Quantitative phase analysis by X-ray diffraction of martensite and austenite in strongly oriented orthodontic stainless steel wires

A method is described for measuring the volume fractions and textures of martensite and austenite in strongly textured stainless steel orthodontic wires using a conventional X-ray diffractometer. These wires display a classic fibre texture with the 〈111〉 of the FCC austenite phase and the 〈110〉 of the BCC martensite phase aligned parallel to the wire axis. The samples analysed consisted of wire cross-sections bundled together and chemically polished in an epoxy disc. In this form the dominant lines in the XRD patterns are the austenite (111) and the martensite (110). On the basis of X-ray diffraction results from these two lines only, procedures are described for, (a) correcting the X-ray intensity data for both the finite size and irregular cross-sectional shape of the specimens in relation to the X-ray beam footprint, (b) separately measuring the texture of the austenite and martensite phases and, (c) correcting the 111 and 110 integrated intensities for texture. These procedures are illustrated using X-ray data from four different orthodontic wires. The factors limiting the accuracy of the phase analysis are discussed.