Fast hardening response of martensitic stainless steel by copper-rich cluster formation under pulsed electric current

Abstract

The pursuit of excellent comprehensive mechanical properties using a rapid aging process is of practical significance in the field of aging strengthening. A pulsed electric current assisted aging process can effectively accelerate aging and reduce the energy consumption caused by long-term thermal aging. The mechanism of a Cu-rich cluster formation promoted by a pulsed electric current was investigated to better understand the fast-hardening response process. The size and structure of the Cu-rich cluster were characterized using transmission electron microscopy (TEM); the results indicated that the pulsed electric current did not change the mechanism of aging strengthening but accelerated the process. The time required to achieve peak aging decreased owing to the athermal effect (interaction between atoms and electrons) induced by the pulsed electric current. The athermal effect decreased as the temperature increased, and a relationship between the athermal effect and temperature was obtained. When a current density of 7 × 107 A/m2 was applied, the precipitation activation energy of the Cu-rich clusters decreased by 91 kJ mol−1; this was owing to the following two aspects. (1) The pulsed electric current lowered the vacancy formation energy and resulted in an increase in vacancy concentration and (2) the current density gradient caused by the difference in conductivity between the matrix and Cu-rich clusters accelerated the substitution of the Cu atoms and vacancies and reduced the atomic migration energy. These results provide insights into the effect of a pulsed electric current on the kinetics of precipitation in the martensitic stainless steel and a deduction for the mechanism of the rapid growth of Cu-rich clusters.