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 c-matrix, c¢-hardening phase, and a high-temperature phase whose structure is compatible with d and g 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 c¢¢ 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 c¢-hardening phase of each sample.

On the Development Concept for a New 718-Type Superalloy with Improved Temperature Capability

“…Nevertheless, a few findings warrant reflection in the context of this paper. Firstly, a constrained γ/γ misfit of 0.48% at room temperature was measured by neutron diffraction on VDM Alloy 780 [40]. This answers the question raised in Section 2.1, in that the alloy development concept followed here indeed leads to a large positive misfit, exceeding values otherwise reported for wrought γ -strengthened superalloys.…”

“…Finally, it is mentioned that solvus temperatures of the precipitate phases were determined by in situ neutron diffraction [40]. The results suggest solvus temperatures of γ and δ/η of about 995 • C-1000 • C and 1020 • C-1030 • C, respectively.…”

“…According to reference [36], the chemical composition of VDM Alloy 780 is Ni-25Co-18Cr-3Mo-2.0Al-0.2Ti-5.4Nb. Further information on the mechanical properties and microstructural evolution of VDM Alloy 780 can be found in References [36,40]. This is not the subject of discussion in this article, which had the objective to outline the history of the alloy development with the underlying condsiderations.…”

“…Noting, furthermore, that the sum of the precipitate-forming elements Al+Ti+Nb is considerably larger in VDM Alloy 780 than in alloy 718, namely, approximately 7.9 at.% instead of about 5.6 at.%, it becomes understandable why V17/VDM Alloy 780 attains high strength levels similar to alloy 718 at ambient temperatures despite the switch from a predominantly γ"-strengthened alloy to a γ -strengthened alloy. Thermodynamic calculations suggest a γ content in VDM Alloy 780 of approximately 25%, while the first analysis of the neutron diffraction data indicates a content as high as 35% [40]. Even though more analysis is needed to determine the exact volume fraction, the available information suggests that the γ content not only exceeds that of alloy 718 but also that of Waspaloy.…”

HRTEM analysis of the high-temperature phases of the newly developed high-temperature Ni-base superalloy VDM 780 Premium

“…We previously reported on the structure of the newly developed alloy VDM 780 Premium by means of neutron diffraction experiments [6,8]. However, the structure of the high-temperature phase/ phases present in the material could not be resolved, as it was found to be compatible with both the d and h phases.…”