The role of pore microstructures on the susceptibility of MAX phases (Ti3SiC2, Ti3AIC2,
TisAIC, Ti2AIN2, Ti4AIN3) to thermal dissociation at 1300-1550°C in high vacuum has been
studied using in-situ neutron diffraction. Above 1400°C, MAX phases decomposed to binary
carbide (e.g. TiCx) or binary nitride (e.g. TiNx), primarily through the sublimation of A-elements
such as Al or Si, forming in a porous surface layer of MXx. Positive activation
energies were determined for decomposed MAX phases with coarse pores but a negative
activation energy when the pore size was less than 1.0 μm. The role of pore microstructures
on the decomposition kinetics is discussed.