Shape memory in nanostructured metallic alloys

Materials with nanoscale dimensions show mechanical and structural properties different to those at the macro scale and engineering their nanostructure opens up potential avenues for designing materials tailored for a specific application. This work is focused on shape memory materials, an important class of active materials with wide variety of applications in medical, aerospace and automobile industries, due to their two important properties of super-elasticity and shape memory. These unique properties originate from a solid-solid transformation called martensite transformation and the main objectives of this research are to i) study the atomic mechanisms of the martensite transformation, ii) study the effect of nano-structure on shape memory behavior and iii) computationally explore avenues through which their performance is optimized. A combination of density functional theory (DFT) and molecular dynamics (MD) simulations is used to achieve this. This approach gives an atomic level description and the effects of size, surfaces and interfaces are explicitly described. Detailed analysis of the atomic mechanisms of the martensite transformation in NiTi using DFT revealed a new phase transformation (B19'–B19'') that sheds light on why the theoretically predicted ground state (BCO) is not observed experimentally and that the experimentally observed martensite phase (B19') can be stabilized by internal stresses. This finding is very important as the theoretically predicted ground state does not allow for shape memory in nanoscale NiTi samples.;The size effects caused by the presence of free surfaces and the role of nanostructure in martensite transformation have been investigated in thin NiTi slabs. Surface energies of B2 phase (austenite), B19 (orthorhombic), B19' (martensite) and the body centered orthorhombic phase (BCO) are calculated using DFT. (110)B2 surfaces with in-plane atomic displacements stabilize the austenite phase with respect to B19' and BCO, thus slabs with such orientations are predicted to exhibit a decrease in martensite transition temperature with decreasing thickness. It has been predicted that a thickness of 2 nm is critical for the transition to occur, below which there will be no shape memory in thin NiTi slabs. The opposite trend is observed in slabs with atomic displacements along the surface normal; the phase transformation temperature increases with decreasing size. Finally, it has also been demonstrated via large scale molecular dynamics simulations that the austenite — martensite energy landscapes of NiAl based thermo-elastic shape memory superlattice structures can be engineered via epitaxial integration. This allows for tuning their thermal hysteresis and transition temperatures. This resulted in materials with very low thermal hysteresis and with minimal degradation in phase transition strain thereby making them ideal for actuating applications.