A novel mechanism to generate metallic single crystals

Generally, the evolution of metallic single crystals is based on crystal growth. The single crystal is either produced by growing a seed single crystal or by sophisticated grain selection processes followed by crystal growth. Here, we describe for the first time a fully new mechanism to generate single crystals based on thermo-mechanically induced texture formation during additive manufacturing. The single crystal develops due to two different mechanisms. The first step is a standard grain selection process due to directional solidification, leading to a pronounced fiber texture. The second and new mechanism bases on successive thermo-mechanically induced plastic deformations and texture formation in FCC crystals under compression. During this second step, the columnar grain structure transforms into a single crystal by rotation of individual grains. Thus, the single crystal forms step by step by merging the originally columnar grain structure. This novel, stress induced mechanism opens up completely new perspectives to fabricate single crystalline components and to accurately adjust the orientation according to the load.

A Computational Study of Structure Development and Texture Formation in Carbonaceous Mesophase Fibers

In this thesis, thermal relaxation phenomena after the melt-extrusion of a rigid discotic uniaxial nematic mesophase pitch were studied using mathematical modeling and computer simulation. The Eriksen and Landau-de Gennes continuum theories were used to investigate the structure development and texture formation across mesophase pitch based carbon fibers. It is found that during the thermal relaxation, discotic nematic molecules stored elastic free energy decays. The distorted nematic molecular profile reoriented to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation resulted in different transverse textures. The rate at which the fibers are cooled is the main factor in controlling the structure development. A slow cooling rate would permit nemiatic discotic molecules to reorient to a well developed (radial or onion) texture. The random texture is a result of rapid quenching. The numerical results are consistent with published experimental observations.