Shravana Katakam

Laser assisted high entropy alloy coating on aluminum: Microstructural evolution

High entropy alloy (Al-Fe-Co-Cr-Ni) coatings were synthesized using laser surface engineering on aluminum substrate. Electron diffraction analysis confirmed the formation of solid solution of body centered cubic high entropy alloy phase along with phases with long range periodic structures within the coating. Evolution of such type of microstructure was a result of kinetics associated with laser process, which generates higher temperatures and rapid cooling resulting in retention of high entropy alloy phase followed by reheating and/or annealing in subsequent passes of the laser track giving rise to partial decomposition. The partial decomposition resulted in formation of precipitates having layered morphology with a mixture of high entropy alloy rich phases, compounds, and long range ordered phases.

In Situ Laser Synthesis of Fe-Based Amorphous Matrix Composite Coating on Structural Steel

Iron-based amorphous materials, owing to their very high hardness, elastic modulus, wear resistance, and corrosion resistance, can be potential materials for surface modification and engineering of many structural alloys. The current study focuses on a novel functional coating, synthesized via laser cladding of an iron-based (Fe48Cr15Mo14Y2C15B) amorphous precursor powder, on AISI 4130 steel substrate, using a continuous-wave diode-pumped ytterbium laser. The coatings were characterized by different techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). SEM and TEM studies indicated the presence of Fe-based nanocrystalline dendrites intermixed within an amorphous matrix. A three-dimensional thermal modeling approach based on COMSOL Multiphysics (COMSOL Inc., Burlington, MA) was used to approximately predict the temperature evolution and cooling rates achieved during laser processing. The mechanisms for the formation of crystalline phases and the morphological changes in the microstructure were studied based on the thermal model developed. Although the thermal model predicted substantially high cooling rates as compared to the critical cooling rate required for retaining an amorphous phase, the formation of crystalline phases is attributed to formation of yttrium oxide, reducing the glass-forming ability, and formation of different oxide phases that act as heterogeneous nucleation sites resulting in the composite microstructure.

Amorphous Coatings and Surfaces on Structural Materials

Metallic glasses show a unique combination of high strength, excellent corrosion, and wear resistances because of their amorphous structure having a short-range order. In spite of excellent properties, the application of metallic glasses is restricted because of their inherent limitations in the bulk form, including limited tensile ductility. Using metallic glasses as the coatings for structural applications is an attractive way of taking advantage of their superior properties. Additionally, metallic glass-based composites having crystalline phases embedded in a amorphous matrix have also shown improved properties. Thus, metallic glasses can be synthesized as the coatings or subjected to surface modification to provide functionally superior surfaces. This article is a review of metallic glass-based coatings and surface modification of metallic glasses to achieve functionally superior surfaces for structural applications. Essential theoretical concepts were discussed which influence the processing. Common ways of processing along with the influence of various processing parameters were explored. Some non-conventional techniques which emerged as a result of continued efforts were also taken into account. Corrosion and wear properties of these materials along with the underlying mechanisms were discussed in detail. Focus was given to the recent product level applications explored in the open literature. Current challenges in the field were reviewed and guidelines for the future developments were provided.

Laser assisted crystallization of ferromagnetic amorphous ribbons: A multimodal characterization and thermal model study

This paper focuses on laser-based de-vitrification of amorphous soft magnetic Fe-Si-B ribbons and its consequent influence on the magnetic properties. Laser processing resulted in a finer scale of crystallites due to rapid heating and cooling during laser annealing compared to conventional furnace annealing process. A significant increase in saturation magnetization is observed for laser-annealed ribbons compared to both as-received and furnace annealed samples coupled with an increase in coercivity compared to the as received samples. The combined effect of thermal histories and stresses developed during laser annealing results in the formation of nano-crystalline phase along the laser track. The phase evolution is studied by micro-XRD and TEM analysis. Solute partitioning and compositional variation within the phases are obtained by Local Electrode Atom probe analysis. The evolution of microstructure is rationalized using a Finite Element based heat transfer multi-physics model.

Stress-induced selective nano-crystallization in laser-processed amorphous Fe–Si–B alloys

It is shown that laser processing results in localized compressive stresses in amorphous Fe–Si–B, leading to homogeneous nano-crystallization at the edges of the laser track. The mechanism can be attributed to enhanced diffusivity at these edges, resulting from a reduced diffusion activation barrier, which has been calculated by coupling the results of a thermal model with microstructural characterization.

Comparison of the crystallization behavior of Fe-Si-B-Cu and Fe-Si-B-Cu-Nb-based amorphous soft magnetic alloys

The role of the solute elements, copper, and niobium, on the different stages of de-vitrification or crystallization of two amorphous soft magnetic alloys, Fe73.5Si13.5B9Nb3Cu1, also referred to as FINEMET, and a Fe76.5Si13.5B9Cu1 alloy, a model composition without Nb, has been investigated in detail by coupling atom probe tomography and transmission electron microscopy. The effects of copper clustering and niobium pile-up at the propagating interface between the α-Fe3Si nanocrystals and the amorphous matrix, on the nucleation and growth kinetics have been addressed. The results demonstrate that while Cu clustering takes place in both alloys in the early stages, the added presence of Nb in FINEMET severely restricts the diffusivity of solute elements such as Cu, Si, and B. Therefore, the kinetics of solute partitioning and mobility of the nanocrystal/amorphous matrix interface is substantially slower in FINEMET as compared to the Fe76.5Si13.5B9Cu1 alloy. Consequently, the presence of Nb limits the growth rate of the α-Fe3Si nanocrystals in FINEMET and results in the activation of a larger no. of nucleation sites, leading to a substantially more refined microstructure as compared to the Fe76.5Si13.5B9Cu1 alloy.

Dynamic crystallization during non-isothermal laser treatment of Fe–Si–B metallic glass

Fe–Si–B metallic glass foils were subjected to non-isothermal laser treatment to induce crystallization, and the effect of laser fluence on crystallite size was investigated. Temperature, and corresponding heating and cooling rates generated during laser processing of metallic glass were estimated using multiphysics computational models. Estimation of the onset and arrest temperatures of crystallization was based on the results obtained using the thermal model. Crystallite size was measured with the aid of x-ray diffraction and transmission electron microscopy. The fraction of crystallization was estimated with a differential scanning calorimetry. Crystallite size increased with laser fluence in the initial stages and saturated later within the laser fluence range (0.6–0.9 J mm−2) explored in the current efforts. The fraction of crystallization steadily increased with the increase in laser fluence. Unlike conventional processes, in the present situation the dynamic effects during laser processing dominated the crystallization and growth process. Rapid heating rates during laser processing led to a shift in the onset of crystallization temperature to a higher level. Faster cooling rates prematurely arrested the crystallite growth yielding much finer crystallite sizes.

Nanocrystallization in spark plasma sintered Fe48Cr15Mo14Y2C15B6 bulk amorphous alloy

Spark plasma sintering (SPS) is evolving as an attractive process for the processing of multi-component Fe-based bulk amorphous alloys and their in-situ nanocomposites with controlled primary nanocrystallization. Extended Q-range small angle neutron scattering (EQ-SANS) analysis, complemented by x-ray diffraction and transmission electron microscopy, was performed to characterize nanocrystallization behavior of SPS sintered Fe-based bulk amorphous alloys. The SANS experiments show significant scattering for the samples sintered in the supercooled region indicating local structural/compositional changes associated with the profuse nucleation of nanoclusters (∼4 nm). For the samples spark plasma sintered near and above crystallization temperature (>653 °C), the SANS data show the formation of interference maximum indicating the formation and growth of (Fe,Cr)23C6 crystallites. The SANS data also indicate the evolution of bimodal crystallite distribution at higher sintering temperatures (above Tx1). The grow...

Tensile behavior of laser treated Fe-Si-B metallic glass

Fe-Si-B metallic glass foils were treated with a linear laser track using a continuous wave Nd-YAG laser and its effect on the overall tensile behavior was investigated. Microstructure and phase evolutions were evaluated using X-ray diffraction, resistivity measurements, and transmission electron microscopy. Crystallization fraction was estimated via the differential scanning calorimetry technique. Metallic glass foils treated with the lower laser fluences (<0.49 J/mm2) experienced structural relaxation, whereas higher laser fluences led to crystallization within the laser treated region. The overall tensile behavior was least impacted by structural relaxation, whereas crystallization severely reduced the ultimate tensile strength of the laser treated metallic glass foils.

Crystallisation behaviour during tensile loading of laser treated Fe–Si–B metallic glass

Effect of tensile loading on crystallisation behaviour of as-cast and laser thermal treated Fe–Si–B metallic glass foils was investigated. Tensile loading lacked any marked influence on the crystallisation behaviour of as-cast and structurally relaxed laser-treated metallic glass foils. Furthermore, the average crystallite/grain size in partially crystallised laser-treated metallic glass foil was nearly equal to the average crystallite/grain size in the region away from the fracture of the same partially crystallised laser-treated metallic glass foil after tensile loading. However, a significant crystallite/grain growth/coarsening of the order of two and half times was observed in the fractured region compared to the region around it for the laser-treated partially crystallised metallic glass foils. The simultaneous effects of stress generation and temperature rise during tensile loading were considered to play a key role in crystallite/grain growth/coarsening.