Department of Metallurgical & Materials Engineering

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The simple shear response of soft polymers under large deformation (&gt;50%) and strain rates spanning 10 −3 – 10 3 s −1 is characterized by developing quasi-static and split-Hopkinson pressure bar based single-pulse dynamic simple shear experiments rooted in continuum mechanics fundamentals. Cross-linked polydimethylsiloxane (PDMS) is chosen as a model material. By examining the evolution of stress, strain and strain rate, the latter two parameters measured using two-dimensional digital image correlation (DIC), it is demonstrated that dynamic simple shear deformation consists of four distinct stages: momentum diffusion, inertia effect, steady-state material response, and strain rate decay. By isolating the unsteady and steady-state deformation stages, inertia-free material response is captured under a uniform strain rate. It is shown that the shear response of PDMS is nearly linear with a weakly rate-sensitive shear modulus in the investigated strain rate range. Further, by analyzing the DIC strain-field and comparing the kinematic experimental results with those predicted by classical continuum mechanics, it is demonstrated that the proposed experiments not only achieve a nearly theoretical simple shear state that is uniform across the specimen, but also allow for post-test validation of individual experiments based on these criteria. © 2019, Society for Experimental Mechanics.

World University root node

Experimental Mechanics

Quasi-Static and High Strain Rate Simple Shear Characterization of Soft Polymers

Functionally graded materials offer potential as a solution to the premature failure of thermal barrier coatings due to thermal residual stresses, however this solution has not been fully evaluated to date. An analytical methodology based on the bending of plates has been used to model the residual stress distribution through the layers of a functionally graded (FG) cermet top coat-bond layer-substrate system after cooling from a high deposition temperature. The model is then applied to two material systems with different trends in variation in the coefficient of thermal expansion from the substrate to the top coat layer. Increasing the number of layers reduces the discrete stress variations across the interfaces through the FG layers and reduces the propensity of interfacial cracking. While the stress in the top coat is always compressive for the Inconel substrate, the stress in the top coat is tensile in nature for the steel substrate when the ceramic volume fraction in the functionally graded layers is low. The stress in the top coat gradually becomes compressive with an increasing volume fraction of ceramic in the functionally graded top coat. The curvature of the TBC system follows a similar trend and changes its sign with increasing ceramic content for the steel substrate. The effect of the number of layers on the curvature gradually diminishes and the curvature becomes constant beyond only a few layers. © 2018

Fulltext

Mechanics of Materials

Residual stresses in functionally graded thermal barrier coatings

Fatigue of structural materials is a leading cause of failure and has motivated fatigue-resistant design to eliminate risks to human lives. Majority of conventional materials, including advanced steels and Ni-based superalloys, exhibit limited fatigue endurance limits at ambient temperature. With this in mind, we designed a Cu-containing FeMnCoCrSi high entropy alloy which exhibits a normalized fatigue ratio of ∼0.62 UTS (ultimate tensile strength). Our design approach was based on (a) engineering the γ phase stability to attain sustained work hardening (WH) through delayed γ (f.c.c.) → ɛ (h.c.p.) transformation to hinder fatigue crack propagation and (b) an ultrafine grained (UFG) microstructure to delay crack initiation. We verified that a UFG γ dominant microstructure could provide opportunities for enhanced fatigue resistance, as sustained WH activity strengthened the material locally in the crack plastic zone, thereby validating our expectation that the combination of UFG and transformation induced plasticity (TRIP) is a path to design the next generation of fatigue-resistant alloys. © 2019 Elsevier Ltd

Applied Materials Today

Extremely high fatigue resistance in an ultrafine grained high entropy alloy

Two high entropy alloy (HEA) compositions were compared to explore the individual effects of Co and Ni on phase stability and resultant deformation response in Fe-Mn-Cr-Si-containing HEAs. It was observed that Co-Si-containing HEA depicted responsive phase evolution upon friction stir processing owing to decreased γ (f.c.c.) matrix stability as against Ni-Si-containing HEA. As a result, the Co-Si HEA showed the presence of dual-phase microstructure with the dominance of ε (h.c.p.) phase (52 pct), whereas the Ni-Si HEA showed single-phase γ microstructure under similar processing condition. Also, the dominant deformation mechanisms were different in the two alloys. Co-Si HEA showed uniform strain partitioning between the f.c.c. and h.c.p. phases. Conversely, single-phase f.c.c. microstructure in Ni-Si HEA accommodated strain by twinning-induced plasticity. © 2018, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Microstructural Evolution and Deformation Behavior of Ni-Si- and Co-Si-Containing Metastable High Entropy Alloys

Two-dimensional material heterostructures show large surface area and unusual electronic properties, being used in nanoelectronic devices, energy storage devices, and sensors. Doping can improve stability of the heterostructures. Structural and electronic properties of B–doped phosphorene/graphene heterostructures are studied using first-principles calculations. B doping in phosphorene enhances the interlayer charge transfer from phosphorene to graphene. Importantly, B doping creates strong chemical bonding at the interface, which improves stability of phosphorene/graphene heterostructures. The band gap is opened up to 0.53 eV by B doping. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Advanced Theory and Simulations

B-Doping-Enhanced Stability of Phosphorene/Graphene Heterostructures

Multipass friction stir processing (MFSP) of the Ti-6Al-4V alloy was carried out at 600 tool rpm and 80 mm/min traverse speed. After first pass, the initial elongated α structure transformed to prior β grains, consisting of a mixture of acicular α'and very fine lamellar α colonies along with α layer grain boundary in stir zone (SZ). This subsequently transformed to equiaxed α grain via dynamic recrystallization (DRX) process. With the increase in the number of FSP passes the fraction of equiaxed α grains was found to increase, reaching almost fully equiaxed α structure in SZ upon completion of the fifth pass. Flow properties of MFSP Ti-6Al-4V alloy were investigated by differential strain rate test carried out at 927°C. There appears no significant variation in the strain rate sensitivity index (m ≥ 0.3) values between as received Ti-6Al-4V alloy and MFSP Ti-6Al-4V alloy specimens. © 2018 Institute of Physics Publishing. All rights reserved.

IOP Conference Series: Materials Science and Engineering

Multipass-friction Stir Processing (MFSP) of Ti-6Al-4V Alloy and Investigation of Flow Properties

AA7075/TiB2 composites have been synthesized through both in situ salt-melt reaction method and ultrasound assisted in situ process. Microstructural studies reveals that ultrasound assisted in situ method improves the dispersion of TiB2 particles and reduces the porosity level. Moreover, the ultrasonic treatment refines the reinforcement particle size along with improvement in particle dispersion. The mechanical property assessment confirms that ultrasonic treatment improves the mechanical properties of composite. The hardness of the AA7075 alloy is increased from 55 HV to 74 HV by the addition of 5% TiB2 particles and it further increased to 82 HV by ultrasonic treatment. A similar trend is also observed when weight percentage of particles increases to 7.5%. Addition of 5% in situ TiB2 particles increases the ultimate tensile strength of AA7075 alloy by 60 MPa and it is further enhanced by 80 MPa upon ultrasound assisted process. Composites have shown a small reduction in ductility when compared to un-reinforced alloy, though 81% ductility of matrix alloy has been retained. Similar trend has been observed in composites fabricated using ultrasonic assisted casting. © 2017, The Indian Institute of Metals - IIM.

Transactions of the Indian Institute of Metals

Synthesis, Characterization and Mechanical Properties of AA7075 Based MMCs Reinforced with TiB2 Particles Processed Through Ultrasound Assisted In-Situ Casting Technique

An attempt has been made to understand the microstructural evolution during semi-solid isothermal holding of liquidus cast Sr-modified A356 alloy. It was observed that the liquidus casting is effective in producing non-dendritic microstructure with average sphericity of 0.8 and 0.82 for unmodified and modified alloys, respectively. Isothermal holding of the samples at a temperature corresponding to solid fraction of 0.3 for 15 min enhanced the sphericity of unmodified and modified alloys to 0.84 and 0.85, respectively. Further increase in holding resulted grain coarsening and reduction in sphericity. The grain coarsening rate (K) of samples was analysed with Lifshitz–Slyozov–Wanger equation, and the values of K were found to be 58.3 and 98.3 µm3/s for unmodified and modified alloys, respectively. The grain coarsening in unmodified alloy was dominated by Ostwald ripening, whereas in case of modified alloy, grain coarsening was governed by grain coalescence. © 2018, The Indian Institute of Metals - IIM.

Microstructure Evolution during Semi-solid Isothermal Holding of Liquidus Cast Strontium-Modified A356 Alloy

In the present study, the liquid fraction of Mg–2Zn–xY (x: 2, 4, 6) alloys as a function of temperature was determined experimentally using thermal analysis and compared with the thermodynamically calculated results. Thermodynamic calculations were made using equilibrium and Scheil solidification model in the Thermo-Calc (TCMG1 thermodynamic database). Thermodynamically calculated phases and their formation temperature for Mg–Zn–Y alloy system were found to be significantly different from the thermal analysis results. This resulted in large difference between the thermal analysis and thermodynamically calculated solidification range and eutectic knee point values. Hot-tearing-resistant alloy predicted by the thermal analysis method alone matched with the results reported in the literature. © 2018, The Indian Institute of Metals - IIM.

Determination of Liquid Fraction in Mg–Zn–Y Alloys: Thermal Analysis Versus Thermodynamic Approach

Copper has been electrodeposited on copper (FCC) and mild steel (BCC) substrates from acidic sulphate bath with and without cetyl trimethyl ammonium bromide at 0.25, 2, 6 and 9 V. It is found that the surface morphology varies with the change in overpotential, but not with the change in substrate. On the contrary, the crystal shape is found to be independent of the applied overpotential, but varies with the bath chemistry or choice of substrate. © 2017 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute.