On the shock response of single crystal tin

Thesis

With its anisotropic crystal structure, tin exhibits a strong orientation dependence of its single crystal properties. How this anisotropy affects the behaviour under the one-dimensional compression of shock loading has not been examined. With implications for the understanding of other, higher symmetry materials, a better understanding of the role directionality plays on this material will also help design and validate computational models, most notably crystal plasticity and molecular dynamics techniques.

In this work, the behaviour of single crystal tin is examined through the lens of several key shock phenomena, and compared to the polycrystalline behaviour. The Hugoniot states at the elastic limit and β → γ phase transition were measured using photon Doppler velocimetry to track the surface velocity histories and shock wave speeds. While the former shows an unexpectedly strong orientation dependence, with an order of magnitude difference in the yield point between orientations, above the elastic limit only qualitative variations are observed, with no measurable differences. The spall behaviour also shows no anisotropy. Instead, the strain rate is the primary governing factor of spall strength, and suggests that the spall failure is more similar to a viscous flow than brittle failure, the latter being common in many other metals.

Recovery experiments are conducted to examine the resultant microstructure. All samples show a refinement of grain sizes, especially around surfaces, suggesting recrystallisation forms one of the key pathways of plasticity. Examination of these recovered crystals through EBSD techniques shows strong textures, and great similarity between samples independent of crystal orientation or whether a sample has undergone the phase transition. The primary mechanism of plasticity aligns with an axis flip in the γ phase, rather than the slip or twinning phenomena observed statically.

Considering these results together, the anisotropy of the ambient single crystal is lost when plastic mechanisms activate. With similarities between recovered textures, the bulk behaviours may occur in similar microstructural environments, and with similar plastic mechanisms, resulting in the lack of observed differences between samples. With such similar mechanisms acting in a drag controlled plasticity regime, the HEL and spall strain rate behaviours may also be explained.

Authors: Threadingham, J

• DOI: 10.5287/ora-2zpdq6ayd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: tin; polycrystal; shock; phase transition; spall; single crystal
• Subjects: Shock (Mechanics); Strength of materials; Tin