Observation and prediction of the deformation and fracture of shape memory alloys

• Main Author: Creuziger, Adam
• Format: Dissertation
• Language: English
• Subjects: applied mechanics; engineering, materials science; engineering, mechanical
• ISBN: 9780549633716; 0549633715

This thesis explores the deformation and fracture behavior of two common shape memory alloys (SMAs), CuAlNi and NiTi. Millimeter, micrometer and nanometer scale features are observed and compared with crystallographic theory of martensite (CTM) predictions and finite element models. Observed fracture behavior, in conjunction with theory and modeling, shed light on the fracture behavior and deformation in shape memory materials. The in plane and out of plane deformations are quantitatively measured and compared with good agreement to predictions from the CTM. For deformation where the stress state was unknown, predicted martensite plates correlated well with observed features. When the stress state could be calculated using finite element analysis (FEA), an available work criterion was used to predict which type of martensite plate would form; with good agreement in arbitrarily oriented, notched CuAlNi samples. The out of plane deformation caused during transformation was quantitatively investigated and agrees well with the average strain of twinned martensite. Using a FEA model of a tapered martensite, the effect of boundary conditions on the out of plane deformation angle was explored.
Some limitations on the available work criterion were found. The direction of the out of plane deformation near the notch is consistently such that the area directly ahead of the notch sinks downward. This effect, and the wide martensite plates observed after fracture occurs, indicate the role boundary conditions have on the transformation observed. These effects are not taken into account in the current available work criterion. In single crystal NiTi, the available work criterion was not predictive of the deformations observed in uniaxial tension or in notched samples. However, available work predictions were useful in predicting the fracture properties of notched single crystal NiTi samples, a capability not previously demonstrated. Investigation into the grain boundary fracture of polycrystalline CuAlNi show that coincident site lattice (CSL) theory can be used to explain why some grain boundaries in CuAlNi do not fail. This indicates that the fracture of polycrystalline CuAlNi is not due to the transformation behavior, but due to the character of the grain boundaries themselves.