A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y / F E B R U A R Y 2 0 2 3 3 3 phase (peak position marked with green sticks). Using computer software packages such as MDI JADE, TOPAZ, or ICDD PDF, quantitative phase analysis can be carried out by using the whole pattern fitting method. In addition to phase detection, quantification of phase fractions can also be conducted using a variety of methods such as reference intensity ratio (RIR), Rietveld, and internal or external standard methods. The reason XRD is considered such a versatile technique is that with proper analysis, one can get a lot more information in addition to the phase analysis. From the XRD spectra, information can be obtained about the crystallite size, micro-strain, and residual stress by using peak profile analysis. MICROSCOPY Phase analysis using microscopy is the most commonly employed technique for phase characterization of materials, in both industry and research. This method uses the contrast generated during the etching process where the etchant preferentially attacks one phase relative to the other to generate the resultant microstructure observed using microscopes. These microscopes can be stereoscopes, optical microscopes, or electron microscopes such as a scanning electron microscope (SEM). Different materials require different etchants to reveal the microstructure present in the material. Sometimes the same material might allow multiple etchants to be used depending on which phase the researcher wants to observe in the material. A comprehensive list of materials and the corresponding etchant solutions can be found in an ASM Handbook[3]. Using microscopy for phase analysis offers a few advantages. First, this method is relatively easy and requires minimal equipment to obtain the desired results. Second, image analysis can be conducted on a large scale with high throughput, which is helpful for industrial environments. Further, visual representation of phases present in the material helps with quantitative analysis of phase composition. Phase Further, the analysis can be carried out at different length scales. The nature and depth of analysis is dependent on the objectives that a researcher wants to accomplish during the study. This article focuses primarily on the analytical techniques used for phase identification in components, and outlines their advantages and disadvantages compared to other methods. X-RAY DIFFRACTION X-ray diffraction (XRD) is one of the most versatile techniques that can be used for both qualitative and quantitative phase analysis of a material. Unlike other techniques, where the results are characterized based on either chemistry or crystal structure, x-ray diffraction can differentiate between materials based on both. The chemical composition determines the lattice parameter and thus the d-spacing between different planes. The crystal structure determines which crystallographic planes will provide a reflection on the XRD spectra. An XRD spectra obtained from a material contains a lot of information. For starters, if the chemical composition of the material is known, it is easier to identify the phases as each individual phase exhibits a set of peaks that are characteristic of its chemical composition and crystal structure. Every crystal structure has a particular set of planes that can appear in the XRD spectra. Based on the d-spacing obtained from Bragg’s law calculations and the presence or absence of certain peaks in the spectra, the phase present in the material can be identified. Materials with two or more phases will provide a spectra containing all the peaks that are permissible for the phases present in the material. The peak intensities might vary based on the phase fraction of the phases present, but with a traditional laboratory system, it is possible to detect a phase with a phase fraction as low as 1%. Figure 1 shows the XRD spectra of a 17-4 PH stainless steel subjected to heat treatment at 620°C for two hours. Note there is a small peak of austenite phase (peak position marked with blue sticks) next to the martensite analysis using this method requires a basic grasp of metallurgy and material microstructure to know what phases can be present in the material such as solid-solution phases, carbides, and intermetallics (Figs. 2 and 3). In most environments, micro- scopic observations are first carried out on large areas using macroscopes, followed by a metallurgical microscope and finally an electron microscope. The advantage of the electron microscope over optical systems is that it delivers high resolution and high magnification, Fig. 2 — Optical micrograph of spheroidal graphite in a martensitic matrix. Fig. 3 — SEMmicrograph showing the microstructure of 17-4 PH stainless steel in solution annealed condition. Fig. 4 — EBSD phase map of 17-4 PH stainless steel after heat treatment at 580°C for 0.25 hours.
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