高梯度定向凝固技术及其在高温合金制备中的应用

Industrial gas turbines (IGTs) are the key equipment to achieving energy strategy, such as energy conservation and clean power generation. When the large and complex IGT blades are fabricated by the conventional Bridgman directional solidification process, the thermal gradients at the solidification front are low and unstable, resulting in some disadvantages: the coarse dendrite structure with severe dendritic segregation, the increased occurrence of casting defects and the poor performance of mechanical properties. These disadvantages provide a good opportunity for rapid development of the directional solidification with high thermal gradient (HG), such as the liquid metal cooling (LMC). In the present work, the physical basis of HG process, the microstructure, mechanical properties, solution heat treatment, and casting defects of the superalloys processed by HG process, have been reviewed. The HG process increases the thermal gradient and the cooling rate, thus permitting microstructural improvements including a more homogeneous fine-dendrite structure with lower elemental segregation and shrinkage porosity, and refinement of carbide, γ' phase and eutectic, reducing the volume fraction of eutectic and shrinkage porosity. During the solution heat treatment, the HG process increases the incipient melting temperature and reduces the residual segregation as well as the content of solution pore. The HG process could effectively inhibit the formation of freckle chains, increase the critical withdrawal rate of the stray grain formation, and decrease the degree of the misorientation of the <001> grain orientation from the casting axis. Moreover, the HG process could improve the mechanical properties including the stress rupture life, lowcycle fatigue (LCF), high-cycle fatigue properties and short-term strength, but the improvement might be reduced at higher temperature or under the oxidation condition.