Creep Behavior And Effect Factors Of Long-life DZ125Ni-base Superalloys

By means of heat treatments at different regimes, creep property measurement andmicrostructure observation, the influence of the heat treatment on the microstructure and creepperformance of DZ125nickel-based superalloy has been investigated. By thermodynamiccalculations, the rafting time of phase has been measured and predicted. And the strainextent of lattices for the?/double phase alloy at different states has been studied by means ofthe calculation of the lattice mismatch. The deformation and fracture mechanisms of the alloyduring creep are studied by microstyructue observation and contrast analysis od dislocationconfiguration. Some main conclusions have be obtained given as follows:The microstructure of the as-cast DZ125nickel-base superalloy is mainly composed of matrix,?phase, eutectic and carbides. And the obvious elements segregation and nonuniformof?and phases in size exist in the dendrite arm and inter-dendrite regions. After the alloly isfull heat treated, the segregation extent of refractory elements between the dendritic arm/interdendritic regions and the lattice mismatch of/phases decrease. But the obviousdifference of?phase in size still exists in the dendritic arm and inter-dendritic regions, thefine cuboidal?precipitates is uniformly distributed in the dendritic regions, while the coarseones are distributed in the interdendritic regions. And the block-like carbides and radial ormesh-like eutectic microstructure distribute in the inter-dendritic regions.During the creep at intermediate temperatures, the?phase in the alloy can not transforminto the rafted structure. While during creep at high temperatures, the cuboidal?phase in thealloy are transformed into the rafted structure along the direction perpendicular to the stressaxis. After crept for3h at1040°C/137MPa, the?phase in the alloy is transformed into theN-type rafted structure. The diffusion migration rate of the elements in the alloy at varioustemperatures can be calculated, by means of thermodynamic calculations, to forecast therafting time of the?phase in the alloy at different conditions. It is indicated according to thecalculation that the rafting time of?-phase prolongs as the creep temperatures decrease.Furthermore, the needed times of the?phase in the alloy during creep at840and760?arecalculated to be400hand3000h, respectively. The deformation mechanism of the alloy during creep at intermediate temperature is thedislocations slipping in the matrix and shearing into phase. Thereinto, the dislocationsshearing into the phase may be decomposed to form the configuration of two Shockleypartial dislocations plus stacking faults. Moreover, the super-dislocations shearing into phase may cross-slip from {111} plane to (100) plane to form the configuration of K-Wdislocation locking, which can effectively hinder dislocation slipping on {111} plane toimprove the creep resistance of alloy. Undef the conditions of high temperature and lowerstress, the dislocations slipping in the matrix and climbing over the rafted phase is thoughtto be the deformation mechanism of the alloy during the steady state creep. Thereinto, duringthe dislocations climbing, the dislocation jogs are easily formed, and the formation anddiffusion of vacancies are the control links for dislocation climbing. At the latter stage of creep,the deformation mechanism of the alloy is the dislocations slipping in matrix and shearinginto the phase. During creep, the hexagonal and quadrilateral dislocations networks locatedat the/interfaces can release mismatch stress of the lattice and delay the stressconcentration to improve the creep resistance of the alloy.In the latter stage of creep at high termperature, the cracks in the alloy are firstly initiatedand propagated along the grain boundaries, and the various damage features display in thegrain boundary regions with different morphologies. Thereinto, the grain boundaries beingabout45angles relative to the stress axis support the bigger shear stress, which is the mainreason for promoting the occurrence of creep damage. The addition of Hf element maypromote the precipitation of the fine carbides along grain boundaries to restrain the boundariessliding, which may enhance the bonding strength of the grain boundaries. This is the mainreason for grain boundaries displaying the non-smooth surfaces after creep rupture of thealloy.Compared to the conventional heat treatment regime, when the solution temperatureenhances to1260°C, the segregation extent of refractory elements between the dendritic/inter-dendritic regions and misfits of/phases decreases obviously, and the coarse phasein the inter-dendritic regions may be completely dissolved. After aging treatment, the fine precipitates with high volume fraction are dispersedly distributed in the dendrite andinter-dendrite regions, and the eutectic structure can be completely eliminated. Moreover, theoriginal blocky-like carbides in the alloy can be decomposed, and the fine particle-like carbides can precipitate along the boundaries to inhibit boundary slipping. Consequently,compared to the conventional heat treatment regime, the high-temperature solution treatmentcan improve the homogeneity of the microstructure in the alloy, which may enhance the creepresistance to prolong the creep life of the alloy.