Low-temperature superplasticity in a nanocomposite iron alloy derived from a metallic glass

The focus of this research was to use the solid state glass devitrification transformation to develop nanocomposite microstructures in iron-based materials. Mechanical deformation studies on the nanocomposite structures at room temperature revealed that although they exhibited high yield strength (975–1200 MPa), they were brittle and exhibited no tensile elongation. A specialized two-step annealing process was employed which involved first holding for extended times below the crystallization onset temperature to devitrify extremely fine matrix grains followed by heating to temperatures above the crystallization onset temperature for short times. Using this approach, nanocomposite microstructures could be formed consisting of α-Fe, Fe₂₃C₆ and Fe₃B type triplex matrix phases of the order of 75 nm in size which were stabilized by dispersions of 2–10 nm α-Fe precipitates along the grain boundaries and embedded in the Fe₂₃C₆ type phase. At 750 °C and a strain rate of 10⁻³ s⁻¹ this stabilized nanoscale microstructure was found to exhibit superplasticity with an ultimate tensile strength of 1800 MPa and a tensile elongation of 230%. Subsequent testing using 'jump tests' revealed that the strain rate sensitivity factor was equal to 0.51, which, consistent with the microstructural observations, revealed that the main mechanism for elevated temperature deformation was grain boundary sliding/rotational processes.