Thermo-mechanical Deformation of Al319 - T7B with Small Secondary Dendrite Arm Spacing

Thermomechanical fatigue and isothermal deformation experiments were conducted on cast Al 319 alloys with small secondary arm spacings (SDAS) in the range of 25 to 35 μm. The alloy was studied in the overaged state designated as T7B. In the case of the T7B treatment the material possesses dimensional stability, but incurs considerable loss of strength with time and cyclic deformation at temperatures exceeding 250°C. A two-state variable unified constitutive model was developed to characterize the stress-strain response for the material. The model handles temperature and strain rate effects and captures the microstructurally induced changes on the stress-strain response. The thermomechanical fatigue response under in-phase (TMF IP) and out-of-phase (TMF OP) conditions was simulated and the material exhibited a decrease in the stress range by as much as 50% with continued cycling. The decrease in strength was attributed to the significant coarsening of the precipitates at high temperatures and was confirmed by transmission electron microscopy.