Thermal expansion of a Spheroidal Graphite Iron: A micromechanical approach

Abstract

Dimensional variations experienced by materials due to temperature changes are described by the Coefficient of Thermal Expansion (CTE), which is strongly dependent on microstructural features, especially on composites materials. Special attention is given in this work to Spheroidal Graphite Iron (SGI), for which the microstructure may be considered as a composite material formed by graphite particles embedded in a continuous matrix. In this work, a micromechanical approach, accounting for the manufacturing process, was used to compute the CTE of an eutectic SGI in an as-cast condition as a function of microstructural features and temperature. A cubic shaped Representative Volume Element (RVE) with Periodic Boundary Conditions (PBCs) was generated to model the microstructure of SGI. RVEs were formed by 12 non-overlapping spherical nodules embedded in a matrix with varying content of ferrite and perlite, and their size was determined by means of a convergence study. Using finite elements analysis, the macroscopic CTEs were computed for cooling and heating the material in the range from 25 °C to 500 °C. Using this micromechanical model, it was found that volumetric fractions of phases and temperature play a key role on the CTE. This coefficient increased by raising the temperature, increasing the volumetric fraction of ferrite, or decreasing the volumetric fraction of graphite. The manufacturing process had also an influence because plasticity occurred in the metallic matrix during the cooling stage of the casting process. Multivariable polynomial regressions were used on results of the micromechanical model to develop a mathematical expression and evaluate the CTE as a function of the volumetric fractions of phases and temperature. Results of the mathematical expression are compared with experimental data, finding a fairly good correlation between them.