Rapid Solidification And Special Coating Of Highly Undercooled DD3 Superalloy

In the present paper, purification mechanisms and processes for multi-component DD3 superalloy melts have been systematically investigated by means of molten salt purification with cyclical superheating. A mould, in which the already pre-purified DD3 superalloy melt and the molten salt purification agent are mixed, was additionally coated with a novel SiO2-ZrO2-B2O3 (i.e. Si-Zr-B) nucleation-inhibitive compound coating, to prevent the purified melt from nucleating on the mould surface. The production processes (Sol-Gel processing, composite method), the thermal stability and the nucleation inhibition for the DD3 superalloy melt of the Si-Zr-B coating were clarified. The microstructure evolution within a wide undercooling range was investigated, and the forming mechanisms for two kinds of grain refinements were highlighted. The precipitation of the strengthening phase, ?′(Ni3Al(Ti)), in the as-solidified DD3 superalloy was studied. A directional rapid solidification of the DD3 superalloy melts could be only realized in the mould coated with the Si-Zr-B coating. The mechanical properties of the ultra-fine directional structures solidified at different undercoolings were measured and analyzed.More detail is given as follows.Due to the alloying elements Al, Ti, the traditional glass fluxing or the pure cyclical superheating can not be used to undercool the DD3 superalloy melts substantially. Molten salt purification, however, is closely related to the reactions occurring between denucleating agent and heterogeneous nuclei in the DD3 superalloy melt. Whether stable and substantial undercoolings can be achieved for the DD3 superalloy melt depends on the component, viscosity and purification temperature of molten-salt denucleating agent. By adopting the denucleating agent composed of 30%CaF2+30%AlF3+10%Na3AlF6+20% KCl+10% NaCl (wt.%) with minimal qualities of CaO, TiO2 and Al2O3, high undercooling up to 250K was achieved for the DD3 superalloy melt under the protecting of argon gas, with superheating temperatures up to 1820K.In the achieved undercooling range 0-250K, the critical undercoolingsΔT1 (30K),ΔT2 (78K),ΔT3 (153K) andΔT* (180K) of the DD3 superalloy and the corresponding solidification microstructures are: (1)ΔT <ΔT1, dendrites solidified at low undercooling, (2)ΔT1 <ΔT <ΔT2, first kind of granular grain, (3)ΔT2 <ΔT <ΔT3, dendrites solidified at high undercooling, and (4)ΔT >ΔT*, second kind of granular grain.Based upon the nucleation and growth theory of the undercooled melt, the solidification morphology of the undercooled melt is determined by (1) nucleation of the undercooled melt, (2) rapid dendrite growth controlled by diffusion field, capillary action, and atomic attachment kinetics, and (3) the influence of recalescence on the primary formed dendrites. IfΔT<ΔT3, the final solidification morphology depends on the primary formed dendrites. The first kind of granular grains originate from the remelting during recalescence of the primary dendrite formed at lower undercooling. ForΔT>ΔT*, however, large stress,σs, on the solidified dendrites due to inter-dendrite fluid flow induced by pressure gradient and solidification shrinkage during recalescence, results in distortion, fragmentation or collapse of the primary dendrite network, and also will be stored as strain energy in the aforementioned dendrite fragments. Forσs, a modified relation has been developed, with gl, gs as the volume fraction of liquid and primary solid (i.e. gscoh < gs < fsR), gscoh as the solid fraction when the formation of the continuous dendrite framework, fsR as the maximum solid fraction after recalescence,βs as the solidification shrinkage of the primary phase,μas the dynamic viscosity of the liquid, a as the length of L/S coexistence zone,λ2 as the secondary dendrite arm spacing, and tf as the total solidification time. High temperature recovery and recrystallization taking place in these fragments during further cooling results in the further grain size decrease.In connection with the classical nucleation and growth theory, the size, morphology, and distribution of the precipitated ?′phase in the as-solidified DD3 superalloy is also controlled by the melt undercooling. With increasing melt undercooling, solid solubility of the ?′formers (Al and Ti) is extended, and the driving force for ?′precipitation is improved, thus the volume fraction and the number of ?′precipitate are enlarged, which leads to a refinement of the final ?′particles. The increase of the melt undercooling can also suppress dendrite segregation, reduce compositional differences, and homogenize the ?′distribution in the? matrix. Furthermore, a high density of crystalline defects (e.g. dislocations) and grain boundaries could be introduced in the primary solidification structure as effective nucleation sites for ?′. This will reduce the critical nucleation energy and favors ?′precipitation.Si-Zr-B nucleation inhibitive compound coating was firstly prepared by using sol-gel process and shell-moulding technique. With the addition of H3BO3 into the Si-Zr matrix, the formation of [BO4]- tetrahedrals (i.e. Si-O-B-O-Si bond) could link the broken bond (e.g. Si-O-//-O-Si) and thus alleviate the tendency of crystallization. The crystalline amount in the Si-Zr-B coating after heat treatment at 1773±5K for 30 min is only 2vol.%. Large undercooling as high as 250 K can be achieved for the highly purified DD3 superalloy melts in the mould coated with the Si-Zr-B coating. In relation to the results from the classical nucleation theory, Si-Zr-B coating was proved to have a good nucleation inhibition for DD3 superalloy melt, and it is a good coating for DD3 superalloy melt to realize high undercooling directional solidification.Directional rapid solidification of DD3 superalloy was realized in the Si-Zr-B nucleation-inhibitive compound coating mould withinΔT2-ΔT3, by artificially triggering the superalloy melt. It was found that the mechanical properties of DD3 superalloy solidified at high undercooling depend on the microstructural morphology, compositional homogeneity, size and distribution of the ?′precipitate. Compared with that obtained by conventional cast processing(elongation,δ=3.0% and strength,σb=346.4Mpa), the mechanical propertyδ,σb of the undercooled DD3 superalloy is respectively 9 and 3 times larger at the undercooling of 145K (δ=27.0%,σb =1040.4Mpa), which is slightly larger than that of industrial DD3 single crystal superalloy prepared by the conventional directional solidification(δ=25%,σb=1024Mpa).