Microstructural characterization of nanosized nodular irons under severe torsional-compressive deformations

Nodular irons are iron-based alloys of carbon, silicon, and small magnesium/cerium additions. These solute elements promote the formation of graphite spherules. As a result, nodular irons have outstanding mechanical strength, ductility, wear resistance, and machinability. However, the increase in tensile strength and yield strength significantly reduces the elongation capacity. Moreover, previous studies on nodular cast irons deformed by tensile deformation between 800 °C to 1050 °C show increased mechanical properties. Since it is known that nano-effects spontaneously appear below critical sub-unit sizes, the present work intends to obtain graphite nodules nano-sized. When nodular iron is plastically deformed by tensile and compressive stresses at high temperatures, the spherules will elongate and fracture. In addition, a spheroidization of the fragments was observed induced by regeneration processes. Multi-steps of compression and pure shear stresses will produce nodules of smaller sizes. This work subjected a nodular iron with about 200 nodules/mm² to a high-pressure torsion process. Cylindrical specimens were machined to deform in a hot torsion machine, with a coupled compression module, at temperatures between 1,050 °C and 1,125 °C, at an equivalent strain of 2.5, and strain rates of 0.5 s^-1 and 5 s^-1. We used an inert argon atmosphere to prevent further specimen surface oxidation. Before and after deformation, specimens were metallographically analyzed. The microstructure, the percentage of nodularity, nodule density/mm², and the distribution of spherical nano-nodules sizes are reported. In addition, some samples were water quenched at different strain magnitudes to observe the evolution of deformed, fractured, and regenerated graphite nano-nodules. High nodule densities and nanometer nodule sizes were founded.