Internal friction at high temperature and mechanical behaviour of Ni-Cr alloys

Internal friction measurements have been performed on various compositions of polycrystalline alloys (Ni-8.6, 20 and 31.4at%Cr) and single crystals (Ni-20 and 33.6at.%Cr), as well as on industrial alloys of Ni-22at.%Cr containing respectively 0, 20, 66 and 180 at. ppm Ce. The high temperature internal friction spectra obtained on these alloys are characterized by the presence of two relaxation peaks (P1 and P3) and a large thermal hysteresis between the curves measured during heating and cooling. The P1 peak appears at about 950K not only in the single crystals, but also in the polycrystalline specimens. The P3 peak, appearing at about 1100K and the large thermal hysteresis occur only in the polycrystalline specimens. We have proved the origin of the P1 peak is in the interior of the grains and that the P3 peak and the hysteresis are due to the grain boundaries. The P1 peak is interpreted as a Zener peak due to the local stress induced ordering. As the P3 peak is accompanied by a large drop of in the shear modulus, it can be interpreted by a significant stress relaxation due to grain boundary sliding. On the other hand, it has been shown that behaviour of this peak is affected by the presence and the stability of chromium carbide precipitates in the grain boundaries. The microscopic mechanism responsible for the P3 peak is probably the reversible movement of grain boundary dislocation segments which interact with these precipitates. The appearance of the hysteresis is due to the dissolution during heating and reprecipitation upon cooling of these discrete carbides in the grain boundaries. A theoretical analysis of carbides stability as a function of temperature has been compared with the results of internal friction and TEM observations. (author) figs., tabs., refs