Department of Metallurgical & Materials Engineering

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An immiscible metal combination of stainless steel and copper was butt-welded using friction stir welding (FSW). A W-Re tool was used for FSW; tool dimensions included flat and featureless tool shoulder diameter of 16.0 mm, conical tool pin diameters are 5.0 mm at the tip and 7.0 mm at the bottom, and pin length of 4.0 mm. Metallographic samples for base as well as welded cross sections were prepared by polishing down to a surface finish till 0.02 lm diamond suspension. Base material was observed by optical microscopy (OM). Two-dimensional (2D) digital image correlation (DIC) was applied for localized deformation measurements during tensile testing. Microhardness was conducted on the cross section of welded joints with load of 200 g and a dwell time of 10 seconds. Processed copper region was weakened by thermal softening and strengthened by finer recrystallized copper grains. The combined effect of the thermal softening and grain refinement-related strengthening was dependent on the welding parameters. Lower rotation rate and higher traverse speed led to lower heat input and resultant higher hardness in stir zone except from the region steel fragments existed. In addition, welded samples fractured in the processed copper region adjacent to steel/copper interface due to the stress concentration during tensile testing.

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Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Towards Obtaining Sound Butt Joint Between Metallurgically Immiscible Pure Cu and Stainless Steel Through Friction Stir Welding

The principle of elastic strain energy minimization is used to predict the possible ranges of allowable lattice misfit for the formation of coherent and semi-coherent interfaces between a Ni superalloy substrate and a cubic yttria stabilized zirconia (c-YSZ) ceramic coating. Depending upon the elastic modulus, failure strength in compression and tension, and the Burgers vector of the film material, the magnitude of allowable lattice misfit can vary over a wide range. It is shown that compressive stresses as high as several gigapascals may exist at the coherent YSZ films, and a higher positive lattice misfit can sometimes be more favorable for the formation of a semi-coherent interface than is a lower negative lattice misfit. Both coherency and the effect of lattice misfit on the interfacial stress decrease with an increase in the thickness of the film. The effect of lattice misfit on the evolution of interfacial stresses during film deposition must be considered during the design of dissimilar materials interfaces and for the prediction of interfacial delamination and the fracture of films. © 2018

Fulltext

Surfaces and Interfaces

On the evolution of stresses due to lattice misfit at a Ni-superalloy and YSZ interface

The design of non-equiatomic high-entropy alloys (HEAs) opens huge compositional space to develop new materials with exceptional properties. Among them, HEAs with flexible microstructures showed an adaptive phase stability that enhanced the work hardening (WH) ability of the material drastically. With the same motive, here we present a new friction stir processed Fe39Mn20Co20Cr15Si5Al1 HEA that demonstrated an unexpected strength–ductility response just upon annealing. The inter-competing precipitation and grain/twin formation events during low-temperature annealing resulted in an unexpected f.c.c. (γ) → h.c.p. (ɛ) transformation. This unusual phase evolution triggered development of refined, ɛ-dominated microstructure coupled with a uniformly dispersed fine γ phase. The controlled 〈c+a〉 slip and twinning in the ɛ phase along with the transformation of a refined γ matrix resulted in higher elongation of 52% with enhanced ultimate tensile strength of 1.12 GPa during deformation. Thus, the metastability-assisted design of ɛ-martensite-dominant HEAs by annealing opens a new path to obtain strong yet ductile alloys, which is otherwise not feasible in conventional steel/HEA designs. © 2018 Elsevier Ltd

Applied Materials Today

Unexpected strength–ductility response in an annealed, metastable, high-entropy alloy

This research work is an attempt to understand the effect of stored strain energy heterogeneity over macro-scale, induced during interrupted annealing treatment, on the micro structural evolution in a heavily cold rolled AISI 304L austenitic stainless steel. The cold rolled AISI 304L stainless steel (SS) samples were subjected to different annealing treatments. The microstructures obtained after each annealing treatment were characterized using optical and electron microscopy techniques. Further, an attempt was made to estimate the stored strain energy by a novel approach of micro hardness measurement both over small scale (area 3600 μm2) and large scale (area ~0.25 mm2). The stored strain energy was determined using hardness values. Using these data, spatial stored strain energy distribution contour maps were created. They were, in turn, used to characterize the local softening behavior both at micro and macro-scale levels after each interrupted annealing treatment. The results showed that, interrupted annealing treatment induces increased stored strain energy heterogeneity after each annealing interruptions. Also, it was possible to determine recrystallization kinetics of the material using stored strain energy map, which helps in designing annealing cycles for achieving ultrafine grained microstructure. © 2018 Elsevier Inc.

Materials Characterization

Effect of stored strain energy heterogeneity on microstructure evolution of 90% cold rolled AISI 304L stainless steel during interrupted annealing treatment

Free standing films of graphene oxide (GO) were synthesized using an improvisation of Tour's method [ACS Nano, 4, 4806-4814 (2010)]. Reaction time and temperature were significantly modified to reduce the presence of graphitic residues in the as synthesized GO. These films were characterized for phase purity and we report their performance for ammonia sensors. © 2017 Elsevier Ltd.

Materials Today: Proceedings

Free standing graphene oxide films for gas sensing applications

Even though transition metal dichalcogenides (TMDCs) are deemed to be novel photonic and optoelectronic 2D materials, the visible band gap being often limited to monolayer, hampers their potential in niche applications due to fabrication challenges. Uncontrollable defects and degraded functionalities at elevated temperature and under extreme environments further restrict their prospects. To address such limitations, the discovery of a new 2D material, α-PbO is reported. Micromechanical as well as sonochemical exfoliation of 2D atomic sheets of α-PbO are demonstrated and its optical behavior is investigated. Spectroscopic investigations indicate layer dependent band gaps. In particular, even multilayered PbO sheets exhibit visible band gap > 2 eV (direct) which is rare among semiconducting 2D materials. The emission lifetime of multilayer PbO atomic sheets is 7 ns (dim light) as compared to the monolayer which gives 2.5 ns lifetime and an intense light. Density functional theory calculations of layer dependent band structure of α-PbO matches well with experimental results. Experimental findings suggest that PbO atomic sheets exhibit hydrophobic nature, thermal robustness, microwave stability, anti-corrosive behaviour and acid resistance. This new low-cost, abundant and robust 2D material is expected to find many applications in the fields of electronics, optoelectronics, sensors, photocatalysis and energy storage. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Alpha Lead Oxide ($\alpha$‐PbO): A New 2D Material with Visible Light Sensitivity

In the present study, Gas Tungsten Arc Welding (GTAW) of Inconel 718 alloy has been carried out at different welding currents with and without ultrasonic vibration. In the absence of ultrasonic treatment, samples welded with a current of 80 A have shown a crack length of 15 mm which is found to increase to 35 mm for a welding current of 120 A. The slow solidification rate associated with high welding current causes increase in hot cracking length. Non-equilibrium Scheil solidification simulation of Inconel 718 based on CALPHAD approach suggests that the long freezing range of more than 175 °C, small fraction of eutectic, i.e., less than 10% in susceptible freezing range (SFR) and segregation of low melting point Laves and γ′′ (Ni3Nb) phases play critical roles in hot cracking behavior of Inconel 718 alloy. Ultrasonic treatment during solidification of weld metal has decreased the crack sensitivity and dendritic arm length of Inconel 718 alloy from 47.5% to 13.3% for the welding current of 120 A. There was a corresponding reduction in dendritic arm length from 1256 to 89 μm. Further, it was observed that the welding current has very limited role in the hot cracking behavior of Inconel 718 alloy in the presence of ultrasonic vibration. Dendritic fragmentation, grain refinement and reduction in solute segregation are recognized as the main factors that mitigate hot cracking tendency of Inconel 718 alloy by ultrasonic vibration. © 2017 Elsevier B.V.

Journal of Alloys and Compounds

Mitigation of hot cracking in Inconel 718 superalloy by ultrasonic vibration during gas tungsten arc welding

Degradable magnesium based implants are predisposed to failure by stress corrosion cracking (SCC) when subject to tensile loads in the human body. Slow strain rate (SSR) tests are used in SCC analysis and the applied strain rate is a critical factor to determine if the material failed by stress corrosion or mechanical overload. In this paper, the SCC susceptibility of Mg-4Zn alloy in simulated body fluid (SBF) is investigated at different strain rates. It was found that elongation to failure (ε) and ultimate tensile strength (UTS) decreased by 80% and 58% respectively when extension rate in SBF decreased from 3.6 × 10−4 s−1 to 3.6 × 10−6 s−1. When the Mg-4Zn alloy is strained in SBF, multiple surface cracks appear on the gauge, initiated at the corrosion pits that induce mechanical overload and failure at higher strain rates. As the strain rate decreases, the increased contact time between the surface and SBF results in hydrogen embrittlement, micro crack formation and progressive transgranular failure that is evident in the fractography. The reduction in UTS and elongation even at the highest strain rate tested indicate that the critical strain rate for stress corrosion cracking of Mg-4Zn alloy in simulated body fluid could be as high as 3.6 × 10−4 s−1. © 2018 Elsevier B.V.

Materials Science and Engineering A

Stress corrosion cracking of biodegradable Mg-4Zn alloy in simulated body fluid at different strain rates – A fractographic investigation

Present study investigates the role of ultrasonic treatment (UT) on microstructure modification, porosity mitigation and their influence on mechanical properties of Sr modified A356 Al alloy. Addition of Sr in trace amount (500 ppm) to A356 alloy reduces the length of eutectic Si from 15.6 µm to 0.65 µm with a concomitant increase in porosity from 0.84% to 3.59%. Ultrasonic treatment of Sr modified A356 Al alloy retains the modification of eutectic Si (0.57 µm) and reduces the specific porosity volume to 0.86%. The effect of microstructure modification and porosity mitigation reflects on the mechanical properties of A356 Al alloy. While the Sr addition to A356 alloy reduces the tensile properties, UT of the Sr added alloy results in improved tensile properties. The mechanism of microstructure modification, porosity formation and UT assisted porosity mitigation are discussed in detail. Ultrasonic cavitation assisted degasification and bi-film breakage is proposed as the governing mechanism for porosity alleviation. © 2018 Elsevier B.V.

Porosity alleviation and mechanical property improvement of strontium modified A356 alloy by ultrasonic treatment

In this article, we report an active-matrix type Sn–SnSb alloy, which is sandwiched between nitrogen doped reduced graphene oxide (N-rGO) sheets in the form of a nanocomposite, as a high rate capability anode for lithium-ion batteries. The alloy nanocomposite is synthesized via a cheap and industrially scalable route of microwave-assisted hydrothermal synthesis, and is coated onto electrodeposited 3D microporous nickel foam current collector. The additional mechanical buffering, along with effective electron conduction and lithium ion diffusion pathways provided by N-rGO nanosheets and nickel foam, result in a specific capacity of ∼300 mAhg−1 at a specific current of 4 A g-1 by preventing both pulverization and delamination of the active material. This combination of properties in N-rGO decorated Sn–SnSb nanocomposite anode (with 40 wt% N-rGO) on nickel foam results in a 2nd cycle discharge specific capacity of 705 mAhg−1, with a stable reversible specific capacity of 500 mAhg−1 after 200 cycles @ 0.1 A g-1. The nanocomposite anode also shows capacity retention of 400 mAhg−1 @ 0.8 A g-1 (1C rate) for 120 cycles. As compared to low N-rGO (10 wt%) decorated nanocomposite, the high N-rGO (40 wt%) nanocomposite shows improved performance with a nominal sacrifice of capacity which is at par, if not superior, to the existing commercial graphitic anodes. © 2018 Elsevier B.V.

Journal of Power Sources

Sandwich architecture of Sn–SnSb alloy nanoparticles and N-doped reduced graphene oxide sheets as a high rate capability anode for lithium-ion batteries