Open-cell metal foams have steadily gained importance and interest for various application areas in industry and research over the recent years, due to their specific property portfolio. They stand out among other materials for their acoustic and mechanical properties such as vibration damping and sound and energy absorption capabilities. Combined with the excellent weight-to-stiffness ratio, metal foams are predestined for the use in lightweight structures e.g. in automotive or aircraft applications. Here, they are exposed to a range of static and cyclic loading conditions. Previous investigations of the macro-scale fatigue and quasi-static failure behaviour of the foams have shown a high scatter of the properties. To better understand local microstructural influences on the mechanical properties, we investigate the quasi-static and fatigue properties on the nano- and microscale.
We performed nanoindentation tests on single struts of an open-cell precision cast aluminium alloy foam. First, the mechanical properties of the different phases of the strut, aluminium matrix, silicon and intermetallics, were characterized by quasi-static indentation tests. The obtained values of hardness and modulus were correlated with the local structural differences, and with the distance to the other phases. The nano-scale fatigue behaviour was investigated by cyclic indentation tests. Changes of the load-depth curves were analyzed as a function of cycle number and correlated with microstructural changes in the vicinity of the indents. The curves showed distinctly different courses, indicating cyclic softening or hardening, which, however, could not be correlated with the microstructure visible on the surface. This suggests that the cause of the differences may be the structure beneath the surface. We therefore performed submicron resolution synchrotron micro tomography experiments of nanofatigued struts to evaluate the distribution of silicon particles and other secondary phases in the volume below the indents. The results were then correlated to the fatigue response. Further, TEM investigations of the matrix close to the indent were performed to characterize the fatigue induced dislocation density and structures.