Ni-based single crystal superalloy has been widely used in turbine blades due to its excellent high temperature mechanical behavior. In order to completely exhibit high temperature mechanical properties, the seed method has been used to produce Ni-based single crystal components for [001] orientation paralleling to main force direction. Stray crystals, which unexpectedly nucleate in the melt-back region, will competitively grow with seed during directional solidification. It is important to profoundly understand the mechanism of competitive growth to find ways of overgrowing stray crystal during producing Ni-based single crystal components. However, within the published research there are conflicting views on the mechanism of competitive growth at converging case. Bi-crystal converging competitive growth was investigated in Ni-based single crystal superalloy with different pulling rates using seed technology. A series of polishing and imaging quenching interface were done for the positional relationship of dendrites near grain boundary in 3D reference. It was found that solidification microstruc tures were different with different crystal orientations. Unfavorable oriented dendrite tilting to heat flux restrained favorable oriented dendrite aligning to heat flux mainly thought inserting into the favorable oriented dendrites channel, and this resulted in unfavorable oriented dendrite overgrowing favorable oriented dendrite at low pulling rate. However, at high pulling rate the unfavorable oriented dendrites mainly blocked by grain boundary favorable oriented dendrite and the grain boundary grew paralleling to favorable oriented dendrite core. Favorable oriented dendrite being depressed and vanished, owning to that unfavorable oriented dendrite inserting into favorable oriented dendrites channel result in adjusting primary dendrite spacing, is the main factor to favorable oriented grain overgrew by unfavorable oriented grain. According to above mechanism, effect of pulling rate on competitive growth at converging case was interpreted. This could broaden our understanding of competitive growth at converging case in 3D reference.
Fund: Supported by National Natural Science Foundation of China (Nos.51331005, 51171151 and 51501151), High Technology Research and Development Program of China (No.2012AA03A511), National Basic Research Program of China (No.2011-CB610406), Natural Science Foundation of Shaanxi Province (No.2014JM6227), Foundation Research Foundation of Northwestern Polytechnical University (No.3102014JCQ01022) and Advanced Aero Engine Collaborative Innovation Center of China
Fig.1 Schematic of spatial orientation of the bi-crystal seeds
Fig.2 Cross sectional (a, b) and longitudinal sectional (c, d) OM images of Ni-based single crystal superalloy with [001] direction (a, c) and [001] direction deviating 20° from heat flux (b, d) at pulling rate V=50 μm/s
Fig.3 Effect of pulling rate on dendrite arm spacing
(a) primary dendrite spacing (b) dendrite spacing at grain boundary direction
Fig.4 Cross sectional OM images of Ni-based bi-crystal superalloy with different distances from the melt-back interface of 15 mm (a1), 25 mm (a2) and 35 mm (a3) at V=50 μm/s; and 35 mm at V=15 μm/s (b), V=75 μm/s (c) and V=100 μm/s (d)
Fig.5 Quenched interfaces of Ni-based superalloy through a series of sequential polishing with the spacing of adjacent layers of 25 μm show the positional relationship of dendrites near grain boundary at V=50 μm/s (a~f) ( A1, A2 and A3 indicate favorable dendrite, dash lines indicate the height of A1, white arrows indicate unfavorable dendrite blocked by A2, and black arrows indicate unfavorable dendrite blocked by A3)
Fig.6 Quenched interfaces of Ni-based superalloy through a series of sequential polishing with the spacing of adjacent layers of 45 μm show the positional relationship of dendrites near grain boundary at V=100 μm/s (a~c) (A1, A2 and A3 indicate favorable dendrite, white arrows indicate unfavorable dendrite blocked by grain boundary favorable dendrite A1, and black arrows indicate unfavorable dendrite blocked by grain inner dendrite)
Fig.7 Effect of pulling rate on competitive growth
Fig.8 Schematic of the positional relationship of dendrites near grain boundary
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