An Experimental and Analytical Study of the Effect of Grain Refinement on Strength and Formability of Metals.

Abstract

This research is about the effect of grain refinement on strength and formability of polycrystalline metals. The study has been performed by analytical and experimental approaches:

In the analytical work, mechanical behavior of polycrystalline materials is modeled by assuming that grain interior behavior is controlled by crystal slip with strain hardening, anisotropy and size effect; and grain boundary behavior under stress described a behavior of grain boundary “Mantle Zone” is controlled by viscous response. For such composite response, FEM simulation was carried out to estimate the stress-strain response for different grain size materials down to nanocrystalline size. Predicted dependence of strength on grain size was in agreement with Hall-Petch relation. The simulation also predicted weakening of materials at very fine grain size when mantle zone viscosity is low or it approaches Newtonian viscous behavior.

In the experimental work, study was conducted primarily on formability of alloys as a function of temperature and strain rate. This was done with alloys which were first prepared in ultra-fine grained condition. The work was performed on Mg alloy, Ti alloy and TRIP steel, representing material systems from hcp to bcc-fcc transformation toughened structures. In all cases, formability was found to be enhanced as a function of temperature, causing significant increase in fracture strain. Grain boundary sliding accommodated by boundary diffusion and possible core diffusion were suggested to be the rate-controlling mechanism for superlastic deformation, while, for TRIP steel at the temperature range (25-400oC), thermal activation of plastic flow was found to be most important.

For magnesium alloy, the mechanism of grain refinement was studied in relation to subdivision of coarse grains by twinning process; slip in the inter-twin volume with twin plane rotation. For titanium alloy, enhanced superplasticity at low temperature was examined in the submicrocrystalline range. For TRIP steel, enhancement in both strain hardening rate and strain rate sensitivity were documented as they relate to the overall formability of this alloy. Strengthening potential at elevated temperature in this material was also examined and was found to be related to different deformation and precipitation mechanisms of the microstructure constituents.