Influence of Small Degree Deviation from [001] Orientation on Material Parameters and Fatigue Damage Evolution Law of Ni-Based Single Crystal Superalloys

The characterization of material parameters and the study of fatigue properties of nickel-based single crystal materials at high temperatures are critical to the reliability and safety of turbine engine blades. In this paper, a long-term high temperature resistant micro-speckle fabrication method is developed, and a constructed Digital Image Correlation (DIC) real-time observation system is utilized to monitor the damage evolution behavior of nickel-based single crystals under high-temperature fatigue. The “equivalent modulus”, “equivalent Poisson’s ratio” and “equivalent thermal expansion” of nickel-based single crystal alloys with different deviation angles (5°, 10°, 15° and 20° from [001] orientation, respectively) were obtained by uniaxial tensile experiments at 980℃ combined with integrated digital image correlation (I-DIC) inversion method. The changing law of each thermodynamic parameter with the increase of temperature was analyzed. That is, both the equivalent Poisson's ratio and equivalent thermal expansion coefficient of nickel-based single crystal superalloys increase with temperature, and the “equivalent thermal expansion coefficient” of the material in two directions is different, [001] direction is always greater than that in the [110] direction; the “equivalent modulus” decreases with increasing temperature and increases with increasing deviation angle. And for the first time, a prediction method using the local “equivalent modulus” as the damage variable is proposed. The variation law of local “equivalent modulus” with fatigue cycles under different deviation angles is analyzed. The local “equivalent modulus” decreases with the increase of fatigue cycles under different deviation angles. When the deviation is 10°, the local “equivalent modulus” decreases the fastest. When deviating from 5°, the local “equivalent modulus” decreases relatively slowly. Finally, the fatigue damage evolution equation with the local “equivalent modulus” of different deviation angles as the damage variable is established. By comparing the fitting curve of the formula with the actual value of the experiment, it is found that the fitting degree is good, which verifies the accuracy and validity of the method of predicting fatigue life with the local “equivalent modulus” as the damage variable.