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

## Abstract

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 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 {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 equivalentmore »

- Authors:

- Publication Date:

- Research Org.:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

- Sponsoring Org.:
- USDOE

- OSTI Identifier:
- 972426

- Report Number(s):
- LLNL-PROC-418369

TRN: US201006%%73

- DOE Contract Number:
- W-7405-ENG-48

- Resource Type:
- Conference

- Resource Relation:
- Conference: Presented at: 2010 TMS Annual Meeting, Seattle, WA, United States, Feb 15 - Feb 18, 2010

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 36 MATERIALS SCIENCE; ALLOYS; CRITICAL PRESSURE; DEFORMATION; DISLOCATIONS; FIGS; FREE ENERGY; HYDROSTATICS; PHASE STABILITY; PHASE TRANSFORMATIONS; SELF-ENERGY; SHEAR; TANTALUM; TEMPERATURE DEPENDENCE; THERMODYNAMICS; TRANSFORMATIONS; TUNGSTEN; TUNGSTEN ALLOYS; TWINNING

### Citation Formats

```
Hsiung, L L, Campbell, G H, and McNaney, J M.
```*Transitions of Dislocation Glide to Twinning and Shear Transformation in Shock-Deformed Tantalum*. United States: N. p., 2009.
Web.

```
Hsiung, L L, Campbell, G H, & McNaney, J M.
```*Transitions of Dislocation Glide to Twinning and Shear Transformation in Shock-Deformed Tantalum*. United States.

```
Hsiung, L L, Campbell, G H, and McNaney, J M. Mon .
"Transitions of Dislocation Glide to Twinning and Shear Transformation in Shock-Deformed Tantalum". United States. https://www.osti.gov/servlets/purl/972426.
```

```
@article{osti_972426,
```

title = {Transitions of Dislocation Glide to Twinning and Shear Transformation in Shock-Deformed Tantalum},

author = {Hsiung, L L and Campbell, G H and McNaney, J M},

abstractNote = {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 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 {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.},

doi = {},

journal = {},

number = ,

volume = ,

place = {United States},

year = {2009},

month = {10}

}