Dendrite growth kinetics and microstructure evolution mechanism of Ni₂TiAl intermetallic compound within substantially undercooled liquid Ni₆₄Al₁₉Ti₁₇ alloy

The dendrite growth kinetics and microstructure evolution mechanism of the Ni₂TiAl intermetallic compound within substantially undercooled liquid Ni₆₄Al₁₉Ti₁₇ alloy were investigated using electrostatic levitation (ESL) and drop tube (DT) techniques. The growth velocity of primary Ni₂TiAl dendrites displayed a power-law relation with liquid undercooling, achieving a maximum of 80 mm/s at the undercooling of 254 K (0.16 T_L). The solidification pathway transformed from primary Ni₂TiAl growth plus subsequent (Ni₃Ti+Ni₂TiAl) eutectic growth into the successive growth of Ni₂TiAl and Ni₃Ti phases as the alloy undercooling increased sufficiently. The high cooling rates of freely falling alloy droplets effectively suppressed the martensitic transformation observed for primary Ni₂TiAl intermetallic compound under typical ESL radiative cooling condition. Both nanoindentation and Vickers hardness performances of the Ni₂TiAl phase formed under these two conditions exhibited conspicuous enhancement due to grain refinement effects. The microstructures with martensitic transformation under ESL condition showed higher hardness than those under DT condition despite their larger grain size.