The strain rate sensitivity of materials is often used as an indicator of the operative deformation mechanism, providing insight into the material’s performance. By measuring the rate sensitivity as a function of temperature, the operando performance of a material can be assessed. In this application note, we perform strain rate jump indentation tests on commercial purity Nickel at temperatures up to 600 °C. Strain rate sensitivity was observed to remain relatively constant until 500 °C, which a significant increase due to dynamic recrystallization of the sample under the indentation was observed in-situ. At 600 °C, significant reactions and adhesion between the Nickel sample and diamond indenter was observed. This provides a guideline maximum homologous temperature of ~45% of melting for testing with sample-indenter combinations that are known to be reactive. This illustrates how strain rate control can be used in combination with high temperature testing to provide insight into the operative deformation mechanisms during indentation.
Dynamic indentation techniques like strain rate jump (SRJ) nanoindentation are excellent methods for characterizing the time dependence of local mechanical properties [1]. As discussed in another application note, strain rate jump nanoindentation measurements offer several advantages over conventional constant strain rate (CSR) indentation tests. To determine strain rate sensitivity, a large number of CSR nanoindentations must be made over an area, requiring on the order of a few hours of testing time. SRJ nanoindentations can measure the same property within a single test, locally on a particular microstructural feature, orders of magnitude faster. This increases the lateral precision and greatly reduces the required time. This is essential for applications such as Mechanical Microscopy.
However, higher-speed, local measurements are also desirable for investigating properties in environmental conditions approximating the service conditions of materials, or operando testing. High temperature testing is an excellent example of this, where faster measurements both reduce any possible influence of thermal drift and allow for a larger number of temperature increments to be investigated during a session.
In this application note, we demonstrate the application of high temperature, strain rate jump nanoindentation on commercially pure Nickel. Nickel alloys are particularly relevant for high-temperature aerospace applications, so Nickel is an excellent case study material for these applications. At high temperatures, chemical reactions between the samples and indenters are often a concern, so in this work we investigate the maximum safe temperature for testing Nickel alloys with diamond indenters. This will allow users to confidently perform tests at elevated temperatures without sacrificing expensive diamond indenters. The resulting hardness, reduced modulus, and strain rate sensitivity values for Nickel are then analyzed and discussed in relation to literature values.
For high temperature testing, several technical features are required to allow testing to be performed in conditions similar to room temperature. Ideally, a system operated at high temperatures should behave identically to how it performs under ambient conditions with a similar degree of ease of use, no changes in dynamics or noise, and stable performance over long test periods. To achieve this, the high temperature module of the NMT04 was designed with a number of features - Figure 1.
To remove any possible vibrations from turbulent fluid flow, cooling of the system is employed purely through passive means using copper braids to conduct the heat away to a large heat sink, which is conductively cooled from the outside of the SEM chamber. This removes any danger of water leaks inside the vacuum chamber, prevents any mechanical vibrations during operation, and provides a constant noise floor, independent of temperature.
Figure 1. NMT04 with high temperature module features labeled. Cooling copper braids not shown.
The low thermal mass of the heated portion of the indenter tip on the MEMS heater allows the tip to be rapidly matched with the sample surface temperature using an automated procedure. This consists of performing a series of surface find procedures, where the indenter is rapidly brought into contact with the surface and briefly held there at a constant force. Any deviations in displacement during the hold period are noted and used to align the tip and surface temperature. The tip is then retracted and the procedure repeats until the user is satisfied with the degree of alignment. This procedure automatically accounts for thermal expansion of the system components, as it effectively follows the sample surface through a series of surface finds and retractions.
This combination of precisely-aligned indenter and sample temperatures and stable thermal gradients within the surface frame are essential for achieving low thermal drift (displacement changes due to thermal expansion) during measurements.
At high temperatures, chemical reactions between indenter and sample materials are more likely to occur due to the available thermal energy. Some combinations of sample and indenter materials are particularly difficult (e.g. diamond indenters and carbide-forming materials like iron/steel) and can lead to rapid degradation of the indenter’s tip shape during testing. A review of this phenomenon and various indenter/sample combinations is available in the literature [2].
In this work, we investigate a sample/indenter combination which is known to be problematic at high temperatures: nickel sample / diamond indenter. While not a carbide-former, nickel presents significant carbon solubility and reactivity at high temperatures, due to its electron vacancies in the s and d orbitals [3]. While this combination is known to react at high temperatures, the “safe” threshold temperature for testing such reactive combinations has not yet been determined, i.e. at what temperature do the reactions become problematic? In this work, we push the limits of high temperature testing to determine this threshold.
Testing was performed on the Nickel side of a Ni—Mo diffusion couple away from the interdiffusion zone. Nickel and Molybdenum bars (99.99%, Zhongnuo New Materials Ltd.), both of about 1 cm in length, were gently compressed at 150 MPa and subjected to 900 °C in vacuum using the Gleeble 3500 dynamic testing system. Afterward, the Ni–Mo junction was encapsulated in an argon-purged Quartz tube and annealed at 1200 °C for 168 h, followed by a water quench. The sample was sectioned using an alumina cut-off wheel (Struers 50A 13), then polished using successively finer diamond abrasives, finishing with a 60 nm SiO2 particle suspension.
Nanoindentation strain rate jump testing was performed on each sample using an NMT04 Femto-Indenter with a diamond Berkovich indenter on a FT-S200,000-HT sensor. Testing was performed in situ in a Zeiss EVO 25 scanning electron microscope (SEM) operating at an acceleration voltage of 10 kV. Each indentation was performed using a continuous stiffness measurement (CSM) method [4] in displacement control. Strain rate jump nanoindentation is best performed using continuous stiffness measurement to provide hardness measurements as a function of depth. This allows strain rate jumps to be performed with the hardness continuously evaluated during the jumps.
Indentation strain rate jump tests were performed using a method similar to that of Maier et al. [5], where the strain rate is abruptly varied during indentation to several different levels. A minimum of 4 indentation strain rate jump tests were performed at strain rates of 0.1, 0.01, and 0.001 s-1. The indentation strain rate was initially held constant at 0.001 s-1 until a depth of 400 nm was reached, ensuring that a constant microstructure was present underneath the Berkovich tip. Afterwards, the applied strain rate was increased to 0.1 s-1, then back to 0.01 s-1, down to 0.001 s-1, and finally to 0.01 s-1 with each segment maintained for 100 nm of displacement, resulting in a maximum indentation depth of 800 nm. By repeating the same strain-rate during multiple segments at different indentation depths, the reversibility of the jumps can be illustrated [5]. This demonstrates that the hardness is constant for a fixed indentation strain rate.
Testing was performed at ambient temperature and increasing increments of 100 °C up to 600 °C. During each temperature ramp, tip temperature matching was performed using the procedure described earlier.
Figure 2. Force, hardness, and reduced modulus vs displacement during strain rate jump nanoindentation of Nickel at temperatures up to 600 °C.
The first results to consider are the measured indentation curves. Figure 2 shows the force, hardness, and modulus as a function of the displacement from strain rate jump indentations on Nickel at various temperatures. Force-displacement curves were observed to be repeatable with abrupt changes in slope occurring at displacements where the applied indentation strain rate was varied. As the temperature was increased, the measured forces for each applied displacement were observed to decrease, indicating a proportional decrease in hardness with temperature.
Strain rate jumps during indentation are observed to have a noticeable effect on the hardness-displacement response in Figure 2, particularly at 500 °C. In general, hardness decreases with increasing temperature, as expected. If the indentation size effect influence is subtracted, the hardness values at each jump reversibly return to approximately the same value when the strain rate returned to that rate, as seen before [5].
Reduced modulus is seen to be relatively constant at each temperature, despite the strain rate variations, however the reduced modulus value does decrease with increasing temperatures.
As indentation testing was performed in situ in a scanning electron microscope. The indentation testing of the Nickel sample was observed and recorded at an observation angle of 10° from the sample plane. Still micrographs from the higher temperatures are shown in Figure 3.
Figure 3. In-situ secondary electron micrographs captured during indentation at high temperatures illustrating the changes in mechanical behavior.
At temperatures up to 400 °C, the residual impressions left after indentation were uniform with a slight sink-in morphology, consistent with room temperature observations. However, at 500 °C, a distinct change in behavior was observed where the indentation appeared almost perfectly plastic with small extrusions of material at the edges of the indentation. By 600 °C, the surfaces of several nickel grains were observed in situ to begin to thermally etch [6].
On performing indentations at 600 °C, indentation curves appeared normal during loading, but upon unloading there was significant adhesion of the Nickel to the diamond indenter (Figure 3) with the material pulling off with the indenter. This resulted in the residual impression being destroyed on removal of the indenter and necessitated several ‘cleaning’ indentations to be performed on the molybdenum sample mounting ring to remove the adhered Nickel before further indentations could be made. As this adhesion indicates the onset of significant chemical reactivity between the diamond and the nickel, higher temperatures than 600 °C were not attempted to minimize damage to the indenter. This indicates that the ‘safe’ temperature threshold for diamond indenters on Nickel samples is between 500 and 600 °C.
Trends in hardness, reduced modulus, and strain rate sensitivity, along with top-view electron micrographs of indentations, as a function of temperature are given in Figure 4. Hardness trends as a function of temperature are observed to be consistent with those of Minnert et al. [7], though their indentations were performed to a depth of several microns. Despite the decreasing trend in hardness with temperature, top-view secondary electron micrographs display a consistent size. This is due to the indentations being performed in displacement control to a fixed depth target. Indentations performed with load control to a fixed load target would display an increasing indentation size with temperature. As noted above on Figure 3, indentations up to 400 °C show a similar deformation behavior, while indentations at 500 °C show extruded, recrystallized grains around the edges of the indentation.
At 600 °C, the adhesion with the indenter distorted the residual impression on removal of the indenter, while the surrounding material display thermal etching. While these thermal reactions may elicit some doubt as to the veracity of values obtained at this temperature, the datapoints for hardness, reduced modulus, and strain rate sensitivity acquired during loading appear consistent with the trends with temperature. This suggests that the influence of the adhesion and reactions may mostly be on the unloading of the sample, not the deformation during loading.
Reduced modulus values are found to be consistent with literature values from Köster [8] measured using acoustic techniques with some additional variation that is attributed to anisotropy between various grains.
![Hardness, reduced modulus, and strain rate sensitivity values for Nickel compared to literature values [7, 8] with top-view secondary electron micrographs of residual indents from each temperature.](https://www.oxinst.com/learning/uploads/inline-images/ni-an22-fig4-750-20250528133649.png)
Figure 4. Hardness, reduced modulus, and strain rate sensitivity values for Nickel compared to literature values [7, 8] with top-view secondary electron micrographs of residual indents from each temperature.
The strain rate sensitivity, m, was be determined using the following relationship [5, 9]:
Hardness is related to strength using the Tabor confinement parameter, the Berkovich hardness value is related to the equivalent flow stress, σf, at 7% representative strain (eBerkovich = 7%) by the relationship, σf = H/2.8 [5]. This makes the strain rate sensitivity exponent, m, the slope of the trend on a log-log plot of stress and strain rate. The hardness values from the SRJ tests were taken after subtracting the influence of the ISE. Strain rate sensitivity trends broadly agree with those observed by Minnert et al. [7], with a slight increasing trend with temperature. SRS Values at lower temperatures near ambient are slightly higher then might be expected, this is attributed to a high defect density within the diffusion couple sample than a typical well-annealed polycrystalline sample. The increase in strain rate sensitivity at 500 °C is attributed to a diffusive process occurring during deformation, producing the extruded and recrystallized features seen in the micrographs. Values at 600 °C return to the trend observed by Minnert et al. However, without a residual impression, the deformation mechanism, though almost certainly including some form of diffusive process, remains unclear.
In this application note, high temperature, strain rate jump nanoindentation testing was demonstrated on commercially pure Nickel. This allowed hardness, reduced modulus, and strain rate sensitivity to be determined at temperatures up to 600 °C. Both hardness and reduced modulus values were found to be consistent with literature values. Strain rate sensitivity values indicated that the operative deformation mechanism was consistent from ambient to ~400 °C. At higher temperatures, both in situ observations and strain rate sensitivity indicate a change in deformation behavior with the activation of additional diffusive mechanisms. This demonstrates how strain rate control can be used in combination with high temperature testing to provide insight into the operative deformation mechanisms at temperature.
A major result of this work is the determination of the ‘safe’ temperature range for investigations on reactive, ferrous metals like Nickel. No significant reactions were observed between the diamond indenter and Nickel sample until temperatures >500 °C, or ~45% of the melting point of Nickel. This suggests that testing of other similarly reactive indenter/sample materials may be feasible without significant indenter damage up to a similar homologous sample temperature. For testing these materials at higher temperatures, use of alternative indenter materials, e.g. sapphire or WC, is recommended.
© Oxford Instruments 2025
