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 ofThermal Expansion (CTE), which is strongly dependent on microstructural features, especially on compositesmaterials. Special attention is given in this work to Spheroidal Graphite Iron (SGI), for which the microstructuremay be considered as a composite material formed by graphite particles embedded in a continuous matrix. In thiswork, a micromechanical approach, accounting for the manufacturing process, was used to compute the CTE of aneutectic SGI in an as-cast condition as a function of microstructural features and temperature. A cubic shapedRepresentative Volume Element (RVE) with Periodic Boundary Conditions (PBCs) was generated to model themicrostructure of SGI. RVEs were formed by 12 non-overlapping spherical nodules embedded in a matrix withvarying content of ferrite and perlite, and their size was determined by means of a convergence study. Using finiteelements analysis, the macroscopic CTEs were computed for cooling and heating the material in the range from25 C to 500 C. Using this micromechanical model, it was found that volumetric fractions of phases and temperatureplay a key role on the CTE. This coefficient increased by raising the temperature, increasing the volumetricfraction of ferrite, or decreasing the volumetric fraction of graphite. The manufacturing process had also aninfluence 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 mathematicalexpression 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 correlationbetween them.