Choosing proper heat sink for a high power relay
Your specified operating conditions don't make sense. The maximum voltage drop across one of these relays is 1.4V at 10A, so you can't have 45V in and 10V out. So, the maximum power dissipation in the relay itself is 14W. The temperature drop from junction to case will be 98C. You need a heatsink with a thermal resistance of (150-98)/14=3.7C/W or less.

If you really want to keep the junction temperature below 50C you must reduce the load current. Suppose you add an infinitely good heatsink, with zero thermal resistance. The thermal resistance from junction to case is 7C/W. If the ambient (and case) temperature is 25C then you can only allow a 25C increase from case to junction, so you are limited to 25/7= 3.5A of current.