Abstract
Alloys of iron, nickel, and cobalt containing 10 wt.-% alumina have been produced by the technique of mixing fine oxides of iron, nickel, and cobaIt (0·1μ particle dia.) with α-alumina (0·2μ particle dia.). The resultant mixtures were reduced in pure, dry hydrogen at f∼ 500°C. After reduction, the powders were quenched in liquid nitrogen to render them non-pyrophoric. The powders were then ball-milled, pressed into slugs, canned, and hot extruded into rod at between 700 and 800°C.
Homogeneous dispersions of essentially unchanged alumina were produced within the metal matrices. The coarsening of the alumina with increasing temperature and time was then followed by hardness measurements and by metallographic means, utilizing both optical and electron microscopy. The kinetics of coarsening was most easily followed in the iron-alumina alloy and so this system was studied in greater detail than the nickel-alumina or cobalt-alumina alloys.
The coarsening of the alumina particles in iron follows a (time)1\3 growth law, essentially that predicted by Lifshits and Slezov (Zhur. Eksper. Teoret. Fiziki, 1958, 35,479; Soviet Physics JETP, 1959, 35, 331) for the coarsening of particles in a supersaturated solution.
It is concluded that the growth of alumina particles in iron is accomplished by coalescence, the large grains growing at the expense of the smaller grains. It is implied that nickel-alumina and cobalt-alumina also show the same growth phenomenon. The ability of such apparently stable dispersion-hardened systems to overage could restrict their use in high-temperature applications, a field where it had been hoped that such alloys would excel.
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