Effect of growth rate on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B alloy

The effect of growth rates (V=2–50 µm·s⁻¹) on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B (at.%) alloy at a constant temperature gradient (G=18 K-mm⁻¹) was investigated. Results indicated that β phase was the primary phase of the directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B alloy. As the growth rate increases, the solid/liquid interface turns from cellular growth to dendric growth. The interlamellar spacing (λ_s) decreases with the increase of growth rate according to the relationship of λ_s=3.39V^−0.31. The solidification segregation occurs due to the enrichment of β-stabilizing element Nb, Cr in primary β phase during solidification; moreover, the degree of the segregation increases with the growth rate, resulting in the emergence of B2 phase in lamellar colonies at high growth rates. The microhardness (H_v) grows with the growth rate based on the equation of H_V=328.69 V^0.072, which mainly attributes to the microstructure refinement.