The Influence of Alloying Elements on Mg Vapor Pressure in Liquid Ternary Aluminum Alloys Studied by Laser-Induced Breakdown Spectroscopy

In-situ laser-induced breakdown spectroscopy (LIBS) was used for measurements on molten aluminum alloys containing 0.6 wt pct magnesium in the melt temperature range 685 °C to 790 °C. With increasing melt temperature, an exponential growth of magnesium LIBS emission signals was observed, a phenomenon that has previously been attributed to the presence of Mg vapor above the melt surface. Here we show how this temperature dependence of the magnesium signal is affected by the presence of a second alloying element in the melt. For dilute ternary aluminum alloys Al–Mg–M, with M = Si, Zn, or Sn, the change in vapor-phase contribution to the Mg signal was found to be linearly correlated with the concentration of the additional alloying element but differing in sign and magnitude. Ternary alloys containing group-II alloying elements (M = Be, Ca, or Sr), known to inhibit oxidation of the melt, were also studied. The presence of these elements had a strongly reducing effect on the vapor-phase component of the Mg LIBS signal. We attribute this decrease to the formation of Be, Ca, or Sr-containing oxides that effectively inhibit the transport of Mg from the melt to the surface and into the vapor phase.