Microstructural evolutions in additively manufactured metallic materials

Abstract

Additively manufactured metals have attracted numerous attention due to their superior mechanical properties compared with metals fabricated by conventional ways. However, the relationships among thermal histories, local microstructures, and mechanical properties have remained unknown. The thesis starts from an introduction of additive manufacturing of metals, which will be introduced in Chapter 1. Two model materials, i.e. a CrMnFeCoNi high-entropy alloys and Ti-6Al-4V, will be introduced in details. Fundamental knowledge on crystal structures and crystalline defects needed for this thesis will be briefly introduced. Chapter 2 describes the electron microscopy techniques, including their working principles and applications, used in this thesis, which includes scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The materials fabrication process using laser powder bed fusion will be discussed in this chapter. Detailed microscopy sample preparation processes will be introduced. Experimental instruments and data settings will be discussed in details. Metallic materials produced by additive manufacturing experience complex stress and thermal gyrations, which has the potential to fabricate heterogeneous microstructures. The variations of microstructures along the build direction lead to local-specific mechanical properties, which has not been investigated yet. My first project choose a CrMnFeCoNi high-entropy alloy as the model material to investigate the microstructural evolution along the build direction, which will be introduced in Chapter 3. Advanced microscopy techniques were used to clarify the structural evolution. The local mechanical properties were revealed by micro-indentation tests. My research clarifies the relationships among thermal cycles, local microstructures, and location-specific mechanical properties. A dual-phase Ti-6Al-4V alloy is chosen as the model material for further investigation of the effect of rapid cyclic thermal loadings. The interface stability under large thermal gradients is a hot topic in additive manufacturing. One typical grain boundary, variant selection II, transform to an energetically more favorable twin boundary with the help of cyclic thermal loadings, which will be introduced in Chapter 4. The structural evolution is revealed. One of the major challenges in additive manufacturing is that light elements, including oxygen, are always unavoidably introduced in the as-fabricated components. As Ti alloys have high affinity to these light elements, intermetallic compounds will form, which impact the ductility of the AM components. Chapter 5 introduces a new oxygen-rich face-centered cubic (FCC) phase, which is formed with the help of cyclic thermal loadings. The structural and chemical information is revealed. In-situ and ex-situ mechanical tests are performed. With the introduction of the FCC phase, the strength, ductility and work-hardening rate are improved simultaneously. Ti-6Al-4V components fabricated by laser powder bed fusion (LPBF) usually consist of fully α' martensite due to the high cooling rate generated during the fabrication process. However, the brittle nature of martensite limits the industrial application use of the as-fabricated components. Therefore, investigating the intrinsic annealing effect of LPBF is essential to decompose brittle α' structure into brittle α + β structure. Chapter 6 introduces the detailed decomposition process of α' to α + β. The phase transformation process follows a two-step transformation.