Plastic deformation in nanoindentation of tantalum: A new mechanism for prismatic loop formation

Abstract

The mechanisms of deformation under a nanoindentation in tantalum, chosen as a model body-centered cubic (bcc) metal, are identified and quantified. Molecular dynamics (MD) simulations and indentation experiments are conducted for [1 0 0], [1 1 0] and [1 1 1] normals to surface orientations. The simulated plastic deformation proceeds by the formation of nanotwins, which rapidly evolve into shear dislocation loops. It is shown through a dislocation analysis that an elementary twin (three layers) is energetically favorable for a diameter below $7 nm, at which point a shear loop comprising a perfect dislocation is formed. MD simulations show that shear loops expand into the material by the advancement of their edge components. Simultaneously with this advancement, screw components of the loop cross-slip and generate a cylindrical surface. When opposite segments approach, they eventually cancel by virtue of the attraction between them, forming a quasi-circular prismatic loop composed of edge dislocation segments. This "lasso"-like mechanism by which a shear loop transitions to a prismatic loop is identified for both [0 0 1] and [1 1 1] indentations. The prismatic loops advance into the material along h1 1 1i directions, transporting material away from the nucleation site. Analytical calculations supplement MD and experimental observations, and provide a framework for the improved understanding of the evolution of plastic deformation under a nanoindenter. Dislocation densities under the indenter are estimated experimentally ($1.2 Â 10 15 m À2 ), by MD ($7 Â 10 15 m À2 ) and through an analytical calculation (2.6-19 Â 10 15 m À2 ). Considering the assumptions and simplifications, this agreement is considered satisfactory. MD simulations also show expected changes in pile-up symmetry after unloading, compatible with crystal plasticity.

Twinning anisotropy of tantalum during nanoindentation

“…[26] to be least sensitive to indenter diameter and indentation velocity. On the other hand, the EAM potential function of Ravelo et al [23] has been tested during nanoindentation [13] as well as shock compression experiments [23] and the outcome suggests that this potential function is better in predicting the high pressure and high temperature phases of Ta. Since the intent of this work is to understand the deformation behaviour of Ta, this study made use of an EAM potential (Ta-1) developed by Ravelo et al [23] primarily to study extreme conditions of temperature and pressure such as during nanoindentation.…”

“…Recent research shows that the plastic deformation in tantalum start with twins, which transforms to shear loops and eventually to prismatic loops whereby the screw components of the shear loop cross-slip and pinch out a prismatic loop in a "lasso" action [13]. Biener et al [1] performed nanoindentation experiments on the (010), (110) and (111) Attributing this as a general trend to all the BCC materials, they speculated that the (010) orientation of Ta is a typical example of surface which shows dislocation multiplication process, i.e.…”

Физико-Механические Свойства И Микромеханизмы Локального Деформирования Материалов С Различной Зависимостью Твердости От Глубины Отпечатка

“…Однако при h < 50−100 nm расчетная величина ρ, следующая из этой модели, становится нереалистично большой (> 10 16 m −2 ). Ав-торы работ [6,7,[42][43][44] объясняют возникновение ISE в микрообласти уменьшением размера зоны деформи-рования до некоторого характерного размера микро-структуры. В общем случае возможны и другие при-чины возникновения ISE в твердости.…”

Atomistic Studies of Nanoindentation—A Review of Recent Advances

“…Consistent with bcc slip systems, the shear loops grow by the advance of their edge components while the screw components undergo limited cross-slip. Eventually the screw segments may annihilate each other since they have dislocation lines with opposite signs, and a prismatic loop is pinched off [32,33]. Figure 1 presents a side view of the indentation of a (001) Ta single crystal by an 8 nm diameter spherical indenter to a penetration of 5 nm, and a detailed view of a prismatic loop produced in the process.…”

“…Remington et al [32] presented a comprehensive nanoindentation MD study of Ta single crystals along the three principal crystallographic orientations. They reported that after the formation and annihilation of planar defects similar to the ones reported by Alcalá et al [31], shear loops form and propagate along 111 directions.…”