Composition design of high entropy alloys using the valence electron concentration to balance strength and ductility

The valence electron concentration (VEC) is an important physical factor for phase formation. A high VEC is conducive to forming an FCC phase and improving an alloy's ductility, while a low VEC is beneficial in forming a BCC phase that improves an alloy's strength. This is demonstrated for two HEAs, CoCrCuFeNi (FCC) and AlCoCrFeNi (BCC), that were designed as matrix alloys, where Ni and Mo are alloyed. The microstructure, phase evolution, and the mechanical properties for (AlCoCrFeNi)_100-xNi_x and (CoCrCuFeNi)_100-xMo_x were systematically investigated. As the phase structure for the (AlCoCrFeNi)_100-xNi_x high entropy alloy (HEA) transformed from a BCC to an FCC crystal structure as the Ni content increased from 0 at.% to 16 at.%, the FCC volume fraction increased from 0% to 85%, its compressive fracture strain increased from 25% to 40%, its VEC increased from 7.2 to 7.6. As the phase structure for the (CoCrCuFeNi)_100-xMo_x HEA transformed from FCC to BCC as the Mo content increased from 0 at.% to 16 at.%, the BCC volume fraction increased from 0% to 65%, its compressive yield strength increased from 260 MPa to 928 MPa, its VEC decreased from 8.8 to 8.3. Selecting an element based upon an alloy's VEC is a practical method for designing compositions for HEAs that balance strength and ductility. According to the needs of practical applications, balancing both strength and plasticity requires the following criteria for selecting an element for incorporation into an HEA system: matrix strength is improved by selecting an element with a VEC lower than the average VEC of the matrix, while ductility is improved by selecting another element with a VEC higher than the average VEC for the matrix.