Abstract
Microstructural changes of the Inconel 718 and ATI 718Plus® base materials and the fusion zone caused by the isothermal hold at 760 °C for up to 500 h were investigated. The evaluation was performed using a scanning electron microscope with backscattered electrons detector and scanning transmission electron microscopy supported by energy-dispersive x-ray spectroscopy. Furthermore, to evaluate the effect of microstructural changes on mechanical properties Vickers microhardness measurements were performed. Exposure of welded joint at high temperature led to coarsening and/or dissolution of main strengthening phases’ particles and nucleation and growth of plate-like precipitates. In the interdendritic region of the fusion zone, the transformation of the Laves phase to a complex cluster of precipitates contained σ phase had taken place. Analysis of microstructure and microhardness measurements showed that ATI 718Plus has better mechanical properties and microstructural stability at elevated temperature than Inconel 718.
Highlights
Inconel 718 (IN718) is the most popular precipitationstrengthened nickel-based superalloy introduced by the Huntington Alloys Division of INCO in 1959 (Ref 1)
These modifications led to replacement of metastable c¢¢ main strengthening phase in IN718 with more stable c¢ phase in 718Plus that caused a increase in thermal stability up to 704 °C
The scanning electron microscope (SEM)-BSE investigations (Fig. 2a-c) and quantitative analysis of the microstructure of IN718 base material showed two major microstructural changes after heat treatment at 760 °C: formation and growth of d phase particles, and coarsening followed by dissolution of main strengthening phases particles (c¢¢ and c¢)
Summary
Inconel 718 (IN718) is the most popular precipitationstrengthened nickel-based superalloy introduced by the Huntington Alloys Division of INCO in 1959 (Ref 1). ATI 718PlusÒ (718Plus) is a precipitation-strengthened nickel-based superalloy which has been developed by ATI Allvac company at the beginning of the twenty-first century. The principal difference in 718Plus chemistry is lower Ti-to-Al ratio as well as lower amount of Fe, which is replaced by the addition of Co and W. These modifications led to replacement of metastable c¢¢ main strengthening phase in IN718 with more stable c¢ phase in 718Plus that caused a increase in thermal stability up to 704 °C. Other constituents of 718Plus microstructure are c matrix and precipitates of hexagonal gNi3Ti, g*-Ni6AlNb or Ni6(Al, Ti)Nb phases distributed mainly at the grain boundaries (Ref [6,7,8,9])
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