A new approach to synthesise high strength nano-oxide dispersion strengthened alloys

Abstract

High oxygen content is usually harmful for metals and alloys. However, in this study, we propose a new approach, which involves direct additive manufacturing of a powder alloy that contains a high oxygen level, to synthesise nano-oxide dispersion strengthened (ODS) alloys. Thus, by selectively laser melting a FeNi alloy powder with an oxygen level of 3000 ppm, a new ODS alloy has been in-situ synthesised. The as-fabricated samples were also heat treated at different temperatures to understand their microstructural stability. The nano-sized oxides in the as-fabricated sample were identified to be face-centered cubic Fe3O4 and are entangled with dislocations. During deformation, dislocations loop around these particles suggesting that they have acted as effective dislocation barriers, which leads to enhanced tensile strengths. Heat treatment at or above 950 °C leads to depletion of oxides in grain interior and to segregation of oxides along grain boundaries (GBs). Transmission electron microscopy study reveals that at increased temperatures dislocations tend to act as bridges between oxides and GBs, promoting dissolution of oxides in grain interior and nucleation and coarsening of oxides at GBs. The results indicate that the proposed approach is an effective new way of synthesising high performance ODS alloys and also give a strong implication that the precipitate-dislocation interaction plays a significant role in microstructural evolution during annealing.