Transitions of Dislocation Glide to Twinning and Shear Transformation in Shock-Deformed Tantalum

摘要

Recent TEM studies of deformation substructures developed in tantalum and tantalum-tungsten alloys shock-deformed at a peak pressure ～45 GPa have revealed the occurrence of shock-induced phase transformation [i.e., α (bcc) → ω (hexagonal) transition] in addition to shock-induced deformation twinning [1, 2]. The volume fraction of twin and ω domains increases with increasing content of tungsten. A controversy arises since tantalum exhibits no clear equilibrium solid-state phase transformation under hydrostatic pressures up to 174 GPa [3-5]. It is known that phase stability of a material system under different temperatures and pressures is determined by system free energy. That is, a structural phase that has the lowest free energy will be stable. For pressure-induced phase transformation under hydrostatic-pressure conditions, tantalum may undergo phase transition when the free energy of a competing phase ω becomes smaller than that of the parent phase α above a critical pressure (P_(eq)), i.e., the equilibrium α → ω transition occurs when the pressure increases above P_(eq). However, it is also known that material shocked under dynamic pressure can lead to a considerable increase in temperature, and the higher the applied pressure the higher the overheat temperature. This means a higher pressure is required to achieve an equivalent volume (or density) in dynamic-pressure conditions than in hydrostatic-pressure conditions. Accordingly, P_(eq) for α → ω transition is anticipated to increase under dynamic-pressure conditions as a result of the temperature effect. Although no clear equilibrium transition pressure under hydrostatic-pressure conditions is reported for tantalum [3-5], it is reasonable to assume that P_(eq) under dynamic-pressure conditions will be considerably higher than that under hydrostatic-pressure conditions if there is a pressure-induced α → ω transition in tantalum. The observation of α → ω transition in shock-compressed tantalum and tantalum-tungsten alloys at ～45 GPa in fact reveals the occurrence of a non-equilibrium phase transformation at such a low pressure. We therefore postulated that the equation of state (EOS) based on static thermodynamics, which asserts that the system free energy (G) is a function of volume (V), pressure (P), and temperature (T), i.e., G = F(V, P, T) is insufficient to rationalize the system free energy under dynamic-pressure conditions. Since shear deformation was found to play a crucial role in shock-induced deformation twins and ω phase, the density and arrangement of dislocations, which can alter and increase the system free energy, should also be taken into account to rationalize the non-equilibrium phase transformation in shocked tantalum.