Tweaking AlNi atomic fraction to enhance the mechanical properties of low‐density AlCuFeNiTi- based high entropy alloys

In this study, three non-equiatomic low-density Al 30 Ni 20 Cu 15 Fe 25 Ti 10 (Al 30 Ni 20 , ρ = 6.11 g/cm 3 ), Al 25 Ni 25 Cu 20 Fe 25 Ti 05 (Al 25 Ni 25 , ρ = 6.91 g/cm 3 ) and Al 15 Ni 30 Cu 20 Fe 25 Ti 10 (Al 15 Ni 30 , ρ = 6.98 g/cm 3 ), high entropy alloy (HEA) was designed using phase formation rules and synthesized via arc melting technique. The Al 30 Ni 20 alloy, after uniaxial compression results, showed lower ductility (∼3 %) of this HEA due to the high fraction of the B 2 /BCC phase microstructure. Reducing the Al atomic fraction is responsible for the evolution of FCC phase microstructure and results in improved ductility (∼9 %) in the Al 15 Ni 30 HEA sample in its as-cast state. All three compositions were meticulously tuned so that alloys exhibited low density, aligning with solid solution phase formation rules. Microstructure and mechanical properties of low-density HEAs have been investigated using SEM/EDS, XRD, EBSD, indentation testing, and uniaxial compression testing. Furthermore, the effect of heat treatment on the microstructure and mechanical characteristics of three HEA specimens has been examined to determine the stability of different phases at elevated temperatures. X-ray diffraction results indicated the phase stability following heat treatment of HEAs at 900 ℃ for 24 h. As-cast Al 30 Ni 20, Al 25 Ni 25, and Al 15 Ni 30 have ultimate compressive strength (UCS) of 1824, 2008, and 2080 MPa, respectively. The compressive strain of the same sample was 3.19, 6.64, and 9.33 %, respectively. Interestingly, the UCS of Al 30 Ni 20 -HT , Al 25 Ni 25- HT , and Al 15 Ni 30 -HT was 1785, 2008 MPa, and 1640 MPa, respectively, and strain of the same samples was increased to 3.69, 10.22, and 22.9 %, respectively. Heat treatment demonstrated an obvious strength-ductility tradeoff for three HEAs, i.e., increased ductility at the expense of UCS. The concurrent increase in UCS and ductility with increasing Al and decreasing Ni fraction is attributed to the unique combination of B 2 /BCC and FCC-based dual-phase microstructure. • Three low-density HEAs with varying Al/Ni fractions (Al 30 Ni 20 , Al 25 Ni 25 , Al 15 Ni 30 ) were designed and made via arc melting. • Al 15 Ni 30 exhibited the highest UCS (2080 MPa) and specific UCS (322 MPa·cm 3 /g) due to low density and high strength. • Heat treatment enhanced phase stability, FCC phase fraction, grain size, and ductility for Al 25 Ni 25 and Al 15 Ni 30 HEAs. • Al 15 Ni 30 -HT displayed the best work hardening and ductility performance with an SDI of 144 MPa.