All ETDs from UAB

Advisory Committee Chair

Amber L Genau

Advisory Committee Members

Charles A Monroe

Robin D Foley

Haibin Ning

Gregory Thompson

Document Type

Dissertation

Date of Award

2018

Degree Name by School

Doctor of Philosophy (PhD) School of Engineering

Abstract

Cast iron solidifies with various morphologies of the graphite phase, which directly influences its mechanical behavior. Hence, it is crucial to control the graphite morphol- ogy in order to tailor the properties of the iron to specific applications. This makes it crucial to understand the effect of solidification velocity and composition, the two main factors affecting graphite morphology. To study the influence of velocity and composi- tion, particularly nodularizing elements (Mg) and cerium (Ce), a number of batches of iron were produced. The majority of analysis was conducted on those batches where the carbon equivalent was very close to the eutectic composition. To obtain precise control over solidification velocity, samples were directionally solidified in a Bridgman furnace with solidification velocities from 10 um/s to 30 um/s. Samples for directional solidification were initially obtained from the melt with very high cooling rate in order to produce white iron, which avoids the problem of graphite flotation and maintains a uniform composition along the sample. The samples solidified with a variety of cementite and graphite structures which were correlated to the solidification velocity. A graphite to cementite transition was observed in samples containing only Ce as the solidification velocity increased. Nodularity increased with increasing velocity for both the batches but was more pronounced in case of samples containing both Mg and Ce. Increasing the silicon (Si) content was found to be effective at suppressing carbide formation at all velocities. Mechanical behavior of the samples was analyzed via compression testing and digital image correlation. Strength, toughness and elongation to fraction were found to scale with velocity. It was found that, for samples with lower Si content, the carbide fraction dominated the mechanical properties, while for the samples without carbide, the effects of increasing nodularity with velocity could be observed. By analyzing local strain, it was found that strain localization is characteristic of ductile failure, but in case of brittle failure, strain is distributed evenly throughout the sample. Finally, the nodules in the directionally solidified samples were observed under scanning electron microscope to study the growth mechanism of nodules. The findings support the model proposed by Amini and Abbaschian that growth occurs in stages, starting with isotropic growth and was followed by circumferential or radial growth.

Included in

Engineering Commons

Share

COinS