The role of temperature and microstructure on the dynamic strength of materials

• Main Author: Chen, Laura
• Other Authors: Eakins, Daniel ; Proud, Bill
• Published: Imperial College London 2015
• Subjects: 620.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.686307

Shock physics is the study of material properties under extreme conditions. It specifically pertains to the material state evolution through high strain-rate, high temperature, and high pressure loading. This area of research allows us to bring into view the processes that govern our universe, including those that created planets, as well as discover and create advancements for mankind, such as improved manufacturing, nuclear fusion, and planetary defence. High rate loading brings a material into a non-equilibrium state via a high pressure deformation wave propagating through the material, and the ability of the material to relax to a dynamic equilibrium state reveals important strength information. This thesis examines the role of temperature and microstructure on the dynamic strength of solid metals. Motivated by developing an understanding of meteoroid break-up and establishing techniques for asteroid deflection, this work studied the dynamic strength properties of materials, specifically looking at metallic systems. Materials of interest included recovered meteorite samples from Canyon Diablo and the Gibeon crater and commercially-fabricated metal samples of iron, various Fe-Ni compositions (including Invar), aluminium, and tantalum. The meteorites studied were predominantly Fe-based compositions, and therefore iron, Fe-Ni and Invar were of particular interest. Aluminium and tantalum were studied for comparison of Invar to another FCC metal (aluminium) and of iron to another BCC metal (tantalum). Fe-Ni samples were found to be a mix of the two crystal structures. Laser-driven shock-loading experiments were performed on each material and dynamic strength was characterised for each material as a function of temperature and strain-rate. To ascertain the temperature dependence, a system was developed for pre-heating and pre-cooling targets for laser-driven shock-loading experiments. These experiments were the first high strain-rates laser-driven shock-loading experiments on metallic meteorites and on Invar, as well as on bulk metallic glasses. Analysis was implemented through the consideration of material deformation at the solid state, microscopic, and bulk levels to account for unexpected material behaviour. The work on aluminium and Invar was important because it re-examined the classical view of categorising material relaxation at high strain-rates by crystal structure and determined that electronic structure is likely a more reasonable metric. Additionally, this work is the first to hypothesize the importance of magnetic structure on material relaxation at high strain-rates through the first dynamic loading experiments of Invar. The work on iron and tantalum furthers the exploration of transitions between microstructural deformation mechanisms (i.e. slip vs. twinning) and specifically has brought up the possibility of the existence of a dynamic temperature-dependent twinning-to-slip transition in tantalum, which has not yet been seen in any material under dynamic loading.