Stress and defects driven phase transformations

S. Meka 1, E. A. Jägle 2

1 Indian Institute of Technology, Roorkee, India
2 Department of Microstructure Physics and Alloy Design (MA)

This partner group (PG) was awarded at the end of 2016 and has formally started its activities from 1st March 2017. It is carrying out research activities independently as well as in collaboration with the MPIE. The focus of the research group is to investigate phase transformation phenomena that occur as a result of metal-gas reactions, which, in turn, generate stresses and lattice defects in the material. Some examples of the research are given below.

Researchers from the MPIE and IIT Roorkee are working jointly on developing nitride dispersion strengthened steels by combining the gaseous nitriding expertise of researchers at IIT Roorkee and the Additive Manufacturing (AM) expertise at the MPIE. In this project, Fe-2.3 wt.% Al alloy powder was subjected to gaseous nitriding treatment to realize the internal precipitation of AlN in ferrite matrix in addition to the development of iron nitrides. After that, the developed iron nitrides were reduced to iron and N2 gas by hydrogen reduction treatment such that AlN particles remain stable, whereas iron nitrides are unstable. The nitrided-plus-hydrogen-reduced powders were consolidated using spark plasma sintering (SPS). Sintered compacts have indicated a significant increase in hardness as compared to the case of no AlN dispersion was created, i.e., SPSed as received Fe-2.3wt.%Al alloy powder. Detailed characterization revealed a complex microstructure beyond the targeted AlN particles in ferrite matrix: During nitriding, iron-oxides have developed which also got reduced during following hydrogen-reduction treatment. Due to the availability of this oxygen, Al-oxynitrides have developed as layers in powder particles. We will further optimize the process to avoid oxygen contamination in the future. It is also planned to carry out ballmilling of Fe-2.3wt.%Al alloy powder to reduce the particle size and to introduce dislocations which will act as nucleation sites for AlN and thus are expected to refine the particles. The final aim of this project is to use the nitrided and hydrogen-reduced powders in additive manufacturing employing selective laser melting (SLM) to realize AlN dispersion strengthened steel components.

Other research activity which the PG is currently focussing on is to understand gas/solid equilibria, especially to understand the possible establishment of metastable states leading to spinodal decomposition of the system. In this context, iron-based alloys which show the possibility of spinodal decomposition during the process of equilibrating with externally imposed gaseous nitriding and oxidising atmospheres are being explored. Further joint work is underway on the effect of nitride inhibitor particles on secondary recrystallization in electrical steels.

Fig. 1: Microstructure of SPS-sintered Fe-2.3Al powder that had been gas nitrided and hydrogen reduced prior to sintering. A shell of Al-rich oxynitrides around the previous powder particles is visible.

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