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

Powder metallurgy was used to fabricate fully dense, unreacted composites c onsisting of a copper matrix containing 50-60 vol% ZrW2O8 particles with ne gative thermal expansion. Upon cycling between 25 and 300 degrees C, the co mposites showed coefficients of thermal expansion varying rapidly with temp erature and significantly larger than predicted from theory. The anomalousl y large expansion on heating and contraction on cooling are attributed to t he volume change associated with the allotropic transformation of ZrW2O8 be tween its high-pressure gamma-phase and its low-pressure alpha- or beta-pha ses. Based on calorimetry and diffraction experiments and on simple stress estimations, this allotropic transformation is shown to result from the hyd rostatic thermal stresses in the particles due to the thermal expansion mis match between matrix and reinforcement.