Metallographic Structure Evolution and Mechanical Property Degradation Analysis of Single Crystal Turbine Blades after Long-Term Service

Abstract:

In order to evaluate the damage of microstructure and the deterioration of mechanical properties after long-term service of single crystal turbine blades. This paper selects the high pressure turbine blades of gas turbines that have served 25000h and 50000h respectively. The material in the middle of the blade body was selected for metallographic structure quantitative analysis and microhardness test. The γ" phase size, γ" phase volume fraction, secondary γ" phase size, γ matrix channel width with different service time and at different positions were measured. And the Vickers hardness were tested. The results show that the γ" phase size of different positions in the same leaf is different; The γ" phase size at the leading edge and the trailing edge which working at high temperature is larger than Pressure side and Suction side;however, the γ" phase volume fraction is slightly smaller than the other two; the γ" phase size of serviced 50,000h blade is greater than the serviced 25000h γ" phase size at the same position; the volume fraction is in the opposite order; secondary γ" phase appears in some positions, the width of the base channel is significantly increase at the secondary γ" phase precipitation position; it is verified by 1100 °C/2h-air+cooling/furnace cooling experiment that the secondary γ" phase precipitation is related to the working high temperature and cooling way; the TCP phase which contains many W, Re elements appears in the abnormally high temperature region of the trailing edge, and the phase is identified as the μ phase; the microhardness of the leading edge and trailing edge position of different service time decrease significantly, and the decrease of microhardness is positively correlated with the increase of γ" phase size and decline of volume fraction.; therefore, the γ" phase size and volume fraction can be used as the evaluation parameters of the tissue damage of the material.

Key words:Turbine blades; Single crystal superalloy; Metallographic structure evolution; Mechanical property degradation; Damage evaluation