Dislocation Relaxation Peak Research Articles

Dislocation Relaxation Peaks in Aluminum

Dislocation Relaxation Peaks in Aluminum

Low-temperature ultrasonic attenuation measurement in very pure gold polycrystals at megahertz frequencies

The ultrasonic attenuation in a polycrystal gold crystal of purity 99.999% has been measured at three different frequencies, 5, 10, and 30 MHz, from 77 to 300 K. Mainly one peak (the dislocation relaxation peak, or Bordoni peak) was observed at 178 - 174, 186 - 185, and 237 - 235 K at 5, 10, and 30 MHz respectively. The highest peak temperature observed was for samples given a low stress (8 MPa) while the lowest was for 130 MPa stress. On further increasing the stress to 259 MPa, the peak temperature increases slightly. The present work shows that the peak height decreases as the frequency of measurement increases while it increases as the mechanical stress increases. The activation energy and the attempt frequency decrease as the compression deformation to the sample increases.

The mechanical energy loss in stressed and -irradiated polycrystalline aluminium

The mechanical energy loss in two very pure (99.9999%) deformed aluminium samples (strain, 2%; stress, 12.6 MPa) was studied using the pulse-echo technique at 10 MHz frequency. These two samples were given several -irradiation doses (up to ) at room temperature and liquid-nitrogen temperature. Two relaxation peaks and , dislocation relaxation peaks, were found at temperatures from 179 to 169 K for peak and from 221 to 210 K for peak , having relaxation strengths of - and - , respectively. Both the peak heights and the peak temperatures decrease as the -irradiation dose increases. These variations are explained in terms of the kink-pair formation model.

The effect of gamma irradiation on the dislocation relaxation peak in gold

The effect of gamma irradiation on the dislocation relaxation peak, i.e. the Bordoni peak, of high purity polycrystalline gold has been studied at frequency of 10 MHz. It was found that the effect of gamma radiation is more significant in specimen irradiation at room temperature (1 A) than that irradiated at liquid nitrogen temperature. The variation of the peak height, and temperature of the dislocation relaxation peak as a function of gamma doses are explained in terms of the Kink-Pair formation model.

Study of the dislocation relaxation peaks in easy glide deformed silver single crystals having different length-to-width ratios

The dislocation relaxation peaks in a deformed silver single crystal of orientation (110), i.e. easy glide, with length-to-width ratios of 8:1, 6.5:1, 4.9:1, 4:1 and 1:1 have been studied. These relaxation peaks have been measured at frequencies of 5, 10 and 30 MHz. The results showed that a crystal with the ratio 8:1 produced the highest dislocation relaxation peaks at higher temperature. As the ratio decreased, the peak height decreased nearly linearly when the measurements were carried out at 5 MHz. At frequencies of 10 and 30 MHz, the peak height fell dramatically, especially for ratios less than 8:1. The large dislocation relaxation peak height which occurred at higher temperature in the sample of ratio 8:1 was attributed to long dislocation loops formed during deformation. These loops seem to become shorter as the length-to-width ratio decreases.

The power spectrum associated with a kink chain oscillating in a non-stokesian atmosphere of paraelastic interstitials

The power spectrum of a built-in kink chain oscillating in an atmosphere of paraelastic interstitials was investigated numerically using a fast Fourier transform technique. Above a sharply defined value of the strain amplitude, odd-harmonic generation is observed only at the low-temperature side of the dislocation relaxation peak associated with interstitial—kink interaction. At moderately high strain amplitudes, a strong enhancement of the harmonics situated in the immediate vicinity of the natural resonance frequency of the chain and complete depression of the rest are clearly seen. Finally, the onset of quasi-chaotic kink oscillations is detected when the system is driven into the super-Snœk regime where atmospheric tearing takes place.

The effect of neutron and gamma irradiation on the dislocation relaxation peak in silver

The internal-friction spectrum has been determined in extremely pure polycrystalline silver crystals after being subjected to neutron and gamma irradiation. The spectra obtained for an applied frequency of 10 MHz show that the mechanical losses due to dislocation relaxation decrease as the doses of neutron and gamma radiation increase. The effect of gamma radiation was found to be more significant than that of neutron irradiation. The relationship between damping neutron and gamma dose was found to be of the formQmax−1 α ΛDη, where η is equal to −1.2 for neutron and −1.45 for gamma irradiation. The variations of the peak height and width, and temperature of the dislocation relaxation peak as functions of neutron and gamma doses are explained in terms of the pair-kink formation model.

Effect of γ Irradiation on Dislocation Relaxation Peaks in Polycrystalline Molybdenum at 10 MHz Frequency

Effect of γ Irradiation on Dislocation Relaxation Peaks in Polycrystalline Molybdenum at 10 MHz Frequency

Effects of neutron and gamma irradiation on the dislocation relaxation peak in very pure copper single crystal of 〈110〉 concentration

The effects of neutron and gamma irradiation on the height and temperature of the dislocation relaxation peak (Bordoni peak) have been studied in four very high purity copper single crystals having the orientation 〈110〉. Two crystals have width to length ratio 1:10 and the other two 1.3:1. Samples with different width to length ratio were used so that different dislocation loop lengths could be achieved by initial prestrains. In all samples the peak height and the peak temperature were reduced by irradiation. The samples with width to length ratio 1:10 exhibited a larger peak, due to the longer loops created in them. Gamma irradiation was found to have more effect on the characteristics of the peak than neutron irradiation. The results were in full agreement with the kink pair generation model.

Effect of neutron irradiation on the dislocation relaxation peak in single-crystal copper of 〈110〉 orientation

Abstract The effect of neutron irradiation on the dislocation relaxation-peak height and temperature has been studied in two copper single crystals having the orientation 〈110〉. The first has a width-to-length ratio the 1.3 to 1 and the other a ratio of 1 to 10. Samples with different width-to-length ratios were used so that different dislocation loop lengths could be achieved by the initial pre-strains. In both samples the peak height and the peak temperature were reduced by irradiation. The sample with the width-to-length ratio of 1 to 10 exhibited a larger peak and was more sensitive to the irradiation. The variations in the peak height and temperature with neutron irradiation are explained in terms of the kink-pair generation model. This model can also account for the differences between the results for the two samples.

Calculation of the activation energy, the attempt frequency and the peierls stress in low purity polycrystalline aluminium

The dislocation relaxation peaks (Bordoni B 2 and Niblett-Wilks B 1) have been studied in 3N purity polycrystalline aluminium at kHz and MHz frequencies. B 1 and B 2 were found to occur at 98 and 138 K respectively at the kHz frequency while they shifted to 143 and 174 K respectively at the MHz frequency. The activation energy W, the attempt frequency f 0 and the Peierls stress τ p were found to be 0.065 eV, 1.99 × 10 12 Hz and 1.74 × 10 6 N m -2 for B 1 and 0.10 eV, 1.58 × 10 13 Hz and 4.11 × 10 6 N m -2 for B 2 respectively. The effect of the purity and deformation on W, frr 0 and τ p is discussed.

Dislocation relaxation peaks and solid solution softening in substitutional iron alloys

Dislocation relaxation peaks and solid solution softening in substitutional iron alloys

DISLOCATION MOTION IN VA METALS STUDIED BY INTERNAL FRICTION

Dislocation motion in Va metals, V, Nb and Ta at low temperatures was studied by internal friction and dynamic modulus measurements. The results on pure and impure Nb and Ta support the assignment that the δ and α peaks are intrinsic dislocation relaxation peak and H cold work peak, respectively. Dislocation motion below the δ peak temperatures at say 9K was discussed based on the observed effect of deformation and H charge on modulus defects at 9K. Modulus defects at 9K were considered to be mainly due to kink diffusion along screw and non-screw dislocations and the δ peak was considered to be associated with double kink formation on non-screw dislocations. H charge suppressed the modulus defects at 9K and seemed to induce a very strong dislocation pinning which is stable up to room temperature. Other types of dislocation pinning observed in V after quenching the specimen deformed and H charged were also shown.

Internal friction peaks in cold-worked and hydrogen-charged nickel

Internal friction measurements are made on deformed and hydrogenated Ni at frequencies of 2 Hz, 800 Hz, and 30 kHz over the temperature range between 4.2 and 373 K. Two kinds of relaxation peaks Pd and PdH accompanied by clear ΔM effects are commonly observed. Pd peak is generated at low temperature only by plastic deformation in uncharged Ni. Electrolytic charging with hydrogen suppresses this peak but instead induces PdH peak at a temperature well above Pd peak. On aging (outgassing) at temperatures between room temperature and 473 K, Pd further diminishes temporarily, but recovers to the original height thereafter; conversely, PdH first grows to a maximum height and then decays. These aging characteristics are related with nonuniform distributions of hydrogen in the samples. A good proportionality exists between the maximum height of PdH and the height of Pd before H charging. In fully annealed samples where Pd is absent, PdH is not generated evidently by H charging. From these results, it is concluded that Pd peak is ascribable to the dislocation relaxation peak of the Bordoni type, while PdH peak to the cold-work peak involves the interaction between hydrogen atoms and dislocations.

Dislocation relaxation in single crystals of molybdenum

The characteristics of the α dislocation relaxation peak in molybdenum have been examined in three orientations of high purity single crystals deformed in tension at 170, 300 and 450 K. For deformation at 450 K, the α peak is absent in crystals oriented for easy glide and deformed into stage 1 of work hardening. For crystals of the same orientation deformed into stages 2 and 3 and for crystals with [001] and [101] tensile orientations, the peak is observed superimposed on a step-like background; it grows rapidly, sharpens and moves to lower temperatures with increasing amounts of plastic deformation. Crystals with the easy glide orientation deformed at 170 and 300 K exhibit rapid work hardening and display small, broad, asymmetrical α peaks. Mild strain aging at 500 K does not greatly affect the α peak, but drastically reduces the step-like background damping. The overall response of the α peak to changes in dislocation substructure is very similar to that of the Bordoni peak in f.c.c. metals. Three widely divergent models are considered: double kink generation on non-screw dislocations, kink diffusion on screw dislocations, and breakaway of dislocations from immobile pinning points. It is concluded that the major features of both peaks are better explained by the breakaway models. For the α peak, a variation of the Hasiguti trapped kink model seems most appropriate.

Dislocation relaxation in molybdenum

Dislocation relaxation peaks have been investigated in the three principal directions [100], [110] and [111] of a single crystal of molybdenum at the frequencies 10, 20 and 50 MHz. The activation energies and the relaxation frequencies calculated from the position of the peak showa high degree of anisotropy from one direction to the other and the lowest values are obtained when the ultrasonic waves are applied parallel to the [100] direction. The comparison of the peak with Zener’s single-relaxation theory shows that the peak in the [100] direction is due to a single relaxation process. In the other two directions, the results favour a spectrum of different relaxation frequencies with a constant activation energy. This result is in qualitative agreement with the data obtained earlier for f.c.c. metals and gives strong support to the idea that dislocation relaxation peaks arise from the same mechanism in both f.c.c. and b.c.c. metals.

Dislocation relaxation in single crystals of bcc metals

The behavior of the α dislocation relaxation peak in bcc metals has been investigated for various orientations of high purity single crystals which were deformed in tension at temperatures in the range 77–450°K. The effects of substitutional alloying, interstitial impurities and strain aging were also examined. These results coupled with results obtained by other investigators support a dislocation breakaway model as a likely mechanism of the α peak.

Study on Dislocation Relaxation Peaks in Gold and Copper

An inverted torsion pendulum apparatus is described for the measurement of internal friction and dynamic modulus of metals over the temperature range 4° to 300°K after deformation in situ at 4°K. A detailed study of low-temperature internal friction peaks in pure gold and copper was carried out with this apparatus. By comparison of the results with other reported measurements, the activation energies for the Niblett-Wilks peak (EN-W) and for the Bordoni peak (EB) in gold and copper were deduced as follows: EN−W was 0.10 eV (for gold) and 0.045 eV (for copper);and EB was 0.19 eV (for gold) and 0.12 eV (for copper).The internal friction below 15°K was found to rise for gold specimens deformed at low temperature. This rise is discussed in detail and suggests a relaxation peak below 15°K in cold-worked gold.

Internal Friction and Young's Modulus in Irradiated Tungsten

Measurements of internal friction and Young's modulus as a function of temperature and 13.7-Mev deuteron bombardment flux are reported for polycrystalline tungsten. A dislocation-relaxation peak in the internal friction as a function of temperature is observed. The peak height is found to be reduced by irradiation. The background internal friction decreases and the modulus increases with bombarding flux. The experimental results for modulus change with bombardment is shown to fit a simple dislocation pinning model reasonably well. The effect of immobile radiation induced vacancies on Young's modulus in tungsten is observed to be −0.44% per atomic percent of vacancies compared with Dienes' prediction of −1.0%.

Second Dislocation Relaxation Peak in Aluminium at Low Temperatures

Second Dislocation Relaxation Peak in Aluminium at Low Temperatures