Structure Refinement And Its Mechanism Of Cast Tial Based Alloys Containing Hafnium

The effect of Hf addition on the solidification structure refinement and its mechanism of cast Ti-46Al based alloys were studied. The results show that cast Ti-46Al based alloys with Hf addition have near lamellar microstructure (α₂/γ) with columnar and dendrite morphology. The B₂ phase with richer Hf forms at the dendrite node. Hf concentration has strong effect on columnar/dendrite spacing andγlamellar spacing refinement. Increasing Hf from 0 to 7at. %, the columnar spacing can be reduced from ¹000 to ⁴00μm,and theγlamellar spacing from ⁰.18 to ⁰.08μm. By analyzing the macro and micro structure, the relationship between Hf addition and structure refinement is well established and the possible phase sequence is given. According to the effect of Hf on the melt structure and viscosity and the KGT model, it is deduced that the refinement mechanism of columnar/dendrite spacing is attribute to the formation of large high melt point clusters rich in Hf and the increase of the melt viscosity, leading to a decrease of Al diffusion coefficient in the melt and consequently refining the columnar/ dendrite spacing.The effect of B addition on cast Ti-46Al based alloys solidification structure was studied. The analyses about the type, quantity, morphology and distribution of borides were performed. B concentration has very strong effect on the solidification structure refinement. When B=1.0at. %, the structure morphology changes into equiaxed dendrite. B can form TiB₂ phase. When B=0.2at. %, TiB₂ exist at the interdendrite with short ribbon-like shape, when B=0.6at. %, TiB₂ mainly exist at the interdendrite with ribbon-like shape while some long ribbon-like TiB₂ exist in the dendrite interior, when B=1.0at. %, most TiB₂ with long ribbon-like shape distribute in the dendrite interior, while a little exist at the interdendrite.The coupling influence of Hf and B additions on the solidification structure refinement and its mechanism of cast Ti-46Al based alloys was investigated. The coupling of Hf and B has very strong effect on structure refinement. With increasing Hf and B additions, the grain morphology changes from coarse columnar to fine dendrite and to equiaxed, then to fine near granular structure with average grain size 20μm when Hf =7at. % and B=1.0at. %. Hf can promote the formation of (Ti,Hf)B₂ and reduce the critical composition for B forming (Ti,Hf)B₂. There exist two kinds of (Ti,Hf)B₂, one is the prior (Ti,Hf)B₂ distributing in the center of fine near granular grain, another is the secondary (Ti,Hf)B₂ growing coupled with the matrix by the L→β+(Ti,Hf)B₂ and L+β→α+(Ti,Hf)B₂ eutectic reactions. By discussing the reason of very strong constitutional supercooling forming and analysing the microstructure, the mechanism of fine near granular structure forming is given as: Hf promote B to form prior (Ti,Hf)B₂ and produce double constitutional supercooling by Al and B, ie. the couple effect of nucleation rate increase and growth speed. The selection map of solidification structure morphology and the schematic diagram illustrating the solidification structure evolution are given.The combined effect of Hf, B and Si on solidification structure refinement of cast Ti-46Al based alloys was studied. It is found that although Si can refine the coarse columnar/dendrite structure to small equiaxial eutectic structure, but when Si=3.0at. %, Ti5Si3 phase in the shape of large hexagoned block, long strip and particle will precipitate and make the mechanical properties of the alloys down.Effect of Hf and B additions on the heat treated structure of Ti-46Al based alloys was studied using double-temperature heat treatment processing. DTA analysis shows that for Ti-46Al-5Hf alloy, the eutectoid temperature (Te)=1110℃, the alpha phase transition temperature (Tα)=1330℃. Applying the metallographic analysis method, the optimum parameters of double-temperature heat treatment processing are given as:1400℃,1h→WQ, 1280℃, 3h→AC. The results show that the heat treated microstructure is B and Hf contents dependent. Hf can narrow the lamellar spacing significantly; B can refine the lamellar colony size strongly due to the decrease ofαplate nucleation rate and the restriction ofαgrowth.