Effect Of Surface And Grain Boundary On Cellular Transformation In Ni-based Single Crystal Superalloys Containing Re

At present,Ni-based single crystal superalloys are used for high pressure turbine blades of advanced aero engines due to their excellent high temperature mechanical properties.The cellular transformation could be induced easily by low angle grain boundaries and surface plastic deformation(such as lathing,shot peening and grit blasting)at the service environment.The service safety of turbine blades is damaged by such microstructural degradation.Therefore,the study for the surface processing and grain boundary configuration on microstructural evolution of cellular transformation in Ni-based single crystal superalloys becomes very important.In this study,the Re-containing Ni-base single crystal superalloys SXG3(3rd generation)and CMSX-4(2nd generation)were surface processed and heat treated.The effect of surface cellular colony on stress rupture life at elevated temperature as well as the relationship between surface cellular transformation and plastic deformation anisotropy in single crystal superalloys was analyzed.Besides,with the bonding technique of Re-containing single crystal superalloys,the bi-crystals of SXG3 and CMSX-4 single crystal superalloys with certain grain boundary configurations were fabricated.The effect of grain boundary plane and misorientation on cellular transformation was investigated quantitatively.The surface cellular transformation of SXG3 alloy was induced by regular surface processing after heat treatment at 1100 ? for 200 h.After grit blasting by 0.3 MPa/1 min and 0.5 MPa/2 min,and then subsequent heat treatment at 1100 ?for 200 h,the width of surface cellular colony in SXG3 alloy was 18 and 49 m,which covered 4%and 11%cross-section area in the sheet specimen with 2.5 mm in length and 1.5 mm in width,respectively.The corresponding stress rupture lives of SXG3 alloy at 980 ? and 250 MPa were reduced by 30%and 70%,respectively.As the width of surface cellular colony was increased,the effective bearing area was reduced and the extended depth of surface cracks was enhanced,which led to the decrease of the stress rupture life.in SXG3 alloy.For the turning + grid blasting specimen,the cyclic variation of the deformation width showed as follows:the maximum at[110]orientation and the minimum at[010]orientation on(001)plane.Whereas,the deformation width of grid blasting specimen was nearly the same under different orientations.After heat treatment at 1100 ? for 200h,the width change of cellular colony was in accordance with the deformation for both turning + grid blasting specimen and grid blasting specimen.It is suggested that this phenomenon was associated with the anisotropy of yield strength,and the appearance of the anisotropic width of cellular colony occurred when the surface plastic deformation was over the critical limit.The SXG3 and CMSX-4 bi-crystal superalloys were achieved by transient liquid phase(TLP)bonding and the self-diffusion bonding techniques.The bonding process was carried out at 1290 ?/10-3 Pa for 12-24 h.The grain boundaries of these bi-crystals were planar and the configurations of grain boundaries were controlled precisely.Since there were no carbides precipitated at grain boundary,the self-diffusion bonding technique was the ideal approach for producing bi-crystal superalloys with the planar grain boundary and certain configurations.The study of microstructures along grain boundary in SXG3 bi-crystal indicated that the threshold of grain boundary misorientation for cellular transformation in SXG3 alloy was in the range of 20?25°.The cellular transformation occurred at grain boundary when the misorientation was larger than 25°,and ?' film enveloped with TCP phases was formed at grain boundary when the misorientation was less than 20°.The cellular colony grew along[010]orientation in(010)-based grain boundary,grew along both sides in(110)-based grain boundary.The discrepancy of growth direction in cellular colony at grain boundary with different base planes was associated with the anisotropic mobility of the crystal plane.After heat treatment at 950?1150 ?,the growth rate of cellular colonies in a series of grain boundaries with different misorientations and base planes were similar.The growth of cellular colony was influenced by the diffusion coefficients of Re and W in SXG3 alloy at 950?1150 ?.The appearance activation energy of the cellular transformation was 260 kJ/mol in grain boundary of SXG3 alloy.The research for cellular transformation on hybrid grain boundary of CMSX-4 and SXG3 alloys indicated that the cellular transformation only occurred in the SXG3 alloy with high supersaturation of the refractory alloying elements.The supersaturation of Re at CMSX-4 and SXG3 hybrid grain boundary was reduced by the self-diffusion of alloying elements,which retarded the growth of cellular colony after heat treatment for 200-400h.This study provides the theoretical and experimental basis for designing the surface processing of the Re-containing Ni-base single crystal superalloys and for optimizing the composition of the blade material.It also provides the physical metallurgy basis for the development for the inspection standard of single crystal blades with low angle grain boundaries as well as the assembled turbine blades.Besides,these results have theoretical significance for the development of high stability alloys used in the single crystal blades of aero engines or land-base gas turbines.