Shear induced transformation toughening in ceramics

Allotropy in ultra high strength materials

Allotropic phase transformations may be driven by the application of stresses in many materials; this has been especially well-documented for pressure driven transformations. Recent advances in strengthening materials allow for the application of very large shear stresses as well – opening up vast new regions of stress space. This means that the stress space is six-dimensional (rather than one for pressure) and that phase transformations depend upon crystal/grain orientation. We propose a novel approach for predicting the role of the entire stress tensor on phase transformations in grains of all orientations in any material. This multiscale approach is density functional theory based and guided by nonlinear elasticity. We focus on stress tensor dependent allotropic phase transformations in iron at high pressure and ultra-fine grained nickel and titanium. The results are quantitatively consistent with a range of experimental observations in these disparate systems. This approach enables the balanced design of high strength-high ductility materials.

Here the authors propose a crystal thermodynamics framework describing the tensor stress induced phase transformations in solids based on nonlinear elasticity and first principles calculations. The proposed approach enables balanced design of high-strength, high-ductility materials.

Transformation toughness induced by surface tension of the crack-tip process zone interface: A field-theoretical approach

We study a crystal with a motionless crack exhibiting the transformational process zone at its tip within the field-theoretical approach. The latter enables us to describe the transformation toughness phenomenon and relate it to the solid's location on its phase diagram. We demonstrate that the zone extends backward beyond the crack tip due to the zone boundary surface tension. This setback engenders the crack-tip shielding, thus forming the transformation toughness. We obtain a quadrature expression for the effective fracture toughness using two independent approaches-(i) with the help of the elastic Green function and, alternatively, (ii) using the weight functions-and calculate it numerically applying the results of our simulations. Based on these findings, we derive an accurate analytical approximation that describes the transformation toughness. We further express it in terms of the experimentally accessible parameters of the phase diagram: the hysteresis width, the phase transition line slope, and the transformation strain.

Evidence of shock-compressed stishovite above 300 GPa

SiO₂ is one of the most fundamental constituents in planetary bodies, being an essential building block of major mineral phases in the crust and mantle of terrestrial planets (1-10 M_E). Silica at depths greater than 300 km may be present in the form of the rutile-type, high pressure polymorph stishovite (P4₂/mnm) and its thermodynamic stability is of great interest for understanding the seismic and dynamic structure of planetary interiors. Previous studies on stishovite via static and dynamic (shock) compression techniques are contradictory and the observed differences in the lattice-level response is still not clearly understood. Here, laser-induced shock compression experiments at the LCLS- and SACLA XFEL light-sources elucidate the high-pressure behavior of stishovite on the lattice-level under in situ conditions on the Hugoniot to pressures above 300 GPa. We find stishovite is still (meta-)stable at these conditions, and does not undergo any phase transitions. This contradicts static experiments showing structural transformations to the CaCl₂, α-PbO₂ and pyrite-type structures. However, rate-limited kinetic hindrance may explain our observations. These results are important to our understanding into the validity of EOS data from nanosecond experiments for geophysical applications.

Morphological transformation of the process zone at the tip of a propagating crack. I. Simulation

Stress concentration at a crack tip engenders a process zone, a small domain containing a phase, different from that in the bulk of the solid. We demonstrate that this zone at the tip of a propagating crack exhibits a morphological transformation with an increase of the crack velocity. The concave zone shape with an invagination in its back that is characteristic of a slow crack transforms into a droplet-shaped convex zone upon exceeding a critical velocity value, v_{G}. In this latter case, a metastable wake follows the propagating zone. We obtained this result by computer simulation of a crack propagating in a solid exhibiting a first-order phase transformation.

Crack-tip process zone as a bifurcation problem

Stress concentration at a crack tip generates a solid structural transformation in its vicinity, the process zone. We argue that its formation represents a local phase transition described by a multicomponent order parameter. We derive a system of equations describing the dynamics of the order parameter driven by an inhomogeneous, time-dependent stress field in the solid and show that it exhibits a bifurcation. The latter corresponds to the emergence of a process zone characterized by the distribution of the order parameter localized in the vicinity of the crack tip. The emergence temperature T_{*} considerably differs from the temperature of the bulk phase transformation T_{c}. We demonstrate that T_{*} exhibits a universal behavior T_{*}-T_{c}∼K_{I}^{4/3}, in terms of the stress intensity factor K_{I}, and that the zone universally vanishes upon achieving a critical velocity. These facts together give rise to a universal dynamic phase diagram.

Crack velocity jumps engendered by a transformational process zone

We study a concerted propagation of a fast crack with the process zone where a rearrangement of the solid structure takes place. The latter is treated as a second-order local phase transformation. We demonstrate that the propagation of such a zone gives rise to a nonlinear frictionlike force exerted on the crack tip, resisting its propagation. Depending on the temperature, it produces three regimes of crack motion, which differ in the behavior of the crack tip process zone: (i) always existing, (ii) only emerging at a high crack speed, and (iii) flickering. We show that the latter regime exhibits crack velocity jumps.