Aluminizing of Nickel-Based Superalloys Grade IN 738 by Powder Liquid Coating

摘要

Nickel-based superalloy grade IN738 has superior creep resistance, however, oxidation resistance is the main limitation of this alloy for practical applications at high temperature. Coating of nickel aluminide compounds which have high oxidation resistance on the IN738 alloys surface can remarkably increase the oxidation resistance by formation of Al_2O_3 film as protective layer. Aluminizing by powder liquid coating methods is applied in this research. Mixed slurries of Al and AI2O3 powders are pasted onto IN 738 samples and heated at 1273 K in argon atmosphere for 3.6 to 14.4ks (1 to 4h). Slurries can be classified into four different ratios of Al: Al_2O_3: 10 : 0, 7 : 3, 5 : 5 and 3 : 7. The microstructure was investigated by scanning electron microscope (SEM) and optical microscope. Phases in the coated layer are characterized by Glancing Incident-angle X-ray Diffractometer (GIXD) and Electron Probe Micro Analysis (EPMA). The results show that the coated layer is formed by dissolution of nickel into liquid aluminum at aluminizing temperature resulting in formation of intermetallic compound layer. The coated layer consists of Ni_2Al_3 as a main phase with small amount of NiAl_3 and AlCr_2. AlCr_2 exists mostly at the layer adjacent to the top surface. For 8.1 and 14.4 ks (2.25 and 4 h) holding time, formation of AlCr_2 at the interface of matrix and coated layer occurs due to diffusion of aluminum from coated layer into nickel matrix. The effect of time shows that longer aluminizing time leads to formation of a uniform coated layer. The Al:Al_2O_3 ratio of either 10 : 0 or 7 : 3 will create a uniform coated layer with thickness more than 200 μm.