Secondary carbide precipitation in a directionally solidified cobalt-base superalloy

摘要

A directionally solidified cobalt-basealloy, DZ40M, was solidified with a columnargrained austenitic matrix with a <001>preferential orientation and primary carbides ofchromium-rich M7C3 and MC at grain boundaries andinterdendritically. Secondary carbides in DZ40Malloy are chromium-rich M23C6 and tungsten-richM6C. The M23C6 carbide has a cube-cube orientationrelationship with the austenitic matrix. Initialprecipitation of secondary carbide, M23C6,occurred on dislocations in the austenitic matrixof the as-cast DZ40M alloy during cooling. Agingtreatment (100 to 1000 hours at 850℃) produced aprofusive precipitation of the M26C6 carbidemainly around the primary carbides. In theinterior of grains, M23C6 precipitatedpreferentially on dislocations and stacking faults.Subsequently, M23C6 grew into laths near theprimary carbides and coalesced into chains. Theprecipitation behavior of M23C6 can be explainedby the following reaction: 23M+6C→M23C6. Theprimary carbides are a carbon reservoir for theprecipitation of M23C6. The M6C carbide was foundonly on the surface of the primary M7C3 carbideadjacent to tungsten-rich MC in the aged condition.The precipitation of the tungsten-rich M6C isattributed to the tungsten segregation, whichresulted from decomposition of the tungsten-richMC and good lattice match between M6C and M7C3.The inhomogeneity of secondary precipitation isdue to the uneven distribution of the alloying elements.