The Effects of Nanostructure on the Strengthening of NiAl
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ittleness and poor high temperature strength. Improved mechanical properties may potentially be achieved by refinement of the grain size to nanoscale dimensions. Nanostructured materials, a subject of great interest for a variety of applications, are polycrystalline solids having crystallite sizes usually less than 100 nanometer that are interconnected by interfaces. Due to the presence of the large volume fraction of interfaces and grain boundaries, these materials possess unique properties relative to materials with conventional grain sizes. In particular, the strength, ductility, and diffusion kinetics, among
other mechanical and chemical properties, may be significantly enhanced [2]. Grain size refinement as a well-known method for improving properties of conventional materials, and is the motivation for the current work on the nanostructured NiAl. The Hall-Petch relationship between the yield strength, oy, and grain size, d, is one of the main models that describes the strengthening mechanism for conventional materials: (•y ---(y0+ kd-2
(1)
where a0 and k are constants, and the yield strength ay, equals one third of the measured Vickers hardness. The increased strength of conventional materials is due to the interactions between the dislocations and solute atoms and the interactions between the dislocations and grain boundaries. It is presently not clear whether the Hall-Petch relationship can be extended to the finest grain sizes in the nanophase regime. It has been reported that Hall-Petch behavior is found for nano-materials but with a change of Hall-Petch slope. Some other works have observed a decrease in strength with decreasing KK8.3.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
grain size in ultrafine materials [3]. Thus, the effect of grain-size on strengthening for nano-materials may not be as strong as that predicted for materials with conventional grain size. Strengthening due to grain boundary blocking dislocation motion may not be possible in nanocrystalline materials which may be one of the reasons that the strengthening is lower than that predicted by the Hall-Petch effect. Grain boundaries may first act as dislocation sources under applied stress, and then as dislocation sinks once the applied stress is removed. Deformation mechanisms operating in nanosized materials may thus be different than those observed in conventional materials. Considering the change in the proportion of atoms at grain boundaries for nanosized materials, it is expected that grain boundary activity may be dominant. In addition, dislocation climbing and shear banding are also possible deformation mechanisms that may operate in nanostructured materials. EXPERIMENTAL Stoichiometric mixtures of elemental Ni and Al powders were mechanically alloyed for 16, 24, and 48 h to a final grain size of 27± 18, 11±6, 9±t6 nm, respectively, measured by the ASTM linear intercept method on the bright and dark field TEM micrographs, which yields an area-weighted average grain size. A detectable quantity of Fe w
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