Role of core polarization curvature of screw dislocations in determining the Peierls stress in bcc Ta: A criterion for designing high-performance materials

Abstract

We use a family of embedded atom model potentials all based on accurate quantum-mechanical calculations to study the relation between Peierls stress and core properties of the 1/2a͗111͘ screw dislocation in bcc tantalum ͑Ta͒. We find that the Peierls stress ( P ) is a function of the core-polarization curvature ͑⌸͒ near the equilibrium core configuration. Our results suggest that the computationally available quantity ⌸ is a useful criterion for designing high-performance materials. DOI: 10.1103/PhysRevB.67.140101 PACS number͑s͒: 62.20.Fe, 61.72.Lk Determining the fundamental atomistic mechanisms that underlie plastic deformations of macroscopic materials is a key, enabling step toward designing materials with improved and tailored properties. This is particularly important for bcc metals, which are the basis of some of the highestperforming alloys, but whose behavior is more complex than fcc and hcp materials. 1 Computer simulations could be used to investigate the individual and collective dislocation motions in these materials and guide the development of highperformance materials. 2 At low temperatures, plasticity in bcc metals is governed by low mobility screw dislocations with Burgers vector b ϭ1/2a͗111͘. It is generally believed that the core structure of these dislocations is a controlling factor of their mobility. 3 Theoretical studies on these dislocations have led to two types of core structures: asymmetric 3-5 core and symmetric core. [5][6][7][8] In differential displacement ͑DD͒ maps, 3 the asymmetric core ͓Fig. 1͑a͔͒ spreads in three ͗112͘ directions on ͕110͖ planes, while the symmetric core ͓Fig. 1͑b͔͒ is compact. The main differences between these two types of cores are the relative displacements in the ͗111͘ direction of the two sets of three atoms in the core ͓atoms ͕A, C, E͖ and ͕B, D, F͖ in Fig. 1͑c͔͒. The ''polarization'' 9 of the dislocation core can be used to distinguish different core configurations and is defined by Eq. ͑1͒,or F͒ is the relative displacement between two neighboring atoms in the two columns denoted as X and Y in Fig. 1͑c͒, and b is the magnitude of the dislocation Burgers vector. Thus a symmetric core leads to pϭ0, while pϭ1 corresponds to a fully asymmetric core. The Peierls stress is the minimal shear stress required to move a dislocation in an otherwise perfect crystal. In bcc metals, the Peierls stress depends strongly on the orientation of the shearing. 3,5 In this paper, we study in detail the twinning ( ϭϪ30°) and antitwinning ( ϭ30°) shearing on ͑112͒ planes, where is the angle between the plane with the maximum shear stress and the neighboring ͑110͒ plane. Two recent atomistic calculations 8,10 of the Peierls stress for Ta show good agreement when the applied stress is in the twinning direction: ab initio density-functional theory ͑DFT͒ calculations lead to 675 MPa and calculations using the model generalized pseudopotential theory ͑MGPT͒ potential lead to 600 MPa. However, for the antitwinning direction the calculated Peierls stresses differ by a factor of...

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