Size Effect in Ultrafine Ti-Fe-(Sn) Lamellar Composites during Micro- and Nanoindentation

The indentation size effect (ISE) has been investigated in a series of (Ti_0.705Fe_0.295)_100-xSn_x (0 ≤ x ≤ 4) ultrafine eutectic composites (UECs) by both the micro- and nanoindentation and has been correlated for a wide range of depth between 300 nm and 10 µm. The lamellar composites are comprised of ultrafine FeTi and β-Ti lamellae phases with lamellae thickness between 0.32 and 0.81 μm. The contribution of the individual lamellae phases and the prior deformation on the ISE have been investigated to study the evolution of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs). X-ray diffraction, transmission electron microscopic studies, and the model calculations have revealed that a significant portion of the lamellae interface in the as-solidified composites aid in the development of high GND density ≈ 10¹⁵/m² and exhibit hardness ≈ 16 GPa, which further enhanced up to ≈ 10¹⁶/m² and ≈ 22 GPa upon severe forging. A model has been established to correlate the micro- and nanoindentation hardness by considering the size effect and the microstructure length scale.