Investigating phase transitions and strength in single-crystal sapphire using shock–reshock loading techniques

This paper focuses on developing a technique that can probe phase transitions in ceramics and evaluating strength properties of single-crystal sapphire. High strength ceramics that undergo solid-to-solid polymorphic phase transitions can display a wide range of volume changes. Materials with large volumetric changes can be easily detected; however, materials demonstrating small volume changes, pose a different experimental problem. Because of the variety of diagnostics tools (for acquiring data), and differing interpretation of the data, materials that undergo small volume changes, make detection of transformation quite challenging. Different experimental test methods are needed to potentially allow detection and characterization of materials undergoing phase transitions with small volume changes. In addition, these results show that single-crystal sapphire has considerable strength loss at approximately 56 GPa on the Hugoniot. In this study, we report loading profile measurements in the form of particle velocity histories for single-crystal sapphire that is shocked to a given stress level directly or through multiple steps from a shocked state of 56 GPa. We probe the region of suggested phase transformations utilizing the simplest type of off-Hugoniot reshock loading. This also allows an assessment of shear strength in the shocked state of single-crystal c-axis sapphire. The present measurements show no evidence of a phase transition, but do suggest considerable strength loss in sapphire.