TEM investigation of rapidly deformed Cu and Mo shaped charge liner material
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The strength and ductility of metals is a vast and important research area in which certain trends are well known, but where it is difficult to predict results with a high level of certainty, especially under extreme conditions e.g. high strain rates and very small grain sizes. Results may also be strongly influenced by impurities. All of the above factors play a vital role in the performance of shaped charge liners. Of particular interest is the material used in the manufacturing of liners. The microstructure and extended defects of copper and molybdenum shaped charge liners were investigated. Samples were extracted from the liners by electric discharge machining, to minimize any microstructural damage. Chemical testing revealed a higher than expected impurity concentration. Samples were annealed under two different annealing conditions, in order to obtain a variety of starting microstructures. Copper samples were annealed at 300°C for 30 minutes and 500°C for 30 minutes. Molybdenum samples were annealed at 1200°C for 30 minutes and 1200°C for 3 hours. These samples were then deformed at high strain rates using a split Hopkinson pressure bar. Two strain rates were used, the higher strain rate being approximately twice that of the lower strain rate. For both the copper and molybdenum the lower strain rate was on average 700 s-1, while the higher strain rate was on average 1550 s-1 and 1650 s-1 in the case of copper and molybdenum respectively. In the case of the molybdenum, the results showed a strong strain rate dependency of the yield strength which is typical of body centred cubic materials, whereas no such strain rate dependency could be detected in the copper results. Both materials show significant softening due to annealing, but relatively small changes between less and more intense annealing procedures. The unannealed samples showed significant variation in the stress-strain results, which is attributed to them originating from different parts of the liner. The uniformity of results after annealing indicates that the stress-strain properties of both materials after annealing are not strongly dependent on their prior straining history. The microstructure of these samples was examined using an optical microscope as well as a scanning electron microscope. The grain size was determined using the Heyn method. The as-received copper material had an elongated and heavily deformed microstructure. The lower annealing temperature produced a recrystallised grain structure, having an average grain size of 5 μm. The higher annealing temperature allowed grain growth with grains averaging 9 μm. The annealed copper samples contained annealing twins. In the case of molybdenum, the as-received material consisted of large (200 μm) grains. Annealing under both annealing conditions produced the same recrystallised, non-uniform grain structure with grains ranging from 47 μm to 92 μm. Transmission electron microscopy investigations of the samples revealed that deformation twinning occurred in the annealed and strained copper samples. This twinning occurred at a lower strain rate than expected. Dislocations in an annealed but unstrained copper sample occurred in entangled networks separated with areas containing no dislocations. These mixed dislocations were found to have Burgers vectors of the type b = a/2<110>. Pure edge dislocations with a  projected direction in the (110) plane with a Burgers vector of the type b = a/2[110 ] were also found. These dislocation arrays appear as ripple like structures. No evidence of twinning was found in the molybdenum samples. Some dislocations with Burger vectors of the type b = a/2<111> were found in the molybdenum samples. There are however exceptions, which is difficult to explain. This is an important observation, and further research would have to be performed.