PhD Thesis Defense: Rachel Osmundsen

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"New Insights on the Work Hardening Mechanisms of Ordered Alloys with Special Attention to Antiphase Boundary Tubes"

Abstract

Intermetallic alloys show a dramatic increase in their work hardening rate (WHR) when ordered. Starting in 1962, antiphase boundary (APB) tubes, which are unique to ordered alloys, were theorized to explain this phenomenon. This thesis shows that APB tubes do not impact the WHR of ordered intermetallics. This was verified in multiple alloy systems and experiments were undertaken to determine what does cause the increase in WHR.

First, single crystals of B2-ordered FeAl were deformed and the hardness was measured with APB tubes present and then after the APB tubes were annihilated (with no additional changes in the deformation microstructure). The hardness did not change, showing that APB tubes do not affect the hardening of FeAl.

Second, single crystals of L1₂-ordered Ni₃Al were lightly deformed and observed in the transmission electron microscope (TEM). In-situ TEM heating was performed to determine the APB tube annihilation temperature. The annihilation temperature was then compared to the existing literature on the mechanical properties of Ni₃Al and it was determined that APB tubes do not affect the strengthening of Ni₃Al.

Third, TEM in-situ straining tests were performed on f.c.c. (disordered) and L1₂ (ordered) Ni₃Fe to compare dislocation motion and determine the mechanisms generating the high WHR without APB tubes. Superdislocations in ordered Ni₃Fe exhibited far more dislocation interactions, which contribute to the WHR, then the dislocations in disordered Ni₃Fe. Tensile tests at temperatures just below the order-disorder transition temperature showed that the WHR remained higher in ordered Ni₃Fe than disordered, which is consistent with superdislocation interactions being the main source of strengthening.

Fourth, the large library of images gathered throughout the course of this thesis were examined for evidence of various APB tube formation mechanisms. Evidence supporting the cross-slip annihilation model was found as well as a new intersection feature that has no counterpart in the literature.

The evidence gathered throughout this thesis supports the conclusion that APB tubes do not contribute to the high WHR of ordered intermetallics. Instead, APB tubes are viewed as evidence of the motion of complex superdislocation interactions which are the source of the high WHR.

Thesis Committee

• Ian Baker, PhD
• Erland Schulson, PhD
• Harold Frost, PhD
• Baptiste Gault, PhD

Contact

For more information, contact Theresa Fuller at theresa.d.fuller@dartmouth.edu.