UBC Theses and Dissertations
Modelling the effect of grain size distribution on the mechanical response of metals Raeisinia, Babak
Recent experimental studies have pointed to the potential for producing metals with improved mechanical properties based on manipulation of the grain size distribution. It is, however, unclear how these improvements are brought about and whether grain size distribution manipulation can effectively be used to tailor the mechanical response of metals. In this work, these issues are examined using two novel grain size dependent self-consistent models, an elastoplastic and a viscoplastic, where plasticity is assumed to occur by dislocation slip. For this purpose, monotonic deformation of a number of model f.c.c. polycrystals with moderate stacking fault energy (such as copper) is examined. Polycrystals with lognormal distributions, having average grain sizes ranging from 100 nm to 50 p.m, and bimodal distributions are considered. It is found that increasing the width of the lognormal grain size distribution, while keeping the average grain size constant, decreases the yield strength of the polycrystal and increases its work hardening rate. This behaviour is attributed to the increasing volume fraction of grains larger than the average, as the distribution is widened, which have lower threshold stresses and higher work hardening rates than the average. The simulation results are summarized in the form of new property maps where the range of ultimate tensile strength-uniform elongation combinations which can be achieved through grain size distribution manipulation are shown. These maps also demonstrate that bimodal polycrystals demonstrate better overall properties as compared to lognormal polycrystals. The observed grain size distribution effects are, however, found to be dependent on the nature of the constitutive relationship assumed for the grains. The developed maps provide a first guide for materials engineers interested in the modification of the mechanical properties of polycrystals through grain size distribution manipulation.
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