Superalloy is a material specially designed for high creep resistance and strength in high temperature conditions and service in high temperature environments. At present, they are made of a variety of complex materials, and these composite materials have a dozen or more components. In order to further improve the performance of superalloys, we usually coat the surface with a thermal insulation coating (TBC) composed of ceramics. This provides a protective appearance for the superalloy, which can protect it from harsh environments and heat.
TBC-superalloy thermal insulation coating bonding is achieved by bonding coating, and the bonding coating is formed by placing the alloy under high temperature conditions and oxygen atmosphere for a long time. When a bond is formed, oxygen enters the alloy at different rates through diffusion, while the various other elements that make up the alloy diffuse outwards accordingly. The resulting microstructure is complex and multilayered, creating a strong and durable bond between the insulating coating oxide and the internal metal substrate.
When the temperature changes, the mechanical properties of all materials change. This change is measurable, which means that a series of temperature parameters must be used to evaluate the properties of all adhesive layers. This is crucial when generating models of some superalloys with better mechanical properties. In most cases, the thickness of the adhesive layer is only a few microns, which makes it extremely difficult or even impossible to use traditional creep testing on a separate barrier layer.
This article describes the results of research on the mechanical properties of the adhesive layer as a function of temperature and time, in order to clarify the complex interactions between the various components of various superalloys and their conditions of use.
Figure 1. SPM image of a cross-section sample surface showing a characterised adhesive layer. Collect images using the same probe tip for performing tests with very precise test positioning
experiment process
In this experiment, a commercially-available nickel-based (CM-247LC) superalloy sample was produced with an adhesive coating graded in composition by heating in air. We take the sample, cut it into cross sections and polish it so that we can observe the microstructure of the layer. As shown in the SPM image in Figure 1, each area has a different surface texture.
The first region (region 1) is composed of a NiAl matrix and an alloy containing W and Cr precipitates. Region 2 consists of a solid solution of Pt, Ni and Al. Creep tests were performed on the two layers by nanoindentation at multiple temperatures between 25 ° C and 750 ° C.
In the experiment, the specific technique we used is dynamic creep test. In this case, the experiment uses a small oscillation at a specific reference frequency of 220 Hz and superimposes it on the load function (quasi-static), so that it can continue through the test. Time to continuously measure contact stiffness. The reference creep test results depend on the relationship between contact area and contact stiffness, which gives the performance values of the materials tested over time.
In this experiment, the creep test of the nanoindentation was performed at 25 ° C, 500 ° C, 650 ° C, and 750 ° C, respectively, in Zone 1 and Zone 2. The device consists of a Hysitron® TI 980 TriboIndenter® equipped with an xSol® heating stage and a Berkovich probe for indentation.
Result analysis
The instrument's in-situ SPM imaging acquisition can be used to select and confirm the location of the creep test nanoindentation. Figure 2 shows an example of an SPM image at 650 ° C. The data values obtained from the creep test at each temperature in the two regions are shown in Figure 3 (above). They reveal that when the quasi-static load is constant, the indentation depth increases with time. Figure 3 (below) shows how the hardness decreases as the depth of the indentation increases.
Since minimum room temperature creep is expected, baseline measurements were obtained by performing the same test at 25 ° C. As the temperature increases, the hardness decreases from the initial value, and the creep rapidly increases. The following equation shows the change in creep in steady state:
Where ε is the strain rate, A is the proportionality constant, m is the stress index, Q is the activation energy, R is the gas constant, and T is the absolute temperature
The creep mechanism shows a change in correlation with the stress index m and / or the activation energy Q stiffness value is continuously available when performing a reference creep test. Therefore, the strain rate is determined continuously, as follows:
Where k is the contact stiffness.
The representative stress is assumed to be hardness or average contact pressure. In order to calculate the stress index m, the slope of logε • log H is used here, as shown in Figure 4 (above). During the initial phase of the test, transient non-linear behavior was observed in the logε • log H curve during the first 100-200 seconds. This is not shown in Figure 4 (above) because this phenomenon is not a steady state observation.
Figure 4 (below) shows how the stress index m changes with temperature, revealing that the entire temperature range tested in Zone 1 is likely to be mainly deformed by the same creep mechanism, but in Zone 2, the value of m Significant changes have occurred, indicating that the region is likely to have more than one mechanism participating in the competition.
Figure 2. SPM image of a dent impression in area 2 collected at 650 ° C
Figure 3. (Top) Creep data at each temperature showing the evolution of the indentation depth during the test. (Below) Decay hardness over time at each temperature correlates with increased indentation depth
Figure 4. (Top) Strain rate and stress in each creep test, showing how to calculate the stress index. (Below) The changing stress index in zone 2 indicates that the creep mechanism has changed, while the consistent results in zone 1 indicate that the mechanism is unchanged
in conclusion
The nanoindentation creep test allows the use of very small samples to study the creep properties of materials, so it is possible to study the creep properties of a layer in a complex system with multiple layers. When the nanoDMA® III test is used with the xSol heating stage on the Hysitron TI 980 device, accurate long-term measurements can be performed at temperatures up to 800 ° C on the layer of choice.