Fatigue crack propagation in iron and two iron binary alloys at low temperatures

Fatigue crack propagation in iron and six FeSi and FeNi binary alloys was studied at temperatures from 123 to 296 K to determine alloy and temperature effects. Crack growth rates were observed to decrease initially with decreasing temperature for all alloys. At some temperature below the ductile-brittle transition temperature the trend reversed, with more prevalent static and/or cyclic cleavage fracture modes enhancing crack growth rates. Fatigue crack propagation exponents corresponding to the enhanced growth rates also increased as the temperature decreased below the ductile-brittle transition temperature. At 100°C below this temperature the exponents were two to four times greater than at 100°C above the transition temperature. The variation of the exponents over this temperature range was substantially less for 1% Ni and 1% Si additions than for 2.5% and 4% additions. A similar trend with alloy addition for the Peierls stress and activation enthalpy, previously observed, indicated that a dislocation dynamics or strain rate sensitivity effect could be involved. A dislocation dynamics model based on cleavage growth steps was derived assuming that dislocations along the rivers of a propagating cleavage crack control cyclic cleavage. The calculated fatigue crack exponents and rates provided good agreement with actual values but showed a weakness in predicting the accelerated growth rates at temperatures well below the ductile-brittle transition temperature.