Narrow Results

Free-standing sub-micron Ti-5Al single crystal square pillars were fabricated along double slip and [0001] twinning orientations using FIB fabrication processes. Samples in range of 0.4 to 2.0µm were compressed. The yield stress increases much higher than their bulk counterpart as the specimen width decreases. The tendency of "smaller is stronger" is displayed in Ti-5Al single crystals loaded along and [0001] orientations. The number of slip systems is restricted by specimen physical size as it declines from 2µm to 0.5µm, when the specimens were subjected to double slip loading. Meanwhile, when sample size is less than 1.0µm, micro-pillars along twinning orientation have to compensate the incomplete twinning deformation via shearing due to geometrical restriction and dislocation starvation effects. This variation of deformation mode could be attributed to the starvation effect of dislocations.

In this paper, molecular dynamics was used to simulate the indentation process of copper–nickel (CuNi) alloy. Its mechanical properties and behaviors were investigated focusing on factors such as indentation velocity, test temperature and crystal orientation. Generally speaking, dislocation generations and slips, stacking faults, extended dislocations and deformation twins, one or more of them come into play the dominant role during plastic deformation, which in return leads to an improved or reduced hardness of CuNi alloy. Specifically, simulations and analyses reveal the following: (1) its hardness $H$ increases with $v_{\mathrm{\text{ind}}}$ increasing, but the reduced elastic modulus $E_r$ is not sensitive to $v_{\mathrm{\text{ind}}}$; when it comes to test temperature $T$, both $H$ and $E_r$ are reduced at elevated $T$; besides, the CuNi alloy along $[111]$ owns the highest $H$ and $E_r$, the value of $H$ along $[001]$ is slightly smaller than that along $[110];$ (2) the dislocation density $\rho$ varies severely in the early stage of indentation and then generally levels off when indentation depth reaches approximately 1.5nm; by and large, its hardness and dislocation density follow the classical Taylor hardening model and the hardening coefficient does depend on the three factors; (3) the plastic-zone size parameter $f$, when $h/a_c\approx 0.6$ or equivalently $h/R\approx 0.53$ can be taken as constant roughly 4.0 except in the case of indentation along $[111]$, in which it is about 5.7.