BHN is a mesurement of the stress-strain curve of the alloy, using a brinell measuring system. Apply force to the material and see how much it (displaces) moves and remove force to see how much 'spring-back' (malleability). The shape of the curve changes greatly depending on the content of the melt. Typically a 'S' type of curve. Top of the 's' is BHN and permanent displacement. Difference in psi vs force is a constant (area) so often referred as psi.