In-Situ High Strain Rate Study of Mg Deformation

     One component of the DoD-funded project "Materials in Extreme Dynamic Environments" is very high strain rate mechanical testing of Mg alloys with concurrent electron microscopy. Electron microscopy provides unequaled resolution of the details required to understand in detail the specific mechanism(s) by which materials plastically deform. In situ mechanical testing in a transmission electron microscope (TEM) is an established technique for identifying deformation mechanisms, but previous efforts have focused on static samples or low strain rates in the 0.01 - 0.1 s-1 range. High-strain rate testing has also been done, but these studies have been limited to bulk samples and bulk characterization of strain and deformation using strain gauges and high speed cameras. The current effort to combine high-strain rate testing with in situ electron microscopy is the first of its kind, and if successful will provide valuable information about the way Mg alloys deform as we approach the loading rates expected in Army applications. This data will be incorporated into a multi-scale model that the Army will use to design and predict the properties of next-generation lightweight armor materials.

     The current approach aims to utilize high-performance piezoelectric actuators to load a thinned sample at strain rates from 102 - 104 s-1. Because traditional TEMs are too slow to capture deformation at these rates, these experiments will make use of the dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory. We are currently constructing and characterizing a new in situ sample holder to go into the DTEM for these tests. Preliminary experiments will use aluminum for ease of handling, after which we will move on to studies of Mg and Mg alloys.


                                      Schematic of in situ TEM holder tip showing piezoelectric actuators and the sample loading scheme.



                                                                           Demonstration of DTEM Straining Holder Operation