Abstract
When polycrystalline materials are tested in tension at elevated temperatures, the flow mechanisms depend upon various parameters including the temperature of testing, the applied stress and the material grain size. The plotting of deformation mechanism maps is a procedure used widely in displaying and interpreting the creep properties of conventional coarse-grained metals but there have been few attempts to date to use this same procedure for ultrafine-grained and nanocrystalline materials produced through the application of severe plastic deformation (SPD). This report examines the potential for using deformation mechanism mapping for materials processed by SPD and presents examples for materials processed using equal-channel angular pressing and high-pressure torsion.
Highlights
The grain size of a polycrystalline material is an important parameter in determining the flow properties at elevated temperatures
When the grain size is large the material deforms through an intragranular dislocation process and there is no dependence on grain size but when the grain size is small additional flow processes become important such as grain boundary sliding, superplasticity and diffusion creep[1]
It is well established that superplasticity becomes an important flow process only when the grain size is smaller than ~10 μm[2]
Summary
The grain size of a polycrystalline material is an important parameter in determining the flow properties at elevated temperatures. Over forty years ago it was proposed that creep mechanisms may be most readily identified through the construction of deformation mechanism maps in which, for a constant grain size, the normalized stress, σ/G, is plotted against the homologous temperature, T/Tm, from absolute zero to the melting temperature[7] These maps contain contiguous fields that correspond to the dominance of a specific creep mechanism and generally strain rate contours are superimposed on the map to provide additional information on the creep rate. Alternative and simpler maps were suggested later including plotting the normalized grain size, d/b, against the normalized stress, σ/G, at constant temperature[8], plotting d/b against the inverse of the homologous temperature, Tm/T, at constant stress[9] and plotting σ/G against Tm/T at constant grain size[10] All of these alternative maps have an advantage because they are easy to construct for high temperature creep since the various field boundaries and strain rate contours appear as straight lines. These domains were inserted using the appropriate constants for each
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