The present study deals with phase evolution of oxide dispersed AlCoCrFe high entropy alloy during mechanical alloying and spark plasma sintering. Mechanical alloying of AlCoCrFe resulted in a single BCC phase. However, ordering of BCC phase with evolution of chromium carbide and sigma phase were observed after spark plasma sintering. High hardness of 1,050 ± 20 HV1 and 1,070 ± 20 HV1 was observed for AlCoCrFe high entropy alloy without and with oxide dispersion, respectively. Significant contribution from solid solution strengthening effect in high entropy alloys appears to have overwhelmed the effect of oxide dispersion on hardness. © 2013 Indian Institute of Metals.

Budaraju Srinivasa Murty

Department of Metallurgical and Materials Engineering

Ravi Sankar Kottada

Ameey Anupam

Teja Sirasani

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

Transactions of the Indian Institute of Metals

Mechanical alloying (MA) followed by sintering has been one of the most widely adopted routes to produce nanocrystalline high-entropy alloys (HEAs). Enhanced solid solubility, room temperature processing, and homogenous alloy formation are the key benefits provided by MA. Spark plasma sintering has largely been used to obtain high-density HEA pellets from milled powders. However, there are many challenges associated with the production of HEAs using MA, which include contamination during milling and high propensity of oxidation. The present review provides a comprehensive understanding of various HEAs produced by MA so far, with the aim to bring out the governing aspects of phase evolution, thermal stability, and properties achieved. The limitations and challenges of the process are also critically assessed with a possible way forward. The paper also compares the results obtained from high-pressure torsion, another severe plastic deformation technique. Copyright © Materials Research Society 2019.

Journal of Materials Research

High-entropy alloys by mechanical alloying: A review

Nanocrystalline CoCrFeNi and CoCrFeMnNi high entropy alloys have been processed by mechanical alloying followed by spark plasma sintering. Grain growth kinetics has been estimated for both the alloys by subjecting them to heat treatment in the temperature range 1073–1373 K. These alloys possess a thermally stable single phase FCC structure along with Cr7C3 contamination. Electron back scattered diffraction (EBSD) has been used to determine grain size of all the heat treated samples. Both CoCrFeNi and CoCrFeMnNi alloys exhibit a grain growth exponent, n = 3, suggesting long-range diffusion-controlled grain growth in these alloys. Activation energies for grain growth are 134 and 197 kJ/mol for CoCrFeNi and CoCrFeMnNi, respectively, which are significantly lower than the activation energy of lattice diffusion in these alloys. Hardness is measured for CoCrFeMnNi alloy as function of grain size and is found to follow the Hall-Petch type relation. The strength coefficient (slope of Hall-Petch relation) is calculated as 1.92 GPa, which is nearly three times that of the value reported in literature for coarse grained CoCrFeMnNi. Presence of carbides enhances the hardness of these HEAs. The maximum contribution to strengthening comes from the FCC-carbide phase boundaries. © 2019 Elsevier B.V.

Journal of Alloys and Compounds

Grain growth kinetics in CoCrFeNi and CoCrFeMnNi high entropy alloys processed by spark plasma sintering

Nano crystalline High Entropy Alloys (HEA) with equi-atomic compositions were synthesized through mechanical alloying followed by Spark Plasma Sintering (SPS). Alloys with nominal compositions of TiAlNiCo, TiAlNiFe and TiAlNiCoFe were selected for the study. Microstructure, hardness and room temperature compressive strengths of the alloys were studied. All the alloys had a single phase BCC microstructure along with TiCx nanoparticles dispersed uniformly in it. The alloys exhibit good hardness of more than 700 HV1.0 and a compressive strength more than 2.5 GPa. © 2019 Elsevier B.V.

Microstructure and mechanical properties of Ti-Al-Ni-Co-Fe based high entropy alloys prepared by powder metallurgy route

Thermal stability of CoCrFeNi high entropy alloy in as-milled and sintered conditions was investigated using X-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and atom probe tomography. Composite microstructure consists of FCC and carbide with a fine dispersion of oxide was observed in the sintered condition. Unsolicited contamination of carbon and oxygen in the as-milled powder due to the milling medium had led to the formation of composite microstructure. An exceptional thermal stability was observed upon exposure of sintered compact to higher temperatures (0.56 Tm to 0.68 Tm) for the prolonged duration of 600 h. Sintered compact exposed to 700 °C (0.56 Tm) for 600 h showed negligible change in hardness and grain size. Analysis based on the modified Hall-Petch model for two phase alloy indicates the phase boundaries act as a strong obstacle while the major contribution to strengthening comes from grain boundaries. © 2017 Elsevier Ltd

Materials and Design

Thermal stability and grain boundary strengthening in ultrafine-grained CoCrFeNi high entropy alloy composite

High entropy alloys (HEAs) represent a new class of materials that present novel phase structures and properties. Apart from bulk material consolidation methods such as casting and sintering, HEAs can also be deposited as a surface coating. In this work, thermal sprayed HEA coatings are investigated that may be used as an alternative bond coat material for a thermal barrier coating system. Nanostructured HEAs that were based on AlCoCrFeNi and MnCoCrFeNi were prepared by ball milling and then plasma sprayed. Splat studies were assessed to optimise the appropriate thermal spray parameters and spray deposits were prepared. After mechanical alloying, aluminum-based and manganese-based HEA powders revealed contrary prominences of BCC and FCC phases in their X-ray diffraction patterns. However, FCC phase was observed as the major phase present in both of the plasma-sprayed AlCoCrFeNi and MnCoCrFeNi coatings. There were also minor oxide peaks detected, which can be attributed to the high temperature processing. The measured porosity levels for AlCoCrFeNi and MnCoCrFeNi coatings were 9.5 ± 2.3 and 7.4 ± 1.3 pct, respectively. Three distinct phase contrasts, dark gray, light gray and white, were observed in the SEM images, with the white regions corresponding to retained multicomponent HEAs. The Vickers hardness (HV0.3kgf) was 4.13 ± 0.43 and 4.42 ± 0.60 GPa for AlCoCrFeNi and MnCoCrFeNi, respectively. Both type of HEAs coatings exhibited anisotropic mechanical behavior due to their lamellar, composite-type microstructure. © 2014, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Plasma-Sprayed High Entropy Alloys: Microstructure and Properties of AlCoCrFeNi and MnCoCrFeNi

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

Multi-component high entropy alloys (HEAs) are observed to form simple solid solutions in contrary to general perception that complex compounds may form in such multi-component equi-atomic alloys. In the present study, alloying behavior was investigated using XRD in AlCoCrCuFe and NiCoCrCuFe equi-atomic high entropy alloys synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). Simple FCC and BCC phases evolved after MA, while Cu-rich FCC and sigma (σ) phases evolved along with FCC and BCC phases after SPS. Further, NiCoCuFe, NiCoCrFe and NiCoFe equi-atomic alloys were investigated to confirm the formation of Cu-rich FCC, and σ phases. The hardness was observed to be 770 ± 10. HV for AlCoCrCuFe and 400 ± 10. HV for NiCoCrCuFe. Phase evolution after MA and SPS indicate that configurational entropy is not sufficient enough to suppress the formation of Cu-rich FCC, and σ phases, and enthalpy of mixing appears to play an important role in determining the phase formation in high entropy alloys after sintering. © 2011 Elsevier B.V.

Materials Science and Engineering A

Alloying behavior in multi-component AlCoCrCuFe and NiCoCrCuFe high entropy alloys

The microstructure, hot hardness, and indentation creep of ultrafine grained (ufg) 9Cr-1Mo and 9Cr-1W based ferritic oxide dispersion strengthened (ODS) alloys prepared by mechanical alloying and spark plasma sintering are investigated. Studies on hot hardness revealed that the hardness of Mo-containing alloys is higher than that of W-containing alloys. This is attributed to the complete dissolution of Mo while W did not dissolve completely during mechanical alloying. Hardness of Ti containing alloys is higher compared to the base and Y 2O 3 containing alloys and this is attributed to the presence of Ti-Y-O based nanoprecipitates. Study of hot hardness behaviour of ufg ferritic ODS alloys revealed a sharp transition at around 700-800K. This is possibly due to the enhanced grain boundary mediated plasticity and pronounced reduction in Hall-Petch coefficient with increase in temperature. © 2012 Elsevier B.V.

Hot hardness behaviour of ultrafine grained ferritic oxide dispersion strengthened alloys prepared by mechanical alloying and spark plasma sintering

In the present paper, microstructural characterization and compression creep properties of nanostructured ferritic oxide dispersion strengthened alloys (Fe-12Cr-2W-0.5Y2O3 (in wt.%)) synthesized by mechanical alloying and vacuum hot pressing are presented. In particular, the effect of starting yttria particle size (20nm and 45μm before mechanical alloying) on microstructure and creep properties was investigated. Study of compression creep properties (at 800°C and 900°C) revealed that the ODS alloy with finer dispersoids showed higher creep resistance. Grain growth was not very significant following creep testing at 800°C (from ∼300nm in hot pressed condition to ∼550nm following creep at 800°C). Analysis of stress exponents indicates that dislocation creep is the dominant deformation mechanism at 800°C and 900°C. © 2011 Elsevier B.V.

Effect of yttria particle size on the microstructure and compression creep properties of nanostructured oxide dispersion strengthened ferritic (Fe-12Cr-2W-0.5Y2O3) alloy

The present study is aimed to investigate the consolidation behaviour and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying. The consolidation was achieved by cold pressing with conventional sintering, vacuum hot pressing and hot isostatic pressing techniques. The microstructure and mechanical properties were evaluated. The hardness and compressive strength of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after vacuum hot pressing are 9.50 and 2.19 GPa and those after hot isostatic pressing are 10.04 and 2.83 GPa, respectively. The wear resistance is found to be higher than the commercially used materials such as Ni-hard faced alloy. © 2010 Springer Science+Business Media, LLC.

Journal	Transactions of the Indian Institute of Metals
ISSN	09722815
Open Access	No