Research Projects

Displacive phase transformations (DPTs) are central in exhibiting extraordinary properties such as supere- lasticity and shape memory effects in shape memory alloys; improved yield strength in various steels, many minerals, and other industrially important materials such as graphite/diamond, Ti, Zr, Nb, Hf and some of their alloys, etc. During the DPTs a parent phase (low-pressure/high-temperature phase) transforms into the product phase(s) (high-pressure/low-temperature phase(s)) with different crystal structures. The plastic strains-induced DPTs are commonly observed in industrially important materials, including iron, various steels, Ti, Zr, Hf, Nb, Mg, and some of their alloys. The yield strength of such materials is much lower than the stresses required for stress-induced transformations. The plastic deformations occur by nu- cleation and dynamics of dislocations and deformation twinning. The local stress fields of the dislocations and their pile-ups assist the parent phase in overcoming the barrier for transformations to the product phase at much lower pressure (pressure reduction up to a factor of 100) than that required in the absence of plastic shear. In polycrystalline solids, the stress concentration within the plastic strain localization near the grain boundaries, triple junctions, isolated shear bands and their intersecting regions promote the phase transformations, which have been observed experimentally and in atomistic simulations. The present proposal aims to develop large strains-based thermodynamically consistent theories to study the strains-induced DPTs at two different length scales, i.e. nano and microscale, and perform high-pressure torsion tests for validation. (i) A nanoscale phase-field model will be developed to study the nucleation and growth of the phases from the dislocations and their pile-ups. The effect of dislocations separated by distance ∼ 10 nm (e.g., under sever plastic deformations) in nanocrystalline solids coupled with the kinetics of the grain boundaries will be studied. (ii) A microscale strain-gradient plasticity model coupled to DPTs (induced by dislocation pile-ups, shear bands and their intersections) will be developed, where the length-scale of the plastic strain inhomo- geneities (thickness of the pile-ups, shear bands) will enter in the theory by incorporating the dependency of the constitutive relations on plastic strain gradient. The transformation criterion in inelastic materials will be derived for both models. The kinetic laws for the coupled plasticity and DPTs will be derived using the thermodynamic laws and solved using finite element methods in an open-source C++-based package deal.ii. Large-scale simulations in polycrystals, which will involve billions of degrees of freedom, require us to use high-performance computers. The in-situ high-pressure torsion test will be conducted on commercially available high-purity titanium and zirconium to validate our model and the simulation results.