Solidification structure of undercooled Ni54.6Pd45.4 alloy

Ni54.6Pd45.4, a solid solution alloy without crystallization temperature range, has been undercooled by glass flux method up to the hypercooling limit. It was found that it solidified as pure metals. As undercooling increases, the secondary dendrite arm spacing decreases monotonously until a undercooling of 135 K, above which the secondary branches considerably degenerate. The solidification with undercoolings higher than 160 K leads to grain-refined structures. In this case, their average grain sizes continuously decrease with increasing undercooling even if the alloy has been hypercooled. The fact that grain refinement still takes place in the solidification of the hypercooled alloy indicates that the solidification time after recalescence is not a factor to dominate the grain refinement at large undercooling. Recrystallization of the solid in the post-solidification period should be responsible for the grain refinement.