Effect of B on the superplasticity of Fe-6.6Mn-2.0Al alloy

The influence of B on the superplasticity of Fe-6.6Mn-2.0Al alloy was investigated by means of high-temperature tensile testing at temperatures ranging from 620 °C to 880 °C with initial strain rates of 1 × 10⁻³ s⁻¹ and 1 × 10⁻⁴ s⁻¹. Since the high-temperature deformation mechanism changed below and above ~750 °C, in-depth analyses were carried out using tensile specimens tested at 650 °C and 800 °C. The addition of B increased peak stress and apparent activation energy (Q_a), but decreased or maintained strain rate sensitivity (m) value, average grain size (AGS) and γ fraction, regardless of tensile temperature and initial strain rate. These results are related to the segregation of B atoms at prior γ grain boundaries and α/γ phase boundaries. When deformed at 650 °C, dominant deformation mechanisms of α and γ phases were dislocation slip and GBS, respectively. Elongation decreased by the addition of B due to the less fraction of fine γ grains undergoing GBS. When tensile temperature was 800 °C, both α and γ grains underwent GBS and elongation increased by the addition of B. The higher elongation of the B-added alloy was most likely due to the finer grains resulting from the suppressed dynamic grain growth. Meanwhile, the addition of B did not provoke the self-healing effect due to the small amount of B and short deformation time.