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

Abstract

In this study, three non-equiatomic low-density Al₃₀Ni₂₀Cu₁₅Fe₂₅Ti₁₀ (Al₃₀Ni₂₀, $\rho$ = 6.11 g/cm³), Al₂₅Ni₂₅Cu₂₀Fe₂₅Ti₀₅ (Al₂₅Ni₂₅, $\rho$ = 6.91 g/cm³) and Al₁₅Ni₃₀Cu₂₀Fe₂₅Ti₁₀ (Al₁₅Ni₃₀, $\rho$ = 6.98 g/cm³), high entropy alloy (HEA) was designed using phase formation rules and synthesized via arc melting technique. The Al₃₀Ni₂₀ alloy, after uniaxial compression results, showed lower ductility (∼3 %) of this HEA due to the high fraction of the B₂/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 Al₁₅Ni₃₀ 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 Al₃₀Ni_20, Al₂₅Ni_25, and Al₁₅Ni₃₀ 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 Al₃₀Ni₂₀-HT_, Al₂₅Ni_25-HT_, and Al₁₅Ni₃₀-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 B₂/BCC and FCC-based dual-phase microstructure.