Multiphase material sample preparation using broad‐beam Ar ion milling for EBSD analyses

Abstract

When compared to monophase materials, multiphase materials can offer an improvement in properties or introduce new properties. For example, tungsten, carbides, and nitrides do not have many applications as monophase materials; these metals are brittle, which can override their other desirable properties, such as hardness, high temperature stability, and wear. However, when these materials are dispersed in a soft matrix, such as low‐carbon steel, nickel, or cobalt, and form a composite, the multiphase materials are then characterized by desirable mechanical properties, such as ductility, strength, and resistance to creep and fatigue [1,2]. Other types of multiphase materials are protective surface coatings, such as tribological and antioxidation applications, and microelectronics devices [3]. Knowledge of the phases present in the material, their distribution, and their fractions is fundamental to materials characterization. A common investigative approach for engineered materials is electron backscatter diffraction (EBSD) combined with energy dispersive spectroscopy (EDS). EBSD provides information about crystallographic orientations or misorientations and allows the study of grain boundaries, deformation, and recrystallization. EDS can define the chemical composition of the materials. When combined, these techniques permit phase identification with high accuracy [4]. Sample preparation of multiphase materials for EBSD and EDS can be challenging, especially when the different phases have very different characteristics. For example, if one phase is hard and brittle and the second phase is soft and ductile, the rate of material removal during mechanical polishing can vary widely. In addition, the hard particles removed from the first phase can act as a grinding medium and tear the soft matrix of the second phase. The result is a surface inappropriate for EBSD analyses. Electrolytic etching is another sample preparation technique that is not ideal for multiphase materials; it often requires the use of toxic or dangerous chemicals, it can cause differential etching of the sample surface, and it can give the sample surface a varied topography, which will produce a shadowing effect in the EBSD/EDS analysis. The goal of this work is to illustrate how the use of low energy, broad‐beam argon ion milling can improve and facilitate multiphase material sample preparation. Several examples are discussed, including hard particles within a soft matrix and a multiphase protective layer on a metal substrate.