Internal friction measurements have been performed on various compositions of polycrystalline alloys (Ni-8.6, 20 and 31.4 at.%Cr) and single crystals (Ni-20 and 33.6 at.%Cr), as well as in industrial alloys of Ni-22 at.%Cr containing respectively 0, 20, 66, and 180 at. ppm cerium.
The high-temperature internal friction spectra obtained in these alloys are characterized by the presence of two relaxation peaks (P1 and P2) and a large thermal hysteresis between the curves measured during heating and cooling. The P1 peak appears at about 950 K both in polycrystalline specimens and in single crystals. The P2 peak, appearing at about 1100 K, and the large thermal hysteresis occur only in the polycrystalline specimens. It is shown that the origin of P1 peak lies in the interior of the grains and that the P2 peak and the hysteresis are due to the grain boundaries. The PI peak could be interpreted as a Zener peak due to local stress-induced ordering. The P2 peak is certainly due to grain boundary sliding, more precisely to grain boundary dislocation motion. These dislocations interact with chromium carbide precipitates located in the grain boundary. The hysteresis is then interpreted as due to the dissolution during heating and the reprecipitation upon cooling of these discrete carbides in the grain boundaries.
The three anelastic phenomena, the P1 and P2 peaks and the hysteresis, appear in the same temperature range where intergranular embrittlement was observed during tension tests. This embrittlement, as well as the P2 peak, is associated with grain boundary sliding and is enhanced by carbide dissolution. On the contrary, pinning of the grain boundaries by carbides leads to the disappearance of P2, and plastic deformation is then intragranular. It is shown that cerium additions stabilize the carbides at the grain boundaries. The penetration of oxygen along the grain boundaries, on the contrary, destabilizes the carbide precipitates and enhances grain boundary sliding.