Thermo-mechanical response and form-stability of a fully metallic composite phase change material: Dilatometric tests and finite element analysis

Composite Phase Change Materials (C-PCMs), such as immiscible Al-Sn alloys are designed to store and release heat as a result of the phase transformation of one of the phases, i.e., Sn in this case. The volume changes induced by melting and solidification of the low-melting Sn phase as well as the different thermal expansion of solid Al and Sn phases induces stress fields during thermal cycles. Repeated experimental dilatometric tests were performed on the above C-PCM to check their microstructural stability as well as its effect on their form-stability, i.e., capability to recover the initial shape under various potential cyclic service conditions.

Analysis of the thermo-mechanical behaviour of the two phases, as well as the overall one, have been investigated under the simple thermal profiles reproducing dilatometric tests. A finite element model of fully dense Al-Sn C-PCM, where a single spherical inclusion of Sn is surrounded by Al matrix, is generated and numerical thermo-mechanical analysis is performed. The thermal-dependence of elastoplastic-behaviour, thermal expansivity and other thermophysical properties of the 2 solid phases has been modelled, together with compressibility of liquid Sn. The simulations illustrate the overall material behaviour as well as the local thermomechanical response. The results for the slopes of elongation vs temperature curves agree well with experimental data from dilatometric tests, for which form-stability is observed from the third cycle. The results also suggested that the plastic deformation of the only regions of Al phase surrounding the Sn inclusion accommodates its expansion during heating and melting. In these latter regions, at the end of a dilatometric cycle, compressive strain in radial direction reaches a maximum value of 0.1 %, higher than overall strains.