Influence of Melt Superheating Treatment on the Solidification Process and Microstructure for A Third-generation Single Crystal Superalloy
Author:Wang Hai Feng
Ni-based single crystal superalloys have been mostly used to manufacture the blades in aircraft engines and land-based gas turbines owing to their excellent high temperature strength and oxidation resistance.The properties can be controlled by optimization of the composition during the development of Ni-based single crystal superalloys.The continuing demand for developing increased temperature capability and improved creep resistance of nickel-base superalloys has caused increasing amounts of refractory alloying to be added in superalloys.However,the excess addition of refractory elements results in the increase in density and cost as well as propensity of formation of harmful phase and segregation of elements.For the alloys undergoing the solidification process,the change of melt structure has significant effect on the melt properties,solidifcaiton process and mocrostructure.By employing the melt superheating treatment technology,the segregation is reduced,the dendrite and γˊ phase are obviously refined,resulting in the enhancement of service performence without the change of the composition.At present,the study on effect of melt superheating treatment on the microstructure is more forcused,but little attention is paid to the relationship of melt structure and characteristic temperature points and the influence of melt thermal history on the nucleation undercooling,solidification range and solute distribution coefficient.In the present works,a third-generation Ni-based single crystal superalloy DD90 was used.The differential thermal analysis(DTA)was employed to ascertain the change of melt structure and the effect of melt thermal history on the nucleation undercooling and solidification range.On this basis,the appropriate melt treatment parameters were selected to investigate the evolution of distribution of solute elements with the melt thermal history.Based on the solidification theories,the evolution of solidification interface morphology and microstructure with the melt superheating treatment paramenters was discussed,and the effect of melt superheating treatment on the competition growth in bi-crystal was analyzed.In addition,the effect of melt superheating treatment and composition on rupture life was studied.The main conclusions are summarized as follows:(1)The viscosity of DD90 superalloy melt decreases nonlinearly with the increase of melt temperature.The DTA results show that when the melt temperature increases from the liquidus temperature to 1780 ℃,the nucleation undercooling increases and reaches the maximum value of 86 ℃ at 1780 ℃,but the solidification range shinkes and reaches the minimum value at 1780 ℃,When the melt is heated to 1800 ℃,the undercooling decreases and the solidification range enlarges.In the temperature range of 1500-1600 ℃ and 1700-1780 ℃,the nucleation undercooling markedly increases.(2)When the superheating temperature is 1600 ℃ and the superheating time exceeds 15 min,the undercooling increases.And under the supherheating temperature and time keep constant,the undercooling increases with the increase of cooling rate,but the cooling rate is higher than that of 15 ℃/min,the undercooling has no change.(3)On the basis of the change of the nucleation undercooling with the increase of melt superheating temperature,the superheating temperature of 1500 ℃、1600 ℃、1700 ℃ and 1800 ℃ and the superheating time of 30 min are selected.When the superheating temperature is less than that of 1700 ℃,the solute element distribution at the solid/liquid interface becomes more uniform and the solute distribution coefficients of major solute elements are approaching to 1.(4)The melt superheating treatment can improve the stability of solidification interface and optimize the microstructure.With the increase of melt superheating temperature from 1500 to 1700 ℃,the stability of solidification interface improves,the dendrite and g￠ phase on dendrite core and in interdendrite zone are refined,and the area fraction of g/g￠ eutectic structure reduces.When the melt superheating temperature is further heated to 1800 ℃,the stability of solidification interface decreases,the dendrite arm spacing and the size of g￠ phase on dendrite core and in interdendrite zone increase,the area fraction of g/g￠ eutectic structure also increase.(5)In the converging dendrite of bicrystal,with the increase of melt superheating temperature,the melt superhating treatment improves the overgrowth rate and the overgrowth rate is maximal at 1700 ℃.(6)With the increase of melt superheating temperature from 1500 to 1700 ℃,the size of g￠ phase after solution and aging heat treatment decreases and the volume fraction of g￠ phase increases.At 1100 ℃ /137 MPa,the average rupture life improves from 51.5 to 64.8 h.The maximum increase of stress rupture life is 26%.The maximum rupture life approaches to that of 2Ru superalloy.When the melt is heated to 1800 ℃,the size of g￠ phases increases and but the volume fraction of g￠ phases decreases.At 1100 ℃ /137 MPa,the rupture life reduces compared with that at 1700 ℃.