Andre Rivière

High-Temperature Mechanical Relaxation due to Dislocation Motion inside Dislocation Networks

Internal friction peaks observed in single or polycrystals are clearly due to a dislocation relaxation mechanism. Because a sample observed by transmission electron microscopy (TEM) often exhibits in the same time various dislocation microstructures (isolated dislocations, dislocation walls, etc.) it is very difficult to connect the observed relaxation peak with a particular dislocation microstructure. Using isothermal mechanical spectroscopy (IMS), it is easier to compare, for instance, the evolution of a relaxation peak with measurement temperature to the microstructural evolution observed by in-situ TEM at the same temperatures. IMS was used to study a relaxation peak in a 5N aluminium single crystal firstly 1% cold worked and then annealed at various temperatures. TEM experiments performed in the same material at various temperatures equal to the temperatures used for the damping experiments made possible to link this internal friction peak with a relaxation effect occurring inside dislocation walls. In two other experiments in a 4N aluminium polycrystal and in a metal matrix composite with SiC whiskers, it is shown that the observed relaxation peaks are connected to the motion of dislocations inside polygonization boundaries in the first case and in dislocation pile-ups around each whisker in the second one. Theoretical models proposed to explain such relaxation peaks due to a dislocation motion inside a dislocation wall or network are discussed.

Mechanical Spectroscopy of the Fe-25Al-Cr Alloys in Medium Temperature Range

Fe3Al intermetallic compounds and several (Fe,Cr)3Al alloys with Cr content from 3 to 25 % have been studied using isothermal mechanical spectroscopy. The Zener relaxation caused by reorientation of pairs of substitute atoms in Fe is observed in all studied alloys and used to evaluate the activation parameters of Al (Cr) atom jumps in Fe. The second internal friction peak at higher temperatures was observed only in Cr containing alloys. Isothermal mechanical spectroscopy (employed frequency from 10-4 to 102 Hz) gives some advantages as compared with ordinary techniques, i.e. study of anelasticity as a function of temperature. It allows to avoid transient effects and to measure materials in a state close to equilibrium. This allows to distinguish clearly between Al atom diffusion in Fe3Al in B2 and D03 states (activation energies for Fe – 26 Al in the B2 range the HB2 ≈ 235 kJ/mol, and in the D03 ordered range the HD03 ≈ 286 kJ/mol). Effect of chromium on the Zener relaxation is analysed.

High-Temperature Mechanical Relaxations in Metals and Metallic Alloys

In spite of numerous works, the relaxation phenomena observed at high temperature (between room temperature and the melting temperature TM) are still under discussion. Because relaxation peaks were observed in single crystals, it is generally considered that the basis of the relaxation mechanism is linked to the dislocation network. The main difficulty for high temperature damping measurements is the great sensitivity of internal friction with several experimental parameters: the heating/cooling rate, maximal applied strain amplitude, sample purity, thermomechanical history of the sample, microstructure, etc. This sensitivity can explain the large scatter in experimental results published by various authors. Moreover, internal friction (IF) measurements performed during continuous heating or cooling and using an apparatus working at a quasi-static frequency, do not allow to completely describe the relaxation phenomena. On the contrary, isothermal mechanical spectroscopy (measurements of internal friction in a large frequency, temperature and maximal strain amplitude ranges) improves the experiments or evidences new relaxation effects. This is illustrated in this paper for various examples: slightly cold worked single crystals, polycrystals after a large cold work and recrystallization, non thermally activated peaks observed in metallic alloys, and relaxation peaks at very high temperature (above 0.9 TM).

Isothermal High Damping in Shape Memory Alloys

Shape memory alloys are often considered as high damping materials. In fact, internal friction spectra measured at a fixed frequency (1 Hz for instance) during a temperature change, exhibit a very large damping effect linked to the forward or reverse phase transformations. This effect increases with the temperature change rate and decreases with the measurement frequency. If the temperature is stabilized at the maximum of the effect, the damping drops very quickly to a very low value. However, in some cases, high damping can be evidenced in these materials at very low frequency during isothermal experiments.