Internal friction associated with grain boundaries in NiCr alloys

Abstract

The internal friction spectrum of Ni-20at.%Cr polycrystals is mainly characterized by two relaxation peaks: P1 at about 950 K upon heating; P3 peak at about 1100 K upon cooling, respectively, and a large hysteresis in the peak P3 temperature range. Under some experimental conditions, another peak (P2) is also observed during heating in the temperature range higher than peak P1. In an Ni-20at.%Cr single crystal, only peak P1 is observed during heating and cooling. However, when the single crystal is recrystallized, an internal friction spectrum identical to that of the polycrystal is observed. Furthermore, metallographical observations show that the large hysteresis in internal friction is related to the presence of discrete carbide precipitates at grain boundaries. When the grain boundary is free of carbide precipitates, only peak P3 is observed during the heating and cooling run. In contrast, when the grain boundary contains a continuous distribution of carbide precipitates, only peak P1 is observed during the heating and cooling run. This experimental evidence leads us to attribute peak P3 to grain boundary relaxation, which is affected by the carbide precipitates, and peak P1 to a relaxation process taking place in the grains. The hysteresis results from the dissolution-precipitation processes of grain boundary carbides.