High Temperature Nano-Indentation/Scratch of Ni Alloys Oxide Layer Under Helium Environment

Md Saifur Rahman¹, Ali Beheshti², Andreas A. Polycarpou¹

¹Department of Mechanical Engineering, Texas A&M University, College Station, TX, 77843

²Department of Mechanical Engineering, George Mason University, Fairfax, VA 22030, USA

INTRODUCTION: The very-high-temperature reactor (VHTR) is a type of high-temperature reactor (HTR) that can conceptually have an outlet temperature of 1000 °C and coolant gas pressure of up to 9MPa. The very-high-temperature reactor (VHTR), or high-temperature gas-cooled reactor (HTGR), is a Generation IV reactor concept that operates in very high temperature (700-950 ^oC) and in the presence of helium (He) coolant. It is known that nickel-base superalloys show relatively good resistance against various types of environmental degradation such as oxidation, carburization and decarburization because the protective external oxide prevents the direct interaction of the metal with the helium coolant gas containing the impurities. However, at the high temperatures in HTR, the oxidation resistance of the nickel-base superalloys decreases dramatically because the surface oxide layer no longer acts as an effective barrier against the potentially damaging environment due to the spallation and evaporation of Cr₂O₃ as well as the high atomic diffusion rate. Inconel 617 and alloy 800HT are the most promising materials for VHTR and HTGR reactor operating temperature.

The objective of this research project is to study the micro/nano scales properties of Alloy 800HT and 617’s surfaces and their oxide layers under high temperature.

HT NANOINDENTATION EXPERIMENTS: Nanoindentation experiments are conducted using a commercial nanoindenter shown in Fig. 1. (TriboIndenter (TI) Premier, Hysitron Inc., Minneapolis, MN). High temperature Berkovich indenter with high temperature stage is used which can measure mechanical and tribological properties up to 800^oC. For nanomechanical testing, the alloy samples are polished to roughness (Rq) of 30nm and cleaned using acetone in ultrasonic cleaner. The indentation system also possesses a scanning probe microscopy (SPM) capability and therefore, in-situ surface topography can be measured using a low contact force of 2 μN. It has the capability of supplying gas in the chamber to create required inert environment.


Fig. 1 Nano indenter with HT Stage

To extract mechanical properties such as H and Er from contact area measurements, the same formulation is used which is based on the well-known compliance method for nanoindentation experiments by Oliver and Pharr (1992).¹ Mechanical properties are calculated based on the contact area the probe tip makes with the sample under a specific load.

Results and DISCUSSION: Nanoindentation experiments are performed on the samples using the load curve with holding time at peak load that is used to identify the creep behavior of the material and another holding time during unloading is used to study the thermal drift of the samples at different experimental conditions.

The peak force of 1mN is used and the average result is based on four indentations across different sites. A standardized testing protocol is followed for all experiments. After each high temperature experiment the tip area function has been recalculated by tip calibration to ensure correct measurement of mechanical properties. After the stage and sample reach desired temperature, the setup was kept inactive for one hour to achieve thermal stability.

Fig. 2 Load control indentation curve on Inconel 617 at different temperature using (a) Diamond tip, (b) Sapphire tip.


Fig. 3 SPM images of Inconel 617 surfaces after different temperature nanoindentation.

Fig. 2(a, b) show sample indentation curves at different temperatures and the associated estimated reduced elastic modulus and hardness of Inconel 617 as a function of temperature are shown in Fig. 2(c, d). Both hardness and reduced elastic modulus show a decreasing trend with the increase of temperature. The drift rate decreased continuously at each experiment as the tip was in contact with the sample for longer duration. Fig. 3 shows the SPM scan of the indentations.


Fig. 4 Nanoindentation test results on cross secion of Inconel 617 oxidized at 950 C in He for 100 hr.

Fig. 5 presents the nanoindentation hardness results done on the cross section of oxidized Inconel 617 at 950^oC in He for 100 hours. The nanoindentation results on the oxide layer of the oxidized sample shows the hardness of around 25GPa. Nanoscratch technique is also implied along with nanoindentation to explore the scratch resistance, deformation and damage behavior of the oxide layer.

An FE model, capable of measuring thermal drift rate of the indenter, was built and A modified extraction algorithm was implemented to obtain the elastic modulus, yield strength and Poisson’s ratio with less than 5% error. The model was also applied to the nanoindentation creep results of Inconel 617.

REFERENCES:  1. Oliver, Journal of materials research (1992).