Aluminum alternative solution?

Which properties are important to you?

Not my field, but I'd start with magnesium alloys and titanium alloys, knowing that either one will be more expensive than Al.

As you probably already know, 6061-T6 would be less expensive, but may not have the properties you are seeking. In some sizes, unidirectional fiberglass pultrusions can be cost and strength competitive with aluminium. Sometimes beams can be replaced with rectangular section tubing, in which case a thin wall steel tube can replace an aluminum I beam, yielding a large improvement in torsional rigidity (if that's important) and, in some cases, with no increase in weight, because the structure is more efficient (because the stock sizes might be a better fit). Consider, for instance, bicycle frames, where the steel tubes can be much smaller than their aluminum counterparts, and frame weights are comparable (if chrome moly steel is used -- which is at least as costly as aluminum).

Then there's wood. As a boat builder, I like wood as a material, and as a material for beams (as in floor joists) it is hard to beat. Steel studs are sometimes used in construction, but have no real advantage over wood (other the termite resistance), and so wood continues to be the material of choice for home construction. In small boats, its virtually impossible to build an aluminum boat that matches a wood one for stiffness vs weight. Home built wooden planes are generally lighter and stronger than aluminum production craft of the same size.

Of course, if space and cost are the main constraints, then a steel beam will always be cheaper and stronger than its aluminum equivalent. There are good reasons why you never see commercial building frames built of aluminum.

Loading? Profile? Length? Why was 7005-T53 choosen?