Metallographic Evaluation of Non-Metallic Inclusions
Introduction
Non-metallic inclusions (NMIs) are an unavoidable impurity in metals, which can have deleterious effects on material properties such as strength and toughness, and are commonly isolated and studied in the laboratory through metallographic evaluation. The main goals of metallographic evaluation of non-metallic inclusions are to identify the type and size of the inclusion, as well as to determine whether they are harmful. In order to properly assess the inclusions, it is important to understand the source of the inclusions, as different sources may include different types and sizes of inclusions.
The Sources of Non-Metallic Inclusions
In a metal melt, NMIs are typically derived from mineral ores or refractory materials, slag or slag-forming by-products, the atmosphere, or intentional additions. Ores and refractories are the sources of basic inclusions, such as oxides (SiO2, Al2O3), sulfides (FeS, FeS2, MnS, Cr2S3) and their compounds, as well as other elements such as Cu2O, MgO and CaF2. The slag or slag-forming materials are the sources of various inclusions, including silicates (CaSiO3, Ca3SiO5, Mg2SiO4) and their compounds, as well as other elements such as NiO, ZnO and TiO2. The atmosphere is responsible for the introduction of complex inclusions, such as graphites and carbides. Intentional additions, such as deoxidizers, are usually added in order to reduce the oxygen content of the melt, and can also introduce chemical species such as Mn, Ti, and Al.
Metallographic Evaluation of Non-Metallic Inclusions
For metallographic evaluation of NMIs, samples should be prepared by polishing both sides of the material, followed by etching with a suitable acid. There are several methods for viewing and analyzing the inclusions, including optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron probe microanalysis (EPMA). Each of these methods provides unique information which can be used to identify the type and size of the inclusions, as well as to determine their potential harm.
Optical microscopy is the most fundamentall technique for evaluating NMIs, as it can be used to visualize the inclusions and determine their sizes, shapes, and morphology. Optical microscopy is also useful for studying the relationships between inclusions, such as the presence and number of clusters, as well as the spacing between individual inclusions.
SEM provides high resolution imaging of the inclusions and can be used to measure the size of individual NMIs. In addition, SEM can be coupled with EDS to analyze the chemical composition of the inclusion and its surroundings, allowing for the identification of specific elements. EPMA is also used for chemical analysis of NMIs, and can provide information about the concentration and distribution of elements, as well as their atomic ratios.
Conclusion
Metallographic evaluation of non-metallic inclusions is an essential part of understanding the quality, properties, and performance of metals. Through proper preparation and analysis techniques, it is possible to accurately identify and quantify NMIs, as well as assess their potential harm. By understanding the source and composition of these inclusions, it is possible to identify steps to reduce their number, size, and harm, as well as to improve the quality and performance of the material.
