No abstract

Manipulating the shape and distribution of strengthening units, e.g. particles, fibers, and precipitates, in a bulk metal, has been a widely applied strategy of tailoring their mechanical properties. Here, we report self-assembled patterns of Gd nano-fibers in Mg-Gd alloys for the purpose of improving their strength and deformability. 1-nm Gd nano-fibers, with a 〈c〉-rod shape, are formed and hexagonally patterned in association with Gd segregations along dislocations that nucleated during hot extrusion. Such Gd-fiber patterns are able to regulate the relative activities of slips and twinning, as a result, overcome the inherent limitations in strength and ductility of Mg alloys. This nano-fiber patterning approach could be an effective method to engineer hexagonal metals.

Electron beam melting (EBM) is a form of additive manufacturing (AM) that offers fast, customizable, near-net production of metallic parts by depositing material "layer by layer." In EBM, an electron beam scans across a bed of metal powder, thereby melting the powder in the shape of a desired part. The melted powder solidifies into a layer, the layer is covered with a new layer of powder, and the process repeats. Consequently, any location within the build could be rapidly heated above solid-solid phase transition temperatures several times as the beam scans the layer(s) above [1]. These beam-dependent, cyclic thermal profiles are known to strongly affect morphology and microstructure, resulting in anisotropic grain sizes, macro-and micro-porosity, and complex phase morphologies [2,3]. This directly impacts the mechanical properties of AM-built parts. While these complex thermal profiles can be controlled with beam scanning strategies, they are not fully understood. Understanding and controlling AM thermal gradients in space and time opens the opportunity to design unique, tailored, desirable microstructures.

No abstract

Eutectic phase formation in the weld metal microstructure of Tantalum (Ta)-bearing Ni-based filler metal was investigated in this work. The eutectic constituents in the interdendritic regions composed of the gamma (γ) phase (Ni-matrix), Ta-rich carbide phase (MC) and the Cr-and Mo-rich Laves phase were identified using scanning / transmission electron microscopy (S/TEM). In addition, the orientation relationship between precipitates and matrix and element segregation were studied by electron diffraction (ED) and X-ray energy dispersive spectrum (EDS) mapping. The atomic structure of each phase and the structure of the interfaces were visualized in high-resolution STEM imaging and the stoichiometric formula of the observed Laves phase and MC carbide were determined.