Size Effects on Strength in the Transition from Single-to-Polycrystalline Behavior
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During the transition from single crystalline to polycrystalline behavior, the available data show the strength increasing or decreasing as the number of grains in a cross section is reduced. Tensile experiments were conducted on polycrystalline Ni with grain sizes (d) between 16 and 140 μm and varying specimen thickness (t), covering a range of λ (=t/d) between ~0.5 and 20. With a decrease in λ, the data revealed a consistent trend of strength being independent of λ at large λ, an increase in strength, and then a decrease in strength. Microstructural studies revealed that lower constraints enabled easier rotation of the surface grains and texture evolution, independent of the specimen thickness. In specimen interiors, there was a greater ease of rotation in thinner samples. Measurements of misorientation deviations within grains revealed important differences in the specimen interiors. A simple model is developed taking into account the additional geometrically necessary dislocations due to variations in the behavior of surface and interior grains, leading to additional strengthening. A suitable combination of this strengthening and surface weakening can give rise to wide range of possibilities with a decrease in λ, including weakening, strengthening, and strengthening and weakening.
KeywordsDigital Image Correlation Sample Thickness Polycrystalline Sample Thin Sample Taylor Factor
This work was supported by the Department of Science and Technology. One of us (AHC) acknowledges with gratitude the support provided by Professor Laszlo Toth at the University of Lorraine (Metz) during a sabbatical leave. We would like to acknowledge useful discussions with Professor Satyam Suwas at Indian Institute of Science (IISc) relating to texture strength and Taylor Factor calculations. PG would also like to acknowledge the help of Prof. Jozef Keckes (Montan University) and Anuj Bristhi (IISc) for X-ray measurements and dislocation density calculations.
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