An improved long-term nanoindentation creep testing approach for studying the local deformation processes in nanocrystalline metals at room and elevated temperatures

Abstract

The strain-rate sensitivity of ultrafine-grained aluminum (Al) and nanocrystalline nickel (Ni) is studied with an improved nanoindentation creep method. Using the dynamic contact stiffness thermal drift influences can be minimized and reliable creep data can be obtained from nanoindentation creep experiments even at enhanced temperatures and up to 10 h. For face-centered cubic (fcc) metals it was found that the creep behavior is strongly influenced by the microstructure, as nanocrystalline (nc) as well as ultrafine-grained (ufg) samples show lower stress exponents when compared with their coarse-grained (cg) counterparts. The indentation creep behavior resembles a power-law behavior with stress exponents n being ∼ 20 at room temperature. For higher temperatures the stress exponents of ufg-Al and nc-Ni decrease down to n ∼ 5. These locally determined stress exponents are similar to the macroscopic exponents, indicating that similar deformation mechanisms are acting during indentation and macroscopic deformation. Grain boundary sliding found around the residual indentations is related to the motion of unconstrained surface grains.

Appendix: Experimental details for the nanoindentation long-term creep testing

Figure A1 shows the results of long-term creep tests at RT on ufg-Al with an initial indentation depth of 1000 nm and various creep durations up to 10 h. Included in the plot is also the temperature variation within the indenter chamber. The temperature fluctuates between 23.2 and 23.6 °C within a 10 h test segment. The displacements recorded by the nanoindenter are nonreproducible and unsteady as it directly reflects the slight changes in the chamber temperature. The temperature fluctuation causes therefore thermal drift effects, which can be easily corrected by using the dynamic stiffness signal [Fig. A1(a) ]. The corrected data based on the contact stiffness measurements show for all measurements a small but steady increase in the displacement with time and a better reproducibility [Fig. A1(b)].

FIG. A1.

ufg-Al—Creep data carried out during nanoindentation long-term creep tests at 1000 nm indentation depth and tested at room temperature (RT) with variation of the applied creep time (1 h, 2 h, 5 h, and 10 h); (a) Indentation depth data from the indentation system and exemplarily corresponding ambient temperature during 10 h experiment, (b) corrected indentation depth data according to Eqs. (2) and (4), (c) resultant $\dot h$. over creep time (with fused silica as a reference material), and (d) hardness over resulting creep-rate.

A good agreement between the different measurements of $\dot h$ over the applied creep time performed for ufg-Al is shown in Fig. A1(c). This demonstrates the consistency of the long-term nanoindentation data and it also confirms that the evaluation based on the contact stiffness is more or less not affected by thermal drift. More generally, the changes in the indentation depth $\dot h$ as well as the resultant creep-rates ${{\dot \varepsilon }}$ clearly decrease with increasing creep time, but never run in a plateau. Accordingly, indentation creep tests were also carried out for 2 h on fused silica. In comparison with ufg-Al, fused silica shows a significantly lower change in indentation depth [Fig. A1(c) ] and thus a lower creep rate [Fig. A1(d) ]. Generally, the increasing indentation depth is mainly caused by plastic deformation processes due to the applied constant load during the creep experiment. However, a pile-up formation or grain coarsening during creep might also lead to an increasing contact area and thus to an increasing creep rate or to a reduced hardness.

The resultant impressions from the long-term nanoindentation were intensively studied using SEM and FIB (Fig. A2). Magnifying the grains along the indenter edges show no significant grain coarsening during the long-term indentation experiments. By evaluating the projected contact areas (A_c−SEM) from these SEM images, a good agreement between the contact areas, determined from the dynamic CSM method and A_c−SEM was found. Therefore, additional pile-up effects or grain coarsening can be neglected here for the evaluation of the nanoindentation creep data.

FIG. A2.

SEM images of different indentations in ufg-Al varying the applied creep time (according to Fig. A1).