Cellular precipitation in Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys

Abstract

The phases present in directionally solidified Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys are identified. These are the nickel-and chromium-rich lamellae, a Widmanstatten precipitation of nickel in the chromium-rich lamellae, grain boundary precipitation of chromium, and scattered discrete particles of chromium oxide. Upon annealing, the amount of the nickel precipitates increases drastically and a well defined cellular precipitation appears in the nickel-rich lamellae. The cellular precipitation conforms in almost every way to typical cellular reaction such as in the Pb-Sn system. Cellular (linear) growth rate, G, and interlamellar spacing, S, were measured on specimens annealed for times ranging from 1/2 to 100 hr at 625°, 700°, 760°, and 850°C. G increased from (average for both Ni-Cr alloys) 2.3 × 10−8 cm per sec at 625°C to 7.7 × 10−7 cm per sec at 760°C and decreased again at 850°C. S varied from 2 to 10 × 10−5 cm as the annealing temperature was increased. The calculated grain boundary diffusivity, DB, representing the diffusion of chromium in fcc Ni-Cr solid solution, increased from 6.7 × 10−11 sq cm per sec at 625°C to 8.6 × 10−8 sq cm per sec at 850°C. The activation energy 64 kcal per mole, is of the order of that obtained for self-diffusion.