Hot deformation behavior and processing map of a γ′-hardened nickel-based superalloy

Abstract

The hot deformation behavior of a γ′-hardened nickel-based superalloy was investigated by means of isothermal compression tests in the temperature range of 1010–1160 °C and strain rate range of 0.001–1 s⁻¹. The results show that the hot activation energy of the alloy is about 427.626 kJ/mol. On the basis of experimental data, processing maps were developed by utilizing the principles of the dynamic materials model (DMM). The processing maps exhibit one domain with high efficiency of power dissipation. At the strain of 0.6, the domain occurs in the temperature range of 1105–1160 °C and strain rate range of 0.02–0.25 s⁻¹ with a peak efficiency of about 42.2%. Microstructure observations reveal that full dynamic recrystallization (DRX) occurred in the domain. In the processing maps, the curvature changes of the isoefficiency contours occur in the temperature range of 1080–1090 °C, which is at the dissolution temperature of γ′ phase in the alloy. Moreover, it can be found that the strain has an effect on the efficiency of power dissipation. The efficiency value increases with increasing strain in the high strain rate domain (0.1–1 s⁻¹), while the efficiency value decreases with increasing strain in the high temperature (1080–1160 °C) and low strain rate (0.001–0.01 s⁻¹) domain. The flow instability is predicted to occur in the high strain rate domain, which is manifested as adiabatic shear bands.

Introduction

The nickel-based superalloy (nominal composition Ni–18.3Cr–6.4Co–5.9W–4Mo) is a kind of γ′-hardened superalloy with low stacking fault energy. In aviation industry, the alloy has been widely used in combustion systems, gas turbines and other related high-temperature applications due to its excellent high-temperature strength, creep resistance and corrosion resistance. In particular, the highest service temperature of the alloy can reach about 1000 °C [1]. In the alloy, the elements of Ti and Al make a great contribution to form γ′ phase (Ni₃AlTi), which is the main strengthening phase. Meanwhile, W, Mo, Co are used as solution strengthening elements, and B, Ce, Mg are used as grain boundary strengthening elements [2]. However, the alloy contains many alloying elements, which leads to the difficulties of controlling microstructures in the hot working process [3], and the γ′ phase in the matrix can greatly influence the hot workability of the alloy. In addition, processing parameters play an important role in controlling the microstructures [4], [5]. Thus, it is very necessary to research the hot deformation behavior of the alloy for optimizing processing parameters and controlling microstructure.

It has been widely accepted that a processing map is very beneficial for optimizing processing parameters and controlling microstructure in hot deformation of superalloys [6]. The processing map is developed on the basis of the dynamic materials model (DMM) by Prasad et al. [7], [8]. According to the DMM approach, the work piece in hot working is considered to be a dissipator of power, and the instantaneous power P is dissipated through temperature rising (G content) and microstructure evolution (J co-content). The total power absorbed by the work piece can be determined as

$$P=\sigma \dot{\epsilon }={\int }_0^{\dot{\epsilon }}\sigma d\dot{\epsilon }+{\int }_0^{\sigma }\dot{\epsilon }d\sigma =G+J$$

The ratio between G and J is decided by the strain rate sensitivity (m). Meanwhile, the ratio of the power used for microstructure evolution is reflected by the efficiency of power dissipation (η), which is a function of strain rate sensitivity. The parameter η can be determined as

$$\eta =\frac{J}{J_{\max }}=\frac{2m}{m+1}$$

The variation of η with strain rate and deformation temperature constitutes a power dissipation map, from which some specific microstructural mechanisms can be preliminarily judged. In general, a large value of η is usually related to DRX in hot deformation. A continuum criterion for the occurrence of flow instability is obtained by using the principle of maximum rate of entropy production and given by

$$\xi (\dot{\epsilon })=\frac{\partial \ln [m/(m+1)]}{\partial \ln \dot{\epsilon }}+m<0$$

When $\xi (\dot{\epsilon })$ becomes negative, the flow instability is forecasted to occur. The variation of $\xi (\dot{\epsilon })$ with strain rate and deformation temperature constitutes an instability map, in which the regions of negative parameter represent the flow instabilities. The instability map can be superimposed on the power dissipation map to construct the processing map, which can be used to optimize processing parameters and control microstructure in hot working process.

In this study, the hot deformation behavior of a typical γ′-hardened nickel-based superalloy was investigated by isothermal compression tests under wide ranges of forming temperature and strain rate. Based on the dynamic material modeling (DDM), the processing maps of the studied nickel-based superalloy were constructed to optimize processing parameters. In addition, the microstructure evolution was analyzed to validate the established processing maps of the studied Ni-based superalloy.