PhD Thesis Colloquium: Mr. Gangavarapu Shashidhar (22/09/26)
Thesis title:
Environmental Degradation of Structural Materials: Hydrogen-assisted degradation and embrittlement of Ni base Single Crystal Superalloy (CMSX-4) and Zr-Nb Pressure Tube alloy (Zr-2.5Nb), Methanol-induced corrosion and mechanical degradation of Al base alloy (5052)
Faculty advisor(s):
Prof. Surendra Kumar Makineni
When?
22nd September, 2026 (Tuesday), 11:00 AM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract
World’s persistent drive to reduce its carbon footprint invoked thrust for alternative fuels and sustainable energy processes; this led to a rapid advancement in energy technologies and process design. Hydrogen, Ethanol, Ammonia, Methanol etc are being looked upon to become the workhorse in achieving these clean and sustainable energy goals and are being extensively tested as alternative fuels. In this backdrop, it is vital to understand the behaviour of existent structural materials under these fuel environments. Although environmental embrittlement of structural materials under alternative fuels is well explored, there is need for further insights into interactions of these fuels and their by-products with the microstructural features. While most of these fuels are constituted by light elements (i.e H, C, N, O), understanding their interactions with microstructural features is the bottle neck because inherently characterizing light elements is challenging. However, the advent of modern characterization techniques like Atom Probe Tomography (APT), integrated Differential Phase Contrast Scanning Transmission Electron Microscopy (iDPC STEM), Electron Energy Loss Spectroscopy (EELS) allows the detection of these light elements with good spatial resolution, enabling the current scope of work. This work constitutes following parts: Hydrogen-assisted degradation and embrittlement of 1) Ni base Single Crystal Superalloy (CMSX-4), 2) Zr–2.5Nb alloy; 3) Methanol-induced corrosion and mechanical degradation of Al base alloy (5052).
Understanding hydrogen embrittlement in single crystal superalloy (CMSX-4):
It is vital to understand the interaction of hydrogen with materials used in aeroengines, especially Ni base superalloys. Studies are mostly confined to few commercial alloys and are sparse in single crystal superalloys like CMSX-4. Hydrogen embrittlement studies in such alloys containing only γ, γˊ phases and higher volume fraction of later phase lack comprehensive understanding of the undergoing phenomena. Hydrogen is found to increase the slip planarity, accumulate dislocations in γ channels causing a macroscopic fracture along {001} γ/γˊ interface. There is long standing enigma on the exact reason behind this γ/γˊ interfacial failure. Few theoretical studies predict the segregation of H to γ/γˊ interface as the cause for this. However, there is no evident experimental proof to show preferential segregation of H to any of the microstructural features. We evaluated the trapping sites of H using APT and iDPC STEM. For detecting hydrogen (H) using APT, deuterium (D) is used as an isotope and a workflow for electrochemical charging using deuterated solutions followed by APT specimen preparation using Focused Ion Beam (FIB) and APT experiments is established; same charging and sample preparation workflow is followed for iDPC STEM (except for deuterated solutions). These results are correlated with other experimental results like uniaxial test, microhardness, fractography. The preferential segregation of H to γˊ phase is established and this is known to decrease the dislocation velocity and stacking fault energy in it, thus hardening the γˊ phase. This leads to the change in mechanism of deformation from that of shearing of γˊ precipitate to that dominated by interfacial dislocations. Hydrogen enhanced localized plasticity (HELP) in γ channels and resistance to further movement of these dislocations into γˊ phase because of its hardening will increase the interfacial dislocations and thus localized hydrogen concentration at the γ/γˊ interface. This H concentration when reaches a critical concentration will lead to failure along the interface following local decohesion (HEDE).
Hydrogen-assisted degradation and embrittlement of Zr-Nb Pressure Tube alloy (Zr–2.5Nb):
Zr-Nb alloy used in pressure tubes of Heavy Water Reactors has severe susceptibility to hydrogen embrittlement under service conditions. As a preventive measure the alloy is autoclaved in heavy water to form an oxide layer over it, given the limited solubility of hydrogen in zirconium oxide compared to zirconium, this oxide is hypothesised to reduce the hydrogen embrittlement susceptibility of the underlying alloy. However, the defects in this oxide layer are found to influence the hydrogen uptake by the underlying alloy. In this context present study focuses primarily on microstructural characterization of the oxide for its phases/defect structure evaluation so as to identify the diffusion pathways for hydrogen into the metal. These oxides are known to be nanoscale, characterizing which is challenging; iDPC STEM in conjunction with Scanning Precession Electron Diffraction (SPED) and 4D STEM are performed to perform this. FIB is used to prepare cross section and plane view TEM lamella of the oxide. The sample is further evaluated using TEM and presence of zirconium oxide is confirmed by STEM-EDS maps; further presence of monoclinic, tetragonal, hexagonal phases of zirconium oxide are confirmed. The exact quantification of these phases and their grain boundary character is performed. The presence of the hydride phase in the underlying alloy was also confirmed by plasmonic studies using low loss EELS, its crystal structure and orientation was further established. With this preliminary understanding from the TEM experiments, we further proceeded with APT experiments to evaluate the diffusion pathways for D. The workflow established for deuterium charging in first part is used for this purpose. Results indicate D segregation at the interfaces of oxide and establishes the role of oxide grain boundaries on H pickup by underlying alloy.
Methanol-induced corrosion and mechanical degradation of Al base alloy (5052):
Methanol has emerged as a promising candidate for marine fuel. To enable retrofitting of existing infrastructure for methanol as fuel, it is essential to understand its interaction with various materials under operational conditions. This study presents a comprehensive evaluation of the corrosion behavior of thin aluminum sheets (60 μm thick), of alloy 5052 commonly used as liners in fuel tanks, when exposed to methanol. The mechanism of corrosion is established through detailed analysis of corrosion products using advanced microscopic and spectroscopic techniques. Under dry conditions, corrosion proceeds via methoxide formation followed by its decomposition to Al₂O₃, leading to localized pitting. Cracks could be observed at the bottom of these pits. Further the effect of this corrosion on the mechanical performance is evaluated by doing cyclic bending test under methanol medium. Role of corrosion on the life to failure is established by failure analysis aided by detailed characterization and correlated with the long-term corrosion studies performed earlier. Effect of water addition on the type of corrosion is also evaluated to understand the change in mechanism of corrosion with water.