Shape memory alloys respond to deformation by indentation with strong two-way cyclic displacements on heating and cooling. Shallow indents can vanish on heating, and deeper indents will change depth when thermally cycled. In the present work, following indentation by an added step of surface planarization can furthermore produce what has been termed "Surface Form Memory" or SFM. The term SFM describes an effect that causes one surface form to reversibly transform to another, under thermal excitation. The term surface form is meant to distinguish between say, flat and "bumpy" surfaces, on a scale much larger than the intrinsic surface roughness. Surface protrusions, or `exdents' may reach an amplitude near 20% of the initial indent depth, and may be created on nano to macroscopic size scales. The surface form memory effect may have special applications into novel optical, microelectromechanical, and tribological systems. Surface deformation of a NiTi shape memory alloy (SMA) was accomplished using spherical, cylindrical, pyramidal, and flat punch indenters. Single indents and indent arrays have been made. The amplitude of SFM after planarization was found to be related to the size of the subsurface deformation zone and can be maximized by thermal cycling, multiple indentations, by changing indent depth and spacing, and by varying indentation temperature parameters. The spatial extent of the subsurface deformation zone was studied by both experimental methods and finite element modeling. Methods such as successive front and back thinning probed the size of the deformation zone directly, while finite element modeling provided stress-strain information supporting the conclusion that the deformation zone responsible for the two-way effect was a slip zone that had plastic strains larger than 7%. NiTi SFM, when studied in constrained recovery experiments, showed an energy density up to 10 MJ/m3, which is about the same as that of a conventional NiTi actuator. This proves SFM's potential to be used in high energy density actuation devices. Finally, Laser Shock Peening (LSP) was demonstrated to make indents in bulk NiTi alloys efficiently. These dynamically made indents have larger two-way response ratio than the quasi-static indents made by conventional indenters. Combining the high energy density and high efficiency of the LSP process, LSP SFM may see special applications into thin film actuators and MEMS devices.
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