Degree Type

Dissertation

Date of Award

2017

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

Major

Materials Science and Engineering

First Advisor

L. S. Chumbley

Abstract

Rare-earth transition metal alloys and compounds exhibit extraordinary magnetic behavior and properties, but are typically difficult to fully homogenize. The presence and persistence of linear features in giant magnetocaloric (Gd5Si2Ge2) materials is well known and their effect on the parmagnetic to ferromagnetic transition has been documented. Similarly, other Gd compounds have been studied for their magnetic properties but their microstructure has not been thoroughly explored. In this work, giant magnetoelastic effect compounds GdNi1−xCox are investigated using electron microscopy techniques in order to determine changes to microstructure as a result of the substitution of Co for Ni.

Initial investigations of the as-cast and annealed microstructures reveal a persistent Gd(Ni,Co)2-type phase as a minor phase (roughly 5 vol.%) from x=0.00 to 0.30, while both x=0.50 and 1.00 contain a significant (roughly 38 and 21 vol.%, respectively) amount of a secondary phases. The secondary phase in x=0.50 is determined via energy dispersive x-ray spectroscopy (EDS) to be approximately Gd4(Ni,Co)3, while the phases in x=1.00 are GdCo2 and Gd4Co3. Electron backscattered diffraction analysis of the x=0.30 specimen indicates an extreme directionality to the grain structure with columnar grains 30-50μm wide by upwards of 1000μm long when viewed parallel to the growth axis. Upon sectioning perpendicular to the growth axis, several new microstructural features are discovered including distinct lines which are surrounded by dendritic arms radiating outward. These distinct linear features are determined through TEM investigations to be mirror twins on the (110) plane and appear to be several tens of μm long. Further, the dendrites are identified as being deficient in Co (compared to the matrix) while the boundary between the light and dark regions exhibits characteristics of an anti-phase boundary. When annealed, the dendrites grow to encompass the entire grain, and the boundaries between these regions straighten out and become more regular. Similar microstructures are noted for the other members between x=0.00 and 0.30 while variations in the Ni:Co ratio for the matrix and 1:2 phases are noted. Other microtstructural features noticed include stacking faults and the persistence of wide linear features with a composition different than that of the matrix. The composition of these wide features is determined via EDS to be between the Gd4(Ni,Co)3 and Gd3(Ni,Co)2-type phases. Atom probe tomography data correlates well with this and indicates the composition of the wide linear features in x=0.30 to be Gd3(Ni,Co)2.

The x=0.50 specimen contains Gd(Ni,Co), Gd(Ni,Co)2, Gd4(Ni,Co)3, and Gd12(Ni,Co)7- type phases. Diffraction analysis of the 1:1 phase is in good agreement with literature values for this composition. The 12:7 phase is fitted to the Ho12Co7 P121/c1 crystal structure with lattice parameters a=7.56Å, b=12.51Å, c=11.46Å, β=108.8°compared to literature values of a=8.41Å, b=11.39Å, c=11.16Å, β=124.2°differing significantly from literature likely due to the presence of Ni. However, the 4:3 phase has not been indexed for this composition before and when fitted to the Gd4Co3 P63/m crystal structure lattice parameters of a=b=11.81Å, c=4.20ÃÂÂ

were calculated compared to literature a=b=11.59Å, c=4.05Å. The GdCo sample contained a eutectic region containing both the 4:3 phase and regions ranging in composition from 12:7 to Gd metal (including regions near the Gd3Co composition).

The instability of Co in the GdNi-type crystal structure seeds the breakdown of the solid solution and may be the cause for the numerous microstructural features seen here. Annealing for long periods has a significant effect, but does not fully remove these features. Effects of these features on the magnetic properties has not been performed and is required to determine if they play a significant role. In the case where they impact the bulk properties, steps should be taken in synthesis of specimen to ensure the bulk properties measured are not in any way due to microstructural inhomogenities such as those seen here.

DOI

https://doi.org/10.31274/etd-180810-5025

Copyright Owner

Timothy Prost

Language

en

File Format

application/pdf

File Size

114 pages

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