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Title: Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation

Abstract

An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to similar to 520 MV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of similar to 630 HV at about 450 degrees C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 MV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA.more » After times as short as 5 min at 450 degrees C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd.« less

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
OSTI Identifier:
1251844
Alternate Identifier(s):
OSTI ID: 1336587
Grant/Contract Number:  
AC05-00OR22725
Resource Type:
Journal Article: Published Article
Journal Name:
Acta Materialia
Additional Journal Information:
Journal Name: Acta Materialia Journal Volume: 96 Journal Issue: C; Journal ID: ISSN 1359-6454
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; High-entropy alloys; Severe plastic deformation; Compositionally complex alloys; 3 dimensional atom probe tomography; Microstructure; HIGH-PRESSURE TORSION; NANOSTRUCTURED MATERIALS; GRAIN-REFINEMENT; PHASE-STABILITY; ELASTIC-MODULI; STRENGTH; METALS; MN

Citation Formats

Schuh, B., Mendez-Martin, F., Völker, B., George, E. P., Clemens, H., Pippan, R., and Hohenwarter, A. Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation. United States: N. p., 2015. Web. doi:10.1016/j.actamat.2015.06.025.
Schuh, B., Mendez-Martin, F., Völker, B., George, E. P., Clemens, H., Pippan, R., & Hohenwarter, A. Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation. United States. https://doi.org/10.1016/j.actamat.2015.06.025
Schuh, B., Mendez-Martin, F., Völker, B., George, E. P., Clemens, H., Pippan, R., and Hohenwarter, A. 2015. "Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation". United States. https://doi.org/10.1016/j.actamat.2015.06.025.
@article{osti_1251844,
title = {Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation},
author = {Schuh, B. and Mendez-Martin, F. and Völker, B. and George, E. P. and Clemens, H. and Pippan, R. and Hohenwarter, A.},
abstractNote = {An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to similar to 520 MV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of similar to 630 HV at about 450 degrees C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 MV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450 degrees C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd.},
doi = {10.1016/j.actamat.2015.06.025},
url = {https://www.osti.gov/biblio/1251844}, journal = {Acta Materialia},
issn = {1359-6454},
number = C,
volume = 96,
place = {United States},
year = {Tue Sep 01 00:00:00 EDT 2015},
month = {Tue Sep 01 00:00:00 EDT 2015}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at https://doi.org/10.1016/j.actamat.2015.06.025

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Works referencing / citing this record:

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