An electron probe X-ray microanalyzer (EPMA) is a powerful equipment to characterize the distribution of the chemical composition in various materials [Rinaldi & Llovet, 2015]. EPMA determines the chemical composition accurately due to its high energy resolution (~ 10 eV) [Williams & Carter, 2009]. The elemental map confirms the relative intensity difference of individual elements in the various phases, including precipitate, inclusion, and matrix phases [Lee et al., 2022; Han et al., 2021]. Although there was an effort to quantify the X-ray intensity without a standard specimen [Trincavlli et al., 2014], the obtained intensity is generally quantified by comparing it with the intensities of standard specimens where their chemical compositions have already been determined. The quantification lines that show the correlation between X-ray intensity and absolute composition for the individual elements should have to be obtained from the standard specimens under the same analysis condition (beam diameter, beam current, accelerating voltage, and detecting crystals) to the observation condition for the actual specimen. Since the chemical compositions of the standard specimens are varied to obtain the correct quantification line, repeated EPMA analyses on the multiple numbers of standard specimens are necessary. The quantification line is generally expressed by a linear equation below;
where Ii is the X-ray counts of element i, Ci is an absolute composition of element i, ki is the proportional constant, and B is a background intensity or an intensity without element i in the matrix. X-ray emission depends on atomic number (Z), the absorption of X-rays (A), and the fluorescence of X-rays within the specimen (F) [Williams & Carter, 2009; Trincavlli et al., 2014; Ziebold & Ogilvie, 2002]. Therefore, the proportional constant ki is inversely proportional to the ZAF correction factor. ki and B values are varied with the corresponding element and measuring conditions (beam diameter, beam current, accelerating voltage, detecting crystals, and so on).
One of the typical difficulties in EPMA quantification is finding a proper standard specimen. Since the chemical compositions of the standard specimens are diverse, the elemental distributions in the standard specimens are nonuniform. These compositional inhomogeneities in the standard specimens bring a significant deviation from the absolute composition of the target specimen because they make an uncorrected quantification line. Since the chemical inhomogeneity of standard specimens is intrinsic and cannot avoid, the proper method to overcome it is necessary from a practical viewpoint.
The multi-phase steels are composed of diverse phases, including α-ferrite, αb-bainite, α′-martensite, pearlite, and retained austenite (γ) [Han et al., 2021; Kim et al., 2022]. Among them, γ controls the mechanical properties in the multi-phase steels. γ changes the work-hardening and ductility of the steel through the transformation-induced plasticity (TRIP) effect during mechanical deformation [Spencer et al., 2009]. The TRIP phenomenon has a strong relationship with the mechanical stability of γ. The chemical compositions of γ that are specifically the contents of γ stabilizer (C, Mn, and so on) should be determined to evaluate the mechanical stability of γ. However, there are several difficulties to investigate γ in the multi-phase steels using transmission electron microscopy (TEM). Firstly, the volume fraction of γ is only a few % order. There is a limitation in finding the retained γ in a TEM specimen. Rare distribution of γ is also an obstacle to fabricating TEM specimens using a focused ion beam. Secondly, statistical analysis is difficult in TEM analysis due to a limited observation of γ.
In this study, we aim for the practical aspect of EPMA quantification of the chemical composition of retained γ in multi-phase steel. The accurate quantification method in EPMA analysis is suggested. Moreover, EPMA analysis's obtained carbon (C) content was compared to the calculated result from the X-ray spectrometry (XRD).