Surface form memory by indentation and planarization of NiTi: displacements and mechanical energy density during constrained recovery

• Published in: Journal of materials science Vol. 46; no. 23; pp. 7401 - 7409
• Main Authors: Fei, Xueling, O’Connell, Corey J., Grummon, D. S., Cheng, Yang-Tse
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01.12.2011; Springer; Springer Nature B.V
• Subjects: Alloys; Base metal; Characterization and Evaluation of Materials; Chemical-mechanical polishing; Chemistry and Materials Science; Classical Mechanics; Compressive strength; Contact pressure; Crystallography and Scattering Methods; Density; Energy recovery; Enthalpy; Flux density; Indentation; Intermetallic compounds; Intermetallics; Martensitic transformations; Materials Science; Nickel base alloys; Nickel compounds; Nickel titanides; Polymer Sciences; Shape memory alloys; Solid Mechanics; Surface roughness; Temperature dependence; Titanium compounds; Base Metal; Shape Memory Effect; Shape Memory; Martensite; NiTi Alloy
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-011-5702-6

Indentation-induced two-way shape memory leads to pronounced temperature dependence of the depth of spherical indents made in martensitic NiTi shape-memory alloys. They are shallower when austenitic, and depth varies during both M → A and A → M transformations. If the impression is planarized, by metallographic grinding at T  <  M f , a protrusion rises at the site when warmed past A f . If cooled again this “exdent” retreats, restoring optical flatness. The cycle is repeatable, and exdent heights can exceed 15% of prior indent depth. Since it maps between macroscopically distinguishable topographies, or forms, at orders greater length scale than the surface roughness, we call the effect “surface form memory”—SFM. Notable regarding potential applications is that, when loaded in compression by planar contact with a strong base metal, exdents exert sufficient pressure to indent the latter, suggesting that subsurface transformational mechanisms operate at volumetric work-energy densities >10 7  J/m 3 , fully ~10% of the M → A enthalpy.