Open AccessInfluence of rapid annealing on the evolution of fractal abnormal grains in nanocrystalline Pd–10 at% Au
Author(s) -
Raphael Zeller,
H. J. Fey,
Christian Braun,
R. Birringer,
Carl E. Krill
Publication year - 2019
Publication title -
iop conference series. materials science and engineering
Language(s) - English
Resource type - Journals
eISSN - 1757-899X
pISSN - 1757-8981
DOI - 10.1088/1757-899x/580/1/012055
Subject(s) - nanocrystalline material , nucleation , materials science , grain growth , grain boundary , annealing (glass) , fractal , grain size , fractal dimension , crystallography , dwell time , chemical physics , metallurgy , microstructure , nanotechnology , thermodynamics , chemistry , mathematics , physics , mathematical analysis , medicine , clinical psychology
During abnormal grain growth, certain grains grow to much larger sizes than their neighbors. Usually, such abnormal grains expand in all directions at roughly the same speed, but in nanocrystalline Pd-10 at% Au, they appear to send forth dendritic offshoots into the surrounding matrix, resulting in highly irregular, fractal-like grain shapes. Exploiting the fast heating capabilities of a salt-bath oven, we have investigated the influence of heating rate on the evolution of fractal grain morphologies in nanocrystalline Pd-10 at% Au. In this material, the nanocrystalline matrix is stable at 170°C, but fractal grains appear within minutes at 200°C and 270°C, followed by fast growth until impingement. Surprisingly, the higher dwell temperature leads to a smaller average grain size in the fully impinged state. For the latter, no additional grain growth is observed at longer dwell times, but the fractal dimension of grain boundaries decreases with time at 270°C. These findings are consistent with abnormal grain formation being governed by a temperature-dependent nucleation process, upon which is superimposed the usual capillary driving force for grain growth; however, the mechanism by which boundaries migrate through the nanocrystalline matrix while retaining fractality remains unclear.