Diamond is the hardest natural material, but its practical strength is low and its elastic deformability extremely limited. While recent experiments have demonstrated that diamond nanoneedles can sustain exceptionally large elastic tensile strains with high tensile strengths, the size- and orientation-dependence of these properties remains unknown. Here we report maximum achievable tensile strain and strength of diamond nanoneedles with various diameters, oriented in , and -directions, using in situ transmission electron microscopy. We show that reversible elastic deformation depends both on nanoneedle diameter and orientation. -oriented nanoneedles with a diameter of 60 nm exhibit highest elastic tensile strain (13.4%) and tensile strength (125 GPa). These values are comparable with the theoretical elasticity and Griffith strength limits of diamond, respectively. Our experimental data, together with first principles simulations, indicate that maximum achievable elastic strain and strength are primarily determined by surface conditions of the nanoneedles.

Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through in situ tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{30}{\mathrm{Co}}_{10}{\mathrm{Cr}}_{10}$, at. %) activates three types of dislocations, i.e., $⟨a⟩$, $⟨c⟩$, and $⟨c+a⟩$. Specifically, nonbasal $⟨c+a⟩$ dislocations occupy a high line fraction of $\sim 31\%$ allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a $c/a$ ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal $⟨c+a⟩$ slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the $c/a$ ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the $c/a$ ratio of hcp phases into regimes promoting nonbasal slip activation.

Gaits and transitions are key components in legged locomotion. For legged robots, describing and reproducing gaits as well as transitions remain longstanding challenges. Reinforcement learning has become a powerful tool to formulate controllers for legged robots. Learning multiple gaits and transitions, nevertheless, is related to the multi-task learning problems. In this work, we present a novel framework for training a simple control policy for a quadruped robot to locomote in various gaits. Four independent phases are used as the interface between the gait generator and the control policy, which characterizes the movement of four feet. Guided by the phases, the quadruped robot is able to locomote according to the generated gaits, such as walk, trot, pacing and bounding, and to make transitions among those gaits. More general phases can be used to generate complex gaits, such as mixed rhythmic dancing. With the control policy, the Black Panther robot, a medium-dog-sized quadruped robot, can perform all learned motor skills while following the velocity commands smoothly and robustly in natural environment.