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Effect of stress in chemical diffusion Behera, Saroj Kumar


Diffusion has been studied in a range of systems having intermediate phases in the diffusion zone. It has been found that in some systems (Ag-Sb, Ag-Se, Ni-Sb, Cu-Se and Cu-Sb) the diffusion rates are very sensitive to compressive stress, with a load of 100 psi making a significant difference to the width of the diffusion zone. In other systems (Cu-Zn, Cu-Sn and Al-Zr) stresses up to the maximum of 1500 psi had no effect on the diffusion rate. The growth rates of all phases in the pressure sensitive systems were found to be parabolic with time indicating diffusion control. In Cu-Se and Cu-Sb there was a nucleation time at the beginning of diffusion. However, growth of the phases in these systems was also found to be parabolic once this effect was accounted for. The effect of compressive stress was, generally, to increase the growth rate of one of the intermetallic phases. In Ag-Sb,- Ag-Se and Cu-Sb, there was a limiting stress above which growth rates of the intermediate phases were constant. Such a limiting stress was not observed in Ni-Sb and Cu-Se and the growth rates of the Ni_Sb_ and Cu^Se phases in these systems increased apparently linearly with applied stress. In experiments in which diffusion took place at low stress following an initial high stress anneal, it was generally found that the growth rate characteristic of the new stress was attained after long times of diffusion. In Cu-Se and Cu-Sb however, it was found that the stress-sensitive phases disappeared on ageing, although a finite growth was observed in the normal growth experiments. From the existing knowledge of diffusion theories, this particular phenomenon could not be explained. However, it is thought that this may possibly be due to a significant decrease in specific volume on formation of these phases. Non-appearance of certain stable phases predicted from the phase diagram has been attributed to their small diffusion coefficients. Hydrostatic tests were carried out to see if there was any difference in growth rates between uniaxial compressive stress and triaxial hydrostatic pressure. It was found in general that the growth rate under hydrostatic pressure was very similar to that for a compressive test of zero psi, indicating that applying a hydrostatic pressure does not have any effect on the growth rate and that only compressive loading is of any significance. All the stress-sensitive systems investigated showed a very large Kirkendall effect. The tungsten markers interfered with diffusion and the width of the diffusion zone adjacent to the markers was less than elsewhere. This gave rise to ledges of the pure metals with the tungsten wires being at the top of the ledges. The development of ledges was much greater in pressure sensitive systems than in other systems and could be attributed to slow lateral diffusion due to the lack of compressive stress in this direction. The experimental results can be explained on the basis of porosity which forms at a single interface in these systems owing to the large Kirkendall effect. This decreases the effective cross-sectional area of diffusion and so reduces the width of the diffusion zone. The effect of compressive stress is to decrease the amount of porosity and hence increase the effective interface area and the atomic flux into the diffusion zone. The limiting stress observed in Ag-Sb, Ag-Se and Cu-Sb was thus attributed to the complete absence of porosity in the diffusion zone. In Ni-Sb and Cu-Se it is believed that the pressure sensitive phases have very high growth rates and the maximum stress of 1500 psi was insufficient to obtain a good interface. All the other results can be explained satisfactorily by the mechanism suggested.

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