Improvement of Shape Memory Effect by Optimizing Thermal and Mechanical γ → ε Martensitic Transformation by Hot Rolling

抄録

We propose a new concept to improve functional/mechanical properties of Fe-high Mn alloys, by suppressing thermal γ → ε martensitic transformation with a proper thermo-mechanical treatment. As an experimental example to demonstrate the validity of this concept, we tried to improve the shape memory effect of a Fe-15.3Mn-5.8Si-0.24C (mass-%) alloy by hot rolling. The alloy showed a dual γ/ε phase at room temperature (M_s = 66°C) and a poor shape recovery strain of approximately 0.6% after the solution-treatment. The pre-existing thermal ε-martensite can act as obstacles against the reversible forward-reverse γ → ε martensitic transformation under loading and subsequent heating. Hot rolling at 800°C with a 70% reduction appeared to dramatically improve the shape recovery strain up to 2.9%. The rolling suppressed thermal γ → ε martensitic transformation during cooling to obtain the single γ-phase at room temperature. Further annealing of the 800°C -rolled specimen at 800 to 1000°C or rolling at 600°C deteriorated shape recovery strain to about 1%. The rolling at 400°C made the alloy brittle.