Temperature-Dependent Indentation Behavior of AZ91-Based Composites: An ECM–FPM Comparative Study of Constraint Factor and Pile-Up

Abstract

The indentation properties of the AZ91 magnesium alloy composites containing 3, 6 and 9 wt.% Al₂O₃ particles are
investigated with respect to their temperature variations. The experiments were tensile testing and static spherical
indentation at room temperature (RT) and 473 K to assess the effect of reinforcement content and temperature on plastic flow behavior, Meyer’s hardness, constraint factor (CF) and the pile-up characteristics. The true stress–true plastic strain curve showed a definite rise in flow stress as the amount of Al₂O₃ was increased. The flow stress values at true plastic strain of 0.05 for both AZ91 and AZ91 + 9 wt.% Al₂O₃ increased with an increase in the temperature from RT to 473 K, being 283.7 and 418.5 MPa, respectively, which is an improvement of around 47.5% for AZ91+9 wt.% Al₂O₃. The hardness of the composite was also observed to increase with the increasing average strain, and the strength coefficient was observed to be 396.66 MPa for AZ91 and 863.88 MPa at RT and 926.81 MPa at 473 K for the composite AZ91+9 wt.% Al₂O₃. The CF showed an initial rise in strain followed by the stabilization beyond a critical transition strain, the value of which decreased from 3.9% at RT for AZ91 to 2.8% for AZ91+9 wt.% Al₂O₃. Comparison of experimental result with Johnson’s Expansion Cavity Model (ECM) and Richmond’s Fully Plastic Model (FPM) indicated good agreement and thus verification of the applicability of these models to the confined deformation behavior of the AZ91 based composites. In addition, lip height analysis showed a decrease from 0.174 mm for AZ91 to 0.095 mm for AZ91+9 wt.% Al₂O₃ at RT, which suggested that the growth of pile up was inhibited when the reinforcement was added. Overall, the AZ91+9 wt.% Al₂O₃ composite showed the best indentation resistance, least pile up tendency and better constrained deformation behaviour of all the materials studied.