In Situ Characterization at Elevated Temperatures of a New Ni-Based Superalloy VDM-780 Premium

Abstract

A new Ni-base superalloy VDM-780 Premium was developed for higher service temperatures (above 650 °C), keeping the good processing characteristics of alloy 718. This article presents, for the first time, the morphology and the microstructure characterization of this newly developed superalloy VDM-780 by means of scanning electron microscopy (SEM) and neutron and X-ray diffraction (XRD), after three different aging treatments performed for setting up different microstructures. Results show the presence of the γ-matrix, γ′-hardening phase, and a high-temperature phase whose structure is compatible with δ and η phases but whose exact crystal structure or the possibility of two different high-temperature phases still remains open. Rietveld refinements have allowed phase identification, determination of the lattice constants, and the weight fractions of constituting phases and shown that the presence of the different phases, amount, and morphology highly depend on the aging treatments. No traces of the γ″ phase are observed regardless of the heat treatment. In situ neutron diffraction (ND) studies at high temperature have allowed the determination of the solvus temperatures of the different phases present in each material after the corresponding aging treatment as well as the study of the evolution of their lattice parameters with temperature. The Vickers hardness (HV) of the three different samples was measured, and the results are correlated with the amount and particle size of the γ′-hardening phase of each sample.