On the accuracy of the X-ray diffraction quantitative phases analysis method in Inconel 718

Abstract

Inconel 718, a widely used Nb-modified nickel-base superalloy, consists of quite a few phases in the matrix such as δ, γ ′, γ ′′ and NbC, as well as small amounts of α-Cr, TiC, NbC, TiC, etc. Because of the determinant effects of the content of δ, γ ′ and γ ′′ phases in the microstructure on the mechanical properties of this alloy, many efforts have been focused in the aspect of quantitative phase analysis [1–4]. In our previous study [5], a simple X-ray diffraction quantitative phase analysis method for Inconel 718 has been proposed based on the relationship between the lattice constant of the γ phase and the content of precipitates, but the accuracy of this method has not been studied in detail. In this work, quantitative analysis was performed using X-ray diffraction method and selective electrolysis method, and the results were compared directly. The composition of as-received Inconel 718 is shown in Table I. Specimens under five different conditions were used. Condition A: solution at 970 ◦C for 1 h, a.c., then cold rolled to 40% reduction in thickness followed by a standard double aging (720 ◦C, 8 h + 620 ◦C, 8 h). Condition B: solution at 970 ◦C for 1 h, a.c., then cold rolled to 40% reduction in thickness, solution at 970 ◦C for 1 h, a.c., again cold rolled to 40% reduction followed by the double aging. Condition C: solution at 970 ◦C for 1 h, ac, cold rolled to 25% reduction in thickness, then solution at 970 ◦C for 1 h, a.c., again cold rolled to 25% reduction followed by the double aging. Condition D, E: sampled from different positions of a turbine disc after forging, then solution at 970 ◦C for 1 h followed by the double aging. The samples for XRD quantitative analysis were mechanically polished followed with chemical etching. Quantitative analysis was performed in D/max-rB X-ray diffractometer with Cu Kα radiation. In detail see [5]. For the chemical quantitative analysis, extracting procedures of δ, γ ′ and γ ′′ phases from the matrix via selective electrolysis are as following. Extracting precipitates of γ ′, γ ′′, δ and NbC from the matrix via