The Role of Thermomechanical Routes on the Distribution of Grain Boundary and Interface Plane Orientations in Transforme
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THERE is an ongoing requirement for material and product innovations to meet the increasing demand of higher performance at minimal cost. The development of these materials requires a deep understanding about the influence of each microstructure constituent, and their interactions, on the property of interest. An important active structural element is the grain boundary whose characteristics control a number of the properties of polycrystalline materials. The manipulation of grain boundary structure is one of the fundamental goals of the materials science and engineering field since the mid-1980s.[1] The main challenge is to control the population and connectivity of certain grain boundary types relevant to the property of interest to enhance material performance. The typical approach is iterative thermomechanical processing (e.g., recrystallization) where the nucleation and growth of grains are controlled to a large extent by
HOSSEIN BELADI, Senior Research Academic, is with the Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia. Contact e-mail: [email protected] GREGORY S. ROHRER, W.W. Mullins Professor and Head, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted March 28, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
low-energy boundary network configurations.[2] Many technologically important metals such as steel and titanium alloys do not, however, maintain the hightemperature microstructure and undergo phase transformation on cooling. Because the resultant microstructure depends on the phase transformation path, this is the most effective way to tailor the microstructure and properties. For instance, for a given steel composition, the austenite state (grain size and density of various defects) and cooling rate are the most important thermomechanical parameters for the control of the different phases at room temperature (e.g., polygonal ferrite, bainite or martensite). The transformation of austenite to ferrite takes place at a relatively high temperature (during slow cooling) where both nucleation and growth processes are controlled by the diffusion/reconstructive mechanism. Alternatively, the displacive shear mechanism occurs during the austenite to martensite phase transformation on rapid cooling. Bainite is usually formed at an intermediate temperature range between those of the reconstructive (ferrite) and displacive (martensite) phases. It was recently found that the grain boundary network is largely controlled by the phase transformation mechanism constraints rather than the relative energies of the interfaces.[3,4] The purpose of this paper is to review and compare the grain boundary character distributions of four microstructures processed in different ways. These distributions have been previously reported in different contexts,[3,5,6] but here we focus on the role of the transformation path on the grain boundary character distribution.
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EXPERIMENTAL PROCEDURE
Thermomech
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