TEM analysis of Cr–Mo–W–B phase in a DS nickel based superalloy

Abstract

Rene 80 is a precipitation hardened nickel-based superalloy, which is widely used to manufacture components of gas turbine engines for aerospace and power generation applications. It is designed to withstand service temperatures of up to 980 C [1] and possesses remarkable hot corrosion resistance, long-term microstructural stability, and hightemperature strength produced primarily by precipitation hardening by ordered intermetallic Ni3(Al,Ti) c0 precipitates. Directional solidification (DS) processing of the alloy results in further improvement of its elevated temperature properties and permits the use of a higher service temperature, which improves the thermal efficiency of gas turbines [2–5]. It has been reported that in DS Rene 80, the tensile strength is increased by about 10–15% and the creep rupture life is extended by 2–4 times relative to conventionally cast material [5]. Boron is an essential minor elemental addition to these heat resistant materials primarily to improve their creep rupture properties [3, 4]. Nevertheless, it has been recognized that the mode in which boron exists on grain boundary regions, either in austenitic solid solution form or selectively partitioned into second phase particles, can significantly influence the creep properties of superalloys [6, 7]. Experimental study is crucial in establishing the presence and nature of intergranular borides in multi-component superalloys due to the influence of interfacial elemental segregation that is not generally considered during thermodynamic equilibrium calculations of theoretical models [8]. The objective of this investigation was to perform transmission electron microscopy (TEM) study of heat-treated DS Rene 80 superalloy to understand the nature of boron rich second phase particles present along its grain boundary regions. The composition of the DS Rene 80 material used in this study was (wt%) 0.2C, 14.1Cr, 9.52Co, 4.0W, 3.98Mo, 0.02Nb, 0.18Fe, 2.9Al, 4.98Ti, 0.028Zr, 0.013B balance nickel. The alloy was subjected to standard solution heat treatment for 2 h at 1200 C and was subsequently aged at 1025 C for 16 h. Thin foils from the aged material for TEM study were prepared by mechanical polishing and dimpling to a thickness of less than 10 lm followed by ion milling in a Gatan precision ion polishing system (PIPS). TEM microchemical and crystal-structure analyses were performed in a field emission gun JEOL 2100F scanning transmission electron microscope equipped with Oxford energy dispersive spectrometer (EDS), Gatan imaging filter (GIF) system, and electron energy-loss spectrometer (EELS). High-resolution TEM micrographs were filtered using an inverse fourier fast transform (FFT) program in Gatan Digital Micrography software. A TEM boron post-edge electron energy loss image of the heat-treated alloy consisting of c0 precipitates and particles of a different phase along the grain boundary and inside the grain identified with arrows is shown in Fig. 1a. Microchemical analysis by X-ray energy dispersive spectroscopy indicated that the particles are rich in Cr, Mo, W, and B (Fig. 1b), and have much different composition from the adjoining regions (Fig. 1c). The presence of boron in the particles was further confirmed by EELS analysis (Fig. 1d) and EELS-based energy filtered transmission electron microscopy (EFTEM) elemental mapping (Fig. 1e). TEM Electron diffraction pattern (EDP) analysis was performed by systematic titling of particle of this phase along certain crystallographic directions to determine its crystal structure. Figure 2a is a [001] zone-axis EDP showing a fourfold symmetry of the phase. Figure 2b and c were obtained by H. R. Zhang O. A. Ojo (&) Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6 e-mail: ojo@cc.umanitoba.ca