Synergetic Effects of Complex Additions of Mo and Ni on the Phase Transformation Behavior and Mechanical Properties of Austenitic Lightweight Steels

Fe–30Mn–10.5Al–1.1C alloys with different Mo and Ni contents are prepared to investigate the phase transformation behavior and mechanical properties of austenitic lightweight steels. Solution‐treated samples have microstructures consisting of an austenite matrix and ordered κ‐carbides, and the size of the κ‐carbides decreases with the addition of Mo or Ni. In addition, the ordered body‐centered cubic (BCC) phases of B2 and DO3 also form when 3wt%Mo and 3wt%Ni are complexly added, resulting in a large decrease of the austenite grain size. Nanoindentation experiments show that the intrinsic strength of the austenite matrix changes depending on the precipitation behavior of the κ‐carbide; i.e., the addition of 3wt%Mo or 3wt%Ni decreases the intrinsic strength of the austenite by suppressing κ‐carbide precipitation, while complex additions of 3wt%Mo and 3wt%Ni increase them again. The results of tensile tests show that the alloy containing both 3wt%Mo and 3wt%Ni exhibits the highest yield (1102 MPa) and tensile strength (1155 MPa) with large tensile ductility (33%) due to the effects of grain refinement of the austenite grains and formation of secondary hard phases of DO3 and B2, as well as κ‐carbides. Finally, the changes in the precipitation behavior of the κ‐carbide are investigated via transmission electron microscopy and atom probe tomography analyses. For environmental concerns, many investigations have focused on the development of lightweight steels. This study investigates the synergetic effects of complex additions of Mo and Ni on the properties of lightweight steels. The results show that complex additions of Mo and Ni promote ordered ferrite phases with fine grain size, exhibiting high yield (1102 MPa) and tensile (1155 MPa) strengths with large elongation (33%).