Bounds to Hardening by Solid Solution, Precipitation and Short Range Order in Mg Binary Alloys

Miedema’s coordinates are used to rank 4 model binary alloys considering the respective values of enthalpy of formation and the tendency to developing random solid solution, precipitation, short range order (SRO) and intermetallic compounds. The terminal solid solubility generally increases whereas the tendency to order decreases with decreasing heat of formation, and hardening by near-random solid solution and/or precipitation is expected to be dominant for solutes with low tendency to order, such as Al. For solutes with an intermediate tendency to order, hence solubility, such as Zn, or to form compounds, such as Gd, SRO is predicted to dominate the hardening. For solutes whose very large heat of formation leads to very high melting point intermetallics forming congruently, such as Sb, the terminal solid solubility is too low for any solute based hardening to be feasible. Implicancies for alloy design and selection regarding solute or precipitation hardening, SRO and creep resistance are discussed.