On the relation between hardness and yield strength in a sand cast AZ91 alloy

The effect of ageing time at 165 degrees C on the mechanical properties of sand cast AZ91 alloy has been studied. The hardness, H-upsilon, and 0.2% proof stress, YS, increase with ageing time while the tensile ductility decreases. It is possible to fit the flow curves of the material to the equation sigma = K epsilon(pl)(n), where sigma is the true stress and epsilon(pl) the true plastic strain, with a single K-value (570 MPa) and varying the strain hardening exponent, n, according to the alloy temper. It is shown that the value of n is related to the Vickers hardness number by n = 1.382 - 0.265 In(H-upsilon) and that this expression can be used to calculate the 0.2% proof stress as YS = 569 (0.002)(n) (the value of 569 MPa used here is because the proof stress is the engineering stress)