Multi-principal element alloys from the CrCoNi family: outlook and perspectives

A great research effort is being employed in the development of Multi-Principal Element Alloys (MPEAs). The major interest on these alloys lies in the vast compositional field in which they exist, which provides a challenging opportunity for designing alloys with improved combinations of properties. Among the MPEAs produced to date, compositions containing large contents of Cr, Co and Ni, including the recently developed and well-studied Cr33Co33Ni33 alloy (in atomic %), are among the most promising for structural application due to their outstanding combination of strength and ductility. The present work is a critical review of some of the important publications on the topic. First, we give a brief history of the MPEA development in recent years highlighting the importance of CrCoNi alloys. Next, a brief explanation of the phase equilibria in this system is given. The third and major part of this work consists in a compilation followed by critical discussion of the main studies explaining the good mechanical properties of these alloys. This compilation includes what is known about grain refinement, solid solution strengthening, twinning and transformation induced plasticity and short-range ordering. The major controversies on the area are pinpointed as well as directions for future research. Several other CrCoNi alloy properties are also reviewed, including what has been published on wear, fatigue, corrosion, oxidation and hot deformation. High Cr-containing alloys have excellent combinations of strength, ductility, corrosion (in saline environments) and oxidation resistance, surpassing some of the best performing Ni-superalloys.