Evaluation of surface-diffusion coefficients from grain-boundary grooving data on Mo,Mo—Re,and Cr

The grain-boundary grooving (GBG) technique has been widely used to obtain surface-diffusion coefficients of metals since this method offers great experimental simplicity in companson to methods such as sinusoidal profile decay. However, the evaluation of grain-boundary groove data has always posed a problem: in many cases, it is found that the same experimental data can be analysed to obtain values for D_s or D_v by assuming only one of the two diffusion mechanisms to be solely responsible for matter transport. The values of D_s and D_v thus obtained are in good agreement with those determined by other techniques. Some workers have used parameters such as the ratio of groove height to groove depth as criteria to determine the predominant transport mechanisms. However, it has been shown by Robertson that the height/depth ratio for grooving under surface-diffusion conditions can approach that for volume-diffusion conditions if the slope of the groove at the root is large. Therefore, such parameters cannot be used to decide on the transport mechanism. In addition, mesurement of groove height or depth by Interference microscopy is not very accurate. Consequently, the conclusions based on GBG experiments are misleading in many instances. The purpose of this work is to analyse some of the data available in the literature where it is felt that the conclusions of the previous worker have to be revised.