Solidification Behavior in Newly Designed Ni-Rich Ni-Ti-Based Alloys

The present investigation reports phase and microstructure evolution during solidification of novel Ni-rich Ni-Ti-based alloys, Ni₆₀Ti₄₀, Ni₅₀Cu₁₀Ti₄₀, Ni₄₈Cu₁₀Co₂Ti₄₀, and Ni₄₈Cu₁₀Co₂Ti₃₈Ta₂ during suction casting. The design philosophy of the multicomponent alloys involves judicious selection of alloying elements such as Cu, Co, and Ta in the near Ni₆₀Ti₄₀ eutectic alloy by replacing both Ni and Ti so that phase mixture in the microstructure remains the same from the binary to quinary alloy. The basic objective is to study the effect of addition of Cu, Co, and Ta on the phase evolution and transformation in the Ni-rich Ni-Ti-based alloys. The detailed electron microscopic studies on these suction cast alloys reveal the presence of ultrafine eutectic lamellae between NiTi and Ni₃Ti phases along with dendritic NiTi and Ti₂Ni phases. It has also been observed that in the binary (Ni₆₀Ti₄₀) alloy, the ordered NiTi (B2) phase transforms to trigonal (R) phase followed by NiTi martensitic phase (M-phase), i.e., B2 → R-phase → M-phase during solid-state cooling. However, the addition of alloying elements such as Cu, Co to the binary (Ni₆₀Ti₄₀) alloy suppresses the martensitic transformation of the ordered NiTi (B2) dendrite. Thus, in the ternary and quaternary alloys, the ordered NiTi (B2) phase is transformed to only trigonal (R) phase, i.e., B2 → R-phase. The secondary precipitate of Ti₂Ni has been observed in all of the studied alloys. Interestingly, Ni₄₈Cu₁₀Co₂Ti₃₈Ta₂ quinary alloy shows the disordered nature of NiTi dendrites. The experimentally observed solidification path is in good agreement with Gulliver–Scheil simulated path for binary alloy, whereas simulated solidification path deviates from the experimental results in case of ternary, quaternary, and quinary alloys.