Abstract
The attractive strain burst phenomenon, so-called “pop-in”, during indentation-induced deformation at a very small scale is discussed as a fundamental deformation behavior in various materials. The nanoindentation technique can probe a mechanical response to a very low applied load, and the behavior can be mechanically and physically analyzed. The pop-in phenomenon can be understood as incipient plasticity under an indentation load, and dislocation nucleation at a small volume is a major mechanism for the event. Experimental and computational studies of the pop-in phenomenon are reviewed in terms of pioneering discovery, experimental clarification, physical modeling in the thermally activated process, crystal plasticity, effects of pre-existing lattice defects including dislocations, in-solution alloying elements, and grain boundaries, as well as atomistic modeling in computational simulation. The related non-dislocation behaviors are also discussed in a shear transformation zone in bulk metallic glass materials and phase transformation in semiconductors and metals. A future perspective from both engineering and scientific views is finally provided for further interpretation of the mechanical behaviors of materials.
Highlights
Materials 2021, 14, 1879. https://Mechanical property testing by indentation-induced deformation has a long history predating 1900
Several papers demonstrated that the segregation of C atoms at the grain boundary increased the critical stress of the grain-boundary pop-in [138,140,142]
Schuh and Nieh [194] performed nanoindentation measurements on bulk metallic glass (BMG) to analyze the fundamental deformation mechanism. They demonstrated that the pop-in phenomenon in BMG corresponded to the formation of a shear band, and the event probability depended on the kinetics of the shearband formation
Summary
Mechanical property testing by indentation-induced deformation has a long history predating 1900. Indentation techniques have been used on various materials, metallic ones, and on relatively brittle materials including ceramics, semiconductors, and intermetallic compounds. Another advantage of the indentation technique is the minimization of the tested area of a material to probe each microstructural component and separate an individual contribution to the mechanical properties for further interpretation of the strengthening factors and mechanisms. Mechanical characterization at the same microstructural scale is critical to improve the guiding principles of material design to obtain better-performing materials. The advanced technique enables us to obtain the statistical, and dynamic behavior of materials, and has other approaches including crystal plasticity with dislocation theory and quantum modeling. This paper reviews the experimental and computational approaches to the pop-in phenomenon and discusses the fundamental mechanical behavior mechanisms of various materials for a deeper understanding of the strengthening factors of the macroscopic properties
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