Plenary: Huseyin Sehitoglu, University of Illinois
8:10 - 8:50 a.m.: Plenary: Huseyin Sehitoglu, University of Illinois
Huseyin Sehitoglu, University of Illinois
“Transformation-Induced Slip in Shape Memory Alloys and its Implications in Iirreversibility and Fatigue”
Huseyin Sehitoglu, Sameer Khan Mohammed
Shape Memory Alloys (SMAs) undergo stress-induced martensitic phase-transformation affording a “superelastic” behavior with functional applications. Such behavior is undermined by the formation and accumulation of irreversible residual strains in each cycle. These residual strains arise from transformation-induced slip-emission and current understanding has postulated the microstructural role of emitted dislocations to accommodate lattice-mismatch while also observing a preference to occur during reverse-transformation. This study develops a thermodynamic framework to offer a causal explanation for slip-emission from Gibbs’ free energy considerations. Superelastic stress-strain curves for a reversible pathway sans emitted-slip and for an irreversible pathway with emitted-slip are derived. It is shown that both pathways obey the first law of thermodynamics but it is the second law of thermodynamics that dictates the path preference. The driving-forces associated with the irreversible pathway are analyzed to establish the cause of emitted-slip and why it is thermodynamically preferred despite offering a higher lattice-friction barrier. Consequently, a new approach to target fatigue-resistant SMA properties is offered, focusing on the interplay of the individual driving-forces coming from the elastic strain-energy, work-interaction, and lattice-friction, as revealed by the thermodynamic framework. We also show the theory of slip emission during reverse loading is in agreement with experimental in-situ loading TEM observations. Our focus is primarily on NiTi but we will also discuss iron and copper based shape memory alloys.