A new constant contact pressure (CCP) indentation creep method is presented, which is based on keeping the mean contact pressure as defined through Sneddon’s hardness constant, until a steady-state strain rate is achieved. This is in contrast to the conventional constant load–hold (CLH) creep experiments, where the load is held constant and relaxation in both hardness and strain rate occurs at the same time. Besides controlling the mean contact pressure, the dynamic stiffness is furthermore used to assess the indentation depth, thereby minimizing thermal drift influence and pile-up or sink-in effects during long-term experiments. The CCP method has been tested on strain rate sensitive ultrafine-grained (UFG) CuZn30 and UFG CuZn5 as well as on fused silica, comparing the results with those of strain rate jump tests as well as the CLH nanoindentation creep tests. With the CCP method, strain rates from 5 × 10−4 s−1 down to 5 × 10−6 s−1 can be achieved, keeping the mean contact pressure constant over a long period of time, in contrast to the CLH method. The CCP technique thus offers the possibility of performing long-term creep experiments while retaining the contact stress underneath the tip constant.

In this work, the influence of Al-solutes on the mechanical behavior of Cu–AlX solid solutions has been studied using indentation strain rate jump tests for single crystalline and ultrafine-grained (UFG) microstructures from high pressure torsion (HPT) processing. Al-solutes in Cu classically lead to a solid solution strengthening, coupled with a decrease in stacking fault energy, which influences also the grain size after HPT processing. For all alloys, a higher hardness is found at lower indentation depths, which can be nicely described by a modified Nix/Gao model down to 100 nm indentation depth. Among the single crystals, the largest size effects are found for the higher solute contents, indicating a stronger work hardening at small length scales for the solid solutions. The dilute UFG solid solutions showed a strong softening after a strain rate reduction test, with a pronounced transient region. Cu–Al15 is, however, quite stable, showing abrupt changes in hardness without strong transients. This indicates that solute solution strengthening does not only influence the indentation size effect and structure formation during HPT processing but also stabilizes the grain structure during subsequent deformation.

The fracture toughness of NiAl single crystals is evaluated with a new method based on the J-integral concept. The new technique allows the measurement of continuous crack resistance curves at the microscale by continuously recording the stiffness of the microcantilevers with a nanoindenter. The experimental procedure allows the determination of the fracture toughness directly at the onset of stable crack growth. Experiments were performed on notched microcantilevers which were prepared by focused ion beam milling from NiAl single crystals. Stoichiometric NiAl crystals and NiAl crystals containing 0.14 wt% Fe were investigated in the so-called “hard” orientation. The fracture toughness was evaluated to be 6.4 ± 0.5 MPa m1/2 for the stoichiometric sample and 7.1 ± 0.5 MPa m1/2 for the iron containing sample, indicating that the addition of iron enhances the ductility. This effect is intensified with ongoing crack propagation where the Fe-containing sample exhibits a stronger crack resistance behavior than the stoichiometric NiAl single crystal. These findings are in good agreement with macroscopic fracture toughness measurements, and validate the new micromechanical testing approach.

For a better understanding of the local fracture behavior of semi-brittle materials, we carried out bending experiments on notched microcantilevers of varying sizes in the micrometer range using NiAl single crystals. Smaller and larger beams were milled with a focused ion beam in the so-called “soft” <110> and “hard” <100> orientation and were tested in situ in a scanning electron microscope and ex situ with a nanoindenter, respectively. The measurements were evaluated using both linear-elastic fracture mechanics and elastic–plastic fracture mechanics. The results show that (i) the fracture toughness is in the same range as the macroscopically determined one which is around 3.5 MPa$\sqrt {\rm{m}}$ for the soft orientation and around 8.5 MPa$\sqrt {\rm{m}}$ for the hard orientation, that (ii) there is a strong influence of the anisotropic behavior of NiAl on the fracture toughness values, and that (iii) the J-integral technique is the most accurate quantification method.

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.

[Meza et al. J. Mater. Res.23(3), 725 (2008)] recently claimed that the correction factor beta for the Sneddon equation, used for the evaluation of nanoindentation load-displacement data, is strongly depth- and tip-shape-dependent. Meza et al. used finite element (FE) analysis to simulate the contact between conical or spheroconical indenters, and an elastic material. They calculated the beta factor by comparing the simulated contact stiffness with Sneddon’s prediction for conical indenters. Their analysis is misleading, and it is shown here that by applying the general Sneddon equation, taking into account the true contact area, an almost constant and depth-independent beta factor is obtained for conical, spherical and spheroconical indenter geometries.

The Oliver and Pharr method for evaluating nanoindentation load–displacement data is based on the measurement of the contact stiffness, which is usually determined at the very beginning of the unloading sequence, or, using dynamic nanoindentation, continuously during the whole loading segment. A new experimental method has been developed to continuously monitor the contact stiffness throughout the unloading sequence. It provides supplementary information about the shape and area of the residual impression, as well as a direct measurement of the shape of the effective indenter previously introduced by Pharr and Bolshakov. The new method was applied to indentations on fused silica, sapphire, nanocrystalline nickel, and ultrafine-grained aluminum. Lastly, the new procedure was adapted to directly measure the epsilon factor used in the Oliver and Pharr method. A value of 0.76 was found from indentation into fused silica, in close agreement with literature values.

A nanoindentation strain-rate jump technique has been developed for determining the local strain-rate sensitivity (SRS) of nanocrystalline and ultrafine-grained (UFG) materials. The results of the new method are compared to conventional constant strain-rate nanoindentation experiments, macroscopic compression tests, and finite element modeling (FEM) simulations. The FEM simulations showed that nanoindentation tests should yield a similar SRS as uniaxial testing and generally a good agreement is found between nanoindentation strain-rate jump experiments and compression tests. However, a higher SRS is found in constant indentation strain-rate tests, which could be caused by the long indentation times required for tests at low indentation strain rates. The nanoindentation strain-rate jump technique thus offers the possibility to use single indentations for determining the SRS at low strain rates with strongly reduced testing times. For UFG-Al, extremely fine-grained regions around a bond layer exhibit a substantial higher SRS than bulk material.

In this work the hardening effect of Ta and Mo in Ni-base alloys was investigated using a combinatorial approach with diffusion couples. Furthermore, the Ni-Fe system was used as a reference system taking advantage of the full miscibility at high temperatures. Ta was chosen, as aside from having a technical relevance in the Ni-base superalloys, it also has a high miscibility in Ni. The main focus of this paper will be solid solution hardening. It will be shown that even though the determination of hardness is subject to varying indentation size effects (ISE) [Durst et al., Acta Mater.55(20), 6825 (2007)], only a few modifications are necessary to describe solid solution strengthening measured by nanoindentations using the Labusch theory [Labusch, Acta Metall.20(7), 917 (1972)]. Moreover, after a careful evaluation of the results, the data can be used to investigate solid solution hardening effects quickly and efficiently with small amounts of material.

The effect of the cobalt content and WC grain size on the deformation behaviour of WC-Co hard metals was investigated by studying materials with a varying WC grain size and cobalt content. The WC grain size ranged from 2.65 to 0.25 µm and the binder content ranged from 6 to 15 wt%. Single and multiple scratch tests were conducted using a Nano indenter with a Berkovich diamond tip and the load ranged from 5 to 500 mN with a tip sliding velocity of 10 µm/s. Several damage mechanisms were observed and these show a combination of ductile and brittle wear. The bulk properties i.e. composite properties of the hard metal determine the wear in the 6 wt% Co samples on the other hand the 15 wt% Co samples exhibited a localised response to the wear i.e. the wear is determined by the individual phases in the hard metal.

Finite element simulations of conical indentations in two-phase elastic–plastic materials were used to investigate the influence of the shape and the aspect ratio of particles embedded in a matrix material on the deformation behavior and hardness during depth-sensing indentation. Starting with single-phase materials, pile-up behavior and its influence on the contact area was studied. Particle–matrix systems were simulated for elastic–perfectly plastic particles embedded in a matrix, with a yield-strength ratio of 2 or 0.5, respectively. The simulations were motivated by indentation experiments in precipitation hardened nickel-base superalloy with a nanoindenting atomic force microscope. In the studied alloys, the matrix formed channels with a thickness of about 100 nm around the precipitates with diameters of about 500 nm. The simulations explained an experimentally observed transition from particle to matrix deformation behavior during indentation. Depth limits in hardness testing in particle–matrix systems were evaluated. Depending on the aspect ratio, soft and hard particles were tested reliably up to a normalized contact radius of about 70% particle diameter.

The determination of plastic properties of metallic materials by nanoindentation requires the analysis of the indentation process and the evaluation methods. Particular effects on the nanoscale, like the indentation size effect or piling up of the material around the indentation, need to be considered. Nanoindentation experiments were performed on conventional grain sized (CG) as well as on ultrafine-grained (UFG) copper and brass. The indentation experiments were complemented with finite element simulations using the monotonic stress-strain curve as input data. All indentation tests were carried out using cube-corner and Berkovich geometry and thus different amount of plastic strain was applied to the material, according to Tabors theory. We find an excellent agreement between simulations and experiments for the UFG materials from which a representative strain of εB ≈ 0.1 and εcc ≈ 0.2 is determined. With these data, the slope of the stress-strain curve is predicted for all materials down to an indentation depth of 800 nm.