The Determination of Hall-Petch Constants in Pure Mg

Flow stress data from specimens tested in tension or compression, with grain sizes ranging from 90 pm to 1440 pm have been used to construct Hall-Petch plots. Three different methods were used to obtain the yield strength data: 0.2% offset proof stress; finding the minimum of the second derivative of the flow stress with respect to the strain; and the stress at which the strain after unloading is 0.2%, thus subtracting the pseudoelastic component of the strain introduced by twinning. It is shown that the last method leads to more self-consistent results. It is also shown that the usual method of finding the Hall-Petch constants through linear regression may sometimes lead to negative values of the friction stress. It is argued that the friction stress should be introduced as a physical constant, calculated from first principles, in order to obtain more reliable values for both the friction stress, sigma(o), and the stress intensity parameter, k.