Forming Nanocrystalline Structures in Metal Particle Impact
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IT is well known that a number of mechanical properties in a polycrystalline metal are improved when its microstructure is refined. Most notably, fine grained metals are usually much stronger than conventional coarse grained ones. The way to ultrafine grain structures in metals and alloys with sufficiently large stacking-fault energy may be via severe plastic deformation, which results in the formation of dislocation cell structure and its transformation to a refined grain structure. A number of surface treatment techniques are based on this concept. For instance, in shot peening, the surface structure is modified through a series of impacts by steel balls imparted on a sample. In some cases, it has been shown that nanocrystalline layers (with an average grain size less than 100 nm) could be obtained at the interface between a particle and the substrate.[1,2] To enhance our ability to fabricate homogeneous fine VINCENT LEMIALE, Research Scientist, is with the CSIRO Division of Process Science and Engineering, Clayton, Victoria 3169, Australia, and the School of Mathematical Sciences, Monash University, Clayton, Victoria, 3800, Australia. Contact e-mail: [email protected] YURI ESTRIN, CSIRO Professorial Fellow, is with the CSIRO Division of Process Science and Engineering, and the ARC Centre for Design in Light Metals, Department of Materials Engineering, Monash University, Clayton, Victoria, 3800, Australia. HYOUNG SEOP KIM, Associate Professor, is with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyungbuk, 790-784, Korea. ROBERT O’DONNELL, Research Group Leader, is with the CSIRO Division of Process Science and Engineering. Manuscript submitted September 6, 2010. Article published online December 22, 2010 3006—VOLUME 42A, OCTOBER 2011
grained layers, it is therefore essential to gain a better understanding of their formation under individual particle impacts. In this context, the use of numerical techniques such as finite element (FE) analysis may provide an insight into the details of mechanical behavior under high speed deformation, which in many cases cannot be captured experimentally. This, however, is only possible if an adequate constitutive description based on microstructure evolution is used in an FE simulation. A number of numerical models of particle impact were reported in the literature. Simulations based on these models were able to reproduce the observed deformation pattern for a wide range of initial velocity conditions.[3–5] However, most previous studies were focused on reproducing the material behavior at a macroscopic level, without accounting for the evolution of the underlying microstructure. Therefore, it was not possible with these models to estimate the characteristic length scale of the microstructure produced. Recently, a dislocation-based model has been successfully applied to dynamic Taylor impact tests.[6] The model, originally developed in the context of severe plastic deformation processes operated at relatively low to
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