arXiv subjects

Surendra Kumar Makineni

Publications and source records attributed to Surendra Kumar Makineni.

6 recordsLinked to original sources

arXiv2025

Exploiting solute segregation and partitioning to the deformation-induced planar defects and nano-martensite in designing ultra-strong Co-Ni base alloys

Single-phase, multi-elements (three or more) with high concentrations show exceptional tensile strength up to ~ 0.8-1.2 GPa. However, they possess a very low 0.2% yield strength (YS), i.e., they can be permanently deformed at very low-stress levels of 300 to 600 MPa. Here, we reveal by exploiting atomic-scale solute interactions with the deformation-induced structures to design ultra-strong single-phase alloys with YS > 2 GPa. This was achieved by controlled thermomechanical processing that introduces stacking-faults (SFs), nano-twins (NTs), and nano-martensite {\epsilon}-laths (NMLs) during cold deformation followed by facilitating solute segregation/partitioning to them by tempering at intermediate temperature. We demonstrate the phenomena in a low stacking faulty energy multi-component (face-centered-cubic, fcc structured) Co-33Ni-24Cr alloy (all in at.%) containing 5at.% Mo as a solute. It is also shown that the degree of strengthening after tempering scales up with the fraction of these structures (before tempering) in the alloy microstructure that can be tuned by the amount and temperature of cold deformation. Cold-rolling with 45% and 65% thickness reduction, followed by tempering at 600{\deg}C for 4 hours, led to an YS of 1.5 GPa and 2 GPa with elongation to fracture (%El) 14% and 7%, respectively. The YS is further enhanced to ~ 2.2 GPa without reduction in %El upon cryo-rolling followed by tempering. The alloy microstructure is stable at 600{\deg}C up to 100 hours and also retains an YS of ~ 1.5 GPa with %El of 18% during tensile test at 600{\deg}C. The derived high YS and high-temperature stability are critically a consequence of solute partitioning to the NMLs that we termed as Solute-Partitioned NMLs (SP-NMLs) in the microstructure.

cond-mat.mtrl-sci↗

arXiv2025

Cobalt-Controlled Interphase Partitioning Regulates Matrix Solute Transport and $\gamma'$ Coarsening in Ti-Rich NiCoCr-Based Superalloys

The microstructural stability and mechanical response of $\gamma/\gamma'$ superalloys at elevated temperatures are governed by solute chemistry and elemental partitioning across the $\gamma/\gamma'$ heterophase interface. Conventionally, a lower $\gamma^{\prime}$ solvus temperature and a larger $\gamma/\gamma^{\prime}$ lattice misfit are expected to increase the coarsening rate. Here, Co-for-Ni substitution is used in Ni--Co--Cr--Al--Ti superalloys to systematically modify the $\gamma$-matrix chemistry and generate medium-entropy-alloy-type matrix environments with Ni:Co:Cr ratios of approximately 2:1:2, 2:2:1, and 1:2:1. Despite a $\sim$65~$^{\circ}$C reduction in $\gamma^{\prime}$ solvus temperature, the activation energy for Ti-rich Ni$3$(Al,Ti)-type $L1{2}$ $\gamma^{\prime}$ coarsening increases from $\sim$156 to $\sim$302~kJ~mol$^{-1}$ with increasing Co content. This enhanced coarsening resistance is attributed to an increase in the multicomponent solute-transport resistance, $S=\sum_i (\Delta C_i)^2/(D_i^\gamma C_i^\gamma)$, from $\sim 8.811\times10^{15}$ to $\sim 1.267\times10^{16}$~s~m$^{-2}$, together with a reduction in the apparent interfacial-energy term from 25.0 to 7.55~mJ~m$^{-2}$. The dominant transport resistance correspondingly shifts from Ni/Cr in B--10Co to Ni/Co/Cr in B--30Co.

cond-mat.mtrl-sci↗

arXiv2025

High-strength and ductile lightweight cast aluminium alloys with superlattice nano-layered fibres (SNL) and core-shell nano-particles

Lightweight, high-strength structural materials are component enablers in transportation and aerospace, reducing carbon footprints and enhancing fuel efficiency. Cast aluminium alloys, mainly based on eutectic compositions, make up 85% of these materials but often fail catastrophically due to inefficient load transfer across the interfaces between the brittle eutectic phase and the ductile matrix. Here, we discovered that promoting a superlattice nano-layer (SNL) around the eutectic fibres, achieved by adding Zr to an Al-Gd near-eutectic alloy, enables excellent load transfer capabilities, resulting in a 400% increase in tensile ductility. The primary Al matrix also contains a high number density of superlattice core-shell nano-particles. This exceptional increase in formability is attributed to the ability of the SNL to prevent dislocations from accumulating at the weak and brittle eutectic fibre/matrix interfaces, thereby avoiding stress concentrations that would otherwise initiate fibre breakage and debonding. The core-shell nano-particles in Al 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. This atomic interface design overcomes the ductility limitations of cast-eutectic alloys, enabling them for structural applications.

cond-mat.mtrl-sci↗

arXiv2023

Chemical heterogeneity enhances hydrogen resistance in high-strength steels

When H, the lightest, smallest and most abundant atom in the universe, makes its way into a high-strength alloy (>650 MPa), the material's load-bearing capacity is abruptly lost. This phenomenon, known as H embrittlement, was responsible for the catastrophic and unpredictable failure of large engineering structures in service. The inherent antagonism between high strength requirements and H embrittlement susceptibility strongly hinders the design of lightweight yet reliable structural components needed for carbon-free hydrogen-propelled industries and reduced-emission transportation solutions. Inexpensive and scalable alloying and microstructural solutions that enable both, an intrinsically high resilience to H and high mechanical performance, must be found. Here we introduce a counterintuitive strategy to exploit typically undesired chemical heterogeneity within the material's microstructure that allows the local enhancement of crack resistance and local H trapping, thereby enhancing the resistance against H embrittlement. We deploy this approach to a lightweight, high-strength steel and produce a high-number density Mn-rich zones dispersed within the microstructure. These solute-rich buffer regions allow for local micro-tuning of the phase stability, arresting H-induced microcracks thus interrupting the H-assisted damage evolution chain, regardless of how and when H is introduced and also regardless of the underlying embrittling mechanisms. A superior H embrittlement resistance, increased by a factor of two compared to a reference material with a homogeneous solute distribution within each microstructure constituent, is achieved at no expense of the material's strength and ductility.

cond-mat.mtrl-sci↗

arXiv2021

Microstructural engineering of medium entropy NiCo(CrAl) alloy for enhanced room and high-temperature mechanical properties

This work demonstrates the development of a strong and ductile medium entropy alloy by employing conventional alloying and thermomechanical processing to induce partial recrystallization (PR) and precipitation strengthening in the microstructure. The combined usage of electron microscopy and atom probe tomography reveals the sequence of microstructural evolution during the process. First, the cold working of homogenized alloy resulted in a highly deformed microstructure. On annealing at 700°C, B2 ordered precipitates heterogeneously nucleate on the highly misoriented sites. These B2 promotes particle stimulated nucleation (PSN) of new recrystallized strain-free grains. The migration of recrystallized grain boundaries leads to discontinuous precipitation of L12 ordered regions in highly dense lamellae structures. Atomic-scale compositional analysis reveals a significant amount of Ni confined to the GB regions between B2 and L12 precipitates, indicating Ni as a rate-controlling element for coarsening the microstructure. On 20 hours of annealing, the alloy comprises a composite microstructure of soft recrystallized and hard non-recrystallized zones, B2 particles at the grain boundaries (GBs), and coherent L12 precipitates inside the grains. The B2 pins the GB movement during recrystallization while the latter provides high strength. The microstructure results in a 0.2% yield stress (YS) value of 1030 MPa with 32% elongation at ambient temperature and retains up to 910 MPa at 670°C. Also, it shows exceptional microstructural stability at 700 °C and resistance to deformation at high temperatures up to 770°C. Examination of deformed microstructure reveals excessive twinning, formation of stacking faults, shearing of L12 precipitates, and accumulation of dislocations at around the B2 precipitates and GBs attributed to high strain hardening of the alloy.

cond-mat.mtrl-sci↗

arXiv2018

On the segregation of Re at dislocations in the γ' phase of Ni-based single crystal superalloys

We report evidence of Re and Mo segregation (up to 2.6 at.% and 1 at.%) along with Cr and Co to the dislocations inside of γ' precipitates in a second generation Ni-based single crystal superalloy, after creep deformation at 750°C under an applied stress of 800 MPa. The observed segregation effects can be rationalized through bridging the solute partitioning behavior across the γ/γ' interface and the pipe diffusion mechanism along the core of the dislocation line. This understanding can provide new insights enabling improved alloy design.

cond-mat.mtrl-sci↗