Characterization of the NiBe nanoprecipitates in CoCr superalloys
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abstract
Cobalt based superalloys were developed for applications in high temperature, corrosive environments and for bio‐implants such as orthodontic devices. CoCrNi alloys have an FCC structure and excellent mechanical properties and their high strength is obtained through a combination of solid‐solution and precipitate strengthening. As such, these alloys are primarily Co with 15–20 wt.% Ni and 15–20 wt.% Cr and contain a cocktail of additives composing of Fe, Ti, W, Mo, and C. Some alloys contain a small amount of Be which is thought to improve mechanical properties via solid‐solution strengthening mechanisms. In copper based alloys, Be additives are well‐known to precipitate into GP zones and improve mechanical propertiesvia precipitate hardening mechanisms. Beryllium having a low solubility in most alloys and an affinity to form intermetallic compound, especially with Ni and Ti [1], and thus secondary precipitation of the Be intermetallics in cobalt superalloys is likely. So the question arises, whether the observed improved mechanical results from GP zone and precipitation hardening rather than the assumed solid‐solution solute strengthening.Using aberration corrected STEM and HR‐STEM EDS, we have characterized wrought and overaged alloys and find that Be forms GP zones and Ni‐Be B2 intermetallic precipitates. These alloys contain roughly 1 at. % Be, and thus detecting its presence in solid‐solution or within precipitates, especially those containing Ni, is challenging. Beryllium cannot be detected with EDS X‐ray analysis nor can it be detected with EELS if the precipitates contain large concentrations of Ni as the K edge of Be overlaps with the M edges of Ni. HAADF‐STEM images (Fig. 1 A)) are indirect proof and suggest that Be forms intermetallic precipitates with Ni that have a much lower intensity compared to the matrix material, which is primarily Co, Ni and Cr. The EDS maps in Figure 1 confirm that the precipitates primarily contain Ni and thus should have similar Z contrast and intensity as the matrix, though it does not. The precipitates must have a high concentration of lighter Z element, the only one having an affinity to form compounds with Ni and Co is beryllium. Furthermore, we have directly confirmed the presence of beryllium using a special imaging technique, integrated differential phase contrast imaging (IDPC), that is shown in Figure 2 B) [2]. IDPC images are generated using a segmented ADF detector and by integrating the 2 components of the center of mass (COM) of the image given by the momentum transfer of electron probe to the specimen. The resulting image from the integration of the differential COM signals gives a direct map of the associated phase shift due to the interaction of the probe with the sample and thus maps both Be and Ni atoms. Using IDPC is an easy method for directly visualizing Be in the precipitates.Crystallographic analysis of the precipitate's orientation using aberration corrected (AC) STEM images (Figure 2 C) ) suggests that they have a Kurdjomov‐Sach relationship ({110}BCC/{111}FCC). Though the precipitates differ in orientation relationship with those previously observed in Ni‐Be alloys[1], their measured lattice constants (0.262 nm) and ordered B2 structure are the same. The coherency strain can be easily observed in the IDPC image in which the first two lattice planes are compressed by ~1% along the <211> direction. In the wrought alloys, we observe Guinier‐Preston (GP) zones with similar orientation relationships as observed in the overaged material. The GP zones generate a large coherency strain along the <211> direction as seen in the ADF‐STEM images (Figure 3 B). The GP zones grow along three <111> matrix directions, two that lie in the [110] Z.A. and one on an inclined plane. Using the strain contrast in the ADF images to locate the GP zones in a thin region of the foil, we can observe their size and structure in AC‐HAADF STEM images. The GP zones are small being only 1 monolayer thick and are elliptically shaped with the major axis being 20–30 nm. The atomic column intensity of the GP zone is much lower than the matrix and similar to what was observed in the overaged precipitates. Though beryllium cannot be directly detected, it can be inferred that the GP zones contain Be from the analysis of the overaged precipitates. It can be speculated that the nanoprecipates form via the ripening of GP zones, and beryllium being observed in them is the first direct link that the improved mechanical properties may result from precipitation hardening rather than the perceived solid‐solution mechanism.
