Elastic-plastic-brittle transitions of potassium dihydrogen phosphate crystals: characterization by nanoindentation
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Elastic-plastic-brittle transitions of potassium dihydrogen phosphate crystals: characterization by nanoindentation Yong Zhang1 • Ning Hou2 • Liang-Chi Zhang3 • Qi Wang1
Received: 19 May 2020 / Revised: 7 June 2020 / Accepted: 23 July 2020 The Author(s) 2020
Abstract Potassium dihydrogen phosphate (KDP) crystals are widely used in laser ignition facilities as optical switching and frequency conversion components. These crystals are soft, brittle, and sensitive to external conditions (e.g., humidity, temperature, and applied stress). Hence, conventional characterization methods, such as transmission electron microscopy, cannot be used to study the mechanisms of material deformation. Nevertheless, understanding the mechanism of plastic-brittle transition in KDP crystals is important to prevent the fracture damage during the machining process. This study explores the plastic deformation and brittle fracture mechanisms of KDP crystals through nanoindentation experiments and theoretical calculations. The results show that dislocation nucleation and propagation are the main mechanisms of plastic deformation in KDP crystals, and dislocation pileup leads to brittle fracture during nanoindentation. Nanoindentation experiments using various indenters indicate that the external stress fields influence the plastic deformation of KDP crystals, and plastic deformation and brittle fracture are related to the material’s anisotropy. However, the
& Ning Hou [email protected] & Liang-Chi Zhang [email protected] 1
School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
2
School of Mechatronics Engineering, Shenyang Aerospace University, Shenyang 110136, People’s Republic of China
3
Laboratory for Precision and Nano Processing Technologies, School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
effect of loading rate on the KDP crystal deformation is practically negligible. The results of this research provide important information on reducing machining-induced damage and further improving the optical performance of KDP crystal components. Keywords Potassium dihydrogen phosphate (KDP) crystal Transition mechanism Plastic deformation Brittle fracture
1 Introduction Potassium dihydrogen phosphate (KDP) crystals are crucial to laser ignition facilities as optical switching and frequency conversion components for inertial confinement fusion [1]. To fabricate the KDP crystal component, the main technology employed is ultra-precision diamond fly cutting [2, 3]. However, machining-induced damage, such as crack and collapse, often emerges on the component surface/subsurface because of the brittleness of KDP crystals, significantly reducing the laser damage threshold of its components. To obtain a damage-free surface, KDP crystals should be machined in plastic mode. To date, however, few attempts have been made to understand the plastic-brittle transition behavior of KDP crystals. This limited understan
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