Dissolution kinetics of δ phase and its influence on the notch sensitivity of Inconel 718
Introduction
The Ni–Fe base superalloy Inconel 718, which is age-hardened by combined precipitation of fine γ′ and γ″ phases in an austenitic matrix, has been widely used for high temperature services such as gas turbine disks [1], [2]. The precipitation of intermetallic phases in Inconel 718 has been intensively studied. It has been found that the major hardening phase is the metastable γ″ phase which goes to the δ phase in equilibrium. All these three phases are of the A3B type: the γ″ and δ phases are based on the composition Ni3Nb and the γ′ phase on the composition Ni3(AlTi). The amount of precipitation phases, together with their shape and distribution, has a determining influence on the mechanical properties. Considerable research has been carried out on the strengthening effects of the γ′ and γ″ phases and the toughening effects of δ phase in order to improve the overall mechanical properties. It is widely accepted that δ phase has important effects on the microstructure and mechanical properties [3], [4]. Recently, grain size control in “Delta Processed” Inconel 718 and the influence of δ phase on high temperature notch sensitivity and impact toughness have been studied [5], [6]. The results indicate that δ phase can form in the alloy during processing or in service, generally appearing in plate-shaped or globular morphology. Controlled precipitation of the δ phase at grain boundaries is believed to have beneficial effect on the stress rupture ductility, and can also inhibit the growth tendency of the austenitic grains during the forging process. The δ phase will partly be dissolved into the austenitic matrix during the hot working. As far as the controlled precipitation is concerned, the precipitation and dissolution behavior are the two most important aspects, but little work has been performed on the latter aspect.
The goals of the present work are to analyze the dissolution behavior of δ phase and to determine its effects on the high temperature notch sensitivity of Inconel 718.
Section snippets
Materials and heat treatment
Hot rolled bars of Inconel 718 with an average grain size of about 20 μm were chosen for the dissolution behavior analysis. The chemical composition is listed in Table 1. Solid solution annealing was performed at 980 °C for 3 h followed by air cooling. A δ phase ageing treatment (890 °C for 20 h) was carried out to insure adequate amounts of δ phase in the austenitic matrix for dissolution analysis. Dissolution treatments were performed at 980 °C, 1000 °C and 1020 °C.
X-ray diffraction quantitative phase analysis
According to the accepted phase
Dissolution process of δ phase during holding at high temperatures
Changes of the amount of δ phase in Inconel 718 during holding at different temperatures, as determined by XRD, are shown in Fig. 2. It can be seen that the amount of δ phase decreases with an increase of holding time at different temperatures. The phase cannot be fully dissolved into the austenitic matrix at 980 °C and dissolution rate is relatively low. An equilibrium state, with a δ phase amount about 3 wt.% will be reached after holding for more than 30 min, and a certain amount of δ phase is
Dissolution kinetics of δ phase in Inconel 718
For further understanding of the dissolution process of δ phase in Inconel 718, the amounts of δ phase during dissolution process are defined as follows: W0 is the δ phase amount before dissolution treatment; Wt is the δ phase concentration at some instant during the dissolution process; ΔW = (W0 − Wt,) is the amount of δ phase dissolved at some instant. Curves of the amount of δ phase dissolved versus time at different temperatures are shown in Fig. 3. Three different dissolution stages are
Conclusions
- 1.
The δ phase can be fully dissolved into the austenitic matrix after holding at 1020 °C for more than 2 h. A certain amount of δ phase still exists in the alloy after holding at 980 °C and 1000 °C for more than 6 h.
- 2.
The dissolution rate keeps at a high level at the beginning stage, and then decreases gradually with the increase of holding time. A dynamic equilibrium state will be approached holding at 980 °C for more than 30 min and at 1000 °C more than 2 h.
- 3.
The alloy has no notch sensitivity with the δ
Acknowledgement
This work is supported by the National Nature Science Foundation of China under contract No. 59971039.
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