Effect of silver on the glass forming ability of MgCuGdY bulk metallic glasses
Résumé
Bulk metallic glasses have been synthesized in the Mg(x)Cu(y)Ag(z)Gd(10) and Mg(x)Cu(y)Ag(z)Gd(9)Y(2) systems by HF melting and injection in conical and cylindrical shape copper moulds in order to determine their critical diameter and to prepare samples for mechanical tests. Silver can be substituted either to the copper or to the magnesium leading to different alloys whose properties have been measured by differential scanning calorimetry (DSC) curves and X-ray analyses. Up to a high diameter (10 mm), they present the structure of bulk metallic glasses. The effect of silver is in general to reduce the glass transition region (Delta T = T(x) - T(g)) but with an increase of the critical diameter which is not in agreement with glass forming ability (GFA) criteria when choosing Delta T as so, however the gamma = T(x)/(T(g) + T(l)) parameter is in agreement with the GFA as the gamma parameter is increasing when the critical diameter is increasing. The effect of silver in the gadolinium-yttrium-based alloys is to decrease the Delta T range from 69 K without silver to 40 K with 9 at.% of Ag. Mechanical measurements made by compression tests at room temperature show that these alloys stay brittle with an elastic limit above 800 MPa but with no improvement of plasticity due to silver