Recrystallization And Its Effect On Mechanical Properties Of DZ40M Superalloy

DZ40M (a directionally solidified cobalt-base superalloy) has been used to fabricate turbine vanes at a working temperature below 1050 oC. In this dissertation, recrystallization (RX) microstructures and its effect on mechanical properties of DZ40M alloy were investigated. The work mainly includes recrystallization behavior, effect of recrystallization on tensile properties and stress rupture properties, as well as thermal fatigue behavior.The DZ40M alloy was indented using a Brinell hardness tester with a load of 1500 kg. RX occurred at dendrite core after annealing at 1040 oC for 90 minutes. The RX grains were coarsening at 1150 oC and continuously growing with the increasing in annealing temperature. Fine M23C6 carbides were dissolved at the moving RX grain boundaries. Moreover, particles with larger size that can hardly be dissolved may prevent the moving boundary. Fully recrystallized microstructure under the indentation was observed when the annealing temperature is 1200 oC. At relatively low temperatures, the RX developed by changing its orientation through twinning in order to reduce the required driving force during growth. Volume fraction of twinning decreased with the rising of the annealing temperature. Large fraction of the M7C3 carbides were dissolved at 1280 oC, but it re-precipitated along the boundaries during air cooling. Maximum RX depth increased with the increasing in the indentation load and the annealing temperature from 1040 oC to 1280 oC.RX has little effect on the tensile strength of DZ40M alloy. For the specimens annealed at 1250 oC, ductility decreased as increasing the depth of RX. All samples were not broken at the RX site. Secondary necking in the tensile samples besides the fracture position has been observed. RX zone between the neckings shows much less deformation. It is believed that during the tensile test at high temperature, homogeneous deformation was inhibited by the M23C6 carbide which precipitated at RX grain boundaries.During the stress rupture test at 750 oC /330MPa, 900 oC /130MPa and 980 oC /83MPa, samples annealed at 1280 oC was easily fractured at the RX zone and the rupture life decreased sharply. The bonding strength between the RX grains was increased by the reduction of annealing temperature. At 650 oC /460 MPa, crack occurred at carbides first. In the recrystallized specimens annealed at 1280 oC, crack not only initiated at, but also propagated along the ribbon-like M7C3 carbide at transverse direction boundaries easily. When testing at 900 oC /130MPa, boundary between the RX region and the matrix was cracking severely. Due to the sharp reduction of the stress concentration at the crack tip when the crack was parallel to the stress axis, the crack can be stopped by the resistance of primary carbide and secondary M23C6 which located at the RX grain boundaries.When the maximum cyclic temperature was 1000 oC, the longer thermal fatigue cracks in the recrystallized specimens was observed when the notches are long the solidification axis. In these specimens, RX has less effect on the thermal fatigue property, because crack propagated easily along the carbides which aligned at the interdendritic regions. In samples with notches perpendicular to the solidification direction, crack propagated along the RX boundaries easily. Stress assisted grain boundary oxygen embrittlement at the crack-tip region was the predominant mechanism of the environmental degradation of the alloy.