• Waitz, Thomas
• Karnthaler, H. Peter
• Fischer, Franz Dieter
• Antretter, Thomas

Abstract

<p>Bulk nanocrystalline NiTi alloys were made by methods of severe plastic deformation. Solid state amorphization of NiTi by high pressure torsion was followed by polymorphous devitrification to obtain stress free nanograins of the B2 high temperature phase. Upon cooling, the transformation from B2 austenite to B19' martensite is suppressed by a transformation barrier that increases with decreasing size of the nanograins. Grains with a size of less than about 50 nm do not transform to martensite even at large undercooling. The analysis of the atomic structures by high-resolution transmission electron microscopy reveals the result that the martensitic transformation is taking place by nanoscale twinning. Low-energy twin boundaries facilitate arrays of compound twins on atomic level to overcome the strain energy barrier. Nanograins were modeled as spherical inclusions containing twinned martensite to calculate the transformation energy and to find a critical grain size below which the martensitic transformation becomes unlikely. An energy minimization criterion enables to predict the morphology of the transformed grain. In grains larger than about 100 nm self-accommodation occurs by a unique "herring-bone" microstructure yielding energy minimization and strain compatibility at invariant interfaces. Calculations using the geometrically nonlinear theory of the martensitic transformation agree with the observed geometry of the "herring-bone" microstructure.</p>

Topics

• impedance spectroscopy
• morphology
• compound
• polymer
• grain
• inclusion
• grain size
• phase
• theory
• laser emission spectroscopy
• transmission electron microscopy
• twinned