Marchese Giulio, Bassini Emilio, Aversa Alberta, Lombardi Mariangela, Ugues Daniele, Fino Paolo, Biamino Sara
Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy.
Materials (Basel). 2019 Feb 5;12(3):486. doi: 10.3390/ma12030486.
Hastelloy X (HX) is a Ni-based superalloy which is employed to produce gas turbine and gas-cooled reactor sectors due to its outstanding oxidation resistance and high tensile strength at high temperatures. This alloy can be processed by laser powder bed fusion (LPBF) fabricating complex geometries in a single step. However, post-processing thermal treatments must be applied to generate a suitable microstructure for high-temperature applications. The investigation reports the microstructure evolution of LPBF HX samples under specific post-processing treatments. A hot isostatic pressing (HIP) treatment can close the internal cracks and reduce the residual porosity (less than 0.1%). Moreover, the HIP-triggered recrystallization generated equiaxed grains, while the slow cooling rate generated a film of intergranular carbides (Mo-rich M₆C and Cr-rich MC₆) and intragranular carbides (Mo-rich M₆C carbides). Therefore, a solution annealing was performed to dissolve the film of carbides which may reduce the ductility. The post solution annealed material consisted of equiaxed grains with ASTM grain size number mainly 4.5-5.5 and inter/intragranular Mo-rich M₆C carbides. The microstructure is highly comparable with solution annealed wrought HX alloy. Finally, after simulating short thermal exposure at 745 °C for 6 h, a significant formation of Cr-rich MC₆ carbides was observed strengthening the LPBF HX alloy.
哈氏合金X（HX）是一种镍基高温合金，因其出色的抗氧化性和高温下的高拉伸强度而被用于制造燃气轮机和气体冷却反应堆部件。这种合金可以通过激光粉末床熔融（LPBF）一步制造出复杂的几何形状。然而，必须进行后处理热处理以生成适合高温应用的微观结构。本研究报告了LPBF HX样品在特定后处理条件下的微观结构演变。热等静压（HIP）处理可以闭合内部裂纹并降低残余孔隙率（小于0.1%）。此外，HIP引发的再结晶产生了等轴晶粒，而缓慢的冷却速率产生了晶间碳化物（富钼的M₆C和富铬的MC₆）和晶内碳化物（富钼的M₆C碳化物）薄膜。因此，进行了固溶退火以溶解可能降低延展性的碳化物薄膜。固溶退火后的材料由等轴晶粒组成，ASTM晶粒度数主要为4.5 - 5.5，以及晶间/晶内富钼的M₆C碳化物。该微观结构与固溶退火的变形HX合金高度可比。最后，在模拟745℃下短时间热暴露6小时后，观察到大量富铬的MC₆碳化物形成，强化了LPBF HX合金。
