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Behavior of TiN inclusions and their influence in random grain formation in Ni-based superalloys

University of British Columbia

Studies on the columnar to equiaxed transition and random grain formation in superalloy turbine blades have suggested that non-metallic inclusions may be a possible nucleation source. Though there is no strong evidence to substantiate the idea, during directional solidification if the inclusions are able to survive in the region ahead of the solidification front, they could act as potential nucleation sites and may aid in the formation of random grains. On the. basis of the Turnbull-Vonnegut lattice dis-registry model, the lattice mismatch between the FCC Ni-matrix and a typical cubic TiN inclusion is determined to be less than 20%. Hence, a TiN particle can act as a potential heterogeneous nucleant in Ni-based superalloys with sufficient undercooling. The undercooling required in the Ni-based superalloys for TiN inclusion is calculated to be about 19 °C, which is comparable to the undercooling found in some single crystal superalloy turbine blades. Therefore, if the TiN particles are stable in the region ahead of the liquidus isotherm, they may be potential nucleation sites and hence cause the formation of random grains. This thesis reports on a quantitative study to verify the hypothesis that the TiN particles are stable in the region ahead of the solidification front during directional solidification. Thermodynamic calculations concerning the solubility of nitrogen and the formation of TiN in alloy IN718 have determined that the equilibrium nitrogen content required for the formation of TiN is about 39 ppm [N] (at Tliq =1340°C). Experimental studies on alloy IN718 at various conditions have indicated that TiN precipitation will not take place once the equilibrium nitrogen content is below this value of 39 ppm at Tliq. In the solid/liquid mushy region, the combined effects factors such as: a) cooling, b) segregation of Ti and nitrogen, and c). rejection of nitrogen into the bulk liquid, reduce the solubility limit to below 39 ppm. Hence T iN precipitation takes place in the segregation zone. The experiments have established that TiN particles precipitated in the solid/liquid mushy zone will not float out of the interface. Therefore, the nitrogen content must exceed the saturation solubility of TiN at Tliq to provide nuclei for random grain formation. Due to nitrogen rejection into the bulk liquid, there is TiN precipitation just above the Tliq. Directional solidification (DS) experiments on IN718 samples under nitrogen pressure and at a withdrawal rate of 2.4μ.m/sec have revealed that TiN particles of size > 10 microns (which have a rise velocity > 28 μ,m/sec) are able to float and eventually get collected at the top of the ingot (which is the final portion to solidify). These might be the TiN particles that are precipitated in the liquid ahead of the solidification front, i.e. above the Tliq , and which subsequently undergo flotation. Smaller TiN particles of size < 4 microns are found at the bottom portions of the ingot which indicates that these particles may be precipitated later during solidification in the solid/liquid mushy zone, and thus didn't "escape" out of the interface. Hence, the withdrawal rate should be at least an order lesser in magnitude than the velocity of rise of the TiN particles for substantial flotation to occur. [Scientific formulae used in this abstract could not be reproduced.]

Thesis/Dissertation

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2009-05-26

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http://hdl.handle.net/2429/8269

Metals and Materials Engineering

University of British Columbia

UBCV

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