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

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Fatigue crack propagation in iron and six FeSi and FeNi 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.