Welcome to Android Central! Assuming you had a good screen lock PIN/password set up, they're not going to get any of your data (and the folks who deal with this aren't going to have the intense hacking skills needed to pry your data out of a dead phone -- forensic data specialists get paid hundreds of dollars to use specialized equipment for that, not people handling RMA). As mustang7757 suggested, use Google's Find My Device to send a remote lock and wipe command to the phone in the event it ever powers on and connects to the web.
If you have an SD card inserted, definitely remove that before sending it in.