Ductility enhancement in ultrafine-grained Fe–Ni–Mn martensitic steel by stress-induced reverse transformation

The effect of large strain deformation on the mechanical properties was investigated in a martensitic Fe–Ni–Mn alloy. After a combined deformation route (cold rolling plus wire drawing), the mechanical properties improved. The tensile strength of alloy in this study reaches to 2540 MPa and total tensile strain to 7% which are much higher than our previous study which after cold rolling and aging tensile strength was 1840 MPa with total tensile strain of 2.8%, although in both cases the improvement are significant for this brittle alloy. The enhancement of ductility after aging might be attributed to the effective role of grain refining down to nanoscale and the formation of austenite during deformation process and stabilization of that austenite in the aged wire. The austenite transforms to ɛ-martensite during tensile testing because of its low stacking fault energy (SFE) and increases the total measured tensile strain more. X-ray diffraction (XRD) analyses and transmission electron microscopic (TEM) images clarified the suggested reason for this ductility enhancement.

Research highlightsFe–10Ni–7Mn age-hardenable martensitic steel in the strain-free, aged condition shows premature fracture and zero ductility. By Large strain deformation, nano-grained wire could be fabricated. Ultrahigh straining improved the ultimate tensile stress up to 2540 MPa and strain up to 7%. X-ray and TEM observations determined reverse transformation of martensite to austenite to epsilon martensite in nanostructured Fe–Ni–Mn steel. As a result and as shown by X-ray diffraction and SFE calculation, deformation of austenite could lead to ɛ-martensite formation by stacking fault mechanism in nano-grains. These transformations could be the reasons for further mechanical properties improvement.