Materials design through phase and microstructure engineering: case studies in metal additive manufacturing and high entropy silicide thin films
The 21st century presents humanity with unique challenges, addressing which requires constant improvements in manufacturing technologies and engineering materials. In this dissertation, two areas of modern material research are explored: additive manufacturing and high entropy materials.
First, the problem of microstructural inhomogeneities within additively manufactured metal parts is addressed. Specifically, the effect of process parameters and feature thickness on the microstructures of 316L stainless steel components fabricated by laser powder bed fusion (LPBF) is examined. A standard benchmark geometry developed by the National Institute of Standards and Technology, which contained walls of 0.5, 2.5 and 5.0 mm in thickness, is used. Optical microscopy, finite element analysis, scanning electron microscopy and electron backscatter diffraction reveal dramatic microstructural differences in features of different thickness within the same component. The feature thickness influences the cooling rate, which in turn impacts the melt pool size, solidification microstructure, grain morphology and density of geometrically necessary dislocations. The relationship between feature size and grain morphology is dependent on the energy input used during LPBF. Such behavior suggested that local manipulation of LPBF process parameters can be employed to achieve microstructural homogeneity within as-printed stainless steel components.
Second, a novel approach toward manipulation of residual stresses in additively manufactured parts is explored. Alloy design involving engineering of solid-state transformations is proposed as a way to change the residual stress state of the as-printed components. To this end, parts fabricated from two metals (pure Fe and Fe-50Cu) are evaluated. Residual stress measurements demonstrate that surface residual stresses can be successfully manipulated by adjusting the alloy composition. It is hypothesized that solid-state transformations experience by the Fe and Fe-rich phases are responsible for the observed differences in residual stresses. This study is the first to suggest using residual stress as a design criterion in alloy engineering for metal additive manufacturing.
Finally, a new high entropy silicide material for applications in electronics is proposed. The CALculation of PHAse Diagrams (CALPHAD) approach is used to identify two candidate single-phase high entropy silicides (HES): the ternary (CrMoTa)Si2 and the quinary (CrMoTaVNb)Si2. Both candidate compositions are experimentally synthesized via electron beam evaporation followed by heat treatment in vacuum, which facilitates the solid-state reaction. Both the ternary (CrMoTa)Si2 and the quinary (CrMoTaVNb)Si2 HES form a single phase with a C40 hexagonal crystal structure, validating our CALPHAD phase formation predictions. This work reports the first experimental realization of a thin film high entropy silicide material.