| High alloying nickel-based superalloys possess integrated properties such as good high-temperature strength, oxidation resistance, corrosion resistance, fatigue resistance, fracture toughness and plasticity while they have good microstructure stability and reliability of service at higher temperatures. Thus they are used widely in the high-temperature rotating components such as rocket turbine disc and aircraft engine.Based on the China National Key Fundamental Research Development Program (973 Program) entitled "Fundamental Research on the Superalloy Design and Preparation", we cooperated with Beijing General Academy of Iron and Steel to study on the project entitled "Microstructure Evolution Mechanism during Plastic Process of High Alloying Superalloys" and achieved following research results:(1) The deformation behavior at elevated temperatures over the temperature range from 900 to 1150℃ with the strain rates from 5×10-4 to 10s-1 in GH79, U720Li and GH4742 alloys were investigated through hot compression tests. The results show that first, the strain rate sensitivity exponent, m, and the activation energy for flow, Q, in aforementioned alloys are within the range of 0.072～0.254 and 341.57-2290.91 kJ/mol, respectively, m value reaches the maximum value and Q value reaches the minimum value at the deformation temperature of 1100℃. Second, on the basis of hot compression experimental data, the high-temperature constitutive equations for three alloys were obtained by means of multiple linear regressions.(2) Grain sizes decrease gradually with increasing strain during hot deformation of GH79, U720Li and GH4742 alloys. Dynamic recrystallization (DRX) occurs easily in the temperature range of 1050～1100℃ and fine equiaxed grains are formed. Phase transformation reactions, MC+γ→M23C6+γ’and MC+γ→M6C+γ’, take place during the deformation at the temperatures of 1100℃ and 1150℃. In the meantime, calculation of the grain size exponent, p, reveals that p value takes on the trend of first increase and then decrease and reaches the peak value at 1100℃. p value is close to 2.0 at the temperatures of 1050℃ and 1100℃. In this case, the alloys have good forming performance.(3) Five instability criteria such as GEGEL, MALAS, PRASAD, MURTHY and SEMIATIN were deduced and analyzed and dynamic processing maps based these five criteria were plotted using BP (Back Propagation) artificial neural network technique respectively for three superalloys. Appropriate processing domains and flow instability regions during hot deformation were predicted and the technological parameters of three superalloys were optimized using the processing maps. Based on above work, the deformation mechanisms in different deformation regions for three superalloys were obtained by means of metallographic analysis.(4) On the basis of hot compression simulation experiment, DRX critical strain models, DRX kinetics models and DRX grain size models during high-temperature deformation were established for three superalloys and DRX mechanisms for typical regions were analyzed for three superalloys. The results show that the DRX mechanisms are twin DRX (TDRX), continuous DRX (CDRX) and discontinuous DRX (DDRX) for GH79 and GH4742 alloys with the increase of deformation temperature. The DRX mechanisms are TDRX, the combination of TDRX and CDRX, and, DDRX for U720Li alloy with the increase of deformation temperature.(5) R-W-S (Ruanon-Wasworth-Sherby) deformation mechanism maps (DMM) incorporating dislocation quantities inside the grain were constructed in the temperature range of 900～1150 ℃ after we analyzed and compared four types of DMMs:Ashby, Langdon-Mohamed, RWS and RWS incorporating dislocation quantity. The deformation mechanisms for three alloys were predicted accurately based on the constructed maps. The comparison of the microstructure observation and high-temperature mechanical behavior reveal that the forecast of DMMs is both true and accurate. |
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