Superalloy Structure at High-Temperatures - An In-Situ Study

Bond coatings are deposited as intermediate layers on superalloy structures to improve several critical performance criteria, including hot corrosion resistance. This limits atomic migration of the base metal and adhesion of a thermal barrier coating.

Synthesis of the Pt-Aluminide bond coating causes a microstructurally and compositionally graded structure, typically comprising of three discrete layers: an interior interdiffusion zone consisting of coarse precipitates in B2-NiAl matrix, an intermediate layer consisting of β-(Ni,Pt)Al and an outer layer consisting of intermetallic PtAl₂ and W-rich fine precipitates.

It is important to understand the mechanical properties of both the superalloy substrate and bond coating at elevated temperatures to advance the development of these material systems for use in extreme situations.

Hysitron SEM PicoIndenter with 800 °C Heating

A Hysitron^® - PI 88 SEM PicoIndenter^® fitted with the 800 °C heating option (Figure 1) was employed to carry out micropillar compression testing inside an SEM to assess the mechanical properties of the superalloy substrate and bond coating at high temperatures.

In-situ mechanical testing enables precise alignment of the tip with the sample and direct, real-time examination of the deformation processes. A side benefit of carrying out these tests in the SEM is that the high-vacuum atmosphere limits the oxidation of the sample, particularly at high temperatures, allowing the measurement of the true mechanical properties of the superalloy and bond coating materials.

Figure 1. Schematic of Hysitron PI 88 with the 800 °C heating option.

Testing Procedure

The focused ion beam (FIB) milling process was used to prepare micropillars in the Pt-rich and base superalloy regions. The Hysitron PI 88 fitted with a flat punch probe was used to compress micropillars.

The system’s displacement-controlled feedback mode was used to compress the pillars to 5-12% strain at a strain rate of 10^-2 - s^-1. Compression tests were performed at room temperature (RT) and various elevated temperatures up to 800 °C. The heating was accomplished through closed-loop resistive heating of both the sample and probe.

For each test, real-time video was recorded together with the load-displacement data. The synchronization of these two pieces of information helped validate the test and facilitated a thorough analysis of the deformation across temperatures.

High-Temperature Uniaxial Pillar Compression

Bond Coating

The microstructure of the deformed pillar shows grain boundary rotation and sliding at temperatures higher than 700 °C. Transgranular cracks can be clearly seen in the microstructure below this temperature.

Furthermore, intergranular surface cracks appear near the top surface at 750 °C and above. As illustrated in the stress-strain curves in Figure 2, plasticity in the bond coat is characterized by major strain hardening after yielding at room temperature and partial strain hardening at higher temperatures. The modulus of the bond coating decreased by ~9% at 800 °C compared to room temperature. A more vital change was seen in the yield strength, which was decreased by ~50% after heating to 800 °C.

Figure 2. Morphology of the bond coating pillars after compression at RT (left) and 800 °C (middle). Transgranular cracking can clearly be seen in the pillars tested at room temperature, while intergranular cracks appear only at high temperatures. The stress-strain curves (right) indicate major strain hardening at RT, which is more limited at higher temperatures.

Superalloy

The major load drops seen in the stress-strain curves of the superalloy (Figure 3) are connected with the formation of slip bands. At 600 °C, a greater number of slip bands with larger step size can be seen on the pillar surface compared to room temperature. At 600^oC the measured yield strength and elastic modulus decreased by ~20% compared to room temperature.

Figure 3. Morphology of the superalloy pillars after compression at RT (left) and 600 °C (middle). The stress-strain curves (right) show significant load drops, which are associated with the formation of slip bands shown in the images.

Figure 4. Variation in elastic modulus and yield strength as a function of temperature for the PtAl bond coating.

Conclusions

Compared to room temperature, the yield strength and modulus of the bond coating were found to decrease by ~50%  and ~9% at 800 °C, respectively. Grain boundary rotation and sliding dominate the plasticity at temperatures above 700 °C.

Several slip bands were seen in the Ni-based superalloy. The severity and density of the slip banding increased with temperature.

The Hysitron PI 88 - with the 800 °C heating option combines quantitative mechanical characterization at elevated temperature with real-time SEM imaging for a detailed analysis of the deformation mechanisms in superalloy bond coating systems.

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

For more information on this source, please visit Bruker Nano Surfaces.