[Kevin Sevcik]

Another good resource is efunda.com. Tooooons of materials and other information. But a limited number of page views per day unless you pay membership.

Agreed with SNelson, though. Alloying materials primarily affect hardness, yield strength and tensile strength. To increase stiffness, you'll need to move to a different material, or change the cross section of your part. Tubes with larger diameters are much stiffer per pound than rods. So if you can get away with a larger diameter tubular shaft/strut/whatever, it will be much stiffer. Though at that point, it's prone to buckling. So you have your choice of lighter, larger, stiffer, etc.