Unusual Functionalities in Shape Memory Alloys
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Shape memory alloys have received extensive attention in the past 40 years for their superelastic and actuator capabilities, but their application potential is much greater than these two fields. In this work, we explore unique processing routes that lead to new functionalities, such as tissue scaffolds; magneto/elasto caloric; magnetic sensing; perfect one-way actuation and tailored thermal expansion coefficients, in materials that undergo martensitic transformation. First, we show that NiMn based metamagnetic shape memory alloys can be processed into foams with a selected pore size using solid state replication. This enhances the material’s mechanical and heat transfer properties which make it attractive for biomedical implants and magneto/elasto caloric applications. We also explore the biocompatibility of these NiMn based alloys. While the base alloys are shown to be cytotoxic, a layer-by-layer assembled polymer passivation layer is used to improve the biocompatibility. Next, we examine two unique types of glass in NiMn based shape memory alloys that can be stabilized with simple secondary heat treatments. Generalized glassy theory has been used to describe rotational/translational frustration of symmetry in many material systems and functional domains. This theory is extended to the strain domain of thermoelestic transitions to explain frustration observed in the diffusionless martensitic transformations. We show that this “strain glass” frustration can be created through secondary heat treatments on a single magnetic shape memory alloy composition and link the responsible defect structures to cluster spin glass transitions and an experimentally measured Kauzmann point. These results show the power of glassy theory to extend beyond the currently known functional domains and can potentially extend shape memory alloys into magnetic sensing and perfect one-way actuation applications. Last, we propose a statistical thermodynamic model to explain the huge thermal expansion anisotropy exhibited by many martensitic alloys. We show that this model can be used to predict thermal expansion anisotropy directions in many martensitic alloy systems. Processing is then used to tailor the macroscopic thermal expansion coefficient for a NiTiPd test material. This provides scientists and engineers with a previously unavailable design tool which can fulfill the need for thermal expansion compensation.
SubjectShape Memory Alloy
Magnetic Shape Memory Alloy
Meta Magnetic Shape Memory Alloy
Negative Thermal Expansion
Monroe, James Alan (2013). Unusual Functionalities in Shape Memory Alloys. Doctoral dissertation, Texas A&M University. Available electronically from