Effects of aluminum content and particle size on volume expansion during the sintering of Fe-Al mixed powders

We investigated the changes of volume, phase composition and microstructure during the sintering of Fe and Al mixed powders. Al powders were mixed with Fe powders in 0–35 at% proportions, and the mixture was then cold-compacted at 500 MPa. Pressureless sintering was carried out at 1100 °C for 1 h to produce sintered samples. As the Al content increased to the values higher than 20 at%, the volume of the sintered samples increased as compared to that of the green compacts. An analysis of the results using a dilatometry and a differential scanning calorimetry shows that a rapid volume expansion occurred near the Al melting temperature at all compositions, and that the expansion rate increased with the Al content. The volume expansion was found to occur because the sample volumes initially occupied by the Al powder were converted into voids by the rapid self-propagating high-temperature synthesis (SHS) process. In the samples sintered at 700 °C, the melting of Al and an exothermic Fe2Al5 formation (2Fe + 5Al → Fe2Al5) was found to proceed simultaneously during the SHS process, and the volumes initially occupied by Al were converted into the voids of sizes similar to those of the Al powder. As the sintering temperature increased, an Fe3Al phase was found to form around the Fe2Al5 phase. At 1100 °C, the Fe3Al phase was found to occupy the whole volume. We also examined the dependence of the volume expansion on the Al particle size in the case of Fe-Al with 28 at% Al content, and we found that, during the sintering process, the volume increased in the samples containing Al particles with size 53–106 μm, whereas it decreased in samples with 3–30 μm Al particle size. We attribute this volume dependence to the fact that a decrease in the volume of the voids created during SHS near the Al melting point promotes densification during the subsequent sintering process. •Using the elemental Fe and Al powder, the formation mechanism of Fe3Al was investigated.•The volume expansion occurred when Al content increased.•The Al particle size is directly related to the volume expansion.•The extent of the volume expansion changed with the size of the Al powders.