PhD Thesis Colloquium: Ms. Shavi Agrawal (25/07/24)
Thesis title:
Thermal and Thermo-Mechanical Response of Additively Manufactured Hastelloy X
Faculty advisor(s):
Dr. Avadhani G S (Co-Supervisor - Prof. Satyam Suwas)
When?
25th July, 2024 (Thursday), 04:00 PM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract:
Additive manufacturing (AM) is one of the emerging technologies to manufacture near-net-shape engineering components layer-by-layer using a suitable heat source. The rapid adoption of AM in the field of aerospace owes to its ability to fabricate intricate parts in one step, sidestepping multiple processing stages, thereby reducing time, costs, and material waste. The blend of AM with nickel-based superalloys enables the production of intricate and durable components for the aerospace industry. The laser powder bed fusion processing (LPBF) of Hastelloy X serves as a prime example of this capability. While Hastelloy X has a long-standing history in aerospace applications, additive manufacturing represents a novel manufacturing technique. To ensure the industrial implementation of additively manufactured Hastelloy X, validating its viability through an extensive array of experimental investigations is imperative. Hastelloy X, known for its exceptional strength, oxidation resistance, and ability to withstand high temperatures, is particularly suitable for demanding applications in aerospace, especially in gas turbine engines. This thesis aims to investigate the microstructural and mechanical properties of LPBF-processed Hastelloy X to establish a correlation between different microstructural states and their corresponding mechanical behaviours at both room and elevated temperatures.
The initial stage of investigation comprises the microstructural characterization of as-built LPBF Hastelloy X. Additionally, the effect of heat treatment processes in incrementally rising temperatures on as-built microstructure, texture and room temperature tensile behaviour has been explored. Elongation was found in the range of 55 -75%, outperforming the wrought counterpart. The exceptional ductility of heat-treated specimens was attributed to combined factors, including the nearly full-density specimen (designed to have no cracks and minimal porosity), the stacking fault, the dislocation-mediated plasticity caused by the formation of microbands, and the twinning-induced plasticity (TWIP) effect.
Further, the elevated temperature tensile behaviour of stress-relieved (1050°C for 1 hour) Hastelloy X ranging from 500°C to 900°C has been investigated in detail. Moreover, the precipitation behaviour of M6C and M23C6 carbides in the above-mentioned temperature range has also been explored. The gradual decline in elongation was linked to the prevalence of intergranular fracture at higher temperatures, driven by the precipitation of carbides along the grain boundaries.
Additionally, the effect of prolonged thermal ageing on microstructural evolution and mechanical behaviour has been investigated. The stress-relieved Hastelloy X underwent prolonged thermal exposure at two distinct temperatures, namely 800°C and 950°C, each for a duration of 500 hours. This investigation aimed to elucidate the distribution and evolution of secondary phases within the material. A comprehensive microstructural analysis utilizing transmission electron microscopy (TEM) revealed the development of M23C6 carbides alongside topologically close-packed (TCP) phases such as µ, R, and P. The room temperature tensile tests demonstrated higher yield strength and lower elongation of the 800°C, 500 h specimen compared to the 950°C, 500 h specimen, which was attributed to the precipitation strengthening provided by the µ phase.
Furthermore, creep and stress rupture behaviour of LPBF Hastelloy X have been investigated in the stress range of 75 – 150 MPa at 800°C, accounting for the microstructural anisotropy in two different directions in stress-relieved conditions. The investigation reveals that the dynamic precipitation of M6C, M23C6, σ and µ phases were found to be the origin of threshold stress. Considering the influence of threshold stress, the stress exponent values were estimated to be ~ 5.1 and ~ 4.5 for vertical and horizontal specimens, respectively, indicating dislocation climb as the underlying deformation mechanism. Notably, vertically oriented samples exhibit significantly superior creep and stress rupture properties compared to their horizontally oriented counterparts. This discrepancy is largely attributed to the columnar grain morphology observed in the vertical specimens, contrasting with the equiaxed morphology in the horizontal specimens.
Our study concludes that the LPBF Hastelloy X is a suitable candidate for replacing its wrought counterpart as both room and elevated temperature properties are comparable to its wrought counterpart.