The Microstructure of Electron-Beam Welds in Inconel 718

Abstract

Electron-beam welds in the Ni-base superalloy, Inconel 718 are freauently found to contain an extensive system of micro-cracks radiating from the weld boundary into the heat-affected zone (HAZ) of the matrix. To study precipitation in cross-sections of weld samples* over a large area adjacent to the weld, extraction replicas were prepared from the electro-polished and etched surfaces. Some preliminary identification of the extracted particles has been made by EDX analysis and convergent-beam diffraction, combined with scanning microscopy and EDX analysis of the electro-polished or ion-eroded surfaces. The welds are 1 cm deep, and near the top surface, where most of the long U+2018cracks’ occur (Fig.1a), the weld is 1.5 mm wide. The FAZ, as defined by a suitable etchant (10% HC1-methanol), extends for a further 1mm on each side. The grain size (50-100μm) is unchanged across the matrix-FAZ boundary. EDX spectra from the large precipitates (2-10μm) in the matrix and HAZ (Fig.la) show a strong Mb peak coupled with a weak Ti peak; the other alloy elements are entirely absent. Additional evidence from convergent-beam diffraction patterns (Fig.3) and measurement of the lattice constant2 (ao = 4.42 + 0.015 Å), proves that the majority of the particles are carbides, (Nb,Ti)C, with a F-centred cubic structure. These precipitates are absent from the weld, and the backbone of the dendritic weld precipitation is formed by thin, branched platelets (Fig.4). These are also carbides, (Nb,Ti)C, which recrystal 1ize from the melt. However, the FDX snectra now show weak additional peaks from most of the alloy elements (Ni, Cr, Fe, Mo).