激光立体成形镍基单晶高温合金显微组织研究

Microstructures of Nickel-base Single-Crystal Superalloy Prepared by Laser Solid Forming

采用激光立体成形(Laser Solid Forming,LSF)技术制备了一种镍基单晶高温合金,对沉积态和热处理态试样进行了显微组织观察。结果表明,沉积态单晶高温合金试样中下部为二次枝晶不发达的微细枝晶,一次枝晶间距随试样高度的增大而略有增加;试样的顶部存在转向枝晶层。枝晶间均匀弥散分布着类圆形显微疏松和小块状MC型碳化物。透射显微镜观察可知,沉积态试样γ基体中分布着大量纳米级γ'颗粒。沉积过程中累积的残余应力在γ基体中萌生了位错,并推动了位错滑移,致使一些位错在γ基体通道中发生缠结,有些位错甚至切入γ'。进行固溶+两级时效热处理后,试样发生了完全再结晶,再结晶晶粒大小极度不均匀,有些再结晶晶粒中还存在孪晶。热处理后,LSF试样中γ'析出相的尺寸分布与传统铸造合金热处理后相似。

Single-crystal superalloys have been commonly used in modern advanced aero engines to produce high-pressure high-temperature turbine blades.In order to extend the lifetime of single-crystal turbine blades,repair of cracked and worn ones is of great interest.Laser Solid Forming（LSF） is a prospective technology that can be used to repair single-crystal blades.In this study,the microstructure of the as-deposited and heat-treated LSF single-crystal superalloy has been characterized through optical microscopy（OM）,scanning electron microscopy（SEM） and transmission electron microscopy（TEM）.The as-deposited LSF sample shows a directionally solidified columnar dendrites growing almost parallel to the building direction.Between the dendrites,nearly round microporosity and blocky MC carbide particles are dispersed uniformly.TEM results indicate that many nano γ＇ particles precipitate in γ matrix.In addition,during LSF deposition process,stress is formed,which generates plenty of dislocations.During heat treatment,the LSF sample experiences full recrystallization.The recrystallization grains are in different sizes,and twins are found in some recrystallized grains.Because the residual stress is fully released during heat treatment,the size and shape of γ＇ precipitates in the recrystallized and un-recrystallized regions are similar to those in conventionally cast samples.