If Delta actually cancelled the flight and it is not operating or there is no reasonable alternative within 90 minutes, you can get a refund. If you just decide not to travel, but the flight is still operating, then you will only get an ecredit. Credits can be used in increments to buy multiple tickets in the original passengers name. I think you can only initiate a credit card charge back if the original charge is within 60 or 90 days (not sure which)