Kinetics of iron α-εphase transition under thermodynamic path of multiple shock loading-unloading

The dynamics of iron under extreme conditions like high temperature and high pressure has been well studied for several decades. But, there have been not many reports about the phase transition kinetics coupled with complicated thermodynamic paths, especially loading-unloading-reloading path, which is closer to the real applications. A three-layer structure impactor with five stages performed in the front-surface experiment is made up to approach the special path. We choose epoxy to be the adhesive as it has low impedance and high strength. Tantalum, the standard material of high impedance which also has single wave structure, is selected for reloading process. The wave profile shows a 3-wave structure in the first unloading period and the inverse phase transition threshold is calculated to be about 11.3 GPa. This onset pressure of reverse phase transition is not consistent with Barker’s result, higher than his result (about 2.5 GPa). By comparing with recalculated result of Jensen’s data, we find that our result is consistent with theirs.In this work the inverse phase transition ends at about 10 GPa, the value from this way which is higher than Barker’s finding, even higher than his result of the threshold pressure of reverse phase transition. And at this state there remains 12%–15% of e phase. So it cannot be seen as the completed reverse phase transformation. The phase transition onset pressure is 10–12 GPa on the reloading path and it is about 1–2 GPa lower than the first phase transition. By simulating the wave profile, the discrepancy of using different phase transformation characteristic time t as 30 ns and 5 ns is analyzed. It can be seen that the phase transition rate of reloading is faster than that of the first loading process. These phenomena may be caused by the twins and the dislocations which are produced by the inverse phase transition. Also, as unloading time becomes longer, the mass fraction of e phase becomes lesser and the onset pressure of a → e phase transition becomes lower. This because with more e phases transforming into a phase, more twins and dislocations will be produced in material. Therefore, it brings the lower onset pressure.


Fig. 2 .
Fig. 2. The phase diagram of iron and the thermodynamic loading path.

Fig. 4 .
Fig. 4. Schematic diagram of sound velocity calculation(A is the impact moment, B is the rarefaction wave arrival time, C is arbitrary time of unloading process).

Fig. 5 .
Fig. 5. Particle velocity of impactor/window interface (A is the impact moment, B is the rarefaction wave arrival time, C is the elastoplastic unloading crutch point, D is the start point of reverse phase transition, E is the ending of unloading, F is the start point of reloading, G is the phase transition point of the reloading process, H is the reloading P 2 wave arrival time).

Fig. 6 .Fig. 7 .
Fig. 6.Schematic diagram of phase and strain wave(the letters on the time axis have the same meaning with Fig.5).

Fig. 8 .
Fig.8.Simulation of interface velocity with a characteristic time of 30 ns(the letters have the same meaning with Fig.5).

Fig. 9 .
Fig.9.Comparison of the measured particle velocity and simulation (the letters have the same meaning with Fig.5).

Fig. 10 .
Fig. 10.The relation between reloading phase transition pressure and mass fraction of e phase.

Table 1 .
Hugoniot parameter of materials.

Table 2 .
Gauges of components.

Table 3 .
The data of experiment shot No.2.

Table 4 .
The mass fraction of e phase in the end of unloading process and reload phase transition pressure on each stage.