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Mechanical Properties
Published in Alan Cottrell, An Introduction to Metallurgy, 2019
In F.C.C. metals the Peierls-Nabarro force is extremely small and the stress to move a dislocation is almost independent of speed, at least up to the order of 103 m s−1. B.C.C. transition metals such as iron behave at room temperature in a manner intermediate between the F.C.C. metals and the non-metals mentioned above.
Dislocation structure and dynamics govern pop-in modes of nanoindentation on single-crystal metals
Published in Philosophical Magazine, 2020
Nian Zhou, Khalil I. Elkhodary, Xiaoxu Huang, Shan Tang, Ying Li
With the nanoindentation on single crystals, the first occurrence of pop-in is believed to be caused by a homogeneous nucleation of dislocations under the indenter [15–17]. Several mechanisms of successive pop-in are adopted. Xia et al. [10] explained the mechanism of the influence of the indenter radius on the pop-in mode. Single crystals in the experiment will inevitably contain some randomly distributed defects. Larger indenters can cause large stress volume. Therefore, these defects have a higher probability to become a source of dislocation during the indentation process. When the dislocations nucleate from these defects in the bulk, successive pop-in is generated. When the indenter is relatively small, the dislocations are mainly dominated by homogeneous nucleation. In this case, the pop-in mode is mainly affected by the lattice type. The slip capability of the dislocations generated by the first pop-in determines whether subsequent pop-in can occur. If these nucleated dislocations can rapidly slip into the bulk of the material, leaving no defects under the tip of the indenter, then the deformation produced from the progressing indentation will remain elastic under the indenter's tip. The accumulating elastic energy hence activates the next pop-in. As the velocity of dislocations is usually related to the Peierls–Nabarro force in a lattice, Xia et al. [10] attribute pop-in mode to the Peierls–Nabarro force. Since FCC metals have a smaller Peierls–Nabarro force when compared to BCC metals, FCC metals prefer a successive pop-in mode. Moreover, experiments conducted under the same conditions for Al and Cu single crystals (both FCC), as reported by Shibutani et al. [18], indicate that successive pop-in in Al is more obvious than in Cu. They explain that successive pop-in in Al is related to the perfect dislocations nucleated in Al, which easily slips away. However, as stacking fault energy in Cu is lower than that in Al, partial dislocations are more easily activated. Partial dislocations tend to react and form immobilised dislocations near the indented surface, such as Lomer–Cottrell (L–C) junctions [19,20]. The easier slip of dislocations in Al also leave no defects under the tip of the indenter, thus facilitates successive pop-in, while the immobilised dislocations in Cu prefer a single pop-in.