Application of numerical calculation methods to the diffusionless transformations in shape memory alloys

SUMMARY PhD. Thesis APPLICATION OF NUMERICAL CALCULATION METHODS TO THE DIFFUSIONLESS TRANSFORMATIONS IN SHAPE MEMORY ALLOYS Soner ÖZGEN Fırat University Graduate School of Natural and Applied Sciences Department of Physics 1997, page : xv//+145 The atomic models of solid-solid diffusionless phase transformations which are essential at the explanation of shape memory effect were investigated by a molecular dynamics simulation technique which allows the system to vary in size and shape. For this purpose, the atomic interactions of binary shape memory Ni-%37.5A1, Ni-%50Ti and Cu- %28A1 alloys were represented by Lennard- Jones potential energy function. For the determination of the parameters of potential energy function, optimisation method was used as well as the theoretical methods. We have reached the following results by the examinations carried out by each set of parameters; the optimised parameters for the binary alloys with different compositions from 50% give more realistic results than the theoretical parameters. Simulation studies carried out using the optimised parameters were investigated from the thermodynamical and structural points of view. At the structural analysis; atomic sites, partially pair correlation functions and diffraction patterns were employed. By the thermodynamic calculations performed depending on the compositional configuration of the austenite phase we have determined that the alloy is more stable and harder, and has a small thermal expansion coefficient in the compositional ordered case whereas it is soft and has an unstable structure and a big thermal expansion coefficient in the clusteral case. A diffusionless phase transformation was observed at Nİ-37.5A1 alloy after the quench. By the structural investigations on the martensite phase it was determined that the martensite phase has a twin-like lattice defect and stacking faults at the atomic planes. Heating-cooling cycle was performed on the same alloy and it was observed that diffusionless reversible phase transformation which has a hysteresis between forward and reverse transformation start temperatures has occured Keywords: Shape memory effect, structural phase transformations, potential energy functions, molecular dynamics simulation, diffraction patterns.