Exterior metal threshold to transition from concrete to tiled entryway?
There is a significant issue with aluminum in concrete - e.g., an aluminum post or conduit embedded in concrete is not a good idea. My hunch is that a piece of aluminum on top of already cured concrete will not have the same issues. However, I am not a metallurgist and so, to play it safe, I would look for a similar piece of steel instead.
On the other hand, a quick search finds steel thresholds to be a bit on the expensive side. I suggest checking with an aluminum threshold manufacturer to see if installation on top of concrete is acceptable.