Effects of ultrafine nanograins on the fracture toughness of nanocrystalline materials
- PDF / 349,798 Bytes
- 8 Pages / 584.957 x 782.986 pts Page_size
- 34 Downloads / 182 Views
anqiu Zhoua) Department of Mechanical Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, People’s Republic of China; and Department of Mechanical Engineering, Wuhan Institute of Technology, Wuhan 430070, Hubei, People’s Republic of China
Tongde Shen High-Tech Research Institute & State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, People’s Republic of China
David Hui Department of Mechanical Engineering, University of New Orleans, New Orleans, Louisiana 70148 (Received 22 March 2011; accepted 31 May 2011)
For evaluating the effects of ultrafine nanograins (UFNGs) on the fracture toughness of conventional nanocrystalline (nc) materials, we developed a composite model composed of UFNGs (with a grain size d between 2 and 4 nm) evenly distributed in the conventional nc matrix (20 nm # d # 100 nm). The UFNGs could be treated as a part of triple junctions, denoted as super triple junctions. In the framework of our model, stress concentration near crack tip initiates intergrain sliding that leads to the generation of edge dislocations at super triple junctions. The dependence of critical crack intensity factors on grain size was calculated. It was demonstrated that the existence of the UFNGs approximately doubles the critical crack intensity factors. I. INTRODUCTION
Nanocrystalline (nc) materials have been a source of great interest currently because of their unusual mechanical and physical properties.1–8 In general, nc materials show superior strength but low tensile ductility and low fracture toughness, which limit their applications.1,3,4,6,9,10 However, the evidence of tensile superplasticity in nc materials has been studied and reported.11–20 Understanding the fundamentals of superplasticity in the nc materials is of great importance to develop new applications. So far, many models have been developed to explain this phenomenon.3,10,21–29 Most of them attributed the superplasticity to the alternative deformation modes such as lattice dislocation slip, intergrain sliding, Coble creep, triple junction diffusional creep, rotational deformation, and nanoscale twin deformation effectively operating in nc materials. One of the several strategies that have been suggested for enhancing ductility in nc materials is to develop a bimodal grain size distribution in which fine grains can provide high strength, whereas coarse grains can enable strain hardening to enhance ductility.30–32 Here, we study the mechanical behavior of an nc material a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.206 1734
J. Mater. Res., Vol. 26, No. 14, Jul 28, 2011
http://journals.cambridge.org
Downloaded: 22 Oct 2015
composed of an exactly opposite inhomogeneous microstructure where ultrafine nanograins (UFNGs) (2–4 nm), instead of micrometer-sized coarse grains, are embedded inside a matrix of nc grains (20–100 nm). The number of grains with various diameters in nc materials can usually be represented by a log
Data Loading...
