Date of Award

12-2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Easo P. George

Committee Members

Dayakar Penumadu, George M. Pharr, Peter K. Liaw

Abstract

Fe3Al is an intermetallic compound which has shown some excellent engineering properties and has been widely studied for this reason. It also shows interesting mechanical phenomenon like yield stress anomaly and pseudoelasticity. Mechanical behavior and diffraction studies on the pseudoelastic aspect of Fe3Al have been presented in this work.

Single crystalline, D03 ordered Fe3Al is known to show pseudoelastic behavior at room temperature. Pseudoelastic behavior was seen in both tension and compression with a distinct tension-compression asymmetry. No strain hardening occurred under tension even at high applied strains as opposed to compression, where the alloy strain hardened continuously. In-situ observations on the surface revealed reversible features indicating activity on the (211) planes.

The tensile stress-strain curve shows notable changes with varying temperature. At very low temperatures (~100 K) shape memory effect is seen for small amounts (~3% in compression) of applied strain. At high temperatures (~393 K) pseudoelasticity is lost and plasticity commences. Between these two extremes, the reverse stress (stress during strain recovery) follows the Clausius-Clayperon type relationship with temperature but the forward stress remains unchanged.

In-situ Neutron Diffraction experiments in both tension and compression show large reversible changes in the diffraction pattern upon loading. Intensities and position of various peaks changed reversibly by large amounts during the load-unload cycle. All changes in the diffraction pattern revert back close to the original pattern upon unloading. These changes are closely correlated to the load-unload stress-strain curve. These large changes in the diffraction pattern point towards major structural changes inside the crystal and cannot be explained by elastic effects alone.

Closer inspection revealed the appearance of new peaks and satellite reflections on loading, which disappeared upon unloading. Diffraction experiments point towards a phase transformation which might be responsible for the pseudoelastic behavior in Fe3Al.

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