Harpreet Singh Arora

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.

Structural relaxation driven increase in elastic modulus for a bulk metallic glass

The change in elastic modulus as a function of temperature was investigated for a zirconium-based bulk metallic glass. High temperature nano-indentation was done over a wide temperature range from room temperature to the glass-transition. At higher temperature, there was a transition from inhomogeneous to homogeneous deformation, with a decrease in serrated flow and an increase in creep displacement. Hardness was found to decrease, whereas elastic modulus was found to increase with temperature. The increase in elastic modulus for metallic glass at higher temperature was explained by diffusive rearrangement of atoms resulting in free volume annihilation. This is in contrast to elastic modulus increase with temperature for silicate glasses due to compaction of its open three dimensional coordinated structure without any atomic diffusion.

Exceptionally high cavitation erosion and corrosion resistance of a high entropy alloy

Cavitation erosion and corrosion of structural materials are serious concerns for marine and offshore industries. Durability and performance of marine components are severely impaired due to degradation from erosion and corrosion. Utilization of advanced structural materials can play a vital role in limiting such degradation. High entropy alloys (HEAs) are a relatively new class of advanced structural materials with exceptional properties. In the present work, we report on the cavitation erosion behavior of Al0.1CoCrFeNi HEA in two different media: distilled water with and without 3.5wt% NaCl. For comparison, conventionally used stainless steel SS316L was also evaluated in identical test conditions. Despite lower hardness and yield strength, the HEA showed significantly longer incubation period and lower erosion-corrosion rate (nearly 1/4th) compared to SS316L steel. Enhanced erosion resistance of HEA was attributed to its high work-hardening behavior and stable passivation film on the surface. The Al0.1CoCrFeNi HEA showed lower corrosion current density, high pitting resistance and protection potential compared to SS316L steel. Further, HEA showed no evidence of intergranular corrosion likely due to the absence of secondary precipitates. Although, the degradation mechanisms (formation of pits and fatigue cracks) were similar for both the materials, the damage severity was found to be much higher for SS316L steel compared to HEA.

Electromechanical behaviour of nanoporous metallic glass

Understanding the electrical and mechanical behaviour of nanoporous materials is critical for their use in energy applications. A palladium-rich nanoporous film, 500 nm thick with pore size ranging from 10 to 50 nm, was obtained by electrochemical dealloying of a Ni–Pd–P–B metallic glass. Nanomechanical and electrical properties were measured simultaneously, as a function of depth, for the nanoporous structure as well as the unaltered metallic glass substrate. The elastic modulus for the nanoporous structure was found to be 22 GPa compared to 131 GPa for the metallic glass substrate. The ratio of moduli scales with the square of the relative density in agreement with linear elasticity models for cellular materials. The electrical resistivity of the nanoporous layer was found to be 2.2 times higher compared to the metallic glass substrate, which was attributed to the tortuosity of current path in cellular structures.

Controlling the length scale and distribution of the ductile phase in metallic glass composites through friction stir processing

Abstract We demonstrate the refinement and uniform distribution of the crystalline dendritic phase by friction stir processing (FSP) of titanium based in situ ductile-phase reinforced metallic glass composite. The average size of the dendrites was reduced by almost a factor of five (from 24 μm to 5 μm) for the highest tool rotational speed of 900 rpm. The large inter-connected dendrites become more fragmented with increased circularity after processing. The changes in thermal characteristics were measured by differential scanning calorimetry. The reduction in crystallization enthalpy after processing suggests partial devitrification due to the high strain plastic deformation. FSP resulted in increased hardness and modulus for both the amorphous matrix and the crystalline phase. This is explained by interaction of shear bands in amorphous matrix with the strain-hardened dendritic phase. Our approach offers a new strategy for microstructural design in metallic glass composites.

Electrochemical and Friction Characteristics of Metallic Glass Composites at the Microstructural Length-scales

Metallic glass composites represent a unique alloy design strategy comprising of in situ crystalline dendrites in an amorphous matrix to achieve damage tolerance unseen in conventional structural materials. They are promising for a range of advanced applications including spacecraft gears, high-performance sporting goods and bio-implants, all of which demand high surface degradation resistance. Here, we evaluated the phase-specific electrochemical and friction characteristics of a Zr-based metallic glass composite, Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5, which comprised roughly of 40% by volume crystalline dendrites in an amorphous matrix. The amorphous matrix showed higher hardness and friction coefficient compared to the crystalline dendrites. But sliding reciprocating tests for the composite revealed inter-phase delamination rather than preferred wearing of one phase. Pitting during potentiodynamic polarization in NaCl solution was prevalent at the inter-phase boundary, confirming that galvanic coupling was the predominant corrosion mechanism. Scanning vibration electrode technique demonstrated that the amorphous matrix corroded much faster than the crystalline dendrites due to its unfavorable chemistry. Relative work function values measured using scanning kelvin probe showed the amorphous matrix to be more electropositive, which explain its preferred corrosion over the crystalline dendrites as well as its characteristic friction behavior. This study paves the way for careful partitioning of elements between the two phases in a metallic glass composite to tune its surface degradation behavior for a range of advanced applications.

Surface Modification of Metallic Glass Composites Through Severe Plastic Deformation

Refinement of crystalline dendrites in a metallic glass composite, Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5, was demonstrated by friction stir processing. The surface hardness of the amorphous matrix as well as the crystalline dendritic phase was found to increase by nearly a factor of two for the processed specimens. Higher hardness for the amorphous matrix was explained by the interaction of shear bands, while that for the crystalline dendrite was explained by grain refinement.

Microwave synthesized complex concentrated alloy coatings: Plausible solution to cavitation induced erosion-corrosion

Surface phenomenon such as cavitation erosion-corrosion limits the working life and durability of the fluid machines through significantly altering the efficiency. Surface modification is an apparent and economical route for improving the sustainability of these components. Recently developed complex concentrated alloys (CCAs) or high entropy alloys (HEAs) possess exceptional properties owing to high configurational entropy. We developed CCA coatings on the stainless steel using a facial and effective microwave processing technique. The effect of Al molar fraction in AlxCoCrFeNi (x = 0.1-3) CCAs on ultrasonic cavitation erosion-corrosion was investigated in 3.5% NaCl solution. For comparison, cavitation erosion and electrochemical corrosion behavior of the pre- and post-tested samples was also performed. Detailed microstructure and mechanical characterization of the developed coatings were also preformed using different analytical techniques. The equimolar CCA coating showed apical degradation resistance under both pure erosion and erosion-corrosion conditions. The observed behavior is attributed to high strain hardening, optimal hardness, fracture toughness, and utmost stability of the passive layer. The phenomenal conjugation of these properties was associated with highest configurational entropy for equimolar composition resulting in sluggish diffusion, and severe lattice straining. Compared to pits, striations and cracks characterizing the morphology of the degraded stainless steel, the equimolar and Al0.1CoCrFeNi CCAs showed TTS (tearing topograph surface) as the dominant failure mode characterized by presence of microplastic deformation. The degradation of the Al3CoCrFeNi CCA occurred mainly through brittle failure mode. The difference in failure mechanism is related to the mechanical properties and underlying microstructure.

Ultrasonic cavitation erosion-corrosion behavior of friction stir processed stainless steel

Cavitation erosion remains the primary cause of material degradation in fluid machinery components operating at high speed. Micro-jets/shock waves caused by implosion of bubbles on material surface results in significant material loss and premature failure of the components. The presence of corrosive medium further exuberates this effect, causing rapid degradation. Here, we demonstrate a novel pathway to control cavitation erosion-corrosion by tailoring the surface properties using submerged friction stir processing (FSP), a severe plastic deformation process. FSP parameters were varied over wide range of strain-rates to generate tailored microstructures. High strain-rate processing resulted in nearly single phase fine grained structure while low strain-rate processing resulted in phase transformation in addition to grain refinement. As-received and processed samples were subjected to ultrasonic cavitation in distilled water as well as in corrosive environment of 3.5% NaCl solution. Individual roles of cavitation erosion, corrosion and their synergistic effects were analyzed. Depending on the microstructure, processed samples showed nearly 4-6 times higher cavitation erosion resistance compared to as-received alloy. Superior cavitation erosion-corrosion resistance of processed samples was attributed to surface strengthening, higher strain-hardening ability and quick passivation kinetics. The results of current study could be potentially transformative in designing robust materials for hydro-dynamic applications.

High temperature mechanics of nanomoulded amorphous metals

Abstract High temperature mechanics of nanomoulded amorphous metals was investigated by in situ nanomechanical testing. Nanopillars of Pd43Cu27Ni10P20 metallic glass were synthesized by thermoplastic forming and their stress–strain response was obtained concurrent with direct observation of their deformation behaviour. This allowed the measurement of mechanical behaviour from nanopillars and the corresponding bulk substrate with identical thermal history. A rise in elastic modulus was seen with increase in temperature for both the nanopillars and substrate, which was explained by diffusive rearrangement of atomic-scale viscoelastic units. The results provide fundamental insights into structural rearrangement in metallic glasses.