Study of GTD-111 Superalloy Microstructural Evolution During High-Temperature Aging and After Rejuvenation Treatments

Abstract

Nickel-based superalloys are normally present in mechanical components requiring resistance to high temperatures and extreme mechanical stresses, e.g., gas turbine blades. The large amount of γ′-precipitates coherent with the γ-matrix in these superalloys ensures excellent properties at high operating temperatures. Nevertheless, after long service periods, the γ′-precipitates grow and lose coherence with the matrix. Intermediate thermal treatments are a potential solution to recover the microstructure, extending the component lifetime. Thermal cycles for γ′-dissolution and reprecipitation define the efficiency of such thermal treatments. The objectives of this work are to analyze the aging kinetics of the nickel-based superalloy GTD-111 (used in gas turbine blades for thermoelectric energy generation) and to study the effects of solution and preprecipitation treatment temperatures on the microstructure and hardness obtained by rejuvenation thermal treatment. For the experiments, samples were previously aged under laboratory conditions during 2,000 h at 1,000 °C. The subsequent rejuvenation thermal treatments consisted of four steps: solution (at 1,150, 1,175, and 1,190 °C), preprecipitation (at 1,055 and 1,120 °C), and reprecipitation (at 845 °C). The resulting microstructures were analyzed using scanning electron microscopy and Vickers hardness measurements. During the aging treatment, the average size of γ′-particles changed from 0.20 µm², as found in virgin material, to 0.99 µm² after 2,000 h of aging; the hardness changed from 417 to 320 HV, respectively. For solution treatment at 1,150 °C, total dissolution of γ′-particles did not occur and they presented a bimodal size distribution of coarse and fine particles. For solution treatment at 1,175 and 1,190 °C, the γ′-particles were fine and homogeneous, and the maximum hardness obtained after reprecipitation was 460 HV for 1,190 °C solution treatment and 1,120 °C preprecipitation.