Phase prediction, microstructure, and mechanical properties of spark plasma sintered Ni–Al–Ti–Mn–Co–Fe–Cr high entropy alloys

Abstract

The effect of mechanical alloying on the development of Ni–Al–Ti–Mn–Co–Fe–Cr high entropy alloys (HEAs) utilizing the spark plasma sintering (SPS) method is the main goal of this study. A bulk sample was fabricated using SPS after the alloys were mixed for 12 h. Thermodynamic simulation, X-ray diffraction, scanning electron microscopy, nanoindentation, and microhardness were used to investigate the microstructure and mechanical properties of the as-mixed powders. The master alloy was made of NiAl and was subsequently alloyed with Ti, Mn, Co, Fe, and Cr at different compositions to develop HEAs at a sintering temperature of 850 °C, a heating rate of 100 °C/min, a pressure of 50 MPa, and a dwelling time of 5 min. A uniform dispersion of the alloying material can be seen in the microstructure of the sintered HEAs with different weight elements. The grain size analysis shows that the Ni₂₅Al₂₅Ti₈Mn₈Co₁₅Fe₁₄Cr₅ alloy exhibited a refined structure with a grain size of 2.36 ± 0.27 µm compared to a coarser grain size of 8.26 ± 0.43 μm attained by the NiAl master alloy. Similarly, the HEAs with the highest alloying content had a greater microstrain value of 0.0449 ± 0.0036, whereas the unalloyed NiAl had 0.00187 ± 0.0005. Maximum microhardness of 139 ± 0.8 HV, nanohardness of 18.8 ± 0.36 GPa, elastic modulus of 207.5 ± 1.65 GPa, elastic recovery (W_e/W_t) of 0.556 ± 0.035, elastic strain to failure (H/E_r) of 0.09.06 ± 0.0027, yield pressure (H³/\(E_{{\text{r}}}^{2}\)) of 0.154 ± 0.0055 GPa, and the least plasticity index (W_p/W_t) of 0.444 ± 0.039 were attained by Ni₂₅Al₂₅Ti₈Mn₈Co₁₅Fe₁₄Cr₅. A steady movement to the left may be seen in the load–displacement curve. Increased resistance to indentation by the developed HEAs was made possible by the increase in alloying metals, which ultimately led to higher nanohardness and elastic modulus.