The hardness of solid solutions

Abstract

It is a well-known fact of metallurgy that the addition of one metal to another produces an increase of strength and hardness. In some alloys, this change of properties is accompanied by the formation of a new micro-constituent or phase, which is itself harder, and also, as a rule, more brittle than either of the constituent metals. In a very large and important group of alloys, however, the addition of the second metal, up to certain limits of concentration, does not lead to the formation of a second phase or constituent. Alloys of this type, when they have attained an equilibrium condition, consist of an aggregate of polyhedral crystals, homogeneous in composition so far as their micro-chemical behaviour indicates, and in most respects entirely similar to the constituent crystals of the pure metal, which forms the basis of the alloy. A typical example of this kind is furnished by the alloys of copper with zinc, containing up to about 30 per cent. of zinc. Alloys of this type are generally described as “solid solutions,” on the ground that the state of intimate mixture which exists in the liquid (molten) solution of the two metals in one another is preserved in these alloys after solidification. In Continental language, such crystals are more frequently termed “mixed crystals” (“Mischkrystalle”), but the present author prefers so avoid this term on account of a possibly misleading interpretation. A further fact in connection with metallic solid solutions is also well known, but appears to require explanation. A solid solution alloy is always harder and stronger than the pure metal of which it mainly consists, and frequently this difference in physical properties is very marked. Thus, annealed pure copper has a Brinell hardness number of 36, and a tensile strength of about 13 tons per square inch. An alloy of copper containing 30 per cent. of zinc in solid solution, on the other hand, in the corresponding annealed condition, has a Brinell hardness number of 57, and a tensile strength of 20 tons per square inch. It is the purpose of the present paper to suggest an explanation for this hardening and stiffening effect of the added metal in solid-solution alloys, and to show that this explanation leads to an inference which is in striking accord with well-known facts.