On the development of Fe-containing high entropy alloys with improved strength at high temperatures
The emergence of new multicomponent high entropy alloys (HEA) has opened up new avenues for development of high performance materials for structural applications . Among all of them, the precipitation strengthened HEAs with either bcc or fcc matrix have attracted significant attraction as they provide solutions to the restriction imposed on conventional alloys [2–4]. The combination of ductile concentrated matrix and hard intermetallic precipitates shows an opportunity to develop alloys with optimum combination of mechanical properties at elevated temperature. In the present work we report a series of CoNiFeCrAlNbTi fcc-based high entropy alloys that are strengthened by the presence of L12 ordered precipitates in the disordered fcc -matrix. The TEM studies on as-solutionized alloys confirm the presence of very fine spheroidal precipitate distributed uniformly within the matrix. Further aging of alloys at 900 C leads to evolution of cuboidal morphology of precipitates. Composition analysis using STEM-ESD reveals preferential partitioning of Ni, Ti Al and Nb to the precipitates, whereas Co, Cr and Fe locate themselves to the matrix. These Ni rich (Ni,Co,Fe)3(Al,Ti,Nb,Cr) precipitates shows negative mismatch of -0.65 (%) with the CoNiCr high entropy matrix at room temperature. These alloys show mass density in the range of 7.9 - 8.0 g/cc and dissolution temperature in the range of 1030-1090 C. An addition of minute amount of Boron (0.01 at. % B) to these alloys increases the high temperature strength. The presently developed HEA’s show high temperature yield strength in excess of 1GPa at 750 C, see Fig 2b. The temporal evolution of average precipitate size and number density of precipitates at 900 C suggest classical evaporation-condensation (EC) mechanism based LSW model. The calculated precipitates coarsening rates are comparable to heavy elements (W, Re, Ta and Ru) containing Co based superalloys and other high entropy alloys.