Fractures in AL 6061-T6511

Need a little help. I built a heat exchanger out of AL 6061-T6511. The design is such that a heater band is placed around 4 blocks of aluminum bolted together with runners machined through it, making an enclosed heat exchanger with 1 entry and exit port for the heated material to flow in and out. The problem is that after a months worth of daily use, the material has developed small fractures and is now leaking. The temperature on the heater band is 650F+ (highest my IR reads) as it heats the material in the block to 350F (measured by a thermocoupler). I'm assuming the fractures are from the excessive heat. If that assumption is correct, what readily available alloy would be better suited for this application?