Department of Metallurgical & Materials Engineering

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IIT Jodhpur

We have studied carbyne as a promising hydrogen storage material. Density functional theory simulations with vdW corrections have been used to investigate lithium sorption on carbyne and the interaction of pristine and Li-functionalized carbon chains with molecular hydrogen. We showed that Li adatoms at small concentrations stay atomically dispersed on carbyne, donating 0.9e to the chain. Moreover, in the presence of Li, hydrogen adsorption energy increases by more than 5 times in comparison with pristine carbyne. Overall, up to three hydrogen molecules per Li adatom have an adsorption energy close to the range of 200-600 meV per H2, which is necessary for effective sorption/desorption cycles. The resulting theoretical uptake (7.1 wt%) is higher than the U.S. Department of Energy's ultimate goal (6.5 wt%). The calculated van't Hoff desorption temperatures exceed considerably the boiling point of liquid nitrogen. Our results confirm the potential of Li-decorated carbyne for hydrogen storage. © 2020 The Royal Society of Chemistry.

World University root node

Sustainable Energy \& Fuels

Li-decorated carbyne for hydrogen storage: charge induced polarization and van't Hoff hydrogen desorption temperature

The Journal of Physical Chemistry C

Critical Assessment of the Thermodynamics of Vacancy Formation in Fe2O3 Using Hybrid Density Functional Theory

Complex concentrated alloys (CCAs) extend the compositional paradigm shift of high entropy alloys (HEAs) to new microstructural opportunities. CCAs provide opportunities for tunable performance by manipulating deformation mechanisms. Fe–Mn–Co–Cr–Si alloys exhibit potential for a combination of phase transformation and twinning. These alloys give greater flexibility for tailoring transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), which have guided design of next-generation steel alloys over the last 20 years to a new level. For TRIP CCAs, the ductility can be extended to as high as 50% while maintaining a strength exceeding 1 GPa. The Fe–Mn–Co–Cr–Si alloys show extensive γ (f.c.c.) to ε (h.c.p.) phase transformation followed by additional twinning in the epsilon phase. Combination of ultrafine grained microstructure and crack tip transformation leads to enhanced fatigue limit in TRIP CCAs. Design of non-equiatomic CCAs provides a vast, and vastly unexplored compositional space for developing new alloys with tunable properties. © 2020 Elsevier B.V.

Journal of Alloys and Compounds

Metastability driven hierarchical microstructural engineering: Overview of mechanical properties of metastable complex concentrated alloys

Development of multicomponent alloys, popularly known as high entropy alloys (HEAs) provides abundant compositional space for designing a variety of HEAs, including equiatomic, dual phase and microstructurally flexible (MF-HEAs). Among them, the design of MF-HEAs further extends the HEA domain in terms of understanding adaptive phase evolution with respect to change in alloy chemistry and processing parameters. This responsive phase evolution enabled near single phase ε (h.c.p., 95%) or γ (f.c.c., 90%) dominant microstructures, depending on imposed processing strain, strain rate and temperature for a selected alloy chemistry. The γ-phase dominated HEAs showed slower yet sustained work hardening (WH) rates (~ 2000 MPa) owing to transformation induced plasticity (TRIP) effect whereas ε-phase dominated microstructure showed exceptionally high and sustained work hardening rates (2300–2700 MPa) due to formation of nano plates or twins in ε-phase upon deformation. Therefore, MF-HEA design leads to exceptional strength–ductility synergy (1100-1250 MPa, 30-43%) irrespective of the dominance of either γ- or ε-phase in the starting microstructure owing to selective operation of deformation mechanisms in the dominant phase. © 2020 The Authors

Fulltext

Materials and Design

Microstructurally flexible high entropy alloys: Linkages between alloy design and deformation behavior

High entropy alloys (HEAs) have attracted scientific interest due to their good mechanical properties and failure resistance, whereas additive manufacturing (AM) has emerged as a powerful yet flexible processing route for advanced materials. However, limitations inherent in both these fields include HEAs display inferior mechanical properties in as cast condition; and AM demands expansion of printable alloys. A new metastable Fe40Mn20Co20Cr15Si5 (CS-HEA) HEA showing stabilized ε-h.c.p. dominated microstructure after laser powder bed fusion additive manufacturing has been evaluated. As-printed CS-HEA showed higher strength due to high work hardenability, whereas substantial uniform ductility is associated with a combination of transformation and twinning induced plasticity during deformation. Additionally, very low volume percent of voids (∼0.1 %) along with high strength-ductility shows excellent printability of the CS-HEA using laser-based additive manufacturing. © 2020 Elsevier B.V.

Additive Manufacturing

Excellent strength-ductility synergy in metastable high entropy alloy by laser powder bed additive manufacturing

Ti-6Al-4V alloy was caliber rolled (CR) to 92% reduction in cross section area in the temperature range of 750-850 °C. Caliber rolling to a strain &gt;2 resulted in grain refinement ranging from 0.53 to 1.04 µm, which led to enhancement in room temperature yield strength and tensile stregth. The work hardening rate (?) showed higher value at the strain level of ~0.13 for CR 800 °C and 850 ° C as compared to CR 750 °C material. Also, differential strain rate test was done at 850 °C, over the strain rate range of 10 -4 to 10 -1 s -1 , to explore the optimum condition for superplasticity. Initial constant strain rate tensile tests at optimized strain rate of 10 -3 s -1 and the test temperature of 850 °C showed the tensile elongations ranging from 530 to 560%, depending on caliber rolling temperature. Copyright © 2018 MS&T18®

Materials Science and Technology 2018, MS and T 2018

Effect of caliber rolling temperature on room and elevated temperature tensile properties of Ti-6Al-4V alloy

The effect of temperature and strain rate on the tensile flow behavior of Fe-0.3 pct C-CrMoV grade steel was studied over a wide range of strain rates (10−4 to 10−1 s−1) and temperatures (700 °C to 950 °C). The flow curves of the steel showed typical dynamic recovery (DRV)-type characteristics at low temperature, high strain rate, and dynamic recrystallization (DRX) type at high temperature &gt; 775 °C. Stress regimes with stress exponent (n) of 3.6 to 5.5 for low–high stresses were observed. The ‘n’ values at temperatures of 850 °C and 900 °C were found to be &gt; 4, which correspond to dislocation climb as the rate controlling mechanism. At 950 °C, ‘n’ value was found to be &lt; 4, where viscous glide is the rate controlling mechanism. The apparent activation energy (Q) was found to be 320 ± 12 kJ mol−1. Hence, the dominant high-temperature deformation mechanism was identified as high-temperature climb of edge dislocations. The strain rate sensitivity index (m) of the steel was evaluated using jump strain rate tests and cyclic temperature and strain rate jump tests over temperatures of 700 °C to 950 °C and strain rates of 10−4 to 10−3 s−1. Although, ‘m’ value as high as 0.5 was observed, cavitation resulted in premature failure during deformation resulting in low elongation. The volume fraction of cavities was inversely proportional to the strain rate at all temperatures. The fine-grained microstructure aids grain boundary sliding, and diffusion thereby favors the cavity growth at low strain rates. Microstructures evolved during the high-temperature tensile tests were analyzed and the optimum conditions for hot deformation i.e., hot rolling/hot forming schedules were determined as the temperature range of 850 °C to 950 °C and strain rate range of 10−3 to 10−4 s−1. The flow stress data for the steel were found to follow the universal Dorn sine hyperbolic equation. © 2018, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Effect of Strain Rate and Temperature on the Tensile Flow Behavior and Microstructure Evolution in Fe-0.3 Pct C-CrMoV Grade Steel

Superplasticity is the ability of certain polycrystalline materials to exhibit exceptionally large elongations by grain boundary sliding and its accommodation by diffusion, dislocation slip, or both. One of the critical requirements for this phenomenon to occur is the fine equiaxed grains, which should be stable during deformation at elevated temperature. Under this condition, there exists a unique relationship between stress and strain rate as a function of temperature and grain size, with the high values of strain rate sensitivity index m of greater than 0.3. However, there is ample evidence of the variation in flow stress with strain along with concurrent microstructural evolution. Often the microstructures are known not to be equiaxed yet exhibit superplasticity by rapid change in morphology and size during early stage of deformation. This leads to flow softening and hardening with strain as a result of the dominance of one type of change over the other. Mechanistically, accommodation of grain boundary sliding by grain boundary migration leads to grain growth, resulting in flow hardening, whereas the absence of the same may cause the formation of cavities because of stress concentration at triple points and at two-phase or particle-matrix interphase boundaries. In view of this, the constitutive relationship needs to be modified by incorporating the dependence of stress on strain and concurrent microstructure changes. The present work critically examines the nature of stress-strain curves and microstructure evolution in an attempt to account for the non-steady-state flow and attain a generalized form of constitutive relationship for superplastic deformation. An attempt is made to quantify the variations in flow stress and microstructures interdependently as well as with strain by exploring the suitable trend lines. These equations can numerically and physically help in correlating the microscopic and macroscopic properties to the mechanisms for superplastic deformation. Copyright © 2019 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.