Fatigue mechanisms in an interstitial free steel: analysis through the behaviour of UHP α-iron doped with C and Mn

The cyclic plasticity mechanisms in body-centred cubic (bcc) materials are strongly temperature and strain-rate dependent. At low temperatures, slip asymmetry (due to low mobility of screw dislocations) occurs and leads to changes of the grain shape, whereas above a temperature transition, symmetric slip is observed and the initiation sites are mainly transgranular. For this reason, it is interesting to decrease this transition temperature. This study has been carried out on an interstitial free steel (IFS) at room temperature and for different strain rates (6×10−5 s−1–10−3 s−1), using the measurements of the activation volume in order to determine the cyclic plasticity mechanisms. Ultra-high purity (UHP) α-iron has been used as reference. The influence of C and Mn on fatigue mechanisms has been investigated. Room temperature is at the transition regime between low and high temperature behaviour for all of the studied alloys, and both intergranular and transgranular cracks are observed. An addition of interstitial carbon or substitutional manganese favours the high temperature regime in UHP α-iron. The activation volume of IFS is greater than that of UHP α-iron. This can be attributed to the action of substitutional atoms (Ti, Al, Mn) on dislocations mobility.