The classical alloy for heat-treatment is, of course, medium and high carbon steel. The hardening of steel, by plunging the metal red-hot into water (so producing martensite), and its toughening, by tempering the quench-hardened metal at a moderate temperature, have been known empirically and used for thousands of years. Until this century it was not realized that other alloys could be substantially hardened by heat-treatment. Wilm’s discovery that aluminium alloys which contain copper could be hardened by quenching and ageing or tempering came in 1906 and this was the starting point for the development of duralumin and innumerable other heat-treatable alloys. It was the realization in 1920, by Merica, Waltenberg and Scott, that this form of hardening, which appears only during the ageing or tempering treatment, is due to the decomposition of a supersaturated solid solution produced by the quench, that opened the way to the large-scale development of age-hardening and temper-hardening alloys. Once this scientific principle was known it became a simple matter to choose, from the phase diagram, likely alloy compositions and heat-treatment temperatures; and, since the essential thing required—a solubility that decreases with decreasing temperature—is a simple and common feature of phase diagrams, there was no shortage of potentially heat-treatable alloys.