MTech Thesis Colloquium: Mr. Hemant Kumar (16/10/25)
Thesis title:
Atomic-scale interface engineering for developing high-strength heat-resistant light-weight alloys
Faculty advisor(s):
Prof. Surendra Kumar Makineni
When?
16th October, 2025 (Thursday), 10:30 AM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract:
Light-weight alloys with high specific strength (strength/weight ratio) are one of the core structural materials used for components in aerospace and automotive applications. These alloys enable the reduction of carbon footprint and improvement of fuel efficiency, thereby providing a solution for designing energy-efficient transport vehicles. Two prominent classes of alloys used are Al-based and Mg-based, which contain solutes that strengthen their soft matrix by inducing strain primarily through solid solution and precipitate formation that shares hetero-phase interfaces (HPIs) with the surrounding lattice, providing resistance to the motion of dislocations under external load. Another widely practised strengthening strategy is thermomechanical processing, which generates a high density of internal defects, including line and point defects, as well as interfaces such as planar defects, grain boundaries (GBs), and twin boundaries (TBs). The interfaces are efficient in obstructing dislocations but compromise ductility and toughness, thereby increasing the brittleness of the alloy. In recent years, particularly in Al-based and Mg-based alloys, promoting solute segregation to interfaces has been extensively exploited to further enhance mechanical strength and thermal stability. Segregation can lead to modifications in the interfacial structure, composition, and interfacial energy. The nature and tendency of segregation depend on the alloy composition and the type of interface, i.e., either GBs, TBs or HPIs. The present thesis exploits interfacial segregation at the atomic scale of these interfaces in the design and development of (1) a cast near-eutectic Al-Gd-based alloy by minute addition of Zr, cast and extruded (2) Mg-Ce-based alloy with and without Gd addition and (3) a Mg-Sn alloy with Zn addition.
In the first part, a near-eutectic cast Al-2.5at.%Gd alloy was investigated with the target of achieving high strength and ductility by promoting Zr solute segregation at the HPIs between the brittle Al3Gd/α-Al soft matrix. In typical eutectic alloys, the brittle intermetallics are prone to easy fracture, and additionally, the incoherent, weak interfaces are unable to effectively transfer load upon the application of stress. These interfaces act as high-stress concentration sites, resulting in a high propensity for crack nucleation/propagation, which leads to a loss of ductility and promotes brittle or early fracture. In this work, 0.15at.% Zr was added to the binary Al-Gd alloy, followed by annealing at 400°C. It was observed that the Zr composition increases with the time of annealing, from ~2.5 at.% after 5 hours to ~25 at.% after 25 hours. An atomic-scale HAADF STEM image reveals that the Zr-rich regions have a superlattice-ordered L12 crystal structure with the stoichiometry of Al3(Zr, Gd), forming a near-conformal dense nano-layer on the brittle Al3Gd fibres. These are termed as superlattice nano-layered (SNL) fibres and prevent the formation of high-concentration sites upon loading, thereby increasing ductility by 400% from ~4% to ~20%. A fine distribution of coherent core-shell nano-particles of Al3(Zr,Gd) also precipitates, strengthening the primary soft α-Al matrix, which enables a tensile strength of ~ 295 MPa at room temperature and retains up to 130 MPa at 250°C. They cause a large number of dislocation cross/multiple-slips on {111} planes, forming ultra-fine (⁓ 12 nm) dislocation networks that leverage substantial plastic strain accumulation.
In the second part, the role of solute (0.1at.%Ce) segregation to the GBs and creep-induced TBs was investigated in enhancing the creep resistance of pure Mg. The alloys were prepared by gravity die casting followed by hot extrusion at 350°C with a ratio of 12:1. Atomic-scale compositional analysis reveals that the Ce composition increases to 1.5 at.% at the GBs, which contribute to Zener pinning during creep deformation. Additionally, creep deformation proceeds with extensive twinning, where the Ce increases to 0.5at.% indicating the occurrence of solute drag on the movement of twinning partials during deformation. Both effects result in reduced creep rate by one order of magnitude (minimum creep rate value of ~3.85 x 10-8 s-1 for the Mg-Ce alloy as compared to the value of ~9.56 x 10-7s-1 for pure Mg) and a higher activation energy (~160 kJ/mol for Mg-Ce alloy, while ~90 kJ/mol for Pure Mg). The Mg-Ce alloy is found not to be precipitation hardenable. Hence, Gd (1.1at.%) was added to the base Mg-0.1%Ce alloy, followed by ageing at 200°C up to 200 hours. A pronounced hardening was observed with an increase in hardness up to ~102 HV after 100 hours from an initial value of 68 HV. The atomic-scale structural analysis reveals a fine distribution of coherent β′ (Mg7Gd) precipitates within the α-Mg matrix, as well as a network of β1 (Mg3Gd) precipitates with a honeycomb-type distribution. Interestingly, compositional analysis across these precipitates reveals that Ce partitions up to ~1.5 at.%. The peak-aged alloy exhibits a tensile strength of ~325 MPa, accompanied by a ductility of ~10%, while maintaining a tensile strength of up to ~215 MPa.
The third part deals with an Mg-Sn alloy with a 3 at.% Zn addition, where the reversal of the strength-ductility trade-off was shown due to the Zn segregation at HPIs. The alloy was produced by gravity die casting and hot extrusion, followed by solutionising heat treatment at 450°C and ageing at 200°C for precipitation hardening. The uniaxial tensile test shows an increase in both tensile strength (up to ~ 219 MPa) and ductility (up to ~ 14.5%) after peak ageing for 50 hours, compared to the solutionised condition. The microstructure reveals the precipitation of Mg2Sn and MgZn phases, which strengthen the matrix. Both the phases appear faceted and have a definite orientation relationship with α-Mg. The compositional analysis across the HPIs between Mg2Sn and α-Mg shows differential Zn segregation up to ~9 at.% at the 110 facet of the phase oriented with the 1-100 plane of α-Mg, while up to ~ 4at.% at the facet oriented with the basal plane of α-Mg. The former interface was found to be semi-coherent with the presence of dislocations at every 14th plane. Gibbsian interfacial excess was estimated from the compositional profile obtained by atom probe, which shows a reduction in interfacial energy by 65 mJ/m² after ageing heat treatment. Hence, the increase in ductility of the alloy was attributed to the higher critical stress required for decohesion at the interface, which reduces with the increase in interfacial energy.
In conclusion, the thesis examines the critical role of interfacial segregation of solutes at interfaces in influencing the mechanical deformation of lightweight alloys, and demonstrates how atomic interface design can lead to the development of stronger and ductile lightweight heat-resistant alloys suitable for structural applications.