High-throughput nanoindentation mapping of cast Inconel™718 nickelbased superalloy: influence of the Nb concentration(3)

Release time: 2022-01-14 15:50:16  Hits: 153

Regarding the reduced elastic modulus, Fig. 5.d, e and f, the values did not follow the trend observed in the hardness maps (Fig. 5.a-c) and they appear to be constant for each grain and less sensitive to chemical segregations at the dendrite scale. In the case of the solubilized state, the entire measurement area comprised just one grain with its surface normal oriented between the [001] and [101] directions (Fig. 2a), and the elastic modulus value was constant, around 182 MPa. Only the second phases present in the interdendritic spaces present much higher values, well over 200 MPa. On the other hand, the peak-aged and the overaged states present areas with different values of elastic modulus, that match closely with the different crystallographic orientations of the grains present in the measurement area in each case. The light blue grain in the peak-aged condition (Fig. 2b), with the surface normal between [101] and [111], presented the highest elastic modulus, around 210 MPa, (Fig. 5e), while the dark orange grain in the overaged condition (Fig. 2c), with its surface normal close to [001], presented the lowest elastic modulus, of around 165 MPa.

1. - Discussion The correlative studied carried out showed that, even though cast IN718 typically shows grain sizes in the order of millimetres, is a very heterogeneous material at the microscale. The dendritic growth during solidification leads to a large heterogeneity at the scale of the SDAS (around 150 μm in this case). As a result, strong chemical segregations occur within each grain at the dendrite scale, with dendrite cores that are richer in Fe and Cr and depleted in Nb, while the interdendritic regions are much richer in Nb and in hard second phases, such as the TiC particles (Fig. 1). The chemical segregation through the dendrite radius occurs due to differences in the diffusion rates of each constitutive element. While the slower elements, Cr and Fe, remain in the dendrite cores (Fig. 3d-f and g-I, respectively), Ti and Nb tend to segregate towards the outer part of the dendrites (Fig. 3p-r and s-u, respectively), and specially into the interdendritic areas. In the particular case of Nb, the presence of this element is crucial for the formation of the metastable �’’-Ni3Nb precipitates responsible for the strengthening of Inconel™718. These precipitates are disc-shaped (Fig. 4) and play a key role as strengthening agents due to their higher volume fraction with respect to �’ precipitates, as well as their coherency with the Ni matrix and the lattice distortion caused by the c-axis of the D022 body centred tetragonal �’’ structure [10,11]. Therefore, the Nb content is expected to play a crucial role on the local mechanical properties of this alloy. Fig. 6a represents the histogram of the Nb distribution for the three thermal treatments obtained from the corresponding EDS maps, excluding the second-phase particles (Fig. 3s-u). The distribution was similar for the three specimens, with a minimum and a maximum content of 2 and 8 wt.%, respectively, which indicates that Nb diffusion does not take place significantly during the temper treatments. On the contrary, the heat treatments are expected to affect the dissolution, precipitation and coarsening of the �’’ precipitates (Fig. 4), that are expected to be heterogeneously distributed at the dendrite scale, as a function of the local Nb content.

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