Due to the distinctive combination of the mechanical properties such as high strength, high toughness, high resistance to wear, a high strain-hardening rate in polycrystalline form, and heavy impact loads, Austenitic Hadfield steel is an excellent substrate of choice in various applications, i.e., excavators, railway crossings, crusher jaws, grinding mill liners, oil well drilling, impact hammers, bullet-proof helmets, and non-magnetic plates for electromagnets [1-4]. The standard austenitic Hadfield steel contains between 10–14% Mn and between 1.0–1.4% C, and it is completely austenitic in the normal quenched condition. Hadfield steel has a superior work hardening ability compared to other carbon steels. This exceptional work hardening capacity is ascribed to the transformation of to or martensitic structures and deformation twins [2, 5-7].
Hadfield steel has a low stacking fault energy (SFE, 23 mJ m–2) [5], which induces the formation of deformation twins. It is reported that twinning is activated in polycrystalline Hadfield steel, especially at strains of the order of 5–10% [5, 8]. Twins can cause a resistance to dislocation motion, and twin-twin interactions result in a high hardening rate [9, 10]. Four deformation mechanisms affect the mechanical properties of Hadfield steel including [2, 6, 11]: (i) lattice friction, (ii) dynamic strain ageing (DSA), (iii) mechanical twinning, and (iv) dislocation accumulations. However, it is challenging in polycrystalline Hadfield steel to experimentally identify the contribution of each deformation mechanism as all four mechanisms are generally active, and due to grain boundary effects [12, 13].
Firstly, the friction stress (lattice friction) or solid solution strengthening depends on the local atomic structure of the material, it occurs as a result of the fluctuations in the solute-dislocation interaction energy [14]. It can be calculated according to Hall-Petch relationship based on the yield strength of the austenitic structures with respect to the corresponding grain size [15, 16].
Secondly, dynamic strain aging (DSA), which is associated with the repeated dynamic interactions of moving dislocations with diffusing interstitial solute atoms after yielding. DSA is supposed to accelerate the work-hardening rate of Hadfield steel, leading to a high uniform elongation and ultimate tensile strength [17, 18]. DSA appears under low-strain-rates conditions because of the sufficient time available for the interstitial atoms to interact with dislocations, which results in locking/unlocking of dislocation movements. However, DSA can be suppressed by increasing the strain rate during tension tests [8]. The DSA's contribution to the stress response of Hadfield steel can be observed as serrated flow on the stress-strain curves.
Thirdly, regarding the mechanical twinning, prior studies revealed that twinning is the primary deformation mechanism at the beginning, and at initial stages of deformation for tensile loading of oriented Hadfield steel single crystals [5, 10, 12, 19, 20]. There is evidence from an earlier study on Cu single crystals that activation of primary twinning alone could lessen the hardening rate. However, the rate of hardening increases when primary twins interact with secondary twins [21].
Eventually, in terms of dislocation accumulations, some low SFE FCC alloys, as well as TWIP steels, have been shown to develop a high density of dislocations [6, 22, 23]. For example, Hutchinson and Ridley [6] revealed that the observed dislocation density of deformed Hadfield steel is one order of magnitude more than that of pure FCC metals, possibly because of interactions between the dislocations and manganese–carbon atomic dipoles. It is expected that dynamic ageing would decrease the mean free length of slip, leading to more dislocations being immobilised in the crystal after a given strain. Furthermore, the interaction could impede dynamic recovery, hence decreasing the rate of dislocation annihilation. As a result, work hardening is predicted to rise [6]. However, Hutchinson and Ridley [6] reported that the high work hardening rate at true strains above about 0.15 is mostly due to mechanical twinning, which can contribute almost twice as much as the effect of dislocation accumulation. Whereas, Zhang et al. [24] reported that the work hardening of Hadfield steel mostly caused by dislocation interactions during the initial stages of deformation, while the initiation of twin plates led to an unusual hardening at the final stages of the deformation under low strain rate conditions. The study advances expressively from the literature survey that research studies are still needed to investigate the contributions of the deformation mechanisms to the work hardening of the Hadfield steel under high strain rates.
Therefore, the present study aims to quantitatively evaluate the contribution of deformation twins and dislocation accumulations to the flow stress of Hadfield steel at a strain rate of unity per second. Such analysis can provide more details about the plastic behaviour of high interstitial C-Mn steels, especially under high strain rate deformation.