The Shock and Spall Response of Kovar

The shock response and spall strength of Kovar®, an alloy of Fe-29Ni-17Co, are obtained using a single stage powder gun to achieve pressures between 6 and 55 GPa by performing symmetric, planar impact tests. Above ~ 6.5 GPa, the behavior shifts from a dispersive, continuous pressure rise to a shock, and it is shown that the material displays anomalous behavior at low pressures. Above 6.5 GPa, the Hugoniot is described by a linear relationship between shock and particle velocity of US = 2.06UP + 3.54 km/s. The spall strength, an engineering description of the spall phenomenon, was calculated and shown to increase from 3.6-7.4 GPa as stress increases from 6-55 GPa, and it is shown that the method of spall strength computation has a large effect. From previous studies on the iron-nickel system, the authors expected to observe evidence of an α → E phase transition in the material but did not. Upon examining the shock and metallurgical literature, high concentrations of Ni and Co will have competing effects on the phase transition, but the effect of Ni will likely dominate and decrease the phase transition pressure. Further experiments on the high alloy Fe-Ni-Co system are needed to understand Kovar's complex low pressure mechanical behavior.