Microstructural evolution, electrochemical and corrosion properties of Al_xCoCrFeNiTi_y high entropy alloys

The microstructure of the AlxCoCrFeNiTiy high entropy alloy (HEA) system was studied using X-ray diffraction, scanning and transmission electron microscopy. A microstructural evolution from single-phase FCC to FCC + BCC + B2 occurred with increasing Al content. The addition of a comparatively small amount of Ti led to the formation of a Fe-Cr sigma phase. The corrosion characteristics of the alloy system were studied across different compositions, with such an alloy system exhibiting a high resistance to general corrosion, superior to stainless steel 304L in 0.6 M NaCl. Cyclic potentiodynamic polarisation suggested that the HEAs studied underwent pitting corrosion following breakdown. From exposure testing, it was seen that very fine pitting, although not extensive in nature, was the principle form of corrosion for Al x CoCrFeNiTi y after prolonged immersion. There was little evidence of microgalvanic corrosion or selective dissolution of a particular phase observed, despite the heterogeneous microstructure and significant elemental segregation in the alloys studied. The composition of the surface films formed upon the AlxCoCrFeNiTiy alloys were elaborated by X-ray photoelectron spectroscopy, which provided new and further insights regarding the surface films of such alloys. The study herein contributes to an emerging understanding of the corrosion characteristics of high entropy alloys.