Hot deformation studies on a precipitate free Ni-based superalloy.

F. LIZZI; K. PRADEEP; G. SCHADEN; A. STANOJEVIC; S. SOMMADOSSI; C. POLETTI

Valdivia

Congreso; CONAMET-SAM-2019. Congreso Internacional de Metalurgia y Materiales; 2019

Facultad de Ciencias de la Ingeniería de la Universidad Austral de Chile, SOCHIM

The nickel-based superalloy Inconel 718 (IN718) is mainly recognized by two characteristics: high resistance to mechanical deformations and corrosive processes at high temperatures [1]. Other important attributes are its weldability, resistance to oxidation, ductility and machinability. Therefore, this alloy is used in aerospace applications such as turbines, combustion chambers or heat exchangers as well as in the chemical and petroleum industry. Although the hot deformation of the IN718 has been studied in different conditions, there is no evidence of the influence of secondary phases on forging and deformation of the material [2]. The present work, studies the evolution of the microstructure due to thermomechanical treatments on an IN718 nickel-based superalloy without precipitates (IN718WP) produced by powder metallurgy. To achieve this, specimens are deformed by compression tests in a temperature range of 900 to 1025 °C every 25 °C and scaling strain rates from 0.001 to 10 s-1 in a logarithmic scale. Tests were carried out in the thermomechanical simulator Gleeble® 3800. Deformed specimens were observed and analyzed with optical and scanning electron microscopy (SEM) using energy dispersive x-ray spectroscopy (EDS) and electron back scattered diffraction (EBSD).The observed mechanical behavior responds to the expected for this class of materials with a characteristic peak of stress followed by softening due to recrystallization and finalizing with a steady state stress [3]. Flow curves were corrected taking into account the increase in temperature of the samples during the deformation process, as can be seen in Figure 1 [4]. The maximum stresses and room temperature hardness of the precipitate free alloy were compared against the IN718 alloy. IN718WP peak stress values are around 1.5 lower on average (at 900 °C ~250 MPa for 0.01 s-1 ~120 MPa for 0.001 s-1 against 375 and 200 MPa of the IN718 at the same deformation conditions [2]). This is a result of the lack of precipitates as inhibitors of dislocation movement [3]. The softening behavior is similar in both alloys for high deformation rates. However, at low deformation rates the softening of the IN718WP alloy is higher than the standard alloy and the deformed specimens show cracks at the barreled edges. This is due to the coalescence of the existing micro pores from the powder metallurgy process. The deformed microstructure was also analyzed, in particular recrystallized grain size and recrystallization percentage. Dynamic recrystallization was analyzed comparing the grain spread orientation (GOS) of each grain with the assistance of OIM Analysis ? v8 software: nucleated and recrystallized grains have small dispersions (~ 5 °) while old and deformed grains have a higher ones (> 5 °), product of the deformation [5]. The percentage of recrystallization increases with temperature and low deformation rates. This tendency is also observed in the IN718 alloy. Finally, grain size and grains recrystallized percentage of dynamically was correlated with the deformation parameters.Figure 1: Stress-strain curves for the whole temperature at 900°C for the whole strain rate range 0.001-10 s-1 and at 0.001 s-1 for the whole test temperatures range.Acknowledgements: The Christian Doppler Society supports financially the CD-Labor for Design of High-Performance Alloys by Thermomechanical Processing. Keywords: nickel, superalloy, IN718, EBSD, recrystallization, hot deformation.