Influence of Section Thickness of Casting on the Microstructure Characteristics and Creep Rupture Properties of Ti-45al-2nb-2mn-1b

Ti-45Al-2Nb-2Mn-1B (atomic percent, 4522XD) cast step plate with different thicknesses was prepared by investment casting, and the variations of microstructure and creep rupture properties with thickness were examined. With decreasing thickness, lamellar colony size, lamellar thickness, and the size of equiaxed γ grains at the colony boundaries decreased, and the colony boundaries varied from smooth to serrated. Boride particles are mainly long flat needles in the region with 12 mm thickness, curvy flakes with considerable length and width in the region with 8 mm thickness, and curvy ribbons with small length in the region with 4 mm thickness. B27-TiB particles were observed in all the thicknesses, Ti3B4+ TiB2 lamellar particles were observed in the region with 8 mm and 12 mm thicknesses, and Bf-TiB+ TiB2 lamellar particles were observed in the region with 4 mm thickness. The smallest creep rupture elongation is obtained in samples taken from the plate with 12 mm thickness, while the shortest creep rupture life is obtained in samples taken from the region with 8 mm thickness. The former is attributed to the smooth colony boundaries which have weaker resistance to grain rotation, while the latter is attributed to the curvy boride particles that tend to nucleate cracks at the corner with small curve radius, providing a fast track for crack propagation along the boride/matrix interfaces, their considerable size makes the crack reach the critical size for fracture more easily. Weak creep rupture properties of some samples are mainly attributed to the aggregation of colonies whose lamellar interfaces are nearly perpendicular to the loading direction.