Dynamic behavior of metals at elevated temperatures and ultra-high strain-rates

This paper presents results of a series of reverse geometry normal plate impact experiments designed to investigate the onset of incipient plasticity in commercial purity polycrystalline magnesium (99.9%) under weak uniaxial strain shock compression and elevated temperatures up to melt.  Strategic modifications made to the existing single-stage gas-gun facility enable dynamic material behavior characterization under extreme conditions, i.e. ultra-high strain-rates (~10⁶/s) and test temperatures up to 1000 °C.  In this custom configuration, thin metal samples (flyer plate) carried by the specially designed heat-resistant sabot are allowed to be heated uniformly across the diameter in a 100 mTorr vacuum prior to impact by a resistance coil heater with axial and rotational degrees of freedom at the breech end of the gun barrel.  Moreover, a compact fiber-optics-based heterodyne combined normal and transverse displacement interferometer is designed and implemented.  Like the standard PDV, this diagnostic tool is assembled using commercially available telecommunications hardware and uses a 1550 nm wavelength 2W fiber-coupled laser, an optical probe, and single mode fibers to transport light to and from the target.  Using this unique approach, normal plate impact experiments are conducted on preheated (room temperature to near melt point of magnesium) 99.9% polycrystalline magnesium using Inconel 718 target plates at impact velocities ranging from 100 m/s to 110 m/s.  The stress at flyer/target interface, as inferred from the measured normal particle velocity history at the free (rear) surface of the target plate shows progressive weakening with increasing sample temperatures below melt; at higher test temperatures, the rate of softening in stress is observed to weaken and even reverse as the sample temperatures approach the melt point of magnesium samples.  Scanning electron microscopy is utilized to understand the evolution of sample material microstructure following the impact event.