Recrystallization (RX) behavior of CMSX-4 and DZ125L alloy was studied by scanning electronic microscope (SEM), electron backscattered diffraction (EBSD) and electron probe microanalyzer (EPMA). RX nucleation mechanisms and grain growth mechanisms were investigated. The effect of microstructure and other factors on RX was analyzed. The RX in DZ4 blades was also observed. Experimental simulation of the RX tendency in DZ4 and DZ125L alloy was also carried out using Gleeble machine.
After local indentation and heat treatment at temperatures near the solution temperature, RX took place in all the samples of CMSX-4 and DZ125L. RX occurred partially in the samples heat treated at lower temperature, while full RX occurred at higher temperatures. Large RX grains could be observed when annealed at temperatures much higher than the solution temperature. RX was a diffusion controlled process according to the RX kinetics curves of both alloys.
Different RX nucleation mechanisms operate at different temperatures: RX nucleation at dendrite core regions is the dominant nucleation mechanism at high temperatures. Dendrite core nucleation and interdendritic nucleation operate simultaneously at medium high temperature. The interdendritic nucleation was induced by particle (carbides, /¢ eutectics, porosities et. al.) stimulated nucleation (PSN) mechanism. At lower temperatures, RX nucleation by PSN mechanism in interdendritic regions plays an important role.
During RX grain growth, the interaction of secondary phase particles (/¢ eutectics, carbides) with RX grain boundary is mainly affected by temperature and deformation. In CMSX-4, RX grain boundary was pinned by /¢ eutectics, or migrated through /¢ eutectics with a large ¢ precipitated behind the boundary. In DZ125L, RX grain boundary was pinned by carbides by different types of interactions: (1) RX grain boundary could cut through or pass by the carbides; (2) in the areas with high level of deformation, scripta-like carbides could be dissolved at the RX grain boundary when annealing at higher temperatures.
It was found that less RX happened with increase of primary dendrite arm spacing (PDAS) at the same level of deformation and same process of heat treatment. In DZ125L alloy, RX tendency was increased when the carbon content increased, and a maximum RX depth was observed at 0.096 wt% carbon. Depth of RX didn’t change much with further increase in carbon content. Contrary to that of the DZ125L alloy, less RX occurred when 300 ppm carbon was added to CMSX-4.
Microstructure evolution of surface RX and local RX during 980 oC/235 MPa creep test was investigated. It was found that: (1) No obvious change occurred in surface RX. (2) In specimens containing local RX, precipitate free zones (PFZs) were formed during creep along the RX grain boundary that vertical to the applied stress. Crack initiated at PFZ during creep. The detrimental effect of local RX on mechanical properties was probably due to the formation of PFZ.
Finally, RX in DZ4 blades was observed. It was found that most of the RX was very large, with the shape of crescent and mainly located at the lower part of the leading and trailing eage, near the root section of the blades. There are two types of RX in the blades, RX areas with single or only several large RX grains and RX areas containing large number of small RX grains. The RX in DZ4 blades was probably happened before service based on the microstructural analysis.
Based on the results obtained from Gleeble simulation of the directional solidification (DS) process, machining and service, it can be concluded that plastic deformation could be induced during processing of the blade, and RX could be induced during the following high temperature annealing.
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