**5.2.2 Phase transition of 45 steel**

Since the quasi-isentropic compression loading technique actually follows the *P*-*v* response of the material under investigation, the actual evolution of the phase trnasition can be observed. The classical polymorphic transtion of iron at 13 GPa has been studied under quasi-isentropic compression. The two free-surface velocity profiles recorded in our experiments are shown in Fig.28. The elastic precursor wave is clearly seen in the lower pressure region of the two profiles. And the plastic wave and phase change wave occur, which show that the polymorphic transition() takes place. The velocity profiles in Fig.29 indicates that the onset of the phase transition is at velocity of 681 m/s, and the pressure of phase transition is also about 11.4 GPa.

Fig. 29. Velocity profiles of 45 steel under quasi-isentropic compression

### **5.2.3 Spallation and elasto-plastic transition of pure tantalum**

Fig.29 shows the results of the spalling experiments for pure tantalum (Ta contents 99.8%). The loading strain rate is 2.53×105 1/s. For the sample with thickness of 1.66 mm, the spallation is not obvious, perhaps the mirco-damage occurs. For the sample with thickness of 1.06 mm, the spallation is obvious, and the pull-back velocity is 129.6 m/s. According to the formular (23), the spall strength is 4.49 GPa.

$$
\sigma\_{\rm spall} = \frac{1}{2} \rho\_0 \mathbf{C}\_{\rm l} \Delta \mathbf{U}\_{\rm pb} \tag{23}
$$

where 0 is the initial density of sample, *C*l is the Larangian sound speed, Upb is the pullback velocity as shown in Fig.4, and spall is the spall strength of materials.

Under quasi-isentropic compression, the elasto-plastic transition are clearly shown in the velocity profiles of 45 steel and pure tantalum in Figure 28 and Fig.30. Here a concept of isentropic elastic limit(IEL, IEL) is introduced. For the 45 steel sample, the IEL is 2.26 GPa at the loading strain rate of 6.73×105 1/s, and for the pure tantalum sample, the IEL is 2.42 GPa

Since the quasi-isentropic compression loading technique actually follows the *P*-*v* response of the material under investigation, the actual evolution of the phase trnasition can be observed. The classical polymorphic transtion of iron at 13 GPa has been studied under quasi-isentropic compression. The two free-surface velocity profiles recorded in our experiments are shown in Fig.28. The elastic precursor wave is clearly seen in the lower pressure region of the two profiles. And the plastic wave and phase change wave occur, which show that the polymorphic transition() takes place. The velocity profiles in Fig.29 indicates that the onset of the phase transition is at velocity of 681 m/s, and the pressure of

Fig. 29. Velocity profiles of 45 steel under quasi-isentropic compression

the loading strain rate of 6.73×105 1/s, and for the pure tantalum sample, the

Fig.29 shows the results of the spalling experiments for pure tantalum (Ta contents 99.8%). The loading strain rate is 2.53×105 1/s. For the sample with thickness of 1.66 mm, the spallation is not obvious, perhaps the mirco-damage occurs. For the sample with thickness of 1.06 mm, the spallation is obvious, and the pull-back velocity is 129.6 m/s. According to

> spall 0 l pb 1 2

0 is the initial density of sample, *C*l is the Larangian sound speed, Upb is the pull-

IEL) is introduced. For the 45 steel sample, the

spall is the spall strength of materials.

*C U* (23)

IEL is 2.26 GPa at

IEL is 2.42 GPa

Under quasi-isentropic compression, the elasto-plastic transition are clearly shown in the velocity profiles of 45 steel and pure tantalum in Figure 28 and Fig.30. Here a concept of

**5.2.3 Spallation and elasto-plastic transition of pure tantalum**

the formular (23), the spall strength is 4.49 GPa.

back velocity as shown in Fig.4, and

isentropic elastic limit(IEL,

where

**5.2.2 Phase transition of 45 steel**

phase transition is also about 11.4 GPa.

Fig. 30. Velocity profiles of Tantalum samples

at the loading strain rate of 2.53×105 1/s. Because of the difference of loading strain rates, the IEL ranges from 2.26 to 2.35 GPa for 45 steel, and from 2.42 to 2.70 GPa for pure tantalum in our experiments, correspondingly, the yield strength ranges from 1.29 to 1.34 GPa for 45 steel and from 1.12 to 1.25 GPa for pure tantalum.