This is part 1 of a 4 part series focused on how metals can be strengthened. This first way is through alloying. Some background is reviewed, stress versus strain, elastic modulus, and then we consider how to "fill out" the rest of a curve for Cu-Ni alloys (which are fully soluble) given the first "half" of the curve from 0% Ni to 50% Ni. Introduction to Materials Science and Engineering, college-level course. Why does substitutional alloying strengthen a material? The different sized atoms from the alloying agent SLOW DOWN dislocation motion and thereby increase the yield stress of the alloy from the pure material.