Grain Size Distribution Effect on Mechanical Behavior of Nanocrystalline Materials
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Grain Size Distribution Effect on Mechanical Behavior of Nanocrystalline Materials A.V. Sergueeva, N.A. Mara, and A.K. Mukherjee Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA ABSTRACT Grain size distribution effect on the mechanical behavior of NiTi and Vitroperm alloys were investigated. Yielding at significantly lower stresses than found in equiaxed counterparts, along with well defined strain hardening was observed in these nanocrystalline materials with large grains embedded in the matrix during tensile deformation at temperatures of 0.4Tm. At higher temperature the effect of grain size distribution on yield stress was not revealed while plasticity was increased in 50% in NiTi alloy with bimodal grain size structure. INTRODUCTION Reducing the grain size in a polycrystalline material is known to have beneficial effects on its mechanical properties [1]. A material's strength is expected to increase in a manner described by, for example, the well-known Hall-Petch relationship. A concurrent rise in toughness is often expected for many engineering alloys after grain refinement. Ductility, the ability of a material to plastically deform without failure under tensile stresses, was predicted to be improved as well [1]. An extrapolation of grain size dependence of important mechanical properties all the way to the nanocrystalline regime has the exciting prospect of achieving unique mechanical properties never before possible. The promised gain in strength (typically 5-10 times) has indeed been realized in nanocrystalline materials (NCM) [2]. Their ductility at room temperature, on the other hand, is often disappointingly low compared with their coarse-grained counterparts [3]. More importantly, the useful uniform tensile deformation, i.e., the plastic strain before localized deformation sets in at or near the peak in the stress-strain curve, is close to zero for almost all NCM and ultra-fine grained (UFG) metals at ambient temperatures [4]. Obtaining a high strength from UFG or NCM seems to be straightforward, but that alone may not be sufficient for many applications if the material suffers from ductility problems. Even in UFG materials that show better ductility than NCM, strain hardening and strain rate hardening remain insignificant for normal strain rates [5]. The lack of a work hardening mechanism is entirely compatible with the lack of accumulation of dislocations, as suggested by both simulation and experiment [6]. It was noticed that for most of the NCM that showed some strain hardening and respectable ductility, the microstructure usually contained a distribution of grain sizes even though the volume fraction of the larger grains may be low [4,7-12]. In this paper the effect of the presence of large grains in a nanocrystalline matrix on the mechanical behavior of NiTi and Vitroperm alloys was investigated. EXPERIMENTAL DETAILS Nitinol containing 50at.%Ni and 50at.%Ti and strips of melt-spun Vitroperm (Fe73.5Cu1Nb3Si15.5B7) ribbon were used in th
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