Pi95 picoindenter

In-situ compression experiments were carried out in both a TEM (JEOL JEM 2100) and an SEM (Zeiss Ultra), while in-situ tensile and bending tests were conducted in the SEM. The JEOL JEM 2100 uses a high-brightness LaB₆ electron source. It is equipped with Xarosa (4 k × 4 k) as well as Veleta Ultrascan (2 k × 2 k) cameras. In the TEM, in-situ compression tests of pillars with diameters around 200 nm were carried out by using a Hysitron PI 95 Picoindenter with a flat diamond tip. As the load applied is limited to 1.5 mN for the PI 95 Picoindenter, the requirement for thin sample in the TEM, we carried out the in-situ compression experiment of the larger pillars by using a Hysitron PI 85 L picoindenter inside an SEM, with a specially designed system for applying loads up to 10 mN. This system allows real-time observation of deformation process (i.e. slip band development, slip planes and slip directions). The load was applied to pillars by moving the indenter toward the pillars in the displacement control mode. The displacement rates were 1 nm⋅s⁻¹ and 2 nm⋅s⁻¹ for compression of pillars of around 200 nm in diameter and from 500 nm ~ 2.1 µm in diameter, respectively. For the tensile test, a displacement rate of 1 nm⋅s⁻¹ was used. For the bending test, a higher displacement rate − 4 nm⋅s⁻¹ was used.

Nano-compression experiments were performed using a Hysitron TI950 nanoindenter and a PI95 PicoIndenter with a diamond punch and a W punch, respectively, under displacement-control mode and at a strain rate of ~2 × 10⁻³s⁻¹. Nano-pillar samples were fabricated using FIB, with 30 kV/7 pA as the final milling condition. The aspect ratio (height/diameter) of the ex-situ pillars was 2, and the taper angle of each pillar was less than 1.5°. The engineering stress σ was calculated using F/A₀, where F is the measured force and A₀ is the original cross-sectional area at 20% of the pillar’s height away from the top. The engineering strain ε was calculated using L/L₀, where L is the measured displacement and L₀ is the original length of the samples. The true stress and true strain were converted by using the equations: ${\sigma }_{\mathrm{T}}={\sigma }_{\mathrm{E}}\left(1+{\epsilon }_{\mathrm{E}}\right)$ ${\epsilon }_{\mathrm{T}}=\ln \left(1+{\epsilon }_{\mathrm{E}}\right)$ where σ_T, σ_E, ε_E, and ε_T are the true stress, engineering stress, engineering strain and true strain, respectively.

Micro-compression and tension were performed using a Hysitron TI950 nanoindenter and a PI95 PicoIndenter with a diamond punch and a W gripper, respectively under displacement-control mode and at a strain rate of ~2 × 10⁻³s⁻¹. The Hysitron TI950 nanoindenter is an ex-situ instrument in air, and thus the electron beam-induced composition and structure changes can be ruled out during compression. Micro-pillar and dog-bone-shaped samples were fabricated using FIB, with 30 kV/7 pA as the final milling condition. The aspect ratio (height/diameter) of the pillar was 2, and the taper angle of each pillar was less than 1.5°. The engineering stress σ was calculated using F/A₀, where F is the measured force and A₀ is the original cross-sectional area at the top of pillar samples or the cross-sectional area of dog-bone-shaped samples. The engineering strain ε was calculated using L/L₀, where L is the measured displacement and L₀ is the original length of the samples.