The influence of Hf and B on the solidifi cation structure of cast Ti-46Al alloys was investigated.The results show that the coupling effect of Hf and B changes the solidifi cation structure morphology and strongl ref ines the grain size.When the Hf+B contents were increased from 0 + 0.0 to 3 + 0.2,5 + 0.6 and 7 + 1.0(in at.%),the solidification structure morphology changed from coarse columnar dendrite to fine columnar dendrite then to equiaxed dendrite,and further to f ine near granular grain whilst the average grain size decreased to 20 μm It is concluded that the columnar dendrite refinement is due to the effect of Hf and B on the decrease of A diffusion coeffi cient in the melt.The f ine near granular grain formation is attributed to the combined constitutiona supercooling formed by Al and B segregation that is strengthened by Hf and B additions at the solid/liquid interface during solidifi cation,and the TiB2 precipitates acting as heterogeneous nuclei.
To investigate the effect of hafnium addition on the solidif ication structure, Ti-46Al alloys with nominal compositions of Ti-46Al-xHf (x = 0, 3, 5, 7) (at.%) were arc-melted into small ingots in an argon atmosphere. The characteristics of the macrostructures and microstructures were studied using a linear intercept method, OM, SEM (BSE), XRD and TEM. The results showed that the ingots with Hf have near lamellar microstructure in columnar and dendrite morphology. The hafnium concentration has a strong effect on the columnar spacing refi nement. Increasing Hf from 0 to 7 (at.%), the columnar spacing can be reduced from~1000 to~400 μm. Constitute phases of the ingots are α2, a small amount of B2 and c. Most of the B2 phases, richer in Hf and leaner in Al and Ti, exist on the node of the dendrite core in block shape and a little across the lamellar colonies in stick shape. The c phases exist on the boundaries of lamellar colonies in small cellular shape. There also exists a segregation of Hf on the columnar and dendrite core. Particularly, both the α- and β-phase form from the melt as prior phases. The possible phase sequencing during solidif ication and solid-state transformations with Hf is given in this paper.
Su YanqingZhang LiGuo JingjieK. MaruyamaLi Zhiming