Grain-size effect on dislocation source-limited hardening and ductilization in bulk pure Ni

Dislocation source-limited hardening and ductilization is an effective strategy to obtain superior strengthductility synergy in some engineering structural metals. Recent works demonstrated that the synergy could be enhanced by grain-size reduction. However, the mechanism of grain-size dependence is still a mystery. In this work, bulk pure Ni produced by electrodeposition and subsequent annealing, with grain sizes ranging from -20 nm to -20 pm, were methodically investigated to unravel the mechanism of the grain-size effect on dislocation source-limited hardening and ductilization. The high-density nano-twinning in the as-electrodeposited nanograined specimens exhibited better thermodynamic stability than the peers with random high-angle grain boundaries, leading to fine recrystallized grains with low-density dislocations. The low dislocation density enabled extra hardening beyond grain boundary strengthening via yield-point behavior with grain sizes ranging from -110 nm to -10 pm and extra ductilization over -500 nm. This work demonstrated that the prerequisite for dislocation source-limited hardening was that the dislocation density of the specimen should be lower than the size-dependent critical value of ((1.1 x 10 7 / d ) m -2 , d is the grain size in unit of the meter) where a transition from forestdominated hardening to dislocation source-limited hardening could occur. On the other hand, dislocation source-limited ductilization only worked when the grain size was comparable to/larger than the theoretical dislocation mean slip distance. Dislocation source-limited ductilization resulted from more room in grains for accumulation of dislocations and deformation nano-stacking faults enabling the higher work hardening rate. This work offered an altogether new avenue to obtain stronger and more ductile metallic materials through utilizing grain-size dependent dislocation source-limited hardening and ductilization.(c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

The mechanism of grain-size dependence on dislocation source-limited hardening and ductilization is still unknown. This study systematically investigated the effect of grain size on these processes in bulk pure Ni samples ranging from -20 nm to -20 pm. The results showed that high-density nano-twinning in the nanograined samples exhibited better thermodynamic stability and led to fine recrystallized grains with low-density dislocations. This work provides a new approach to obtain stronger and more ductile metallic materials through grain-size dependent dislocation source-limited hardening and ductilization.