Deformation behaviour of additively manufactured Inconel 718: Microstructure, plastic inhomogeneity and temperature dependence
File(s)
Author(s)
Al-Lami, Jalal
Type
Thesis
Abstract
This PhD research project aims to establish a fundamental understanding of the spatial development and distribution of the microstructure in Inconel 718 processed by laser powder-bed fusion (L-PBF) and directed energy deposition (DED) additive manufacturing (AM) technologies, and examine the influence of the microstructure on the deformation and damage behaviour at different temperatures of the material. Special focus was placed on studying the plastic inhomogeneity, which is one of the main mechanisms leading to strain localisation and failure in engineering alloys.
It is demonstrated that the scanning strategy in AM is highly influential in controlling the arrangement and spatial distribution of fine-scale microstructure. The bidirectional scanning strategy with no rotation resulted in highly ordered arrangements of fine Cube {100}<001> textured grains containing a very high dislocation density alternated by coarse Goss {110}<001> textured grains with a lower density of dislocations that were aligned perpendicular to the scanning direction. In-situ tensile testing coupled with microscopy showed that the line rows of fine, dislocation-dense Cube grains were particularly influential in intensifying the plastic inhomogeneity, but this can be minimised by introducing a layer rotation in the scan strategy. High-temperature tensile testing of as-built Inconel 718 revealed that the yield strength decreases with temperature until 450 °C due to thermally-assisted softening, but the strength is regained for further temperature increase to 650 °C. This strength is imparted by nanoscale solute clustering of Al, Ti and Nb atoms, which did not develop further into mature γ' and γ" precipitates. Also, a strong correlation was observed between Ti clusters and dynamic strain ageing (DSA) behaviour, indicating that Ti is most influential in the DSA mechanism...
It is demonstrated that the scanning strategy in AM is highly influential in controlling the arrangement and spatial distribution of fine-scale microstructure. The bidirectional scanning strategy with no rotation resulted in highly ordered arrangements of fine Cube {100}<001> textured grains containing a very high dislocation density alternated by coarse Goss {110}<001> textured grains with a lower density of dislocations that were aligned perpendicular to the scanning direction. In-situ tensile testing coupled with microscopy showed that the line rows of fine, dislocation-dense Cube grains were particularly influential in intensifying the plastic inhomogeneity, but this can be minimised by introducing a layer rotation in the scan strategy. High-temperature tensile testing of as-built Inconel 718 revealed that the yield strength decreases with temperature until 450 °C due to thermally-assisted softening, but the strength is regained for further temperature increase to 650 °C. This strength is imparted by nanoscale solute clustering of Al, Ti and Nb atoms, which did not develop further into mature γ' and γ" precipitates. Also, a strong correlation was observed between Ti clusters and dynamic strain ageing (DSA) behaviour, indicating that Ti is most influential in the DSA mechanism...
Version
Open Access
Date Issued
2023-03-12
Date Awarded
01/10/2023
License URL
Advisor
Pham, Minh-Son
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)