Investigation of the mechanical behaviour of AISI 316L stainless steel syntactic foams at different strain-rates

The mechanical behaviour of stainless steel AISI 316L based syntactic foams containing either 40/60 vol.% of hollow glass microspheres (S60HS) or 40 vol.% of Fillite cenospheres was investigated. In these materials, the hollow particle shells as third phase besides matrix and voids can provide a strengthening effect with the potential of raising mechanical performance above that of conventional, two phase steel foams. Samples were produced by means of metal powder injection moulding (MIM) and subjected to characterization under compressive load, with special attention dedicated to strain-rate sensitivity. Four strain-rate levels were investigated, covering 6 orders of magnitude from 10^-3 to 10^3 s^-1. For the highest, a Hopkinson Bar apparatus was used. The influence of density on strength was determined for samples containing glass microspheres and described by a power law relationship. The foams mechanical strength was found to increase with strain-rate in accordance with the behaviour observed for the reference material without hollow particles. The data were compared with those obtained in a previous work, in which Fe99.7 matrix syntactic foams containing similar levels of glass microspheres were investigated. The higher strength of the AISI 316L materials is associated with differences in matrix properties. Differences in strain-rate dependence of mechanical properties between both materials can be explained qualitatively based on the fcc (AISI 316L) and bcc (Fe99.7) lattice structure. The introduction of Fillite cenospheres induced a further increase of specific strength. Under quasi-static conditions, samples of this type were found to reach the same yield strength as the reference material despite the reduction in density. The significantly lower strength of glass microsphere based AISI 316L foams can be related to the observed microstructures: due to the high processing temperature (1200 C), glass microspheres are destroyed during sintering, their remainders forming glass inclusions, whereas thermally more stable cenospheres remain intact and can thus stabilize the pores. Finally, an empirical strain-rate sensitive model was adopted to reproduce the experimental data: the fitting procedure used to obtain the model parameters is explained and the influence of the strain-rate discussed. The model allows property prediction for additive content and strain-rate levels further to those evaluated experimentally.