Role of Molybdenum in High-Temperature Uniaxial Tensile Deformation Behaviour of Fe-30Mn-5Al-1C-xMo Lightweight Austenitic Steels

The present work investigates the high-temperature uniaxial tensile flow behaviour of lightweight austenitic steels (LWAS) that could be potential high-temperature structural materials. The emphasis is on understanding the role of molybdenum in Fe-30Mn-5Al-1C (wt pct) LWAS, consisting of austenitic single-phase with recrystallized and equiaxed grains of similar to 80 +/- 5 mu m, prepared by induction melting followed by hot-deformation and cold-rolling with solution annealing. The uniaxial tensile tests were conducted in the temperature range of 300-973 K and at an initial strain rate of 10(-3) s(-1). Post deformed microstructural observations are correlated with the flow behavior through EBSD, ECCI and TEM analyses. The yield and ultimate tensile strengths up to 673 K, revealed the dominance of interstitial solid solution strengthening by carbon over substitutional strengthening by molybdenum. However, beyond 673 K up to 0.5 wt pct Mo solid solution by Mo and precipitation hardening by Mo-enriched carbides was dominant in 2 and 3-Mo alloys. A transition in fracture mechanism from purely ductile to quasi-ductile with grain boundary cracking at 973 K led to a significant drop in fracture strain with increased wt pct of Mo. Further, the serrated flow was observed which was attributed to dynamic strain ageing due to carbon at low temperatures and Molybdenum at high temperatures.