History Dependence of the Microstructure on Time-Dependent Deformation During In-Situ Cooling of a Nickel-Based Single-Crystal Superalloy

Chinnapat Panwisawas,Neil D’Souza,David M Collins,Ayan Bhowmik,Bryan Roebuck

doi:10.1007/s11661-018-4703-3

Abstract

Time-dependent plastic deformation through stress relaxation and creep deformation during in-situ cooling of the as-cast single-crystal superalloy CMSX-4® has been studied via neutron diffraction, transmission electron microscopy, electro-thermal miniature testing, and analytical modeling across two temperature regimes. Between 1000 °C and 900 °C, stress relaxation prevails and gives rise to softening as evidenced by a decreased dislocation density and the presence of long segment stacking faults in γ phase. Lattice strains decrease in both the γ matrix and γ′ precipitate phases. A constitutive viscoplastic law derived from in-situ isothermal relaxation test under-estimates the equivalent plastic strain in the prediction of the stress and strain evolution during cooling in this case. It is thereby shown that the history dependence of the microstructure needs to be taken into account while deriving a constitutive law and which becomes even more relevant at high temperatures approaching the solvus. Higher temperature cooling experiments have also been carried out between 1300 °C and 1150 °C to measure the evolution of stress and plastic strain close to the γ′ solvus temperature. In-situ cooling of samples using ETMT shows that creep dominates during high-temperature deformation between 1300 °C and 1220 °C, but below a threshold temperature, typically 1220 °C work hardening begins to prevail from increasing γ′ fraction and resulting in a rapid increase in stress. The history dependence of prior accumulated deformation is also confirmed in the flow stress measurements using a single sample while cooling. The saturation stresses in the flow stress experiments show very good agreement with the stresses measured in the cooling experiments when viscoplastic deformation is dominant. This study demonstrates that experimentation during high-temperature deformation as well as the history dependence of the microstructure during cooling plays a key role in deriving an accurate viscoplastic constitutive law for the thermo-mechanical process during cooling from solidification.

Highlights

HIGH-TEMPERATURE deformation during cooling from solidification in single-crystal superalloys is significant for rationalization and prediction of the stress and strain development in turbine blade investment casting
A decrease in stress of 105 MPa within 15 minutes was observed at the initial stages of cooling in the temperature range, 984 °C < T £ 998 °C
The strain drops quickly from when the temperature is in 984 °C < T £ 998.5 °C, where ecð20 00Þ decreases from 5 9 10À3 to 3.3 9 10À3, resulting in the difference in lattice strain, Deel, of À 1.7 9 10À3

Summary

Introduction

HIGH-TEMPERATURE deformation during cooling from solidification in single-crystal superalloys is significant for rationalization and prediction of the stress and strain development in turbine blade investment casting. Thermo-mechanical deformation following solidification progressively occurs in the solid metal as a function of local cooling rate and specific geometrical features.[1,2] This gives rise to localized stress evolution in the casting. In order to capture the thermo-mechanical response induced during cooling from solidification, the time-dependent deformation at high temperature is studied. Isothermal stress–strain curves under constant strain rate or constant stress and temperature describe the material behavior during processing and in-service conditions.[3,4,5,6,7] While non-isothermal creep properties under such high-temperature/lowstress conditions have been studied extensively in nickelbased single-crystal superalloys, e.g., MC2,[8,9,10] CMSX-4[11] and MC-NG,[12,13] such testing typically represented the VOLUME 49A, SEPTEMBER 2018—3963 in-service conditions for creep life assessment at temperatures up to 1200 °C but neglect the history dependence of the microstructure, i.e., the role of accumulated deformation at a higher temperature on subsequent deformation at the lower temperature.[14,15] Including this aspect is critical for an accurate prediction of thermo-mechanical processes during investment casting.[16]

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Discussion

Conclusion