CHANGE OF THE STRUCTURE OF A HEAT-RESISTANT ALLOY DOPED BY RHENIUM AND LANTHANUM DEPENDING ON THERMAL PROCESSING

The properties of high-temperature nickel alloys for modern engineering were determined by thermal stability of structure, size, shape and amount of hardening γ′-phase, and the strength characteristics of γ′-solid solution. These alloys are strengthened by alloying with rhenium and lanthanum. The aim of this work was qualitative and quantitative study of the alloy structure and the phase composition of a nickel heat-resisting alloy additionally doped with rhenium (0.4 % at.) and lanthanum (0.006 % at.). The investigations were carried out by two methods: the method of transmission diffraction electron microscopy and the scanning electron microscopy method. Investigation of the alloy’s structure was carried out in three states: sample 1 – initial state (after directional crystallization (DC)); sample 2 – DC, annealing at 1150 °C for 1 hour, annealing at 1100 °C for 480 hours;sample 3 – DC, annealing at 1150 °C for 1 hour, annealing at 1100 °C for 1430 hours. The studies showed that the phases observed in the superalloy can be classified into primary and secondary phases. The main phases are γ′ and γ. They form structure of the alloy and are present as quasi-cuboids of γ′-phase separated by γ′-phase interlayers. The remaining phases are secondary. It was found that doping with rhenium and lanthanum leads to the appearance of secondary phases, namely: β-NiAl, AlRe, NiAl 2 Re; σ; χ; Ni 3 La 2 . The formation of secondary phases introduces a serious violation into the structure of quasi-cuboids (γ + γ′)-phases. Rhenium and lanthanum do not fill the entire volume of the alloy uniformly but are present only in local areas. Therefore, in three states of the alloy only a part of the volume of quasi-cuboids (γ + γ′)-phases was affected. The morphology of the secondary phases was studied. It was found that the particles of the σ-phase are thin needles, while the Ni 3 La 2 particles have an internal structure with a characteristic contrast and have a finite thickness. An interesting feature is that the σ-phase and Ni 3 La 2 phase are distinguished in the same places. It was established that the introduction of lanthanum and rhenium changes phase composition of the alloy, suppressing the formation of γ-phase. Particles of secondary phases are localized in individual sections of the alloy with a certain periodicity. The resulting secondary phases are refractory: the melting point of the β-phase is approximately 1600 °C, for the σ-phase it is 2600 °C, and for the χ-phase it is 2800 °C. The formation of secondary refractory phases and their periodic distribution in the structure contributes to hardening of the superalloy doped with rhenium and lanthanum.