Investigation of the reduction of NiAl₂O₄ - I: Neutron diffraction studies

In situ metal-ceramic composites consisting of Ni particles embedded in alumina matrices were obtained by the partial reduction of NiAl₂O₄. The volume shrinkage that accompanies the reduction reaction generates residual stresses. Neutron diffraction studies were performed for the first time at various temperatures to study the evolution of phases in situ during reduction and to determine their stress state. It was determined that compressive stresses of several hundred MPa in magnitude can be generated inside the unreduced part of spinel. It was also found that the stress generation is strongly influenced by material and processing variables such as reduction temperature and the initial density of spinel. The diffraction results were then compared to finite element calculations and a reasonable agreement was obtained.