Vacancy release upon heating of an ultrafine grain Al-Zr alloy: In-situ observations and theoretical modeling

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. [Display omitted] •It is demonstrated that grain structure of a severely deformed Al-alloy store large amounts of vacancies.•Vacancy release by the rearrangement of near equilibrium grain boundaries is followed by in-situ annealing in STEM.•The kinetics of pore formation and dissolution has been followed by in-situ STEM observations.•Theoretical models are provided that reproduce the formation and dissolution of pores formed by vacancy condensation.•A proportion of stored vacancies may be responsible for faster precipitation kinetics in severely deformed alloys.