Modeling mechanical properties – linking atomistics to continuum

A combination of atomistic simulations and experiments is used to study the microscopic deformation mechanisms of materials. Atomic scale simulation tools include molecular dynamics and density functional theory. The experiments involve uniaxial tension and compression tests on: (a) single crystals of low stacking fault energy Co-33%Ni alloys, (b) polycrystalline nano-twinned Ni-x%Co alloys (x = 1, 1.62, 2.9 and 5.6), and (c) equiatomic NiTi shape memory alloys. The reason these materials are chosen as candidates for study is that these materials have very distinct microstructure and mechanical properties. The single crystals of Co-33%Ni alloys demonstrated a competition between slip and twinning based deformations. Depending on the single crystal orientation and loading direction (tension/compression), either mode is activated. A Schmid factor analysis demonstrated that slip or twinning is triggered when the resolved shear stress on the respective systems is the maximum. In other words, the critical resolved shear stress (CRSS) for slip and twinning are found to be around 15 MPa and 38 MPa respectively. To rationalize the experimental observations, the fault energy surfaces (generalized stacking/planar fault energies) are computed. Using a Peierls-Nabarro based modeling framework, the CRSS levels for slip and twinning are predicted on a physical ground. On the whole, the results indicate the inherent material propensity to select either mode of plasticity originates from the underlying fault energetics. Using the fault energy considerations, the anomalous composition dependence of Ni-x%Co alloys (x = 1, 1.62, 2.9 and 5.6) is clarified. The experimental stress-strain curves demonstrated a non-uniform strengthening attributes with respect to the composition. This effect is traced back to the atomic level fault energy surface as a direct consequence of alloying. The existence of competing mechanisms (solid solution hardening versus Suzuki segregation) was discovered. The presence of nano-sized twins in the microstructure (as unveiled by electron backscatter diffraction and transmission electron microcopy) serve to enhance the atomic scale energetics in the form of slip characteristics. Using molecular dynamics simulations, various slip transfer mechanisms at the coherent twin boundary is categorized. The extrinsic levels of the energy barriers to the dislocation slip for individual mechanisms are computed. The corresponding frictional stresses are extracted to eventually arrive at CRSS values. These CRSS values for each alloys represents those for the nano-twinned microstructure. A reasonably good agreement between the experiments and theory lends credence to the modeling approach. Using similar atomistic simulations, we resolve the mechanistic origin of the NiTi superelasticity. Using a newly developed pair potential, the compressive load/unload behavior of austenitic NiTi (consisting of B2 lattice) is examined. The material is demonstrated to undergo full strain recover upon 5% of straining. The origin of such deformation recoverability is traced back to the existence of a reversible phase transformation between the austenite and martensite crystals. The existence of a tension-compression asymmetry is observed in general agreement with the earlier experimental trends in the literature. Such behavior is attributed to the uni-directional nature of phase transformation phenomena (which is found to be governed by deformation twinning of the martensite phase). Inspired by the model output on the pristine (i.e. defect-free) NiTi, we set out to examine the similar attributes in terms of stress-strain response as well as the crystallographic mechanisms in presence of a precipitate. A nano-sized precipitate of rhombohedral lattice type is constructed with a lenticular geometry. Such shape is assumed on the basis of literature electron microscopy observations. Then, the precipitate is embedded in an austenite B2 lattice and then the composite system is energetically minimized. Upon the energetic relaxation, an elastic disturbance field is observed to generate as a direct consequence of the local atomic misfit at the precipitate-matrix periphery. The observed disturbance is quantified in terms of the stress profile along a line. The profile is found to be in good experiment agreement with high resolution electron microscopy measurements as reported earlier in the literature, however, in disagreement with Eshelby based continuum predictions. Subsequently, simulations of compressive deformation reveals that the presence of precipitate reduces the transformation stress, strain and the overall hysteresis of the load-unload hysteresis curve