INTERNAL-FRICTION ASSOCIATED WITH GRAIN-BOUNDARIES IN NI-CR ALLOYS

The internal friction spectrum of Ni-20at.%Cr polycrystals is mainly c haracterized by two relaxation peaks: P1 at about 950 K upon heating; P3 peak at about 1100 K upon cooling, respectively, and a large hyster esis in the peak P3 temperature range. Under some experimental conditi ons, another peak (P2) is also observed during heating in the temperat ure range higher than peak P1. In an Ni-20at.%Cr single crystal, only peak P1 is observed during heating and cooling. However, when the sing le 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 re lated to the presence of discrete carbide precipitates at grain bounda ries. When the grain boundary is free of carbide precipitates, only pe ak P3 is observed during the heating and cooling run. In contrast, whe n the grain boundary contains a continuous distribution of carbide pre cipitates, only peak P1 is observed during the heating and cooling run . This experimental evidence leads us to attribute peak P3 to grain bo undary relaxation, which is affected by the carbide precipitates, and peak P1 to a relaxation process taking place in the grains. The hyster esis results from the dissolution-precipitation processes of grain bou ndary carbides.