625 Alloy

Alloy name: 625
Diagram No.: 817
Type of diagram: TTP
Chemical composition in weight %: 22 wt% Cr, 9 wt% Mo, <5 wt% Fe, 3.5 wt% Nb, balance is Ni
Alloy group: Nickel-based alloys
Note: Time-Temperature-Precipitation (TTP) diagram corresponding to Gamma” and Delta precipitation in Alloy 625 [98].
The long term ageing study was carried out and the mechanical properties of Alloy 625 were determined for samples exposed 8,000 hours at 649 C, 760 C and 871 C. It was found that an increase in tensile strength was observed after exposure at 649 C (812.2 Mpa compared to 456.4 Mpa in mill annealed condition) but that tensile strength dropped significantly to about 671.6 Mpa in samples aged at 760 C and further decreasing happen to about 439.2 Mpa at 871 C. The reported X-ray diffraction results of this study showed that two M6C phases and a Ni3Nb phase were present in the material aged at 871 C, but the increased strength at 649 C was not explained by the X-ray data.
Further investigations in this regard showed that strengthening at intermediate temperatures (between 600 to 700 C) in Alloy 625 occurred by precipitation of the metastable Gamma'' phase. This phase has a body centred tetragonal D022 structure while the equilibrium intermetallic phase is the Delta phase with D0a structure. It is believed that the metastable Gamma'' phase [Ni3(Nb,X)] forms on ageing at temperatures above 600 C and transforms to the orthorhombic Delta phase [Ni3(Nb,Mo)] upon prolonged ageing.
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