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Brittle, ductile and brittle-ductile material removal modes are artificially categorized during the machining processes according to the surface topography, subsurface damage and chip morphology. The material removal mode changes with the change of manufacturing conditions. The brittle-ductile transition (BDT) widely exists in the manufacturing with extremely small deformation scale, thermally assisted machining, and high speed machining. A comprehensive understanding of the BDT mechanisms is significant to improve the integrity of the machined surface, machining efficiency and prolong product life. Joint PhD student Tao Zhang from Huaqiao University and University of Wollongong，Prof. Feng Jiang, Hui Huang, Jing Lu, Yueqin Wu, Xipeng Xu from Huaqiao University, China and Prof. Zhengyi Jiang from University of Wollongong, Australia, wrote a review " Towards understanding the brittle-ductile transition in the extreme manufacturing" on IJEM. In this article, the deformation scale and deformation temperature induced brittle-to-ductile transition, and the reverse transition induced by grain size and strain rate were reviewed, as shown in Figure 1. The nature of BDT was analyzed. And the further improvement of machinability and machining quality based on the BDT were discussed.
Figure 1 The BDT involving the extreme manufacturing
2. Recent Advances
In this review, the BDT involving the manufacturing with extremely small deformation scale, thermally assisted machining, and high speed machining were presented. Then, the BDT mechanisms were discussed including the deformation scale and deformation temperature induced brittle-to-ductile transition, and the reverse transition induced by grain size and strain rate.
Deformation scale dependence of brittle-to-ductile transition
Some ionic and covalent bond materials have higher bond energy, which makes plastic deformation more difficult. These materials usually show obvious brittle failure characteristics at conventional deformation scales. The decline or absence of the natural defect population because of the decreased deformation scale increases the strength of these materials, which enables the plastic deformation mechanisms could be activated. At the same time, as the deformation scale decreases, the stress intensity factor will be smaller than the fracture toughness, which also induces BDT.
Deformation temperature dependence of brittle-to-ductile transition
Covalent bond and ionic bond materials that exhibit brittle fracture failure at room temperature will exhibit obvious plastic behavior when the deformation temperature increases. The BDT induced by the increase in deformation temperature is related to the thermal activation. The increase of deformation temperature intensifies the vibration of the atoms, and it will become easier for the atoms in the deformed region to overcome the energy barrier to slip. With the increase of stress, plastic deformation will occur before brittle fracture.
Grain size dependence of ductile-to-brittle transition
White layer is a common phenomenon in the manufacturing processes, especially for high-speed machining. The obvious grain refinement in the white layer region has been found, and the grain size is usually greater than 100 nm. Generally, as the degree of grain refinement increases, the yield strength increases, but the failure strain decreases, that is, as the degree of grain refinement increases, the material becomes more brittle. Grain refinement is an important reason for the BDT during high-speed machining.
Strain rate dependence of ductile-to-brittle transition
The stress wave theory provides an explanation for the BDT induced by the increase of strain rate. Due to the existence of the unloading wave, the material subjecting to impact loading will show obvious deformation localization. When the impact velocity is higher than the plastic wave velocity, the pseudo-brittle phenomenon will occur. The unloading wave usually has two forms, including the elastic wave induced by the unloading of impact load and the reflected elastic wave.
Figure 2 The effect of unloading waves on the deformation distribution of (a) the elastic wave induced by reflection and (b) the elastic wave induced by the unloading of impact
Dislocation kinematics provides another perspective to explain the brittle-to-ductile transition when materials subject to impact. The velocity of the dislocation relative to the loading tends to zero with the increase of loading velocity. Therefore, the dislocation seems to be fixed when the material is in the high enough strain rate, which leads to the ductile-to-brittle transition.
Finally, it is proposed that the nature of the mutual transition between brittleness and ductility is the competition between the ease of the occurrence of plastic deformation and propagation of crack. It can be represented by the Ouroboros pattern shown in Figure 3. The pattern is inspired by the "snake" of the Chinese zodiac stamp issued in 1989，which means that brittleness and plasticity both reflect the intrinsic properties of the material and that they can transform into each other under certain conditions. The brittleness or ductility of machined material should benefit a specific manufacturing process, which could be regulated by the deformation scale, deformation temperature and machining speed.
Figure 3 The mutual transition between brittleness and ductility
The BDT could be governed by coupled factors like the thermal-mechanical coupling because most of manufacturing processes involve coupled multi-factors. However, the majority of studies aim to analyze the BDT single factor, even trying to avoid multiple-factors through the design of experiments or numerical simulations, which leads to the existence of a gap between the BDT mechanisms and its application in the manufacturing processes.
Moreover, the extreme manufacturing processes involving the high-temperature, high-speed and energy fields makes the study of BDT face more challenges. Traditional quasi-static micro or nano tests could not simulate these extreme conditions. New test technologies should be developed to promote the study of BDT in high-speed and high temperature conditions and in energy fields. Specifically, the study of BDT in energy fields such as chemical field, magnetic field etc. should be paid more attention since it might be highly beneficial to the development of energy assisted manufacturing technologies.
5. About the Authors
Xipeng Xu is a full professor at Huaqiao University (HQU), where he is also the secretary of Huaqiao University Committee of CPC, Dean of Institute of Manufacturing Engineering, HQU, Director of the National & Local Joint Engineering Research Center for Intelligent Manufacturing Technology of Brittle Material Products, Director of the Engineering Research Center of Brittle Materials Machining, MOE. He is also the recipient of many prestigious national awards, including the Second Prize of China National Science and Technology Progress Award, the First Prize of Natural Science Award of the Chinese Ministry of Education, the First Prize of Science and Technology Progress Award of the Chinese Ministry of Education, the First Prize Science and Technology Award of Fujian Province and the first prize of Technical Invention of Fujian Province. He has served as the chairman of International Committee of Abrasive Technology (ICAT), director of China Abrasive Technology Committee, Chairman of Mechanical Engineering Society of Fujian Province, Associate editor of Intl J. Abrasive Tech., and the deputy director of editorial committee of "Chinese Journal of Mechanical Engineering" and "Diamond and Abrasives Engineering".
Professor Zhengyi Jiang is currently a Senior Professor in the School of Mechanical, Materials, Mechatronic and Biomedical Engineering (MMMB) at University of Wollongong (UOW), NSW Australia. He received his BE (1987), ME (1990) and PhD (1996) degrees from the Northeastern University, China. He has over 580 publications (more than 420 journal articles), more than 130 refereed conference papers and 3 monographs in the area of advanced metal manufacturing. He has been awarded over 37 prizes and awards from Australia, Japan and China, including ARC Future Fellowship (FT3), Australian Research Fellowship (twice), Endeavour Australia Cheung Kong Research Fellowship and Japan Society for the Promotion of Science (JSPS) Invitation Fellowship. He is currently leading a highly motivated research team at UOW on rolling mechanics, advanced micromanufacturing, computational mechanics and multi-scale simulation in metal manufacturing. He also has extensive experience in managing large research projects where he is project leader. He was deputy director of the State Key Laboratory of Rolling Technology and Automation (1996-1998), the only State Key Laboratory in rolling and automation area in China, and has accumulated broad knowledge and extensive interdisciplinary experience through his work in Australia, Japan and China.
Hui Huang is a full professor at (HQU)，where he is also the dean of College of Mechanical Engineering and Automation, HQU, associate dean of Institute of Manufacturing Engineering, HQU. Hui received his Ph.D. degree from Nanjing University of Aeronautics and Astronautics in 2002. In 2012, he was funded by the China Scholarship Council for a half-year visiting study at the University of Queensland, Australia. He has served as the member of International Committee for Abrasive Technology (ICAT), senior member of Chinese Mechanical Engineering Society, secretary general of Mechanical Engineering Society of Fujian Province，editorial board members of "Diamond and Abrasive Engineering" and "Superhard Materials Engineering". Hui’s research focuses on brittle material processing and superhard material tools. He is also the recipient of many national awards, including the Second Prize of China National Science and Technology Progress Award. He presided over more than 10 various scientific research projects including 3 National Natural Science Foundation of China, and published more than 100 academic papers, of which more than 40 were indexed by SCI and EI.
Feng Jiang is a full professor and PhD tutor at (HQU)，Feng received his Ph.D. degree from Shandong University in 2009. Feng studied for one year at the Materials Processing Center of Worcester Polytechnic Institute in the United States as a joint Ph.D. from 2007 to 2008, and worked as a postdoctoral researcher in the Department of Precision Instruments and Mechanics, Tsinghua University, from 2010 to 2012. His research focuses on precision ultra-precision machining technologies and material removal mechanisms, numerical simulation of cutting processes and tool design technologies. He has served as the editor of "Modern Manufacturing Engineering" magazine, technical consultant of Xiamen Tungsten Industry Co., Ltd., technical consultant of Xiamen Jinlu Special Alloy Co., Ltd. Feng presided over 1 general project of the National Natural Science Foundation of China, 1 youth fund of National Natural Science Foundation of China, 1 postdoctoral science fund special funded project, 1 postdoctoral science fund general project, 2 national key laboratory open fund projects. He won 1 Second Prize of Shandong Science and Technology Progress Award. He has published more than 50 papers as the first author or corresponding author, of which 30 are included in SCI and 1 academic book. He also been authorized 65 Chinese invention patents.