Multicomponent Co20Cu20Fe20Ni20Ti20 eutectic high entropy alloy (HEA), processed by vacuum arc melting cum suction casting technique was studied. The microstructure of this eutectic HEA consists bcc (β) and fcc (α2) solid solution dendritic phases and eutectics (fcc (α1) plus Ti2(Ni, Co)-type Laves phase). Serrations in the flow behaviour are attributed to multi substitutional solutes in the multi-phase microstructure. Based on the detailed analysis of mechanical data together with deformed microstructural characterization, the optimum thermo-mechanical processing conditions for hot working are identified as T=930-990 °C (1203-1263 K) and strain rate range of 10-3 s-1-10-1 s-1. © 2016 Elsevier B.V.

Ravi Sankar Kottada

Department of Metallurgical and Materials Engineering

Phanikumar Gandham

Sumanta Samal

M. R. Rahul

entropy

Eutectics

microstructure

Nickel

Strain rate

Titanium alloys

Vacuum applications

Deformation behaviour

Flow behaviours

Gleeble

HIGH ENTROPY ALLOYS

Micro-structural characterization

Multiphase microstructure

Processing maps

Thermo-mechanical processing

Hot Working

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Materials Science and Engineering A

Multicomponent Co25Fe25Mn5Ni25Ti20 eutectic high entropy alloy (EHEA), synthesized by vacuum arc melting followed by suction casting method resulted in two types of solid solution phases (fcc CoFeNi-rich (α) and bcc Ti-rich (β)) and Ti2(Ni, Co) type Laves phase. The hot deformation behaviours have been investigated at a temperature ranging from 1073 to 1273 K and different strain rates (10−3,10−2,10−1and 1 s−1). The optimum hot workability conditions of EHEA lies in temperature range 1073–1273 K and strain rate range 10−3 s−1-10−1.6 s−1 as well as temperature 1130–1225K and strain rate 10−0.5-1 s−1. The constitutive relation is established to understand the plastic deformability at high temperature. Finite element simulation has also been used to predict the deformation behaviour during the hot working process. © 2020 Elsevier B.V.

Journal of Alloys and Compounds

Hot workability of Co–Fe–Mn–Ni–Ti eutectic high entropy alloy

High entropy alloys with multiple phases are taken for undercooling studies and established the microstructure evolution with respect to undercooling. The melt fluxing technique was used for the current study to achieve undercooling. Predictions on the equilibrium phase formation of these systems was performed using CALPHAD approach and compared with experimental observations. Metastable microstructures were observed during undercooling and the morphological changes could be correlated with the currently established mechanisms. The detailed microstructure evolution in FeCoNiCuX0.5 (X = Al, Mo, Ti, W, Zr) shows the minute addition of these elements results in variation in microstructure evolution during as cast as well as undercooled condition. The studied systems show a maximum undercooling of more than 0.18 TL. and maintained crystalline nature even in deep undercooling. The segregation nature of elements was studied and correlated with the phase field simulations obtained. © 2019 Elsevier B.V.

Metastable microstructures in the solidification of undercooled high entropy alloys

Studies on solidification of equiatomic multi-component alloys are essential in developing melt-route processing of high entropy alloys. The extent of undercooling during processing controls microstructure evolution. In this study, we present solidification studies on undercooled equiatomic FeCuNi alloy carried out using melt fluxing technique. The alloy shows a two-phase (FCC + FCC) microstructure even at deep undercooling of more than 200 K. The dendrite growth velocity measured using high-speed video imaging shows a nonlinear increase in growth velocity with the increase in the extent of undercooling. A modified dendritic growth model was able to predict the growth velocity. The segregation behaviour and microstructures at different extents of undercooling can be predicted using phase field simulation. © 2019 Elsevier B.V.

Solidification behaviour of undercooled equiatomic FeCuNi alloy

In the design of high-entropy alloys (HEAs) with desired properties, identifying the effects of elements plays an important role. HEAs with eutectic microstructure can be obtained by judiciously modifying the alloy compositions. In this study, the effect of Nb addition to FeCoNiCuNbx (x = 0.5, 5, 7.5, 11.6, 15) alloys was studied by varying the Nb concentration (at.%). FeCoNiCuNb0.5 HEA shows liquid phase separation to form Cu-rich and FeCoNiCu-rich phases. Detailed solidification paths are proposed for these alloys, which show eutectic, peritectic, and pseudo quasi-peritectic reactions. Increasing Nb content promotes the liquid phase separation tendency and causes the formation of Cu-rich spheres. The effect of Nb on the FeCoNiCu-rich phase was studied based on the nanoindentation and correlated with nanohardness. The compressive deformation properties of these alloys are studied at room temperature and high temperature and correlated with microstructure. Fractography results show the mode of fracture and are correlated with the microstructure obtained. Copyright © Materials Research Society 2019.

Journal of Materials Research

Effect of niobium addition in FeCoNiCuNbx high-entropy alloys

A eutectic high-entropy alloy with seven components (Fe, Ni, Cr, V, Co, Mn, and Nb) was designed via the melt route guided by CALPHAD predictions. Configurational entropy estimated for the two-phase microstructure qualifies it to be referred to as a high-entropy alloy. When the Nb content exceeded 9.7 at. pct, the microstructure changed from hypoeutectic with primary face-centered cubic phase to hypereutectic with primary Laves phase. © 2019, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Design of a Seven-Component Eutectic High-Entropy Alloy

Nanocrystalline CoCrFeNi high entropy alloy, synthesized by mechanical alloying followed by spark plasma sintering, demonstrated extremely sluggish grain growth even at very high homologous temperature of 0.68 Tm (900 °C) for annealing duration of 600 h. Mechanically alloyed powder had carbon and oxygen as impurities, which in turn led to the formation of two-phase mixture of FCC and Cr-rich carbide with fine distribution of Cr-rich oxide during spark plasma sintering. Sluggish grain growth is attributed to the Zener pinning effect from the fine dispersion of oxide, mutual retardation of grain boundaries in the presence of two phases, and sluggish diffusivity because of cooperative diffusion of multi-principle elements. © 2015 Elsevier B.V. All rights reserved.

Exceptional resistance to grain growth in nanocrystalline CoCrFeNi high entropy alloy at high homologous temperatures

AlxCoCrCuFeNi (x = 0.45, 1, 2.5 and 5 mol) multi-component high entropy alloys synthesized by mechanical alloying were spark plasma sintered to produce high dense compacts. X-ray diffraction and scanning electron microscopy studies reveal that these sintered alloys exhibit varying microstructures from single phase to three phases depending on Al content. The thermal stability studies carried out in the temperature range of 400-600 C for duration of 2-10 h in Ar atmosphere suggest that these alloys exhibit excellent thermal stability in terms of phases and crystallite size. Highest specific hardness of 160 (HV/g cm-3) is achieved in the sintered Al5CoCrCuFeNi alloy and there is no significant change in the hardness after heat treatment of Al 0.45CoCrCuFeNi and AlCoCrCuFeNi alloys. Hall-Petch analysis based on hardness measurements carried out on sintered samples reveals that solid solution strengthening seems to increase with increase in Al content. © 2013 Elsevier B.V. All rights reserved.

Alloying, thermal stability and strengthening in spark plasma sintered AlxCoCrCuFeNi high entropy alloys

Alloying behavior and phase transformations in Al xCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) multi-component high entropy alloys that are synthesized by mechanical alloying were studied. Two FCC phases along with a BCC phase were formed in Al 0.45CoCrCuFeNi and AlCoCrCuFeNi, while a single B2 phase was observed in higher Al containing alloys Al 2.5CoCrCuFeNi and Al 5CoCrCuFeNi. DSC analysis indicates that BCC phase present in the alloys could be Fe-Cr type solid solution. A detailed analysis suggests that two melting peaks observed during DSC in lower Al containing alloys can be attributed to that of Cu-Ni and Fe-Ni FCC solid solutions. The BCC phase disappears in Al 0.45CoCrCuFeNi and AlCoCrCuFeNi at high temperatures during DSC. However, Al 5CoCrCuFeNi retains its B2 structure despite of heating in DSC. Further, phases present in these alloys retain nanocrystallinity even after exposure to high temperatures. A critical analysis is presented to illustrate that solid solution formation criteria proposed for high entropy alloys in the literature are unable to explain the phase formation in the present study of alloys. Besides, these criteria seem to be applicable to high entropy alloys only under very specific conditions. © 2012 Elsevier Ltd. All rights reserved.

Intermetallics

Phase formation in mechanically alloyed Al xCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys

The alloy, Ti-6Al-4V is an α+β Ti alloy that has large prior β grain size (∼2mm) in the as cast state. Minor addition of B (about 0.1wt.%) to it refines the grain size significantly as well as produces in-situ TiB needles. The role played by these microstructural modifications on high temperature deformation processing maps of B-modified Ti64 alloys is examined in this paper. Power dissipation efficiency and instability maps have been generated within the temperature range of 750-1000°C and strain rate range of 10-3-10+1s-1. Various deformation mechanisms, which operate in different temperature-strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate four distinct deformation domains within the range of experimental conditions examined, with the combination of 900-1000°C and 10-3-10-2s-1 being the optimum for hot working. In that zone, dynamic globularization of α laths is the principle deformation mechanism. The marked reduction in the prior β grain size, achieved with the addition of B, does not appear to alter this domain markedly. The other domains, with negative values of instability parameter, show undesirable microstructural features such as extensive kinking/bending of α laths and breaking of β laths for Ti64-0.0B as well as generation of voids and cracks in the matrix and TiB needles in the B-modified alloys. © 2010 Elsevier B.V.

Journal	Data powered by TypesetMaterials Science and Engineering A
Publisher	Data powered by TypesetElsevier Ltd
ISSN	09215093
Open Access	No