Recrystallization Behavior of a Directionally Solidified Co-base Superalloy DZ40M
|Keywords||recrystallization DZ40M Co-base directionalloy solidification superalloy compressive deformation shot peening carbide low cycle fatigue|
DZ40M alloy is a directionally solidified cobalt-base superalloy, which was independent developed in China. It is mainly applied for countervane materials and the countervane bears some stresses during foundry, countervane manufacture crafts (shot penning, correct shape, burnish, grinding) and service conditions. These stresses will possibly lead the appearance of recrystallization phenomenon. Transverse grain boundary would become the cracking initial point to lead the failure. Recently, some reports for the recrystallization behavior of nickel-base superalloy have been made, while a few researches have been repoted for the directionally solidified cobalt-base superalloy. The purpose of the present work is to investigate the recrystallization behavior of DZ40M alloy on both the theoretic and pratical application to control the recrystallization of DZ40M alloy.Indentation method was used to investigate the phenomenological behavior of recrystallization in DZ40M alloy. The results show that recrystallization nucleate near the primary carbides and at dendrite core. Both the recrystal grains size and the depth of the recrystallization increase with increasing annealing temperature, and there are strong increasing tendency when the annealing temperature is higher than 1473K, because of the decreasing of the pinning force of M23C6.For the compressive deformed DZ40M alloy, the recrystal grains initiation around the primary carbides, especially at the joint of primary carbides and secondary carbides. In another word, the growth of recrystal grain may be restricted by dendrites, but the grain boundary of recrystallization could break the restriction of dendrites when the dendrite is dissolved. With the growth of recrystal grain, the grain boundaries migrate and combine into larger grain. With the increasing of compressive loading, the incubation time of recrystallization becomes shorter and final area fraction of recrystallization becomes larger. The kinetics of recrystallization in DZ40M is agreed with the JMAK equation.The effects of shot peening on recrystallization in directionally solidified Co-base superalloy DZ40M were studied. It was found that shot peening process generates a plastic deformation layer, and it leads surface recrystallzation in the surface region of DZ40M alloy. A surface recrystallization layer occurred after annealing treatment. Annealing treatment at 823 K could relax the residual stress, so than to reduce the driving force of recrystallization.The recrystallization of DZ40M alloy was also influened by the carbides. Recrystal grains could nucleate near the primary carbides, but it could be restrained by the secondary carbides near the primary carbides. The migration of recrystal grain boundary is pinned by secondary carbides. Only few of coarsen carbides far from primary carbides can be broken through by recrystal grain boundary. When the annealing temperature is lower than 1473K, the precipitation occurs before recrystallization, and the amount of secondary precipitation is higher, size is smaller and spacing is shorter. In such case, the secondary carbides could effectively retard the recrystallization in DZ40M alloy.The effects of recrystallization on low cycle fatigue behavior of DZ40M alloy are also investigated. The mechanical testing results demonstrated that the low cycle fatigue property of DZ40M significantly decreased with the partly recrystallization. Fatigue cracks initiate near the carbides and the grain boundaries with slid-bands. Both the fatigue crack initial and propagation can be accelerated with the occurrences of recrystallized grain boundaries.The recrystallization nucleation in DZ40M alloy can be retard by electric-field treatment. The recrystal grain growth is easy in the electric-field treatment than in the normal annealing treatment. It is suggested that the recrystallization process could be controlled by the electric-field treatment when the treating temperature is below 1473K.