Alloy Design and Microstructure Stability of Third Generation Ni-Based Single Crystal Superalloys
Ni-based single crystal superalloys are widely used in the manufacture of aero engines turbine blades and ground-based gas turbine blades because of their superior high-temperature comprehensive performance.In the development process of Ni-based single crystal superalloys,the degree of alloying is continually increased with higher content of refractory elements in order to improve the temperature capability and high temperature performance of the superalloys.The sum of amout of refractory elements in the alloys reaches up to 20 wt% and the content of Re is up to 5~6 wt% in typical third-generation single crystal superalloys.However,a great deal of refractory element addition makes more precipitation of the topologically close-packed(TCP)phases during thermal exposure or in the service,which leads to the decrease of microstructural stability and severely hurts service performance of alloy.The contradiction between improvement of high temperature performance and precipitation of TCP phase promoted by large of refractory elements additon is particularly serious in the third-generation and higher generation single crystal superalloys.In consequence,how to balance the high temperature performance and microstructural stability of alloy becomes one of very important and urgent problem for the third-generation single crystal superalloys.In this paper,in order to develop third-generation Ni-based single crystal superalloys with high-performance and high-microstructural stability,the effect and interaction of alloying elements on solidification characteristic,microstructure as-cast and with heat treatment,microstructure stability have been tried to make clear,the way that could improve microstructure stability in the system of alloys with high content of refractory elements has been explored.Thus,providing scientific basis for the composition design and optimization of advanced Ni-based single crystal superalloys.At first,a large number of calculations for physical parameters of 9240 kinds of alloys with the aid of thermodynamics calculation software JMatPro.Eighteen kinds of alloys which have different content of Re,W,Mo,Cr and Co with the total content of refractory elements between 19.4 wt% and 21.4 wt% has been selected.Then master alloy melting,single crystal sample preparation,heat treatment,thermal exposure had been processed in sequence for above alloys.On this basis,the effect and interaction of alloying elements of alloys have been researched from the analysis of solidification characteristic,microstructure as-cast,microstructure with heat treatment and thermal exposure.At last,one kind of alloy with excellent microstructure stability has been chosen for the test of high temperature stress-rupture property.The main conclusions are presented as follows:(1)After combining the published experimental results and calculation results of exist third generation Ni-based single crystal superalloys and self-designed single crystal superalloys that obtained by means of thermodynamics software JMatPro,the optimum ranges of alloying elements have been confirmed as: Co less than 16 wt%,Cr less than 6 wt%,Mo less than 3 wt%,Re less than 6 wt%,W less than 9 wt%,total content of W and Re less than 12 wt%,total content of refractory elements less than 23 wt%.(2)Re can strongly promote the solidification segregation of W,Al,Ta and Re itself,the segregations of Re and W to dendrite core and Al and Ta to interdendritic zone were dramatically increased.With increasing Co content,the segregations of Re and W to dendrite core and Ta to interdendritic zone were slightly increased,the segregations of Al had no change.With increasing Cr or W content,the segregations of alloying elements had no obvious change except Re.However,the increasing of Mo hadn’t affect the segregations of alloying elements.(3)The size of γ′ phase in alloy with full heat treatment was obviously decreased with increasing Re,comparatively obvious decreasing with increasing Cr or Mo,and slightly decreased with increasing Co.However,W hadn’t affect the size of γ′ phase.In alloys with the same total content of W and Re,the size of γ′ phase in alloy with higher W and lower Re content was larger while in alloy with lower W and higher Re content was smaller,which indicated that the size of γ′ phase would be increased in some degree after W instead of Re.(4)Thermal exposure experiments shown that the precipitation of TCP phase was dramatically increased with increasing Re or Mo content,and also increased in some degree with increasing Cr.W had not promoted the precipitation of TCP phase,but Co could effectively impede TCP phase precipitation.In alloys with the same total content of W and Re,the area fraction of TCP phase in alloy with higher Re and lower W content was larger while in alloy with lower Re and higher W content was smaller,which indicated that the area fraction of TCP phase would be decreased obviously after W instead of Re.(5)The results of experimental alloys after thermal exposure shown that TCP phase precipitated in the early period of thermal exposure was σ phase,but precipitated in later period was comparatively complicated,σ phase,μ phase and R phase were coexisted.According to the morphologies of TCP phase,acicular TCP phase was mostly σ phase,but rod-like and blocky TCP phases might be σ phase,μ phase or R phase.(6)For one optimized model alloy with 4 wt% Re content,no TCP phase precipitated after thermal exposure at 1000 °C for 1000 h,meanwhile,the stress-rupture life reached up to 100 h in the condition of 1100 °C/150 MPa.