The present work provides a first experimental assessment of the product of thermodynamic factor (Φ) and the cross correlation term (S) for high entropy alloys. Following a pseudo-binary approach, the concentration-dependent interdiffusion coefficients are determined for CoCrFeNi and CoCrFeMnNi high-entropy alloys at 1423 K using four different diffusion couples. Combining the data with the measured previously tracer diffusion coefficients, the value of ΦS for the equiatomic compositions is found to be close to unity for CoCrFeNi while a value of 3 to 5 is obtained for CoCrFeMnNi indicating deviations from the ideal solid solution. The interdiffusion coefficients are higher for CoCrFeMnNi substantiating the fact that an increased configurational entropy need not result in decelerated rates of the atomic transport. © 2018 Acta Materialia Inc.

Budaraju Srinivasa Murty

Department of Metallurgical and Materials Engineering

Mayur Vaidya

G. Mohan Muralikrishna

Cobalt alloys

Diffusion

entropy

Interdiffusion (solids)

Iron alloys

Manganese alloys

Ternary systems

Concentration-dependent

CONFIGURATIONAL ENTROPY

Experimental assessment

HIGH ENTROPY ALLOYS

INTER-DIFFUSION COEFFICIENTS

PSEUDO-BINARIES

Thermodynamic factors

TRACER DIFFUSION COEFFICIENTS

Chromium alloys

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

Scripta Materialia

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

High entropy alloys (HEAs) have emerged as a new class of multicomponent materials, which have potential for high temperature applications. Phase stability and creep deformation, two key selection criteria for high temperature materials, are predominantly influenced by the diffusion of constituent elements along the grain boundaries (GBs). For the first time, GB diffusion of Ni in chemically homogeneous CoCrFeNi and CoCrFeMnNi HEAs is measured by radiotracer analysis using the 63Ni isotope. Atom probe tomography confirmed the absence of elemental segregation at GBs that allowed reliable estimation of the GB width to be about 0.5 nm. Our GB diffusion measurements prove that a mere increase in number of constituent elements does not lower the diffusion rates in HEAs, but the nature of added constituents plays a more decisive role. The GB energies in both HEAs are estimated at about 0.8-0.9 J/m2, they are found to increase significantly with temperature and the effect is more pronounced for the CoCrFeMnNi alloy. © 2017 The Author(s).

Fulltext

Scientific Reports

Radioactive isotopes reveal a non sluggish kinetics of grain boundary diffusion in high entropy alloys

High entropy alloys (HEAs) are multicomponent alloys in equiatomic or nearly equiatomic composition. Anticipated sluggish atomic diffusion is reported to be one of the core effects in HEAs which is presumably responsible for their many unique properties. For the first time, in the present study, tracer (Ni) diffusion in CoCrFeNi and CoCrFeMnNi alloys is measured by the radiotracer technique in the temperature range of 1073–1373 K using the 63 Ni isotope. Chemically homogeneous alloys of equiatomic composition were prepared by a vacuum arc melting route. The microstructure and phase stability of the alloys in the given temperature range is confirmed by differential thermal analysis and X-ray diffraction. Ni diffusion in both CoCrFeNi and CoCrFeMnNi alloys is found to follow Arrhenius behavior. When plotted against the homologous temperature, a tendency to a successive slow down of the tracer diffusion rate with an increased number of components in equiatomic alloys is unambiguously established. Both the entropy term as well as the energy barriers is revealed to contribute to this trend. The current results indicate that diffusion in HEAs cannot a priori be considered as sluggish. © 2016 Elsevier B.V.

Journal of Alloys and Compounds

Ni tracer diffusion in CoCrFeNi and CoCrFeMnNi high entropy alloys

Tracer diffusion in single crystalline CoCrFeNi and CoCrFeMnNi high entropy alloys

Diffusion Foundations

A Mystery of "Sluggish Diffusion" in High-Entropy Alloys: The Truth or a Myth?

Materials Chemistry and Physics

Analysis of diffusion in high entropy alloys

Experimental assessment of the thermodynamic factor for diffusion in CoCrFeNi and CoCrFeMnNi high entropy alloys

Journal	Scripta Materialia
Publisher	Acta Materialia Inc
ISSN	13596462
Open Access	No