Systematic study of the effect of Cr on the microstructure, phase content and hardness of the AlCr_xFeCoNi alloys

Microstructure, phase content and hardness of homogenized multicomponent AlCr_xFeCoNi (where x = 0, 0.2, 0.5, 1 and 1.8) alloys was studied systematically using light, electron microscopies and microhardness methods. The goal of this research was to understand the effect of Cr on the microstructure and the reason for the variations in mechanical properties, such as hardness, as a function of Cr concentration. Quaternary AlCoFeNi alloy (i.e., x = 0) was found to be single phase B2 (CsCl structure type) with elongated grains. Alloys containing Cr, solidified in dendritic regime. Phase content has also changed. Addition of Cr promoted the formation of the Body Centered Cubic (BCC) phase at the expense of the B2 and coarsened the grains. At x = 1.8, BCC exceeded the B2 phase volume. Furthermore, in the dendrite region, reversion of the microstructure occurred: when 0 <x < 1.8 - BCC spherical particles were embedded uniformly in the B2 matrix, while at x = 1.8 cuboidal B2 particles were found to be homogenously spread in the BCC matrix. Complex behavior of the microhardness value was noted as a function of Cr content. These results were explained by solid solution strengthening mechanism.