Research of the microstructure and precipitation strengthening in a high-temperature Fe-Ni superalloy

The paper describes the relationships between the kinetics of precipitation and growth of the intermetallic phase gamma' - Ni-3(Al,Ti) and the strengthening magnitude obtained in a high-temperature Fe-Ni superalloy of the A-286 type. In order to accomplish the goal of the study, the author used the LSW coagulation theory and Brown and Ham's conventional analysis of strengthening by ordered particles. The samples were subjected to a solution heat treatment at 980 degrees C/2h/water and then aged at 715, 750 and 780 degrees C, with holding times 0.5-500 h. The heat-treated samples were subjected to structural analyses (TEM, X-ray diffraction) and analyses of mechanical properties (hardness test, static tensile test). Direct measurements on the electron micrographs allowed to calculate the structural parameters of the gamma' phase, i.e. mean diameter, volume fraction and mean distance between particles. In accordance with the LSW theory, linear dependencies of changes in mean diameter as a function of ageing time (t(1/3)) were elaborated and the activation energy (E) of the gamma' phase cogulation process was determined. The author carried out analyses of strengthening and flow stress (Delta tau(o)) increases as a function of the particle size of the gamma' phase and determined the value of the antiphase boundary energy (gamma(apb)) for the analyzed Fe-Ni alloy. It was found that the value of APB energy of phase gamma' depended significantly on the alloy ageing temperature. Such a dependence of the quantity gamma(apb) on the ageing temperature for the investigated alloy was explained by an increase in the degree of internal arrangement of phase gamma' and by increasing sizes of ordered domains as a result of coalescence.