The Mechanical Properties And Damage Behavior Of Dual Structure Superalloys At Intermediate Temperature

Blisk,an integral turbine of blades and disk,has been widely used in small gas turbines.To meet the demand of the creep properties of blades as well as the low cycle fatigue properties of disk,the dual structure blisk with directionally solidified(DS)blades and fine-grain disk is designed.However,there is a mixed grain structure between columnar-grain blade and fine-grain disk,which has significant effects on the performance of the dual structure blisk.Moreover,there are large differences in heat treatments,strength and ductility of fine-grain and columnar-grain superalloys.Therefore,the aim of the present paper is to optimise the heat treatment of the dual structure superalloys,and clarify the deformation and fracture behavior of dual structure superalloys at intermediate temperature.In this dissertation,effect of heat treatment on the microstructures and stress-rupture properties of the K417G fine-grain,coarse-grain and columnar-grain superalloys,and the tensile,creep and low cycle fatigue properties of dual structure superalloys at 600? and 760? were systematically investigated.Scanning electron microscopy(SEM)equipped with an energy dispersive spectroscopy(EDS),transmission electron microscope(TEM),X-ray computed tomography(XCT),electro-probe microanalyzer(EPMA)and electron back scattered diffraction(EBSD)were used to characterize the as-cast and heat treated microstructures of the K417G superalloys,the morphology of dislocations,the information of micro-pores in three dimensions and to identify the chemical compositions of alloy elements.The mechanisms were discussed in detail.The coarsening of ?' phase in fine-grain,coarse-grain and columnar-grain K417G superalloys after hot isostatic pressing(HIP)was observed.The coarse y' dissolved partly during further solution at 1120? and secondary fine ?' formed in the y matrix channels during air cooling.All coarse ?' dissolved in fine-grain superalloys during solution at 1230?,but only partially dissolved in coarse-grain and columnar-grain superalloys.In addition,transition of grain boundary(GB)morphology from wavy to flat during solution at 1230? was observed.Interestingly the volume fraction of micro-pores increased during high temperature solution heat treatment.The creep properties of fine-grain,coarse-grain and columnar-grain superalloys are mainly influenced by on ?' and GB microstructures.After high temperature solution heat treatment at 1230?,950?/235MPa creep deformation of fine-grain specimens was controlled by the grain boundary sliding.Comparing to that of the as-HIPed specimens,stress-rupture properties of fine-grain specimens increase slightly,but lower than that of coarse-grain specimens.Creep deformation of columnar-grain specimens was controlled by the dislocation bypass of the ?'.The stress-ruptrue properties of columnar-grain specimens increased significantly after solution heat treatment,although residual coarse y' were observed in the specimens.At 760? and 645MPa,creep deformation was controlled by the dislocation shearing of the ?'.The stress-ruptrue properties of columnar-grain specimens increased after high temperature solution heat treatment for the precipitation of fine ?'.However,the stress-ruptrue properties of fine-grain and coarse-grain specimens decreased significantly,due to the formation of flat GB induced by the high temperature solution.After solution heat treatment at 1120?,stress-rupture properties of fine-grain,coarse-grain and columnar-grain specimens increased at 950? and 235MPa.Stress-rupture properties of columnar-grain specimens at 760? and 645MPa increased significantly,while stress-rupture properties of fine-grain and coarse-grain specimens decreased slightly.To blance the stress-rupture properties of columnar-grain and fine-grain structure at intermediate and high temperature,the heat treatment of 1120? is used for the dual structure superalloys.Dual structure samples solidified by directional solidification process consist of fine grain zone,thermal affected zone,pseudo columnar grain zone and columnar grain zone.The pseudo columnar grains are misoriented from<001>direction.During transition from fine grains to columnar grains,morphology of MC carbides changed from blocky to script-like,and the size and volume fraction of micro-pores decreased.At 600? the tensile strength of dual structure specimens is slightly lower than that of the columnar-grain specimens.Fracture of dual structure specimens occurs near the columnar-grain region.At 760? the tensile strength of dual structure specimens was slightly lower than that of the fine-grain specimens.The fracture of dual structure specimens occured near the fine-grain region.The different deformation behaviors of fine-grain and columnar-grain region in dual structure specimens were observed.The deformation incompatibility resulted in the stress concentration near the transition zone.However,the mixed grain structures in transition zone avoided the abrupt change from fine grain to columnar grain,and the reduction of tensile ductility of dual structure superalloys at 600? and 760? was not observed.Creep life of columnar-grain specimens at 600? and 961 MPa was markly lower than that of fine-grain specimens.The creep life of dual structure specimens was lower than that of columnar grain specimens,and the fracture of dual structure specimens occured near the columnar-grain region.The creep deformation was dominated by intragranular deformation at 600? and 961 MPa.The degree of local palstic deformation of columnar-grain zone was larger than that of fine-grain zone for the low modulus of columnar grains.Moreover,rotation direction of columnar grains was parallel to the<001>direction,while rotation direction of fine grains was random.Creep life of fine-grain specimens at 760? and 645MPa was lower than that of columnar-grain specimens.The creep life of dual structure specimens was comparable to that of the fine-grain specimens,and the fracture of dual structure specimens occured near the fine-grain region.At 760? and 645MPa,grain boundary sliding was the main deformation mode.The intergranular fracture was dominated in the fine-grain zone.The dual structure specimens exhibited cyclic strain hardening during low cycle fatigue(LCF)deformation at 600? with 0.4%,0.5%and 0.6%total strain amplitudes.With increasing total strain amplitude,the number of cycles to failure decreased gradually,and the transition of fracture site from columnar-grain zone to fine-grain zone was observed.In the low strain amplitude,LCF life was mainly controlled by the elastic strain.Compared to the fine-grain zone,the columnar-grain zone exhibited a large number of slip bands and strong intrusion and extrusion due to the lower elasitic modulus.In the high strain amplitude,LCF life was mainly controlled by the plastic strain.Slip bands were observed in both fine-grain region and columnar-grain region,but cracks preferentially initiated at grain boundary due to the large local deformation imcompatibility.A preliminary heat treatment of the dual structure superalloys is optimised in present work to blance the creep properties at high and intermediate temperature.The deformation imcompatibility of fine-grain region and columnar-grain region results in the lower strength of dual structure specimens.However,the mixed grain microstructures in transition zone provide the reasonable tensile ductility of dual structure superalloys.