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EE4.8.1

NANOSCALE TWINNING AND MARTENSITIC TRANSFORMATION IN SHOCK-DEFORMED BCC METALS Luke L.M. Hsiung, Lawrence Livermore National Laboratory, P.O. Box 808, L-352, Livermore, CA 945519900, USA ABSTRACT Shock-induced twinning and martensitic transformation in BCC-based polycrystalline metals (Ta and U6wt%Nb) have been observed and studied using transmission electron microscopy (TEM). The length-scale of domain thickness for both twin lamella and martensite phase is found to be smaller than 100 nm. While deformation twinning of {112}-type is found in Ta when shock-deformed at 15 GPa, both twinning and martensitic transformation are found in Ta when shock-deformed at 45 GPa. Similar phenomena of nanoscale twinning and martensitic transformation are also found in U6Nb shock-deformed at 30 GPa. Since both deformation twinning and martensitic transformation occurred along the {211}b planes associated with high resolved shear stresses, it is suggested that both can be regarded as alternative paths for shear transformations to occur in shock-deformed BCC metals. Heterogeneous nucleation mechanisms for shock-induced twinning and martensitic transformation are proposed and discussed. INTRODUCTION The combined effects of high strain rate, high pressure, and high plastic strain on Ta (a group V transition metal) can generate a broad range of microstructural features such as dislocation cells/walls, subboundaries, shear bands, recrystallized grains, and deformation twins... etc. [1-6]. A recent TEM study of deformation substructure developed within shock-deformed tantalum has revealed that shock-induced phase transformation can also take place in tantalum under dynamic pressure conditions [7]. Besides high-density dislocation substructures and deformation twins, plate-like omega phase was also observed in tantalum shock-loaded at 45 GPa as a result of the β (bcc) → ω (hexagonal) displasive or marttensitic transition. The orientation relationships (one variant) between the shock-induced ω and parent β phases are: (10 1 0) ω || (211)β, [0001] ω || [111]β, [1 2 10] ω || [0 1 1]β. The lattice parameters of ω phase are a ω ≈ 2 a β = 0.468 nm and c ω

=( 3 /2)a β = 0.286 nm (c/a = 0.611). It has been well documented that omega phase can be formed in numerous group IV transition metals (titanium, zirconium and hafnium) and their alloys, as well as ordered β-type alloy systems [8-10]. In general, both athermal and isothermal omega phases have been reported [8]. For group IV metals [which have hcp (α) structure at room temperature but transform to bcc (β) structure at high temperatures] athermal to forms either under quenching from the β phase field (i.e. β → ω) or under static and dynamic pressures (i.e. α → ω), whereas isothermal ω forms under high temperature aging (i.e. β → ω). It is noted here that tantalum has a bcc structure and exhibits no phase transformation under ambient and static pressures up to 100 GPa [11, 12]. The occurrence of β → ω transition in tantalum shock-loaded at 45 GPa suggests that the shockinduced phase cha