1-20 alloy question

[bigborefan]

Can anyone answer this for me? If tin has a BHN of 7 and pure lead has a BHN of 5, how do they come up with a BHN of 10 when you mix them together at a 1-20 ratio? I can see when you mix linotype (22BHN) with WW (9-12BHN) or pure lead (5BHN), you can come to a #2 alloy of 15BHN because you are diluting the linotype with pure lead or WWs. What causes the combination of two alloys to harden more than their original BHNs singularly?

[felix]

BBF, that is a hard question to answer in simplistic terms. In the alloy situation, where the various chemicals do not combine to share micro-elements, electronics if you will, is most like painting a steel ball to protect the steel from rust. Paint and steel are completely different dealies, and when both are combined in some practical way they together make up something more "useful" than when used as seperate components. Going into more detail would require some heavy duty background on what, when, and where: the goals of a scientist; and background on for what purpose: the goal of an engineer. In reality, all together, this is called research and development (R&D). ... felix

[Glen]

In short, it's because in one case you're talking about pure elements and in one case you're talking about mixtures.

When you add lead to linotype, you are starting with a mixture and simply diluting it. The tin and antimony are already in a lead matrix. The chemical environment around the tin/antimony doesn't change that much, the concentration just changes a little bit, so the hardness tracks with tin/antimony concentrations.

When you add lead to pure tin, that tin is going from an environment of nothing but tin atoms, and by adding that lead you are changing the fundamental nature of the matrix. Tin and lead are both Group IV elements, of roughly similar size (but not identical) and electronegativity, so they alloy very easily, and (more importantly for this discussion) they "play well together" to form a solid solution that has greater strength than either of the pure components. It has to do with how the little "balls" (i.e. lead and tin atoms) settle in together and form a lattice in the alloy.

[bigborefan]

Thank you gentlemen, you explained it very well. That's why I come to this forum. Very knowledgeable people.