Solid State Phenomena Vol. 184

Abstract: Aluminium alloys have very useful properties. Because they are light, easily formed, machined and cast, they are widely used in aircraft industry and automotive industry. Their mechanical properties are mostly influenced by former mechanical and thermal treatment. An extruded aluminium alloy EN AW – 2007 was used in the as-received state and the specimens were thermally cycled with increasing the upper temperature step-by-step. After each thermal cycle the stress-amplitude dependence of internal friction was measured at room temperature. The quality factor Q^-1 was measured by a resonant method at a frequency of 20 kHz. Thermal and mechanical treatments influenced the anelastic properties of the material. The thermal treatment affects the internal friction in a positive way, while the mechanical treatment decreases Q^-1. The microstructural changes were observed by optical microscopy.

Abstract: The Fe-Mn-N alloys used in the present study showed an unexpected increase of both the yield strength and the ultimate tensile strength after prestraining and strain aging. In order to understand the mechanism of this phenomenon, which is due to solute N, static strain aging tests were carried out in combination with resonant impulse excitation internal friction analysis. It is shown that the interaction between thermal kinks and solute N atoms plays an essential role in the increase of the tensile strength.

Abstract: The temperature dependence of the dynamic Young’s modulus, E, and the damping, Q^-1, of Fe-C-N and Fe-17%Cr-C-N alloys with different C, N contents were studied in the temperature range of 25°C to 600°C by the impulse excitation internal friction technique at 1KHz. Hot rolled samples were cold rolled to a thickness of 1.2mm and recrystallization annealed at 820°C for 30sec then gas jet cooled (-50°C/sec). Samples were subsequently tensile strained 6% and 16%. A Snoek peak was observed at 120°C in the undeformed sample and a Snoek-Koster peak was observed at 400°C after deformation. In the case of the Fe-Cr-C-N alloy, a high damping background due to magneto-mechanical damping was observed in the temperature range of 25°C to 400°C. A broaden Snoek peak in FeCr was observed at 300°C. The internal friction peaks observed in this study showed the strong dependence of the amount of interstitial contents and deformation and were well correlated to previous internal friction studies measured mainly by torsion pendulum and inverted torsion pendulum.

Abstract: The effect of Cu-precipitation on the dislocation dynamics is studied by internal friction and magnetic after-effect measurements of thermally aged Fe1%CuC. We found that the copper precipitation in these alloys is accompanied by carbon redistribution. The results of both experiments showed that the hardening regime is governed by an increase of the dislocation density due to the growth of copper precipitates, while in the softening regime carbon redistribution plays a major role.

Abstract: The evolution of martensitic carbon steel during low temperature tempering was studied using internal friction. The steel containing 0.71wt.% carbon was heated at 1093K for 5min and then rapidly cooled into water (quenched), and tempered for 10 min at 340, 380, 420 and 460K. Additionally, other samples were tempered at 380K for 1 and 20 hours. Internal friction was measured by using a forced vibration pendulum, in a temperature range from 300 to 600K, with deformation amplitude 3 x 10^-6 and temperature rate of 0.8 K/min. The internal friction spectrum is decomposed into three Debye peaks: P1 at 380K, P2 at 420K and P3 at 480K, for 3 Hz. P1 is attributed to the epsilon carbide precipitation. P2 and P3 are associated to the dislocation relaxation process. P2 appears when dislocations are pinning with epsilon carbide and P3 appears when dislocations are pinning with cementite carbide.

Abstract: Mechanical loss of pre-strained and hydrogen-charged specimens of 316L austenitic stainless steel has been measured by the free-decay method using an inverted torsion pendulum at frequencies around 1 Hz. It is found that in addition to known hydrogen Snoek-like relaxation process observed at about 240 K a new peak of mechanical loss appears around 270 K. The peak has a relaxation origin with enthalpy close to 0.78 eV and pre-exponential factor of relaxation time is found to be about 10^-12.5 s. Effects of the pre-strain and parameters of electrochemical hydrogen charging on the hydrogen peak in deformed austenitic stainless steels are measured. The obtained results are discussed in terms of the Snoek-Köster mechanism and specifics of hydrogen atomic distribution in a multi-component substitutional alloy of austenitic stainless steel. Based on the analysis the binding energy of hydrogen to dislocation in 316L steel is evaluated to be about 0.16 eV.

Abstract: We have studied the crystal defects in severely deformed pure iron, by combining mechanical spectroscopy with measurements of Vickers hardness and electrical resistivity. Iron of 99.99% purity and a material in which the C content is further reduced to below 1 at. ppm were deformed to strains up to 5.6 by accumulated roll-bonding. Mechanical loss and dynamic shear modulus of as-deformed samples were measured over the temperature range from –195°C to 800°C. Effects due to dislocations and grain boundaries have been identified. Thermal stabilities of the defects have been examined through variations in the mechanical loss during heating-run and cooling-run measurements. It is found that the small difference in the carbon content influences the densities of defects and microstructure, as well as the thermal stability. They are consistent with the recovery behaviour observed through hardness and resistivity in isochronal annealing experiments.

Abstract: The influence of deep cryogenic treatment (DCT) on the microstructure of a bainitic steel is investigated by means of internal friction and transmission electron microscopy (TEM). Two relaxation peaks (P_c1 and P_c2) are observed during cooling and one relaxation peak (P_h) during heating from 100 to 320K. Peak P_c1 may be related to dislocation pinning. Peak P_h is attributed to dislocation-carbon atoms interaction. The decreasing of peak P_h after cycles deep cryogenic cooling indicates that soaking time under the deep cryogenic temperature is not contributed to the precipitation of carbides, while the cycles cryogenic treatment lead to more fine carbides precipitation.

Abstract: The strain-amplitude dependence of internal friction in Cu-0.41Ni-0.11P (mass%) alloys has been evaluated to reveal the relation between the amplitude-dependent internal friction and the stress relaxation performance. Annealing at 250°C after cold rolling causes a suppression of the strain-amplitude dependence with increasing annealing time in the range between 10 s and 10⁴ s. Analysis of the amplitude-dependent internal friction reveals the plastic strain of the order of 10^-9 as a function of effective stress on dislocation motion. It is found that the microflow stress at a constant level of plastic strain increases with increasing annealing time. This result is in line with the improvement in the stress relaxation performance but disagrees with a decrease in the tensile strength and yield stress after annealing. We believe that the increase in the microflow stress after annealing is caused by inhibition of dislocation motion due to Ni-P clusters, which were revealed by three-dimensional atom probe (3DAP) experiments.

Abstract: An equiatomic CuZr alloy quenched from 1073 K was studied by isothermal mechanical spectroscopy and X-Ray diffraction. Experiments were performed using a very large frequency range (10^-4 Hz – 50 Hz) at different temperatures. For each temperature of measurement, experiment started after complete microstructure stabilization of the sample. At room temperature, the X–Ray diffraction spectrum shows that there are two CuZr monoclinic phases as a consequence of a martensitic transformation. These structures are characterized by the existence of twinning defects for the first one and a high dislocation density for the other. Both monoclinic phases disappear at higher temperatures and first transform into the cubic CuZr phase, then this cubic phase transforms into Cu₁₀Zr₇ and CuZr₂ phases above 763 K. Internal friction spectra exhibit two relaxation peaks (P₁, P₂), at low and high temperatures, respectively. After rapid cooling of the sample from 1273 K, the first peak P₁ appears from room temperature and disappears after annealing above 673 K. The P₂ peak appears at about 800 K and increases for measurements at higher temperature up to 880 K. This temperature range corresponds with the existence of both Cu₁₀Zr₇ and CuZr₂ phases. These two peaks are associated with a relaxation linked to the dislocation microstructure in the two CuZr monoclinic phases for P1 and in the Cu₁₀Zr₇ and CuZr₂ phases for P2.