Positron Lifetimes and Microhardness in Thermal Fatigued 4Cr5MoSiV Steel

Positron lifetimes and microhardness have been measured as a function of the thermal fatigue cycle number N in 4Cr5MoSiV steel. It is found that with increasing N: (a) the positron lifetime parameters t1, t2, I1, I2 and ot and the microhardness parameter MH all exhibit quasi-periodic up-and-down variation; (b) these parameters have the same period of variation; and (c) the period of up-and-down variation becomes gradually longer. The variation of MH reveals that fatigue hardening and fatigue softening occur alternately in the process of thermal fatigue. The variations of positron lifetime parameters reveal variations of defects in the fatigued materials. These variations are attributed to microdeformation and dynamic recovery dominating alternately in the process of thermal fatigue.