The influence of disperse alumina inclusions on some characteristics of sintered nickel

Conclusions

• 1. Curves of the relative density of Ni-Al₂O₃ specimens sintered in hydrogen at temperatures above 1475°K, plotted against aluminum oxide content, have maxima which correspond to 5% Al₂O₃ in the case of slow heating and 15% Al₂O₃ in the case of rapid heating. The presence of these maxima is attributed to the fact that, on the one hand, aluminum oxide promotes shrinkage by facilitating the removal of the water vapors formed during the reduction of oxides remaining in the powder; on the other hand, at high Al₂O₃ contents, shrinkage is reduced owing to disrupted nickel particle contacts.

• 2. The experimental curve of shrinkage vs. aluminum oxide content for specimens sintered in hydrogen at temperatures exceeding 1475°K lies much above the curve calculated with the aid of B. Ya. Pines' formula, which is also ascribed to facilitated removal of water vapors formed during the reduction of residual nickel oxides, from the compacts during sintering.

• 3. Aluminum oxide inclusions prevent the growth of nickel grains during sintering at temperatures up to 1475°K (for compositions with 5 and 10% Al₂O₃) and up to 1525°K (for compositions with 15 and 20% Al₂O₃).

• 4. Increasing the amount of aluminum oxide is accompanied by a decrease of submicrograin sizes attained during sintering.

• 5. Specimens sintered in hydrogen are characterized by the presence of microdistortions in the nickel lattice. The greater the density of the sintered specimens, the greater the microdistortions. The microdistortions are due to the evolution of hydrogen from nickel during cooling after sintering, brought about by the decreased solubility of hydrogen in nickel.