In Situ X‐Ray Observations and Microstructural Characterizations for Understanding Combustion Foaming and Reaction Processes to Synthesize Porous Al₃Ti Composites From Al–Ti–B₄C Powders

The combustion foaming behaviors to synthesize closed‐cellular porous Al₃Ti composites are observed in situ using X‐ray radioscopy. The in situ observation of the thermal explosion (TE) mode foaming reveals that the combustion foaming process consists of six steps. The shape of the sample changes uniformly in the first three steps before the sample foams. First, the sample expands uniformly 1) gradual expansion) and maintains an almost constant volume for a fixed period 2) temporary steady state at a constant volume). During these stages, bubbles are not generated inside the sample. Subsequently, the sample shrinks uniformly 3) slight shrinkage). After these three steps, the bubbles are rapidly generated inside the sample 4) rapid foaming). Immediately after foaming, the sample shrinks drastically while bubbles are ruptured and combined 5) severe shrinkage and bubble coarsening). Finally, the sample solidifies as a porous metal 6) solidification). The sample fabricated under the self‐propagating high‐temperature synthesis (SHS) mode includes graded microstructures corresponding to steps (1)–(4). The microstructures of the samples foamed under the TE and SHS modes are characterized. These results are used for discussing the elementary reaction steps arising at steps (1)–(4).