The effects of cooling on the flow strength of metal matrix composites

The flow strength of a metal matrix composite is affected by the properties of each of its constituents (matrix and reinforcement) and how they interact with each other. Three important effects, which are believed to influence the composite strength, were investigated: the back stress, thermal residual stress and generation of dislocations in the matrix. The back stress is defined as the stress developed in the matrix as a result of the particulates resisting plastic deformation. The thermal residual stress is introduced upon cooling from an elevated temperature by the difference of coefficient of thermal expansion between the matrix and reinforcement. Dislocations are formed near the particulates as a result of high local thermal stress.; The work hardening behavior of a particulate metal matrix composite was studied in terms of the Bauschinger effect. The difference and sum of the flow stresses were predicted analytically using Eshelby's equivalent inclusion method. The analytical predictions agreed well with experimental data. The difference of flow stresses increases linearly with plastic strain up to some limiting strain and then bends to a non-linear curve. The relation between the sum of the flow stresses and the plastic strain gives rise to a flat curve. The experimental data confirmed that work hardening is mainly due to back stress and not to forest dislocations.; Eshelby's equivalent inclusion method was used in conjunction with dislocation theory to predict the yield strength of hybrid metal matrix composites containing aligned or misoriented short fiber, disk-shaped and/or spherical particulates. The analytical model considered dislocations to be punched-out away from the particulate into the matrix. These dislocations partially relax the residual stress and also increase the strength of the matrix. The analytical prediction agreed well with experimental data.; A parametric investigation revealed that the particle size, temperature differential and particle volume fraction are major variables influencing the composite strength. While matrix strengthening (due to the formation of dislocations) and back stress increase the composite strength, the thermal residual stress may either increase or decrease the strength depending upon the reinforcement geometry and relative orientation with respect to the loading direction.