TY - JOUR

T1 - Vacancy release upon heating of an ultrafine grain Al-Zr alloy

T2 - In-situ observations and theoretical modeling

AU - Lefebvre, W.

AU - Skiba, N. V.

AU - Chabanais, F.

AU - Gutkin, M. Yu

AU - Rigutti, L.

AU - Murashkin, M. Yu

AU - Orlova, T. S.

N1 - Funding Information: In situ STEM experiments have been performed on the GENESIS platform supported by the R?gion Normandie, the M?tropole Rouen Normandie, the CNRS via LABEX EMC3 and the French National Research Agency as a part of the program ?Investissements d?avenir? with the reference ANR-11-EQPX-0020. N.V.S. and M.Yu.G. are thankful to the Institute for Problems in Mechanical Engineering of the Russian Academy of Sciences, and T.S.O. is thankful to the Ioffe Institute of the Russian Academy of Sciences for their support (within the corresponding state tasks) in development of the theoretical models figuring in the paper. Publisher Copyright: © 2021 Elsevier B.V. Copyright: Copyright 2021 Elsevier B.V., All rights reserved.

PY - 2021/5/5

Y1 - 2021/5/5

N2 - The present study demonstrates the direct observation of pore formation by rapid release of vacancies in ultrafine grain (UFG) Al-0.4at%Zr alloy during in-situ annealing in a scanning transmission electron microscope. The ultrafine grain structure was preliminary obtained by high pressure torsion processing at a hydrostatic pressure of 6 GPa up to 10 revolutions. In-situ annealing reveals the rapid formation of pores in triple junctions (TJs) of grain boundaries (GBs) during the first few minutes followed by their subsequent slow resorption with the complete disappearance of some of them. During annealing, no noticeable displacement of GBs is observed. By considering the evolution of non-equilibrium GBs inherited by the severe plastic deformation, a theoretical description is suggested which describes: (i) the pore formation at disclinated TJs as a thermodynamically driven process of free volume dissolution through generation of vacancies, which then migrate to the TJs and coagulate at them with growth of the pores diminishing the strain energy of the TJ disclinations, and (ii) further decrease of the TJ disclination strain energy through the climb of extrinsic GB dislocations towards the disclinated TJs, accompanied with dissolution of the TJ pores by emission of vacancies which provide the dislocation climb. The rapid release of excess vacancies during the early stage of heat treatment is consequently identified as a phenomenon responsible for accelerated atomic mobility. This work hence provides a new perspective for understanding the accelerated precipitation kinetics observed in severely deformed alloys.

AB - The present study demonstrates the direct observation of pore formation by rapid release of vacancies in ultrafine grain (UFG) Al-0.4at%Zr alloy during in-situ annealing in a scanning transmission electron microscope. The ultrafine grain structure was preliminary obtained by high pressure torsion processing at a hydrostatic pressure of 6 GPa up to 10 revolutions. In-situ annealing reveals the rapid formation of pores in triple junctions (TJs) of grain boundaries (GBs) during the first few minutes followed by their subsequent slow resorption with the complete disappearance of some of them. During annealing, no noticeable displacement of GBs is observed. By considering the evolution of non-equilibrium GBs inherited by the severe plastic deformation, a theoretical description is suggested which describes: (i) the pore formation at disclinated TJs as a thermodynamically driven process of free volume dissolution through generation of vacancies, which then migrate to the TJs and coagulate at them with growth of the pores diminishing the strain energy of the TJ disclinations, and (ii) further decrease of the TJ disclination strain energy through the climb of extrinsic GB dislocations towards the disclinated TJs, accompanied with dissolution of the TJ pores by emission of vacancies which provide the dislocation climb. The rapid release of excess vacancies during the early stage of heat treatment is consequently identified as a phenomenon responsible for accelerated atomic mobility. This work hence provides a new perspective for understanding the accelerated precipitation kinetics observed in severely deformed alloys.

KW - Annealing

KW - Cavities

KW - Disclination

KW - Grain boundary

KW - In-situ

KW - Pore

KW - Severe plastic deformation

KW - UFG

KW - Vacancy

UR - http://www.scopus.com/inward/record.url?scp=85099478143&partnerID=8YFLogxK

UR - https://www.mendeley.com/catalogue/83587000-2f3b-3081-a594-70c70aa967ad/

U2 - 10.1016/j.jallcom.2020.158455

DO - 10.1016/j.jallcom.2020.158455

M3 - Article

AN - SCOPUS:85099478143

VL - 862

JO - Journal of Alloys and Compounds

JF - Journal of Alloys and Compounds

SN - 0925-8388

M1 - 158455

ER -