  The ability to create complex geometries with tailored material properties
has brought interest in using additive manufacturing (AM) techniques in various
industrial applications. However, the complex relationship between AM process
parameters, microstructure, and resultant properties of metals needs to be
fully understood for the widespread use of metal AM. In this study, selective
laser melting is used to print Ti-6Al-4V. In-situ tensile tests with concurrent
detailed microstructural analysis using electron backscatter diffraction,
electron channeling contrast imaging, and digital image correlation are
performed to understand the damage mechanism and its relation to the
microstructure. Our results show that the as-printed part develops a
hierarchical microstructures, consisting of primary, secondary, and tertiary
{\alpha}^' martensite. This hierarchical structure is formed as a result of
cyclic heat treatment during the course of selective laser melting. Upon
deformation, strain localization within primary {\alpha}^' martensite results
in microscopic ductile micro-void formation and coalescence, as well as
macroscopic brittle fracture. In addition to localization inside primary
{\alpha}^', surface steps at the boundaries of these high aspect ratio grains
are formed which reveal the contribution of interfacial plasticity to the
overall deformation of the material.
