Age hardening characteristics and high temperature strength of Mg-Gd and Mg-Tb alloys

Magnesium alloys with Gadolinium or Terbium, whose content 20mass% was near the maximum solid solubility, were melted by means of a fluxless method and their age hardening behavior, mechanical properties at elevated temperature and physical properties were investigated. Both alloys exhibit considerable age hardening and the age hardening playes an important role for strength of Mg-Gd alloy even at 573K. Hardness of the aged specimens decreases steeply with rise of temperature beyond 473K, whereas that of the solution-treated Mg-Gd alloy shows positive dependence on temperature up to 573K, which is considered to be probably due to the precipitation. Tensile strength increases as temperature increases and reaches the maximum strength at 523K, namely 320MPa for Mg-Tb alloy and 330MPa for Mg-Gd alloy. The values are far superior to those of the conventional heat-resistant magnesium and aluminum alloys. Elongation of the aged specimens is less than 1% below 473K, but above that it increases in accordance with a change of fracture mode from transgranular to intergranular fracture with increasing temperature. Density of both alloys is three quarter of conventional aluminum alloys. As compared with practical magnesium alloy, their density is about 15% bigger, thermal conductivities are superior, but coefficients of thermal expansion is higher somewhat.