The nonlinear wave motion equation is solved by the perturbation method. The nonlinear ultrasonic coefficientsβandδare related to the fundamental and harmonic amplitudes. The nonlinear ultrasonic testing system is used to detect received signals during tensile testing and bending fatigue testing of GH4169 superalloy. The results show that the curves of nonlinear ultrasonic parameters as a function of tensile stress or fatigue life are approximately saddle. There are two stages in relationship curves of relative nonlinear coefficientsβ′ andδ′ versus stress and fatigue life. The relative nonlinear coefficientsβ′ andδ′ increase with tensile stress when tensile stress is lower than 65.8% of the yield strength, and they decrease with tensile stress when tensile stress is higher than 65.8% of the yield strength. The nonlinear coefficients have the extreme values at 53.3% of fatigue life. For the second order relative nonlinear coefficientβ′, there is good agreement between the experimental data and the comprehensive model. For the third order relative nonlinear coefficientδ′, however, the experiment data does not accord with the theoretical model.

The scanning acoustic microscope is used to detect the properties of film or coating materials. The ultrasonic wave propagates in the materials with thickness h, sound velocity c, acoustic impedance Z₂ between medium with acoustic impedance Z₁. The echoes from different interfaces overlap and interfere. The interference phenomena are observed in spectrum of echoes. The spectrum has periodic maximums at integral multiples of base frequency f₀. When thickness h is known, speed c of the specimen can be calculated by the relation of f_n = nc/2h. According to the principle, the properties of copper films such as thickness, acoustic impendence and elastic modulus are detected by scanning acoustic microscopy. The experimental results are accorded with the actual properties of specimens.

The existence mode of Y is an important factor affecting the oxidation resistance of NiCrAlY coating. The traditional thermal sprayed NiCrAlY coating had some problems such as internal-stress-induced coating peeling, when it was used at high temperature for a long time. In this paper, Y-modified NiCrAlY coatings were prepared on the surface of thermal-sprayed NiCrAlY coatings by halide-activated pack-cementation technology. The microstructure and oxidation resistance of the coatings after thermal insulation at 1100 °C for 100 h were analyzed. The results show that the oxidation resistance of Y-modified NiCrAlY coatings was improved significantly. When the oxidation temperature and time were 1100 °C and 100 h, the oxidation weight gain of thermal sprayed NiCrAl0.5Y and NiCrAl1.0Y coatings was 3.8 and 2.2 times that of Y-modified coatings, respectively. In addition, the NiCrAlY coatings prepared by pack cementation had better oxidation resistance than thermal sprayed coatings when the content of Y was equal. The oxidation resistance of the pack-cemented NiCrAlY coatings were increased with the increase in Y content when it was in a range of 0~1 wt.%. When oxidized at 1100 °C for 100 h, the oxidation weight gain of the pack-cemented NiCrAl0.5Y and NiCrAl1.0Y coatings was about 1.8 and 0.9 mg/cm², respectively. The formation of highly stable intermetallic compounds Ni₅Y in the pack-cemented Y-modified coatings is concluded to be the reason for improving the oxidation resistance of the coatings. This work can provide a solution to improve the oxidation resistance of traditional MCrAlY coatings.