Meso-to-Nano-scopic Polycrystal/Composite Strengthening
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Q3.6.1
Meso-to-Nano-scopic Polycrystal/Composite Strengthening R.W. Armstrong*+, H. Conrad** and F.R.N. Nabarro*** *AFRL/MNME, Eglin Air Force Base, FL 32542 **Materials Science & Engineering, North Carolina State University, Raleigh, NC 27695 ***School of Physics, University of the Witwatersrand, Johannesburg, SA ABSTRACT A great challenge associated with current investigations of the structural strength properties of nanometrically-scaled polycrystals and/or composite materials relates to quantitative description of the continuous transition in mechanical behaviors occurring when going over to such materials from those microstructural, and larger, scale material deformation properties seemingly well-understood and modeled during the research investigations of the previous century [1]. The consideration relates to whether the same dislocation generation/interaction mechanisms operating within the grain volumes of a conventional microstructured material, say, as compared in the same case for local deformations, dislocation or otherwise, at the grain boundary regions, are either additionally restricted from operation, or enhanced, for nanostructured materials. The deduced indication is that there should be a relatively smooth mechanical property transition, if any change at all, to be demonstrated here on the basis of available experimental and theoretical modeling results. The predicted smooth transition in behavior, however, may be upset, possibly, by failure to achieve the demanding quality control predictably needed for the combined considerations of: (1) structural characterization; (2) mechanical testing methods; and, (3) model computations, so as to allow quantification of the finer-scaled material behaviors. The combined property behavior is illustrated by application of effective low temperature grain boundary strengthening models at larger grain (or particle) sizes, then, transitioning to effective grain size weakening at the smallest grain sizes, with such weakening thought to occur because of otherwise normal high temperature diffusional or grain boundary weakening mechanisms being promoted somehow to become controlling in the practical “thought-tobe” low temperature regime.
INTRODUCTION Application of the Hall-Petch (H-P) relation, when shown graphically for average grain diameters differing by more than 100X, is advantageously assessed on a log-log scale. Figure 1 illustrates the point for a compilation of certain iron and steel material results [2]. For large grain size polycrystals, the (intercept) friction stress is the dominant quantity. At very small grain diameters, a straight line asymptote is approached for the exponential (-1/2) slope dependence --- sooner as the friction stress is lower. In this grain size region, the linear dependence is shifted up or down as the stress intensity coefficient of the inverse square root of grain size dependence is greater or smaller.
Q3.6.2
Figure 1. Compiled iron and steel yield strength results, after Smith, Armstrong, Hazzledine, Masumura and Pande [2].
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