss Spectra Research Articles

Internal Friction and Creep of Titanium Aluminides with Different Microstructure

AbstractATi-46.5 Al-4 (Cr, Nb, Ta, B) alloy (at.%) with different microstructures (fine-grained primary annealed or coarsegrained fully lamellar) was examined both by mechanical-loss (internal-friction) and creep experiments. The mechanical-loss spectra (0.01– 10 Hz), ranging from 300 up to 1300 K, show two phenomena: (i) A loss peak at about 900 K (0.01 Hz) which occurs only in samples with a lamellar microstructure. The activation enthalpy, determined from the frequency shift, is 3.0 eV. The Debye-type peak is related to thermally activated reversible local movement of dislocation segments which are pinned at the lamellae interfaces and within gamma lamellae, (ii) A high-temperature damping background above 1000 K which is controlled by an activation enthalpy of 3.8 to 3.9 eV. This value agrees well with the activation enthalpy determined by creep tests and is in the range of values reported for self-diffusion of Al. These results indicate that both properties (high-temperature background and creep) are controlled by volume diffusion-assisted climb of dislocations.

Mechanical Loss Spectrum of the Fe-20%Ni-C Alloys in the Temperature Interval 300 K–700 K

Mechanical Loss Spectrum of the Fe-20%Ni-C Alloys in the Temperature Interval 300 K–700 K

Ab initio calculation of total energy of a bcc iron cell containing three dissolved carbon atoms, and internal friction in Fe–C solid solutions

Carbon interstitial atoms, when dissolved in bcc Fe, induce elastic dipoles with tetragonal symmetry. Mechanical loss spectra indicate the presence of interstitial atoms by Snoek peaks. The magnitude of the peaks is proportional to the concentration of interstitial atoms in solution and the dipole strength. The ab initio computer simulation of lattice parameters and local structure distortions caused by the Fe–C system with three carbon atoms in different octahedral positions of one sublattice has been carried out using WIEN2k software. The simulation results allowed us to propose a way to estimate the dipole strength. The value found are λ1- λ2 = 0.58.

Mechanical Loss Spectrum of the Fe-20%Ni-C Alloys at Negative Temperatures

Mechanical Loss Spectrum of the Fe-20%Ni-C Alloys at Negative Temperatures

Coherent-Incoherent Transition of ε-Carbide in Steels Found with Mechanical Spectroscopy

Although a coherent-incoherent transition in the e-carbide precipitated in steels is supposedly linked to hardening and microstructural changes, the existence of this transition has not yet been confirmed. In this paper, we investigate this subject using mechanical spectroscopy. By measuring mechanical loss spectra below room temperature of quench-aged Fe-C alloys, mild steel, and pearlitic steel, we reveal a new broad peak (NBP). This peak is related to thermal activation, and its line shape obeys the equation of the Debye peak with a distribution in relaxation time. The Arrhenius plot yielded a large activation energy and gigantic pre-exponential factor. Its intensity grew by aging at temperatures where precipitation of e-carbide has been reported. However, it starts to decay at duration far too early for e-carbide to transform to cementite. For isothermal aging at 393 K (120 °C), the intensity sharply decreased at durations over 3 hours. This decay was accompanied by appearance of another similar peak (NBP′), which had a peak frequency two orders higher than that of NBP. These peaks had comparable intensity. We attribute NBP and NBP′ to coherent and incoherent e-carbides, respectively. We produced a model that attributes the relaxation peaks to reorientations of extra carbon pairs in the e-carbide. The extraordinary values of the Arrhenius parameters may be interpreted by using this model. Based on these results, we assert that mechanical spectroscopy can detect the coherent-incoherent transition in carbon steels. This method will be powerful in studying problems related to the coherency in carbon steels.

Mechanical Spectroscopy of MgB2 Containing Sic / Spektroskopia Mechaniczna MgB2 Zawierającego Sic

The compound magnesium diboride (MgB2) has been well-known since the 1950s; however, its superconducting properties were unknown. Intrinsic characteristics of MgB2 make this material a promising candidate for technological applications, although the low value of the irreversibility field and the decrease in critical current density with the increase in the magnetic field considerably reduce its utility. The present work aimed to study the effect of carbon-based doping on anelastic properties of MgB2 as measured by mechanical spectroscopy. The samples were prepared by using the powder-intube method. The samples were made with 5, 7.5, and 10 wt.% of silicon carbide (SiC). The results reveal complex mechanical loss spectra caused by the interaction between point defects and surface defects in the crystalline lattice of MgB2.

Grain boundary relaxation in yellow gold bi-crystals

The mechanical loss spectrum of a yellow gold bi-crystal is presented and analyzed in detail. The relaxation strength is monitored as a function of several geometrical parameters such as sample width, length and thickness. It is found that the relaxation strength is proportional to the GB density (the inverse width), whereas it depends linearly on the sample thickness. The experimental findings are compared to finite elements (FE) simulations, where the material can glide frictionless along the grain boundary. The simulations show the same dependencies as the mechanical loss measurements. The relaxation peak in the loss spectrum can be interpreted as due to GB sliding accommodated by the elastic deformation of the grains.

Relaxation of Bulk Metallic Glasses Studied by Mechanical Spectroscopy

The relaxational dynamics in metallic glasses (MGs) is investigated by using mechanical spectroscopy. The spectra show that in MGs there are two relaxations: (i) the α relaxation, linked to the glass transition, as observed in other classes of amorphous materials; and (ii) the β relaxation, well observed below the glass transition, with an intensity strongly dependent on the MG composition, the nature of which has been linked to the local microstructure of MGs. For the investigated MGs we find that the intensity and relaxation time of the β relaxation depends, in a reproducible fashion, on the thermal history of the samples. During aging experiments, the intensity decreases (as well as the τβ) with a time dependence described well by a stretched exponential, with an exponent β(aging) independent of the driving frequency. Moreover, we find that the activation energy Uβ and the peak temperature Tβp of the β relaxation follow the approximate relationship: Uβ ≈ 31.5RTβp (for driving frequency 1 Hz), indicating that the high temperature limit of the peak frequency is approximately the same for all the MGs investigated. Finally, the frequency separation of the α and β processes in the mechanical loss spectra for La-and Pd-based metallic glasses is tested against the prediction of the Coupling Model.

Recrystallization and Grain Boundary Formation in an 18-Carat Gold Alloy Studied by Mechanical Spectroscopy

Heavily deformed 18-carat yellow gold samples show a recrystallization peak at 700 K during the first heating. The mechanical loss spectrum of polycrystals shows a relaxation peak at about 780 K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal from 2 % to 10 % causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak. At 8 % deformation the high temperature peak disappears and the peak that is normally observed in polycrystals appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the high temperature peak is attributed to a relaxation mechanism in the sub grain boundaries. The peak of polycrystalline samples located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. The peak temperature can be related to the mean grain size.

Grain boundary formation stages in a deformed yellow gold single crystal studied by mechanical spectroscopy

A monocrystalline 18-carat yellow gold sample is deformed stepwise from 2% to 12% at room temperature. The mechanical loss spectrum is recorded and analysed. At low deformations the exponential high temperature background increases. At a critical deformation of 8% an intermediate temperature peak appears. Such peak typical of polycrystals is related to grain boundary sliding. The comparison of a polycrystal formed of low angle grain boundaries (LAGB) only, and samples deformed between 2% and 8% evidence the existence of an intermediate stage. The high temperature peak P3 characterizes the mobility of LAGB that is controlled by the movement of dislocations.

Turning renewable resources into value-added polymer: development of lignin-based thermoplastic

Productive uses of lignin, the third most abundant natural polymer, have been sought for decades. One especially attractive possibility is that of developing value-added products including thermoplastics based on lignin. This possibility warrants special attention due to growth of the modern biofuel industries. However, the polydisperse molecular weight and hyper-branched structure of lignin has hindered the creation of high-performance biopolymers. Here, we report the preparation and characterization of novel lignin-based, partially carbon-neutral thermoplastics. We first altered the molecular weight of lignin, either by fractionation with methanol, or by formaldehyde crosslinking. Crosslinking of lignin increases the molecular weight, exhibiting Mn = 31000 g mol−1, whereas that of as-received lignin is 1840 g mol−1. Tuning the molecular weight of lignin enabled successful preparation of novel lignin-derived thermoplastics, when coupled with telechelic polybutadiene soft-segments at proper feed ratios. Characteristic to thermoplastic rubbers, free-standing films of the resulting copolymers exhibit two-phase morphology and associated relaxations in the dynamic mechanical loss spectrum. To the best of our knowledge this article is the first report to demonstrate phase immiscibility, melt-processibility, and biphasic morphology of soft and hard segments in a lignin-based copolymer for all feed ratios of two macromolecular components. The use of higher molecular weight lignin enhanced the resulting shear modulus due to efficient network formation of telechelic polybutadiene bridges. The storage modulus in the rubbery plateau region increased with increasing lignin content. The successful synthesis of novel lignin-based thermoplastics will open a new pathway to biomass utilization and will help conserve petrochemicals.

Grain Boundary Relaxation in 18-Carat Yellow Gold

Au60Ag30Cu10(in at%) gold alloy exhibits a mechanical loss spectrum composed of a Zener peak due to Cu atoms in the solid solution and of a second relaxation peak at higher temperature or lower frequency. It is shown that this second peak is related to the presence of grain boundaries as it is absent in the spectrum of a single crystal. This mechanical loss peak, which is stable and reproducible in heating and cooling cycles, is thermally activated with an activation enthalpy of 2.35 eV and an apparent limit relaxation time of 9.6·10-17s. As it is hard to imagine that a whole grain should slip at once along a touching grain, the relaxation peak is interpreted by a dislocation model, which may account not only for the activation parameters but also for the stress amplitude dependency of the peak.

Quantitative Analysis of Carbon in Ferrite in Dual-Phase Steels by Mechanical Loss Measurements

To establish the method for determining the amount of carbon in the ferrite phase in ferrite + martensite dual-phase low-alloy steels, mechanical loss measurements have been performed on a series of Fe–C alloys with varying constitution. The observed mechanical loss spectra of two-phase alloys turned out to be simple superposition of those of single phase alloys, of ferrite and of martensite. The concentrations of carbon in solution evaluated from the magnitude of the Snoek relaxation in the two-phase alloys agree well with those expected from the Fe–C phase diagram. It is thus possible to selectively analyse the carbon dissolved in the ferrite phase in the complex structure, at least in simple binary alloys.

Analysis of Microstructural Changes with Temperature of Thermally Sprayed WC-Co Coatings by Mechanical Spectroscopy

Thermally sprayed hardmetal coatings can be used to improve the wear or fatigue resistance of mechanical parts. Depending on the deposition conditions, their microstructure and phase composition are out of equilibrium at different levels due to the extreme heating/cooling rates. In the present study, the changes that occur with temperature variation are monitored by mechanical spectroscopy. Requirements to specimen of mechanical spectroscopy created the need to prepare WC-17%Co coatings of 1.2 mm thickness by high velocity oxy-fuel (HVOF) spraying. The coatings, separated from the substrate by spark erosion, were tested in a forced torsion pendulum between room temperature and 1570 K at a temperature scanning rate of 1K/min. The mechanical loss spectrum shows different features. At 800 K, a maximum M1 is observed in coincidence with a sudden increase of the elastic modulus. The change of the elastic modulus is due to a densification of the material possibly related to cobalt recrystallization. A relaxation peak located at about 1100 K is typically found in WC-Co hardmetals. It is attributed to the movement of dislocations in the cobalt phase. A sharp peak is observed at 1510 K on heating and at 1410 K on cooling. Such peak is due to the reversible transition from W3Co3C at high temperature to W6Co6C at low temperature as proven by X-ray diffraction. The reversibility of such transformation was observed for the first time.

Grain boundary relaxation and grain growth in 18-carat yellow gold alloy

A high-temperature anelastic relaxation peak in the mechanical loss spectrum of 18-carat yellow gold polycrystals is analysed and interpreted as due to grain boundary sliding accommodated predominantly by elastic deformation of the grains. The characteristic time scale of the relaxation process tvaries with the grain size d, and the grain size exponent is found to be m = 1.09 +/- 0.10 which is in perfect accord with the Zener model that attributes a grain boundary relaxation to viscous sliding along the grain boundaries. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

High-Temperature Mechanical Spectroscopy of Nitrogen-Rich Ca-α-SiAlON Ceramics

Nitrogen-rich Ca–α-SiAlON ceramics made with different starting powder compositions have been studied by high-temperature mechanical spectroscopy in parallel with compressive deformation in a spark plasma sintering equipment. The mechanical loss spectra measured upon heating show a relaxation peak at about 1150 K and a high-temperature exponential background at higher temperatures (>1400 K), which are attributed to the α-relaxation in the glassy phase and to grain-boundary sliding, respectively. A theoretical interpretation of the results shows that the peak position is mainly a function of glass viscosity. The amplitude of the peak is not only affected by the glassy phase quantity but also depends on the restoring force due to grain elasticity. Therefore, despite a higher amount of glassy phase specimens containing elongated grains may show a lower peak. The amplitude of the internal friction peak corresponding to α-relaxation can be used to predict the compression creep of silicon-nitride-based ceramics.

Mechanical spectroscopy of rolling oil thin films on cold-rolled steel sheets

Fine traces of lubricating oil can be discovered in the form of fine debris left on the surface of cold-rolled steel sheets. High resolution mechanical spectroscopy is a sensitive technique for detecting such extremely fine oil traces. It is shown that a characteristic mechanical loss spectrum occurs in the low temperature range (from 180K up to 280K) only in the two following cases, namely (i) if traces of the rolling oil are left on the surface of sheets and (ii) if the clean sheets are covered with a thin film of the rolling oil (or any other natural or mineral oil). It is clearly demonstrated that similar mechanical spectra induced by the presence of oil can be observed in the sub-resonant mechanical spectroscopy if the oil film is deposited on a very soft cellulose neutral substrate. Therefore the behaviour of the oil alone can be obtained with a high resolution. It will be demonstrated that it is possible to assign the mechanical loss phenomena to the steel sheet (like the Snoek-Koster effect above 500K) and oil separately. In particular, it is shown that the low temperature relaxation phenomena do not originate from a specific interaction of the steel surface with oil, as frequently reported in the literature.

Influence of the carbon content on dislocation relaxation in martensitic steels

Mechanical-loss measurements were performed in three grades of martensitic high-carbon tool steels. These steels were treated under carburizing gas before quenching, in order to vary the concentration of interstitial carbon in the martensite, as assessed by thermoelectric-power measurements. The mechanical-loss spectra present a relaxation peak due to interaction between dislocations and carbon interstitials, similar to the Snoek–Köster peak found in ferrite. The amplitude of the Snoek–Köster peak is expected to be independent of the concentration of interstitials. However, the amplitude of the corresponding peak in martensite is found to increase with the concentration of interstitial carbon. This effect is attributed to an increase of the dislocation density with the carbon content.

Implications of the similarity principle relating creep and attenuation in finely grained solids

The Raj–Ashby model of diffusionally accommodated grain-boundary sliding implies the existence of similitude in the mechanical loss spectrum of finely grained solids: the quality factor Q of a shear wave propagating with angular frequency ω is related to the viscosity η for steady-state Coble creep and the (unrelaxed) rigidity μ through a dimensionless relation of the form Q = F ( ω η / μ ) . Using existing measurements of η and Q obtained respectively from uniaxial creep tests, and forced torsional oscillation tests, we test that prediction. Though the similarity principle appears to hold, definitive proof will require further measurements of Q and η over a larger range of grain sizes. With that proviso, we conclude that the Raj–Ashby model includes the basic physical mechanisms acting in the experiments underlying two existing data sets on attenuation in the geological material olivine.

Mechanical spectroscopy of 18-carat AuCuPd white gold alloys

The mechanical loss and dynamic modulus temperature spectra of two 18-carat white gold alloys, Au 58%Cu 24%Pd 18% without (grade A) and with alloying additions (grade B), were measured in the 400–1000 K temperature range. A peak interpreted as a Zener relaxation peak is found at about 630 K in both alloys. In the as-quenched state the peak shifts to lower temperatures due to excess vacancies, creating a transient peak at 520 K visible only upon the first heating. In grade A, a relaxation peak is observed at 750 K and is attributed to grain boundary relaxation, since it is absent in monocrystalline sample. In the same temperature region, the grade B features a peak with a large hysteresis between heating and cooling. A sharp peak observed in heating at 830 K is accompanied by a strong modulus decrease. In cooling, we observe a peak at 750 K, below which the mechanical loss abruptly decreases, accompanied by a modulus recovery. The peak hysteresis is interpreted as being due to dissolution or precipitation, respectively. The internal friction spectrum is correlated with the alloy's mechanical properties, namely the hardening of the grade B upon tempering at a temperature below the dissolution peak.