Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals.
When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. Here we demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. This "cyclic healing" of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising from increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. These results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen.
Center for Advancing Materials Performance from the Nanoscale and Hysitron Applied Research Center in China, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China;
Zhang-Jie
ZJ
Wang
Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218;
Qing-Jie
QJ
Li
Applied Mechanics Laboratory, School of Aerospace, Tsinghua University, Beijing 100084, China;
Yi-Nan
YN
Cui
Applied Mechanics Laboratory, School of Aerospace, Tsinghua University, Beijing 100084, China;
Zhan-Li
ZL
Liu
Center for Advancing Materials Performance from the Nanoscale and Hysitron Applied Research Center in China, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China; Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218;
Evan
E
Ma
Center for Advancing Materials Performance from the Nanoscale and Hysitron Applied Research Center in China, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China; Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
Ju
J
Li
Center for Advancing Materials Performance from the Nanoscale and Hysitron Applied Research Center in China, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China;
Jun
J
Sun
Applied Mechanics Laboratory, School of Aerospace, Tsinghua University, Beijing 100084, China;
Zhuo
Z
Zhuang
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; mingdao@mit.edu zwshan@mail.xjtu.edu.cn suresh@cmu.edu.
Ming
M
Dao
Center for Advancing Materials Performance from the Nanoscale and Hysitron Applied Research Center in China, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China; mingdao@mit.edu zwshan@mail.xjtu.edu.cn suresh@cmu.edu.
Zhi-Wei
ZW
Shan
Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 mingdao@mit.edu zwshan@mail.xjtu.edu.cn suresh@cmu.edu.
Subra
S
Suresh
cyclic mechanical healing; dislocation motion; fatigue; pristine materials; yield strength