W. D. Cao, R. Kennedy, T. Bayha, ATI Allvac, Monroe, NC

Summary:

Allvac® 718Plus^TM is a new Ni-base superalloy developed at ATI Allvac. This alloy shows excellent mechanical properties and high thermal stability, equal to or better than Waspaloy, at temperatures up to 1400°F but with castability, hot workability and weldability approaching that of alloy 718. The total cost of alloy 718Plus components should be significantly lower than competing g’-strengthening alloys due to lower intrinsic raw material costs and better processability, making the alloy an excellent candidate to replace Waspaloy type alloys.

The mechanical properties and processability of alloy 718Plus have been evaluated in several studies, including the MAI “Low-Cost, High Temperature Structural Material” program sponsored by the United States Air Force. In parallel with these alloy evaluations, research work has been conducted to understand the physical metallurgy of this alloy and relevant results will be introduced in this presentation.

Phase transformations during solidification were studied by DTA and SEM/EDS techniques in comparison with modeling calculations and used to interpret as-cast microstructure and microsegregation. Chemistry, crystal structure and size distribution of precipitation phases responsible for age hardening were identified by TEM and X-ray analysis. Special attention was paid to grain boundary precipitates. The nature, morphology and kinetics of GB precipitates were studied. The effect of GB precipitation on mechanical properties, especially rupture ductility and hold-time fatigue crack growth were investigated. Transformation during long-term thermal exposure was also studied and the implication of this transformation to mechanical properties addressed. The role of chemistry on phase transformation and resultant mechanical properties will also be introduced.