Transition of Dislocation Glide to Shear Transformation in Shocked Tantalum
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Transition of Dislocation Glide to Shear Transformation in Shocked Tantalum Luke L. Hsiung and Geoffrey H. Campbell Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550-9900, U.S.A. ABSTRACT A TEM study of pure tantalum and tantalum-tungsten alloys explosively shocked at a peak pressure of 30 GPa (strain rate: ~1 x 104 sec-1) is presented. While no (hexagonal) phase was found in shock-recovered pure Ta and Ta-5W that contain mainly a low-energy cellular dislocation structure, shock-induced phase was found to form in Ta-10W that contains evenly distributed dislocations with a stored dislocation density higher than 1 x 1012 cm-2. The TEM results clearly reveal that shock-induced (bcc) → (hexagonal) shear transformation occurs when dynamic recovery reactions which lead the formation low-energy cellular dislocation structure become largely suppressed in Ta-10W shocked under dynamic (i.e., high strain-rate and high-pressure) conditions. A novel dislocation-based mechanism is proposed to rationalize the transition of dislocation glide to twinning and/or shear transformation in shock-deformed tantalum. Twinning and/or shear transformation take place as an alternative deformation mechanism to accommodate high-strain-rate straining when the shear stress required for dislocation multiplication exceeds the threshold shear stresses for twinning and/or shear transformation. INTRODUCTION Previous TEM studies of deformation substructures developed in tantalum and tantalumtungsten alloys shocked at a peak pressure 45 GPa have discovered the occurrence of shockinduced shear transformation [i.e., (bcc) → (hexagonal) transition] in addition to shockinduced deformation twinning [1]. The volume fraction of twins and phase-domains increases with increasing content of tungsten. A contradiction 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 is stable. For pressure-induced phase transformation under hydrostatic-pressure conditions, tantalum may undergo phase transition when the free energy of a competing phase, say , becomes smaller than that of the parent phase () above a critical pressure (Peq), i.e., the equilibrium transition may occur when the pressure increases above Peq. However, it is also known that material shocked under the Hugoniot (or the dynamic adiabat) conditions can lead to a considerable increase in temperature [6]. This means a higher pressure is required to achieve an equivalent volume (or density) in dynamic-pressure conditions than in hydrostaticpressure conditions. Accordingly, Peq for transition is anticipated to increase under dynamic-pressure conditions as a result of the temperature effect. Although no clear equilibrium transition pressure under hydrostatic-pressur
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