Abstract
The deformation behavior of additively manufactured Alloy 718 in as-built condition and after annealing was studied in situ under tensile loading along the build direction. Pre-characterization by synchrotron X-ray diffraction and electron microscopy revealed a significant amount of γ″ precipitates in the as-built samples, whereas the γ″ phase was entirely consumed and needle-like δ precipitates appeared in the annealed sample. In situ neutron diffraction (ND) and acoustic emission (AE) enabled indirect observation of the role of the precipitates on the mechanical behavior. ND provided information on the load accommodation in the matrix, while AE detected a strong signal from the interaction of dislocations with the δ-phase precipitates during deformation of the annealed samples. The results imply that in the annealed samples the matrix sheds the load to the precipitates, while in the as-built material the matrix bares a significant load.
Highlights
Alloy 718 is a Ni-based superalloy that is widely used for components in gas turbines in jet engines because of its good mechanical properties and corrosion resistance at elevated temperatures.[1]
The Rietveld refinement gives a good fit to the experimental data and it was observed that the as-built specimen contained a mixture of matrix c phase with 20.4 ± 0.8% of c¢¢ phase, 1.8 ± 0.1% of Laves phases, and 0.2 ± 0.1% of metallic carbides, whereas no d phase peaks were found
After annealing at 850°C for 8 h, the c¢¢ phase disappeared, which is seen as a vanishing of the peak ‘‘shoulder’’ shown in the diffraction pattern of the as-built material, i.e., the (111) and (200) peaks of the c matrix become sharper, as shown in Fig. 2b, and c¢¢ is not included in the Rietveld fitting
Summary
Alloy 718 ( known as Inconel 718) is a Ni-based superalloy that is widely used for components in gas turbines in jet engines because of its good mechanical properties and corrosion resistance at elevated temperatures.[1]. For Alloy 718 a few recent examples are contained in references.[10,11,12,13,14,15,16,17] It has been observed that Alloy 718 exhibits a heterogeneous microstructure in its as-built form, containing a large amount of segregating phases, such as MC-type carbides and c/ Laves phase eutectics within the interdendritic regions.[18] Typically a two-step heat treatment process is undertaken to improve its mechanical properties: (1) solution annealing to dissolve the segregation particles and strengthening phase into the matrix, followed by (2) ageing heat treatment to precipitate fine particles of c¢ or c¢¢ phases in the matrix.[19,20,21,22,23] Besides c¢ and c¢¢ phases, d phase forms at the grain boundaries and inside the grains, and Laves phases as well as carbides appear in the matrix during ageing.[24] Despite the significant work done on wrought 718 alloys, the complex thermal cycles and fast cooling rate of the AM processes result in different phase distributions, and grain and precipitate morphologies As such, their effect on the mechanical behavior of these materials is still not very well understood. It is obvious that the signal to background noise ratio is much higher for XRD than for ND, while the peaks from precipitates are beyond the resolution of ND
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