Abstract
Phase transformations play an important role in the mechanical behavior of materials subjected to extreme loading conditions. A series of shock-reshock experiments were fielded to determine whether the phase transitions in materials are significantly enhanced or inhibited by preexisting microstructural features. Polycrystalline zirconium samples were shock loaded using gas-gun plate impact and soft recovered to examine microstructure using electron backscatter diffraction (EBSD). Drive conditions were varied to study the (hcp) alpha to (hexagonal) omega solidsolid phase transformation. Recovered samples were then subjected to a second shock loading event to determine the change in material behavior as a function of pre-shock microstructure. Crystallography of phase fragments in the final microstructure showed that prior twinning (formed during the shock to a peak stress below the transition threshold) appeared to suppress omega formation/retention after reshock. Conversely, when a material was initially shocked into the omega phase field, retained-omega was not found to have a large impact on subsequent omega formation during reshock. This suggests that nucleation and growth of omega phase are important processes, and the relative activity of nucleation vs. growth processes is modified by a pre-existing substructure. Additionally, orientation relationships reveal a reverse transformation pathway (omega to alpha) dominates the final microstructure, suggesting significant grain growth in the omega phase field is possible even for dynamic timescales.
Highlights
Phase transformations play an important role in the mechanical behavior of materials subjected to extreme loading conditions
The lowest stress is known to be below the alpha-omega phase transition threshold for shock-loaded Zr of 7 GPa [1], while the medium and high stresses are above the transition stress
The 4 and 8 GPa shock microstructures were used as the initial microstructure for subsequent shock loading, two of which are shown at the right of Fig. 2
Summary
Phase transformations play an important role in the mechanical behavior of materials subjected to extreme loading conditions. Both newly-generated material phases and deformation mechanisms such as twinning have a large effect on the material at a microstructural level, which in turn drives observed strength and damage phenomena during shock loading. If some phase reversion is observed, as is the case with Zr, a more careful analysis is necessary to determine the pathway that the microstructure traversed between ambient, shocked, and unloaded conditions. As an extension of this, the effect of initial microstructure on the mechanism and kinetics of the a-w phase transition is poorly understood, but the hypothesis is that the initial microstructure (prior to shock loading) plays an important role in the transformation behavior and kinetics. The resulting two-phase microstructures are characterized with respect to plasticity (i.e. twinning) and phase transformations, with special attention to phase fractions, orientation, morphology, and texture to determine the effect of the initial microstructure and test conditions on phase transformation
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