Grain Growth in Dilute Tungsten-Boron Alloys
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CLEVELAND OH LEWIS RESEARCH CENTER
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The grain-growth behavior of arc-melted tungsten and several binary tungsten-boron alloys 0.005 to 0.67 atomic per6ent B was studied in the tern peraurerange 36000 to 42000 F. The alloys were prepared by arc melting sintered electrodes of tungsten and elemental boron powders, followed by extrusion and swaging. Specimens cut from the swaged rod were recrystallized at 3200 F to insure that only grain growth would take place in the subsequent annealing treatments. The grain-growth kinetics were followed by measuring the decrease in grain-boundary surface area beta as a function of time at temperature. At 36000 and 38000 F, the decrease in beta obeyed a power law of the form beta kt-n, where t is time and n and k are constants. At 4000 and 4200 F, however, the power law was no longer obeyed because of the onset of discontinuous grain growth. In the latter case, discrete grains in the structure, termed secondaries, consumed the primary grain matrix resulting in the formation of a duplex structure. The incubation period for the onset of discontinuous grain growth increased with increasing boron content. Discontinous grain growth was postulated to be due to a combination of the dissolution of the tungsten boride phase that freed the pinned grain boundaries and the coalescence of two adjacent primary grains to form a large secondary grain.
- Organic Chemistry
- Properties of Metals and Alloys