Nanovoid induced martensitic growth under uniaxial stress: Effect of misfit strain, temperature and nanovoid size on PT threshold stress and nanostructure in NiAl

Abstract

In the present work, the effect of a pre-existing nanovoid on martensitic growth under uniaxial stress in NiAl is investigated using the phase field theory. In order to create a pre-existing nanovoid in the model, a single nanovoid has been stabilized in the center of the computational domain using a phase field approach. The coupled system of Ginzburg-Landau and elasticity equations are then solved for the evolution of martensitic nanostructure in the presence of the obtained nanovoid. The finite element approach and the COMSOL code are used to solve the above systems of equations. Even though the Ginzburg-Landau model includes two martensitic variants, only the first variant grows under uniaxial stress. The phase transformation (PT) threshold stress is found for different nanovoid radii and different temperatures. The dependence of the misfit strain on the nanovoid radius is considered which results in proper surface stress. The PT threshold stress linearly increases as the temperature increases for any nanovoid radius. The effect of temperature on the PT is found more pronounced for smaller nanovoids. It is also revealed that the PT threshold stress exponentially reduces as the nanovoid radius increases, in agreement with previous experimental/molecular dynamics results. Neglecting the size dependence of the misfit strain leads to a smaller PT threshold stress especially for lower temperatures and larger radii. For any misfit strain, the effect of nanovoid on the PT is more pronounced for larger temperatures. Under the uniaxial stress, the first variant started to grow vertically from the upper and lower surfaces of the nanovoid which evolution is explained based on the distributions of the stress and transformation work. The martensitic growth is more pronounced for larger radii. The martensitic growth is also studied for different temperatures. As the temperature increases, the growth is slower and this is explained using the phase concentration and its time rate. As the temperature increases, the phase concentration decreases and the growth rate decreases at the same phase concentration. The stress and transformation work distributions for different temperatures and their corresponding PT threshold stresses for a constant radius are studied for the earlier stage of growth. As the temperature increases, a larger uniaxial stress is required for the martensitic growth. The effect of mechanical driving force on the nanovoid-induced martensitic growth is studied by applying different uniaxial tensions. As the loading increases, the phase concentration increases and the growth rate increases at the same phase concentration. The obtained results allow for a better interpretation of the effect of nanovoids on the PT at the nanoscale and will help to develop an interaction model between nanovoids and multiphase structure at the nanoscale.