Influence of Ni addition on strain-induced martensite in austempered ductile iron

This study systematically investigates the effects of nickel (Ni) additions on strain-induced martensitic transformation in austempered ductile iron (ADI). The matrix of ADI containing no more than 4 wt% Ni was composed of acicular ferrite and austenite. Increasing Ni content elevated the austenite volume fraction but reduced its carbon concentration. Furthermore, Ni additions suppressed the nucleation rate of the α-phase, resulting in an increase in both the quantity and size of blocky austenite as Ni content was increased. Strain-induced martensite was observed within the blocky austenite for ADI containing 2 wt% and 4 wt% Ni. Notably, approximately 40 % of the austenite in the ADI containing 4 wt% Ni transformed into martensite following the tensile test. This substantial phase transformation effectively mitigated stress concentrations and significantly enhanced the work-hardening capacity. Moreover, concurrent deformation twinning was observed within the blocky austenite. Consequently, an exceptional strength-ductility balance with an ultimate tensile strength of ∼1100 MPa and a total elongation of ∼10 % was achieved.