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Title: Transition of dislocation glide to shear transformation in shocked tantalum

Abstract

A TEM study of pure tantalum and tantalum-tungsten alloys explosively shocked at a peak pressure of 30 GPa (strain rate: ~1 x 10 4 sec -1) is presented. While no ω (hexagonal) phase was found in shock-recovered pure Ta and Ta-5W that contain mainly a low-energy cellular dislocation structure, shock-induced ω phase was found to form in Ta-10W that contains evenly distributed dislocations with a stored dislocation density higher than 1 x 10 12 cm -2. The TEM results clearly reveal that shock-induced α (bcc) → ω (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. Lastly, 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.

Authors:
 [1];  [1]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1347662
Report Number(s):
LLNL-JRNL-710038
Journal ID: ISSN 2059-8521; applab
Grant/Contract Number:
AC52-07NA27344
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
MRS Advances
Additional Journal Information:
Journal Volume: 2; Journal Issue: 27; Journal ID: ISSN 2059-8521
Publisher:
Materials Research Society (MRS)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Hsiung, Luke L., and Campbell, Geoffrey H.. Transition of dislocation glide to shear transformation in shocked tantalum. United States: N. p., 2017. Web. doi:10.1557/adv.2017.236.
Hsiung, Luke L., & Campbell, Geoffrey H.. Transition of dislocation glide to shear transformation in shocked tantalum. United States. doi:10.1557/adv.2017.236.
Hsiung, Luke L., and Campbell, Geoffrey H.. Tue . "Transition of dislocation glide to shear transformation in shocked tantalum". United States. doi:10.1557/adv.2017.236. https://www.osti.gov/servlets/purl/1347662.
@article{osti_1347662,
title = {Transition of dislocation glide to shear transformation in shocked tantalum},
author = {Hsiung, Luke L. and Campbell, Geoffrey H.},
abstractNote = {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 ω (hexagonal) phase was found in shock-recovered pure Ta and Ta-5W that contain mainly a low-energy cellular dislocation structure, shock-induced ω 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 α (bcc) → ω (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. Lastly, 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.},
doi = {10.1557/adv.2017.236},
journal = {MRS Advances},
number = 27,
volume = 2,
place = {United States},
year = {Tue Feb 28 00:00:00 EST 2017},
month = {Tue Feb 28 00:00:00 EST 2017}
}

Journal Article:
Free Publicly Available Full Text
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  • Recent TEM studies of deformation substructures developed in tantalum and tantalum-tungsten alloys shock-deformed at a peak pressure {approx}45 GPa have revealed the occurrence of shock-induced phase transformation [i.e., {alpha} (bcc) {yields} {omega} (hexagonal) transition] in addition to shock-induced deformation twinning. The volume fraction of twin and {omega} 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. 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 thatmore » 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 {omega} becomes smaller than that of the parent phase {alpha} above a critical pressure (P{sub eq}), i.e., the equilibrium {alpha} {yields} {omega} transition occurs when the pressure increases above P{sub 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{sub eq} for {alpha} {yields} {omega} 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, it is reasonable to assume that Peq under dynamic-pressure conditions will be considerably higher than that under hydrostatic-pressure conditions if there is a pressure-induced {alpha} {yields} {omega} transition in tantalum. The observation of {alpha} {yields} {omega} transition in shock-compressed tantalum and tantalum-tungsten alloys at {approx}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 {omega} 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. Typical arrangements of high-density dislocations formed in pure tantalum shocked at {approx}45 GPa are shown in Figs. 1a and 1b. Figure 1a reveals a cellular dislocation structure but no twins or {omega} phase-domains were observed in this region. The formation of low-energy type cellular dislocation structures indicates the occurrence of dynamic-recovery reactions to reduce dislocation density in this region. Figure 1b shows an evenly distributed dislocation structure with a local dislocation density ({rho}) as high as {approx}5 x 10{sup 12} cm{sup -2} according to {rho} {approx} 1/l{sup 2}, where l ({approx}4.5 nm) is the spacing between two dislocations. Here shock-induced twin plates and {omega} phase-domains can be readily seen. These observations provide us a clue that dislocation arrangement and density population, which can alter system free energy through the changes of dislocation self-energy (E{sub s}) and dislocation interaction energy (E{sub ij}), are relevant to the occurrence of shock-induced twinning and phase transformation in tantalum. The objective of this paper is to report new results obtained from pure tantalum and tantalum tungsten alloys shocked at {approx}30 GPa in order to clarify the correlation between dislocation structure (i.e., density and arrangement) and shock-induced twinning and {alpha} {yields} {omega} transition. Emphasis is placed especially on the {alpha} {yields} {omega} transition. Physical mechanisms are subsequently proposed to rationalize the shock-induced twinning and non-equilibrium phase transformation.« less