Seyedeh Mohadeseh Taheri Mousavi1 Haofei Zhou2 Guijin Zou2 Huajian Gao2

1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
2, Brown university, Providence, Rhode Island, United States

The recent synthesis of twin interfaces in two covalent-bonding materials, cBN and diamond, has improved their thermal stability and fracture toughness and meantime introduced a new record for material's hardness. The continuous hardening of these materials by decreasing the twin-spacing even lower than the critical thickness (about 15 nm), which was a turning point to softening behavior in Cu, is mysterious. Here, we show a similar observation of hardening in nanotwinned Pd polycrystalline samples at room temperature and reveal that there exists a softening temperature for materials, below that the softening will be replaced by hardening behavior. Our large molecular dynamics and finite element simulations show that below this transition temperature, thermally-activated source-controlled plasticity will be substituted by the stress-driven one. Since the stress-concentration at grain boundary-twin intersections for nucleation of partial dislocations gets a higher value by increasing the twin spacing, twinning migrations are progressively observed in grains with thicker twin interfaces by decreasing the temperature. The Higher amount of stress concentration is caused by lower elastic-field interactions of close intersections when twin interfaces become far from each other. The higher the bond's strength, the higher transition temperature is captured by our simulations and predicted by our theoretical modeling.These results give an insight for observing hardening in covalent-bond materials in which the transition temperature is anticipated to occur at values higher than the room temperature similar to Pd.