The effects of intergranular sliding on the fracture toughness of nanocrystalline materials with finest grains
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Jianqiu Zhoua) Department of Mechanical and Power Engineering, Nanjing Tech University, Nanjing, Jiangsu 210009, China; and Department of Mechanical & Electrical Engineering, Wuhan Institute of Technology, Wuhan, Hubei 430070, China
Hongxi Liu, Xuming Pang, Shu Zhang, Ying Wang, Lu Wang, and Shuhong Dong Department of Mechanical and Power Engineering, Nanjing Tech University, Nanjing, Jiangsu 210009, China (Received 18 October 2013; accepted 4 April 2014)
A new physical model of plastic deformation in nanocrystalline (NC) materials with finest grains (whose grain size is 2–4 nm) is suggested and theoretically described. The model represents the effect of the finest grains located at triple junctions on the fracture toughness of NC materials in the case that there are multiple dislocations pile-up at grain boundaries (GBs). The maximum number n of the pile-up dislocations is determined by both the capacity of dislocations emitting associated with the crack propagation and the capacity of dislocations pile-up due to the existence of the finest grains. The calculation indicates that the parameter n increases with increment of the grain size and decreases with the finest grain size increasing. The results theoretically reveal that the triple junctions with finest grains can significantly improve the fracture toughness of NC materials compared with the normal triple junctions in wide ranges of their structural parameters.
I. INTRODUCTION
Nanocrystalline (NC) materials have been studied with great interest by many researchers and applied in various fields because of their unique mechanical properties. In comparison with traditional coarse-grained materials (whose grain size is above several micrometers), NC materials usually have lots of outstanding performance, such as superior strength, strong hardness, and better wear resistance. Unfortunately, most of NC materials show low tensile ductility and fracture toughness, which limits their application. However, there are some NC materials with good tensile ductility that have been produced and reported. For instance, Youssef et al.1 reported that their nanostructured Cu sample had a uniform tensile elongation of 14% and an elongation-to-failure of 15.5%. Shan et al.2 observed experimentally that there are no porosities or intergranular microcracks in Ni sample with an average grain size of 9.7 6 3.9 nm after deposition. It is well known that the property of corresponding bulk materials is dependent on their microstructure, hence, understanding of the fundamentals of superplasticity, especially the deformation mechanisms during the loading process, is of great importance to extend the application of NC materials. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.89 1086
J. Mater. Res., Vol. 29, No. 9, May 14, 2014
http://journals.cambridge.org
Downloaded: 07 Apr 2015
In conventional coarse-grained metals, the lattice dislocation slip plays a dominant role during the deformation process. However, when the grain size decreases down t
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