Dislocation nucleation in bcc Ta single crystals studied by nanoindentation

Incipient plasticity of single-crystal tantalum as a function of temperature and orientation

“…The core of screw dislocations in BCC-Ta is also found to degenerate into different planes of a common zone axis which significantly reduces the mobility of screw as compared to edge dislocation segments [31,32]. [20,27]). Similar approaches were adopted by Nemat Nasser et al in setting continuum crystal plasticity models for BCC metals [34] and in the work by Naka et al for the thermodynamical treatment of temperature effects in enhancing mobility of screw dislocations through mechanisms such as kink-pair formation [35].…”

“…Most recent experimental studies focusing on the defect evolution in BCC-materials at small scales so far were carried out on nanopillars [23,24,26,28,29] with a few studies using nanoindentation [20,27]. A recent simulation study by Li et al defined a critical pop-in load in nanoindentation experiments based on the Schmid factor for the different crystallographic planes, which could then be considered as the load required for homogeneous dislocation nucleation using similar arguments as those employed for FCC metals.…”

“…Compared to face centered cubic (FCC) systems, where the correlation between incipient plasticity and dislocation motion has been well established [5,6,8,[14][15][16][17][18][20][21][22]. BCC systems are much more complex and, despite several reports [11][12][13][23][24][25][26][27][28][29], there are still many unanswered questions regarding our understanding of BCC-plasticity in nanocontacts encountered in indentation.…”

“…While this approach is experimentally straightforward, the interpretation becomes complicated by simultaneously changing elastic and plastic properties, including different screw dislocation core arrangements. In this manuscript, in order to avoid this complication, the tests are performed on only one material, Ta, and the test temperature is increased up to 200 °C, which is above the critical temperature of Ta (177 °C) [20,27]. The experiments were accompanied by finite element simulations that account for anisotropic elasticity to evaluate the resolved shear stresses in all slip systems under fixed (applied) indentation loading conditions, as well as with large-scale with a radius of curvature of ~190 nm was used to obtain quantitative load-displacement data, and a spherical tip with a tip radius of ~1 µm was used to study material flow under the indenter .…”

Plastic anisotropy of γ-TiAl revealed by axisymmetric indentation

“…Already Osmond and Cartaud observed that the deformation patterns around a conical indent in single crystals is characteristic of the structure of the indented crystal, as well as for the orientation of the indented plane. More recently, the atomic force microscopy (AFM) technique enabled precise characterization of pile-up topographies and facilitated the study of deformation mechanisms during indentation [32,33].…”