Mechanism of Ductile-to-Brittle Transition in Hexagonal Close-Packed Zinc

The ductile-to-brittle transition (DBT), a well-known phenomenon in body-centered cubic metals, has inconclusive evidence suggesting hexagonal close-packed (HCP) zinc (Zn) may also exhibit such a sharp transition. To confirm this behavior and uncover its mechanisms, we systematically studied DBT in Zn using small-punch test. We compared recrystallized Zn with hot-rolled Zn, which have different initial dislocation densities. Recrystallized Zn shows a sudden DBT with a DBT temperature (DBTT) of -20 °C, while hot-rolled Zn exhibits significantly higher toughness, with its DBTT lowered to -90 °C. Mechanistic analysis reveals that below DBTT, limited mobility of <c+a> edge dislocations restricts plastic deformation, suppresses twinning, and limits strain along the <c>-axis. Above DBTT, coordinated motion of <c+a> edge and screw dislocations and active {101¯2} twinning enable ductile deformation. The improved low-temperature toughness in hot-rolled Zn stems from its pre-existing mobile <c+a> screw dislocations. These results not only link the DBT of Zn to <c+a> dislocation characteristics and twinning propensity, but also propose a dislocation engineering method to enhance toughness in HCP metals.