Effect of Time and Temperature on the Microstructural Evolution of Wide-Gap Brazed MAR-M247 Nickel Superalloy Using BNi-9 Braze Alloy

Abstract

Wide-gap brazing has been widely utilized as one of the go-to alternatives to welding in the repair of turbine components in the aerospace and power generation industries. In this study, differential scanning calorimetry, electron microscopy, and thermodynamic calculations were used to determine the influence of brazing time and temperature on the microstructural evolution for a layered wide-gap brazing process using a MAR-M247/BNi-9 system. Once liquefied, rapid braze infiltration into the MAR-M247 skeleton occurred via capillary action. During infiltration, partial and complete dissolution of the MAR-M247 skeleton occurred, which lead to diffusional solidification at 1068 \(^\circ \)C. Upon further and complete infiltration, it was found that rapid densification was achieved prior to isothermal brazing temperatures. The post-braze microstructure contained \(\gamma \)-Ni matrix grains, precipitated Cr, W, Mo-rich M\(_{x}\)B\(_{y}\) borides, athermal solidification products along matrix grain boundaries and triple junctions, as well as internal porosity. It was found that brazing temperature dictated the athermal solidification products with binary eutectic (CrB + \(\gamma \)-Ni) at 1150 \(^\circ \)C and ternary eutectic (Cr + \(\gamma \)-Ni + Ni\(_{3}\)B) at 1180 \(^\circ \)C and 1205 \(^\circ \)C. These findings agreed with Scheil–Gulliver predictions. Brazing time influenced the compositional homogeneity of the braze liquid, altering solidification behavior. This resulted in higher and lower solidification ranges for shorter and longer brazing times, respectively. Further, it was found that liquid fraction within the brazement increased with both brazing temperature and time, suggesting a persistent liquid phase. This finding was accompanied by an increase in volume fraction of athermally solidified intermetallics, consistent with an increase in liquid phase with increased brazing time and temperature. Lastly, \(\gamma \)-Ni grain growth occurred, although heterogeneity between the upper and lower regions of the brazement was observed. The upper region displayed larger grains on average when compared to the lower region. This was attributed to boride migration during liquid infiltration, which may have hindered grain growth via a grain boundary pinning mechanism.