Q: Will Hyper-V Replica automatically fail over to the replica VM if the primary VM becomes unavailable?

A: No. Hyper-V Replica doesn't have built-in failover functionality and instead focuses on the technology to replicate the primary virtual machine (VM) to the replica location.

If an automated site failover process were required (which isn't that typical, and generally some kind of human "switch" to confirm would at least be desired), then this could currently be achieved using Windows PowerShell, which can perform all Hyper-V Replica functions. Or you could use System Center Orchestrator, after SP1 for System Center 2012 is released.