The Beta To Alpha Transformations In Nickel Aluminum Bronzes At Low Undercoolings
Open Access
- Author:
- Meinert, Kenneth C
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 19, 2012
- Committee Members:
- Paul Raymond Howell, Dissertation Advisor/Co-Advisor
Paul Raymond Howell, Committee Chair/Co-Chair
Richard Martukanitz, Committee Member
James Hansell Adair, Committee Member
Zi Kiu Liu, Committee Member - Keywords:
- Nickel Aluminum Bronze
Sympathetic Nucleation
BCC-FCC Transformation
3-D Reconstruction
Inclusion Nucleation - Abstract:
- The results presented in this dissertation represent the most detailed account of the bcc⇄fcc (β→α) transformation in a system that is non-ferrous. As such, it provides a benchmark against which the multitudinous studies on phase transformation in steels may be gauged. The primary focus of the experimental program has been on the development of intragranular microstructures, although grain boundary related transformations were also examined. The only fundamental intragranular precipitate shape that we encounter at low undercoolings was a Widmanstätten rod with a 〈111〉_β growth direction. The primary nucleation sites for these Widmanstätten rods are inclusions. The complex nature of the intragranular microstructures necessitated the development of a quick, reproducible method for assembling three-dimensional reconstructions from a sequence of serial sections that sample a large volume. The method developed used a laser in a novel way for providing fiduciary markers, and the use of freely accessible software permitted a straightforward method of generating the 3-D reconstruction. In terms of the intragranular bcc→fcc transformation, the results cover the spectrum of nucleation, growth, and coarsening and we have proposed a single mechanism that, at the very least, is operative in all three stages; that mechanism is sympathetic nucleation. For example, it has been found that following the initial inclusion nucleation event, the entire transformation proceeds via sympathetic nucleation of two particular types, facet-to-facet and end-to-end. Somewhat surprisingly, we are the first to recognize the equivalence of facet-to-facet sympathetic nucleation and the homogeneous nucleation of growth ledges. We may thus model both nucleation and growth in terms of a single mechanism. What is even more intriguing, we may exploit facet-to-facet and end-to-end sympathetic nucleation to propose a mechanism of precipitate coarsening, the replacement of fine distributions of small precipitates by a much coarser distribution of large precipitates. This coarsening reaction occurs as the sympathetically nucleated rod-like particles start to tile space, creating large (20-50µm wide) rod-like colonies from the initial (≃1µm wide) inclusion nucleated rods. Our research on sympathetic nucleation in this system has an impact outside the narrow confines of the nickel-aluminum bronze that we used as a model material. For the first time, sympathetic nucleation has been unambiguously identified. Even though Aaronson and Wells first proposed the existence of sympathetic nucleation over 50 years ago, the evidence presented in its favor was always suspect, there was always at least one counter-claim that could explain the observed experimental images; in terms of impingement, polygonization, and the proximity of grain boundaries. We have shown, unequivocally, that impingement cannot produce the microstructures that we developed. Likewise, polygonization is not a problem, and the large beta grain size that we employed in the early stages of the reaction obviates any worries with regard to the interference of grain boundaries. Our crystallographic data from the orientation imaging microscope (OIM) are fully consistent with our model, which predicts that large rod-based colonies, which comprise several hundred sympathetically nucleated entities is, to all intents and purposes, a single crystal. We have documented two fundamentally distinct grain boundary morphology types, the allotriomorph and the secondary Widmanstätten side-rod. The only distinguishing feature of the grain boundary allotriomorph is the fact that its growth rate within the boundary is much greater than its growth normal to the boundary and an approximately lenticular shape is not uncommon. Secondary Widmanstätten side-rods develop with the same growth direction as the intragranular Widmanstätten rods, i.e. 〈111〉_β. Thus far, we have been unable to distinguish fully between the two predominant models for the formation of Widmanstätten side-arms; that due to Townsend and Kirkaldy which is based on interfacial fluctuations that eventually develop into side-arms, and that due to Phelan and colleagues which is based, somewhat coincidentally on sympathetic nucleation. However, we do propose that the thickening of the grain boundary side-rods occurs by heterogeneous facet-to-facet sympathetic nucleation and that the lengthening of the grain boundary side-rods occurs by end-to-end sympathetic nucleation.