A MICROMECHANICAL STUDY OF RESIDUAL-STRESSES IN FUNCTIONALLY GRADED MATERIALS

A physically based computational micromechanics model is developed to study random and discrete microstructures in functionally graded mater ials (FGMs). The influences of discrete microstructure on residual str ess distributions at grain size level are examined with respect to mat erial gradient and FGM volume percentage (within a ceramic-FGM-metal t hree-layer structure). Both thermoelastic and thermoplastic deformatio n are considered, and the plastic behavior of metal grains is modeled at the single crystal level using crystal plasticity theory. The resul ts are compared with those obtained using a continuous model which doe s not consider the microstructural randomness and discreteness. In an averaged sense both the micromechanics model and the continuous model give practically the same macroscopic stresses; whereas the discrete m icromechanics model predicts fairly high residual stress concentration s at the grain size level (i.e., higher than 700 MPa in 5-6 vol% FGM g rains) with only a 300 degrees C temperature drop in a Ni-Al2O3 FGM sy stem. Statistical analysis shows that the residual stress concentratio ns are insensitive to material gradient and FGM volume percentage. The need to consider microstructural details in FGM microstructures is ev ident. The results obtained provide some insights for improving the re liability of FGMs against fracture and delamination. (C) 1997 Acta Met allurgica Inc.