Effect of thermal cycling on microstructure evolution and elements diffusion behavior near the interface of Ni/NiAl diffusion couple

The interdiffusion behavior of pure Ni vs. NiAl alloy was investigated via diffusion couple after diffusion annealing treatment under two different thermal cycling (20 and 60 cycles) conditions and isothermal condition. A combination of scanning electron microscope, transmission electron microscope and electroprobe microanalysis was used to characterize the microstructures and measure the concentration profiles across diffusion zones in the Ni/NiAl couples. A thick Ni-rich NiAl layer and a thin gamma'-Ni3Al layer were formed near the interface in each couple. The martensite layer with Ni concentration of 63-67 at.% was formed in the Ni-rich beta-NiAl layer near the Ni/NiAl interface upon cooling. Meanwhile, the Kirkendall plane was formed close to the initial Ni/NiAl interface due to the different diffusion rate of Ni and Al near the interface. Additionally, results indicated that thermal cycling influenced the microstructure evolution behavior of Ni/NiAl diffusion couple. The gamma'-Ni3Al/gamma-Ni and gamma'-Ni3Al/beta'-martensite interfaces were rumpled after the diffusion annealing treatment under thermal cycling conditions and the rumpling became more severe under thermal cycling condition with 60 cycles. Furthermore, the slope of interdiffusion coefficients vs. Al concentration curve under thermal cycling condition with 60 cycles was higher than that under cycling condition with 20 cycles and isothermal condition, suggesting that thermal cycling might have influence on the interdiffusion behavior near the Ni/NiAl interface. (C) 2015 Elsevier B.V. All rights reserved.