Phase transformation and thermal expansion of Cu/ZrW₂O₈ metal matrix composites

Powder metallurgy was used to fabricate fully dense, unreacted composites consisting of a copper matrix containing 50–60 vol% ZrW₂O₈ particles with negative thermal expansion. Upon cycling between 25 and 300 °C, the composites showed coefficients of thermal expansion varying rapidly with temperature and significantly larger than predicted from theory. The anomalously large expansion on heating and contraction on cooling are attributed to the volume change associated with the allotropic transformation of ZrW₂O₈ between its high-pressure γ-phase and its low-pressure α- or β-phases. Based on calorimetry and diffraction experiments and on simple stress estimations, this allotropic transformation is shown to result from the hydrostatic thermal stresses in the particles due to the thermal expansion mismatch between matrix and reinforcement.