Monitoring the kinetics of the γ’ phase in the N18 superalloy using in situ electrical resistivity measurements

Abstract

In nickel-based superalloys, temperatures related to the formation or the dissolution of the different types of γ' precipitates are important parameters for optimizing the mechanical properties of components but also for developing models which can reproduce the kinetics of their phase transformation. We showed that the electrical resistivity variations during heat treatment of the N18 superalloy was sufficient to monitor the kinetics related to secondary and tertiary γ' precipitates. In particular, the effects of the heating rate and the initial microstructure on the dissolution kinetics of the γ' phase were investigated. Experimental results were also compared to outputs of a precipitation model developed for the N18 alloy showing that in situ electrical resistivity measurements can be used for calibration and validation purposes.

In-Situ Monitoring of Phase Transition and Microstructure Evolution in Ni-Based Superalloys by Electrical Resistivity: Direct Comparison With Differential Scanning Calorimetry and Application to Case Studies

“…The number density of c¢ precipitates is expected to be higher when the material has been previously cooled with a very rapid rate (Figures 9(b) vs (c)), [56] and this potentially created nano-scale c¢ precipitates that become a source of large electrical resistance. [24,25] The major c¢ dissolution (second peak at 780 °C) started at a lower temperature with higher resistivity for the material cooled at 50 K/s. Similar precipitation reactions were previously observed in an additively manufactured Ni-based superalloy without c¢ phase and in supersaturated state.…”

“…These seem to indicate a microstructural event potentially caused by dissolution or coalescence of very fine c¢ precipitates with diameter less than 50 nm according to other studies. [24,50] ThermoCalc TTNi8 database predicts a l phase transition at 664 °C for Waspaloy, however, this is unlikely to occur since carbides are only possible minor phase in Waspaloy unless it is aged for more than 2000 hours. [51] SX superalloys, STAL15 (Figure 6(c)) and CMSX-4Ò (Figure 6(d)), have less complicated DSC thermograms and resistivity profiles compared to those of the polycrystalline (PX) superalloys.…”

“…It has been used to assess material properties in different metallic systems such as Al, Ti, and Zr alloys. [18][19][20][21] In the case of Ni-based superalloys, it can be used to characterize grain growth, [22] volume fraction of the c¢ phase, [23,24] and dissolution and precipitation kinetics of c¢ phase. [24,25] However, electrical current can accelerate recovery and recrystallization, and it can, in principle, affect the phase transformation process in metallic materials.…”

“…[18][19][20][21] In the case of Ni-based superalloys, it can be used to characterize grain growth, [22] volume fraction of the c¢ phase, [23,24] and dissolution and precipitation kinetics of c¢ phase. [24,25] However, electrical current can accelerate recovery and recrystallization, and it can, in principle, affect the phase transformation process in metallic materials. [26][27][28] Several studies have made attempts to extract microstructural information from Ni-based superalloys through electrical resistivity measurement.…”

Precipitation kinetics in metallic alloys: Experiments and modeling

“…Many other physical parameters can also be used to monitor precipitation, some of which can be measured in-situ, such as electrical resistivity [51,52]. Electrical resistivity is mostly sensitive to the matrix solid solution, so following its evolution provides indirect information on the progress of a precipitation reaction.…”