Studying the Impact of Solid Solution Hardening on Nickel-Based SuperAlloys

Materials used for making gas turbine blades are exposed to extreme operating conditions, as the blades need to withstand centripetal forces during long periods of operation at the temperature range of 700-1100°C.

Dimensional tolerances in the turbine demand a material free from creep and capable of withstanding a highly corrosive environment. As the turbine efficiency improves with increasing operating temperatures, development of superalloys capable of performing at higher temperatures is an area of intense research.

Significance of Understanding Hardening Mechanisms in Alloys

Nickel-base superalloys are complex alloys that are often composed of over 10 elements. Alloying with an increasing proportion of refractory elements is one method of imparting improved creep resistance and high temperature capability to the alloys. Re and Ru are two such alloying elements, of which Re is used to improve the creep resistance of the alloy and Ru is used to minimize the formation of topologically close-packed phases.

Gaining insights into hardening mechanisms in the alloy and the influence of alloying elements on the y matrix and y' precipitates is essential in designing alloys with optimized properties. Nanoindentation in conjunction with SPM imaging provides the suitable method to perform direct measurement of the properties of the individual phases.

Sample Preparation

One control alloy based on CMSX-6 (wt.%: Al 4.9, Ti 3.9, Cr 8.2, Co 4.1, Mo 2.5, Ta 0.6, Ni 74.8) and three alloys composed of 3 wt.% Re, 3% Ru, and 3% of each, were directionally solidified in a Bridgman furnace, followed by annealing. The samples were then cut along the {001} crystallographic plane of the y matrix at right angles to the solidification direction, followed by polishing with SiO₂ to a roughness below 2nm. Figure 1 illustrates that polishing in this orientation helps observing the cuboidal y' precipitates clearly as rectangular areas separated by thin channels of y matrix. The alloy with 3% Re and 3% Ru had the smallest precipitates, and for that sample, the average y' size was 790nm and the y channel width was 250nm.

Figure 1. In-situ SPM image of indentations on the Ni-base alloy showing tests positioned on specific phases.

Experimental Procedure

The y' precipitates were slightly recessed caused by the polishing procedure, which is a characteristic effect from polishing with SiO₂. Several nanoindentation tests were conducted on the different phases by applying a maximum load of 250µN utilizing a diamond cube corner indenter probe. Figure 2 provides the comparison of a load-displacement curve from an indent on a precipitate to an indent on the matrix. The "pop-in" events, observable in both curves, indicate the onset of plastic deformation during the indentation. The similarity between the curves before the pop-in reveals that the phases had the same elastic properties. The force needed to produce the pop-in demonstrates that the y material yielded much before the y' precipitates.

Figure 2. Load-displacement curves on alloy Re with nominal 3 wt.% Re.

Experimental Results

The hardness tests on the alloys showed that the addition of Re and Ru had different effects on the y matrix and y' precipitates. Figure 3 delineates the results from tests on the four alloys. The matrix hardness increased significantly with the addition of Re, while the hardness of both phases increased with the addition of Ru. The hardness increase in the precipitates was slightly more when adding both elements compared to adding Ru alone.

Figure 3. Hardness of the γ-matrix and γ´-precipitates for the different alloys.

In all cases, the hardness increase was caused by a solution strengthening mechanism. It is essential to take into account the partitioning behavior of the alloying elements Re and Ru to gain insights into the hardness change in the respective phases in the different alloys.

Energy Dispersive X-ray Spectroscopy (EDS) measurements taken on the alloys in a TEM showed the uneven distribution of the alloying elements. Re was present predominantly in the matrix, while Ru was more uniformly distributed between both phases. The partitioning behavior of the two elements can qualitatively describe the trends in hardness seen in Figure 3. Nevertheless, the presence of Re and Ru elements has affected the distribution of other elements in the alloy.

Conclusion

Hardness testing in conjunction with local chemical analysis facilitates exploring the effect of a single alloying element on different parts of the microstructure in nickel-base superalloys. The large number of elements used for these high performance alloys needs an in-depth analysis to gain knowledge about the complex interactions between the different components of the alloy.

This analysis showed that mechanical measurements with high spatial resolution and in-situ SPM imaging are crucial to gain knowledge about the underlying principles influencing the overall performance of a material. This data will be helpful in designing future alloys capable of operating under increasingly challenging environments.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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