Study on the Solution Heat Treatment and Processing Optimization of Third Generation Ni-base Single Crystal Superalloys
Author:Zhang Zuo Bin
Ni-base single crystal superalloys are widely used in the key components of the aero engine and the land-based gas turbines due to their excellent high temperature properties.During the development of alloys,a large number of refractory elements are added to improve their mechanical performance.However,those refractory elements have detrimental effects on the solution heat treatment in two ways.On the one hand,due to the low diffusion coefficients of refractory elements,a full homogenization is difficult to achieve.On the other hand,refractory elements increase the segregation which would low down the the incipient melting temperature and increase the amount of eutectics.Although these problems could be relieved to some extent through currently used step solution treatment processing,there still exist residual dendrite segregation and eutectics in the alloys even after quite a longer treatment time.Therefore,solution heat treatment is urgent to be optimized.In this paper,the third generation nickel base single crystal superalloys were used.The methods of ramp solution heat treatment,melting solution heat treatment and fast solution heat treatment were used to optimize the solution heat treatment condition.Optimizations of solution heat treatment were made via tuning the time,temperature and the diffusion distance.The effectiveness of solution treatment was gradually improved.The mechanism of solution effects on the creep properties was clarified.The relationship among the solution heat treatment-structure-creep properties was established.The main conclusions are as follows:(1)According to the dissolution process of the eutectic during solution heat treatment,a criterion of the incipient melting was proposed.Based on the composition,measured by EPMA and Thermo-Calc calculation,a simulation method on the composition of micro-area is proposed.The calculated incipient melting temperature of DD33 is 1265 ~oC,and the incipient melting temperature is verified by laser confocal microscopy.(2)Matrix point method and DICTRA dynamic calculation software were used to characterize the residual segregation after the solution heat treatments.As the solution time increased to 1330 ~oC,the residual segregation became slightly smaller.When the solution time was prolonged from 30 h to 35 h,the residual segregation ratio of Re was reduced from 2.15 to 1.93.In addition,when the residual segregation was reduced,the average size and volume fraction of theγ′was slightly reduced and the size distribution of theγ′was more uniform after ageing treatment.(3)A ramp solution heat treatment method was designed to change the step-wise solution heat treatment into a virtuous cycle.During the virtuous cycle,more time was spent at higher temperature,the solution effectiveness increased,and the incipient melting was restrained to some extent.Therefore,the solution temperature was raised from 1330 ~oC to 1337 ~oC,and the residual segregation was reduced,e.g.,the segregation ratio of Re was reduced from 2.15 to 1.89.(4)A melting solution heat treatment method was proposed.In the melting solution heat treatment,the solution temperature was directly rised above the solidus,and the incipient melting was ignored.The incipient melting structure was eliminated by the following solution heat treatment.Compared with the step wise solution heat treatment,the solution temperature was raised from 1330 ~oC to 1360 ~oC and the solution time was shortened from 30 h to 28.6 h during the melting solution heat treatment,thus,the residual segregation is reduced,e.g.,the segregation ratio of Re was reduced from 2.15 to 1.87.(5)The effect of the dendritic structure size on the solution heat treatment was investigated,and a fast solution heat treatment was proposed.During the fast solution heat treatment,the primary dendritic arm spacing was shortened from 260μm to 83μm by the liquid metal cooling technique.Accordingly,the solution temperature was raised from 1330 ~oC to 1340 ~oC and the solution time was shortened from 30 h to 25 h.The results show that the residual segregation after the fast solution heat treatment was significantly reduced,e.g.,the segregation ratio of Re was reduced to 1.17 due to the high solution temperature and short diffusion distance.(6)The diffusion flux between the dendritic and interdendritic areas was used to calculate the homogenization porosity during the solution heat treatments.It is found that the amount of homogenization porosity increased first and then decreased during the solution heat treatment.In addition,when the primary dendritic arm spacing was shortened from 260μm to 83μm,the degree of homogenization of elements increased,thus,the homogenization porosity decreased from 0.41%to 0.12%.(7)After the different optimized solution heat treatments,the amount of the residual segregation of alloy decreased differently.The effectiveness was the best for shortening the diffusion distance,then raising the solution temperature,and the worst for prolonging the solution time.(8)Optimized the solution heat treatments could improve the mechanical performance.As the amount of the residual segregation decreased,the uniformities of theγ′size andγchannel widths increased.Therefore,the creep/rupture life was increased from 105.3 h to 141.1 h after step-wise solution heat treatment at 1100 ~oC/150 MPa.