Using Time-resolved Wave Profile Measurements to Determine Elusive Phase Transitions in Molybdenum

The purpose of this study was to investigate the dynamic strength of molybdenum (Mo) at high pressures, particularly at the location of a solid-solid phase transition within Mo prior to entering the melt regime. The intent was to build on previous work by conducting both symmetric and non-symmetric impact experiments using a two-stage light gas gun and VISAR diagnostic system to examine molybdenum behavior up to pressures of 305 GPa. The approach required compensating for the wave interaction due to the low impedance LiF window, but provided detailed information regarding the release state. The main effort consisted of a series of Mo symmetric impact experiments having a fixed ratio of impactor thickness (1 mm) to target thickness (4.25 mm) and variation of the impact velocity which resulted in changes in both shock and release wave velocity. Based on the geometry, any structural change resulting in a change in release wave velocity would be noticeable through changes in the amount of time at the Hugoniot state or dwell time. As expected, the shock velocity increased proportionally with increased impact velocity, but a noticeable change in dwell time occurred at approximately 190 GPa. Additional analysis of the strength variation of Mo showed an increase in strength from 1.3 GPa to 3 GPa at approximately 190 GPa the location of phase transition, followed by a dramatic drop in strength for stresses above 190 GPa. Strength increased again for stresses of 300 and 305 GPa. The data acquired in this effort indicate the phase transition occurs at a slightly lower pressure than previously indicated and there appears to be a mixed phase region. This paper describes the strategy, experimental method, and corresponding results which are used to draw conclusions about the dynamic strength of molybdenum at high pressure.