Microstructure And Creep Behavior Of A 4.5 %Re/3 %Ru Nickel Based Single Crystal Superalloy

In this paper, a 4.5 %/3 %Ru containing single crystal nickel-base superalloy was designed, prepared, and heat treated under two different heat treatment regimes 1 and 2. By means of creep-property tests, microstructure observation and analysis by SEM and TEM, dislocation confifuration analysis and composition analysis by SEM/EDS, the influence of heat treatment on the elements segregation extent and creep properties of the alloy, and the deformation, strengthening and fracture mechanisms of the crept alloy were investigated.The results show that, in the as-cast alloy with severe elements segregation, the elements(Cr、Mo、W、Re)/(Al、Ta、Co、Ru) are enriched in the dendrite/inter-dendrite regions, respectively. Wherein, the strongest positive/negative segregation element are(Ru、Al)/Re, respectively. As the solution temperature rises and the solution time is prolonged, the segregation extent of the elements in the dendrite/inter-dendrite regions of the alloy is obviously reduced, and the creep resistance of it at elevated and intermediate temperatures is obviously improved. After full heat treated by regime 2, the alloy displays a better creep resistance and longer creep lifetime in the temperature ranges of 1040 ~ 1115 oC, and the activation energies and stress exponents of the alloy during steady-state creep at elevated and intermediate temperatures are measured to be QH = 493.7 kJ/mol, nH = 4.0 and QM = 576.32 kJ/mol, nM = 12.29, respectively.During intermediate-temperature creep, the ?? phase in alloy still remains the cuboidal morphology. During steady-state creep, the deformation mechanism of the alloy is dislocations slipping in ? matrix and shearing into the cuboidal ?? phase. At the latter stage of creep, the alternative activation of primary/secondary slipping systems leads to the initiation and propagation of cracks at ??/??interfaces up to the fracture of it, which is thought to be the main fracture mechanism of the alloy during creep at intermediate temperatures. During the primary stage of creep at elevated temperatures, the ?? phase in the alloy have been rafted. And the deformation mechanism of the alloy during steady-state creep is identified to be dislocations slipping in ? matrix and climbing over the ?? rafts, while that of it at the latter stage of creep is the dislocations shearing into the rafted ?? rafts,and it can decomposeed to form K-W locks with non-plane structure and the configuration of partial dislocations plus APB, which can restrain the the slipping and cross slipping of other dislocations, and these are thought to be the main reasons of the alloy having the higher creep life time at elevated temperature. At the latter stage of creep at elevated temperatures, the alternated slipping of significant amount of dislocations leads to the initiation and propagation of cracks at ??/??interfaces up to the fracture of the alloy, which is thought to be the fracture mechanism of the alloy during creep at elevated temperatures.