Elastic-Plastic Transition under Uniaxial Stress BCC Tantalum

Abstract

Classical molecular dynamics and lattice dynamics were utilized to study the elastic-plastic transition underuniaxial stress in defect free body center cubic (BCC) Tantalum crystals. We demonstrate that the nucleation of defects at the time scales of molecular dynamics in tantalum is due to dynamical instabilities (soft-phonons). Uniaxial compressions test were simulated along the crystallographic directions (100), (110) and (111). The results show that the nucleation of defeccts in the direction (110) is due to crystal twinning.

Twinning anisotropy of tantalum during nanoindentation

“…(100) orientation of tantalum and (ii) the maximum shear (Tresca) stress was found to exceed the theoretical shear strength of tantalum on the (010) orientation but was well within for the (110) and (111) orientations. This unique observation on the (010) plane seems to be in accord with a recent study [6] where the elastic-plastic limit of Ta was observed to go past the theoretically predicted critical shear stress. Figure 6: P-h plots obtained from the MD simulation for the three cases using rigid tantalum indenter where H is material's hardness at micro or nanoscale, h is indentation depth, l 1 ,l 2 and l 3 are the material's length scale for region I, II and III respectively, H 1 , H 2 , H 3 , n 1 , n 2 , n 3 are material constants while δ u and δ 1 are the indentation depths that separate regions, I, II and III respectively.…”

“…Guerrero et al [6] carried out MD simulation of uniaxial compression of nanocrystalline tantalum using their in-house developed EAM potential function to study the elastic-plastic transition on three crystallographic directions namely, (100), (110) and (111). They monitored the variation of phonon with respect to the applied strain rate and based on this, they concluded that the nucleation of defects along (110) of Ta was due to crystal twinning arising out of dynamical instabilities (soft phonons).…”

“…An exception to this observation has recently come from recent experiments [5], where unlike other BCC metals, the plastic deformation of Ta during its nanoindentation was found to be dominated by the deformation twinning. Another area where Ta has shown deviation from the classical knowledge is that its elastic-plastic transition on the (110) crystallographic direction occurs beyond the theoretical critical value of shear stress [6], thereby pointing to the fact that the parameter critical resolved shear stress may not be applicable at atomic scale where the role of dislocation mechanics is more prevalent. This has in fact now been clarified that at atomic scale, a more fundamental criterion for nucleation of plastic deformation is that the work done over the displaced surface should be greater than the line energy of the new dislocation loop [7] and hence an understanding of the dislocation mechanics is important.…”

Kohn Anomaly and Phase Stability in Group VB Transition Metals

“…Tantalum is often used for testing purposes, for study of yield strength, and for comparing theoretical predictions of elastic and plastic properties under extreme pressure and temperatures [41][42][43]50,[54][55][56]58,73,83,84,87,88]. As was mentioned above, tantalum possesses the BCC structure at ambient conditions and this structure remains stable up to at least 1.74 Mbar [42,43].…”

“…Antonangeli et al [73] have presented a description of the elastic-plastic phase transitions in terms of an anomalous behavior of the elastic modulus under compression above 1 Mbar pressure. Guerrero and Marucho [84] have utilized classical molecular dynamics (MD) to study the elastic-plastic transition under uniaxial stress in defect-free BCC tantalum crystals. They demonstrated that the nucleation of the defects at the time scale of MD is due to dynamical instabilities (soft phonons) along the [100] direction.…”