Effect Of Substituting Mo For W On High Temperature Creep Properties Of Second Generation Ni Based Single Crystal Superalloy

At present,Ni based single crystal superalloys have been widely used to prepare key hot-junction components（single crystal turbine blades）in aerospace engines.In order to further improve its high temperature mechanical properties,a large number of refractory elements,e.g.Re and W elements were added to the advanced single crystal superalloys,which significantly increased the density of single crystal superalloys.However,when turbine blades rotate at high speed,the high density of single crystal superalloys will lead to large centrifugal force,and thus high density had became one of the bottlenecks in the development of high-generation single crystal superalloys.Meanwhile,single crystal superalloys undergone slow creep deformation at high temperature.Therefore,high temperature creep performance of single crystal superalloys was an important factor to determine its temperature bearing capacity.In summary,it is necessary to develop low density single crystal superalloys with excellent high temperature creep properties.As compared with W element(19.25 g·cm^-3),Mo element(10.28 g·cm^-3)had the characteristics of lower density.Meanwhile,both W and Mo were typicalγphase strengthening elements,while Mo was more likely to be distributed inγphase,and thus it had stronger solid solution strengthening effect onγphase,and Mo addition could effectively decreasedγ/γ’misfit andγ/γ’interfacical dislocation spacing.In summary,substituting Mo for W is expected to achieve the composition design of single crystal superalloys with low density and excellent high temperature creep properties.In this paper,W-rich second-generation single crystal superalloy Alloy W and Mo-rich second-generation single crystal superalloy Alloy Mo were taken as the research objects,and the influence of substituting Mo for W on 1100℃and 1150℃high temperature creep properties of single crystal superalloy was analyzed.Meanwhile,the main controlling factors of 1100℃/130 MPa and 1150℃/100 MPa high temperature creep properties were clarified.The main research results are as follows:（1）Substituting Mo for W increased creep rupture life of 1100℃/130 MPa from 147.07h to 332.08 h,while substituting Mo for W only increased creep rupture life of 1150℃/100MPa from 80.74 h to 102.76 h.The results showed that the beneficial effect of substituting Mo for W on creep properties obviously decreased with increasing temperature.（2）In-situ X-ray diffraction analysis of Alloy W and Alloy Mo at 1100℃and 1150℃showed that both alloys obtained negativeγ/γ’misfit,and substituting Mo for W effectively reducedγ/γ’misfit.At the same time,γ/γ’misfit of Alloy W and Alloy Mo at 1100℃were more negative than that at 1150℃,which was due to the significant remelting ofγ’phase at higher temperature（1150℃）.（3）Due to substituting Mo for W significantly reducedγ/γ’misfit of single crystal superalloy,a denserγ/γ’interface dislocation network was formed after creep rupture at both1100℃/130 MPa and 1150℃/100 MPa,which hindered the movement of dislocations and was beneficial to improvement of high temperature creep strength of single crystal superalloy.（4）After 1100℃/130 MPa creep rupture,volume fraction ofγ’phase in Alloy W was much higher than the optimalγ’phase volume fraction（～70%）,which reduced the total number ofγ/γ’interfaces,and thus it was not conducive to the improvement of high temperature creep life.However,substituting Mo for W reducedγ’solvus temperature of single crystal superalloys,resulting in relatively low volume fraction ofγ’phase in Alloy Mo after creep rupture at 1150℃/100 MPa,which reduced the total number ofγ/γ’interfaces and strengthening effect ofγ/γ’interfaces.Therefore,the favorable influence of more negativeγ/γ’misfit on high temperature creep performance was weakened,resulting in limited improvement of 1150℃creep life by substituting Mo for W.（5）After creep rupture at 1100℃/130 MPa and 1150℃/100 MPa,substituting Mo for W reduced the thickness ofγ’phase raft and improved the integrity ofγ/γ’raft.At the same time,substituting Mo for W reduced the maximum diameter and area fraction of holes in both solution treated sample and the total area fraction of holes in creep rupture sample.The effects of substituting Mo for W onγ/γ’raft and holes were beneficial to improve high temperature creep properties of single crystal superalloys.（6）After creep rupture at 1100℃/130 MPa,the topological inversion ofγ/γ’phase occurred in both Alloy W and Alloy Mo.Meanwhile,substituting Mo for W reduced the topological inversion ofγ/γ’phase,and thus it was beneficial to improvement of high temperature creep property.After creep rupture at 1150℃/100 MPa,the obviousγ/γ’topological inversion occurred in Alloy W,but the topological inversion did not occur in Alloy Mo,which was caused by relatively low volume fraction ofγ’phase in Alloy Mo at1150℃.（7）Under the creep conditions of 1100℃/130 MPa and 1150℃/100 MPa,substituting Mo for W improved the anti phase boundary energy ofγ’phase,which further increased the difficulty of dislocation cutting intoγ’phase,and thus improved high temperature creep resistance of alloy,which was beneficial to improve the high temperature creep performance of single crystal superalloy.（8）Comprehensive analysis of the above results indicated thatγ/γ’misfit was the main controlling factor for high temperature creep properties of single crystal superalloys at1100℃/130 MPa,while under the creep condition of 1150℃/100 MPa,the combined action ofγ/γ’misfit and volume fraction ofγ’phase affected the high temperature creep properties of single crystal superalloys.