Abstract:Effects of three dimensional direction of DD8 nickel base single crystal superalloy on its competitive grain growth were investigated by seed crystal method.Experimental results showed that〈100〉×〈100〉(the first direction parallel to temperature gradient direction, the second direction perpendicular to temperature gradient direction and the contact  plane of two half column seed crystal) grain competed with〈110〉×〈110〉grain for growth under the conditions ofG=250 K/cm,V=47.6μm/s, and the former eliminated the later; however at the same experimental conditions,〈100〉× 〈110〉grain competed with〈110〉×〈100〉grain for growth, and the later eliminated the former. Metallographic observations of cross sections of specimens showed that the migration velocity at various locations of grain boundaries was different in the process of competitive grain growth. It is indicated that the three dimensional direction of second dendrite is a key factor in the competitive grain growth, and the nonequilibrium grain boundary migration in the competitive grain growth is associated with the stagger between the grain dendrites.

Key words: second dendrite　nickel base superalloy　three dimensional direction　competitive grain growth