Indentation creep testing of superalloys

Great progress has been made over the last years in high temperature nanoindentation testing and quite reliable test systems are available to operate at temperatures up to 800°C. With such systems the high temperature strength is measured via the hardness of materials. However, for high temperature materials especially the creep strength is of interest and therefore also many attempts have been undergone to probe also the creep properties with high temperature nanoindentation. In most cases pointed indenters as Berkovich or conical indenters have been used for this. A major challenge, however, then is, how the nanoindentation data are converted into uniaxial creep properties, i.e. those which are needed for constructional purposes. Although, it seems that the stress exponent can be derived quite successfully with such indenters, an evaluation of a full creep curve for materials with significant primary creep does not seem possible, since the strain a pointed indenter is inducing is fixed by the indenter shape and stays more or less constant during the whole test [1]. Please click Additional Files below to see the full abstract

Local fatigue characterisation of ARB processed copper sheets by dynamic micropillar compression

Local fatigue experiments on microscale samples offer the opportunity to isolate microstructural contributions to the mechanical deformation behavior. In contrast to macroscopic fatigue testing, it is therefore possible to independently characterize the effect of individual defects, as for example grain boundaries. In this study accumulative roll bonding (ARB) architectured copper sheets with a bimodal microstructure were analyzed (Figure 1). Micropillars were fabricated by FIB milling inside individual layers of the material. Due to the bimodal microstructure, they exhibit two extremely different grain sizes, which results in a change of the respective fatigue properties. Additionally micropillars were fabricated at the interface in order to study the interfacial contribution to the fatigue behavior. The investigations were performed by a novel approach that combines dynamic nanoindentation and micropillar compression [1]. With this technique the high cycle fatigue range is easily accessible for microscale samples. Observation of the underlying deformation processes was performed by recording SEM micrographs of the deformed samples. FIB cross-sectioning of the deformed samples was used to investigate the deformed microstructure in the bulk of the specimens. Please click Additional Files below to see the full abstract

Small‐scale insights into superplasticity using micromechanical testing methods

In this work, the superplastic deformation behavior was investigated at the microscale as a function of temperature, strain-rate and grain-size. In detail, the superplastic alloy Zn22Al was characterized by nanoindentation at elevated temperatures, pillar compression experiments and in-situ micro tensile testing. Nanoindentation strain-rate jump tests show that the resulting strain-rate sensitivity is significantly affected by the applied strain-rate and testing temperature. The combination of these findings with the corresponding apparent activation energies evidences three different rate-controlling deformation processes, which are correlated with microstructural investigations of the residual imprints. However, significant differences regarding the deformation kinetics are observed when the size of the plastic zone is successively reduced and finally gets in the order of a few grains, giving rise to a minimum size of the plastic zone for superplastic material behavior. Via a combination of pillar compression experiments and EBSD analysis it is further suggested that superplasticity is the manifestation of a complex interaction between inter- and intra crystalline deformation processes. This behavior is discussed in detail, by taking the influence of the local phase characteristics and pillar dimension into account. Please click Additional Files below to see the full abstract

The influence of pre-deformation on the fracture toughness of chromium, studied by microcantilever bending

Cr is bcc metals, which has a high melting point and high strength. However, its fracture toughness at room temperature is low. This is due to their rather high ductile to brittle transition temperature. At room temperature the fracture toughness is limited by dislocation mobility or by the inability to activate nucleation sources. While this behavior is well characterized for W, there are only few studies for Cr. Please click Additional Files below to see the full abstract

Fracture behavior of metallic thin films as evaluated by bulge-tests and in situ TEM deformation experiments

Metallic thin films generally show a fracture toughness which is considerably lower than that of bulk samples. Although this has been evidenced by several groups, a conclusive understanding of this low fracture toughness is still missing and open questions related with the difficulty of reliably testing very thin films often remain. Bulge testing is a very suitable method allowing reliable investigations of the fracture toughness of thin films by introducing a slit in a freestanding membrane by focused ion beam (FIB) milling. With such tests the fracture toughness of silver and gold films in the thickness range of 100 nm have been determined to be around 2 MPa m1/2 confirming earlier results obtained with other testing techniques on similar metallic thin films. Recent investigations by Preiss et al. [1] gave an explanation for this extremely low fracture toughness based on in-situ observations of the crack tip region by atomic force microscopy (AFM). The AFM scans show stable crack growth mainly along grain boundaries and sliding of grains. Plastic deformation is localized in a very narrow corridor in front of the crack tip and a large plastic zone, as one would typically expect under plane stress, is not observed. We conclude that the spatial confinement of the plastic deformation is the primary reason for the low fracture toughness of metallic thin films. More detailed observations of the deformation mechanisms are of particular interest and are enabled by in situ transmission electron microscopy (TEM). For this a new flexible method for the preparation of thin film samples for in situ mechanical testing in a TEM has been developed [2], which is based on a combination of focused ion beam (FIB) shadow milling and electron-beam-assisted etching with Xenon difluoride precursor gas. Loading of the specimens is performed by a TEM Nanoindenter combined with a Push-to-Pull conversion device. In contrast to existing FIB-based preparation approaches, the area of interest is never exposed to ion beam irradiation and a pristine microstructure is preserved. With this method nanotwinned Cu and Cu-Al thin films were tested in situ in the TEM. Al is an effective element to reduce the stacking fault energy in Cu alloys and leads to increased amount of twinning and detwinning events. The films are tested until final fracture and different deformation mechanism as sliding of grains, twinning and dislocation activity can be correlated with the captured stress-strain curves from the experiment. The fracture behavior of these films will be discussed in the presentation and compared to the bulge-test results. References [1] E. I. Preiß, B. Merle, M. Göken; Understanding the extremely low fracture toughness of freestanding gold thin films by in-situ bulge testing in an AFM; Mat. Sci. Eng. A691 (2017) 2018-2025 [2] J.P. Liebig, M. Göken, G. Richter, M. Mačković, T. Przybilla, E. Spiecker, O.N. Pierron, B. Merle; A flexible method for the preparation of thin film samples for in situ TEM characterization combining shadow-FIB milling and electron-beam-assisted etching; Ultramicroscopy 171 (2016) 82–8

Mechanical testing of twinned copper and copper alloy micropillars

Nanotwinned metals are a promising class of modern materials combining a very high strength and ductility with excellent electrical properties. Their remarkable strength is connected to the high effectiveness of twin boundaries as obstacles to dislocation motion. In order to further characterize these interactions, micropillars containing single coherent twin boundaries with different orientations were compressed with a flat punch and subsequently investigated in the scanning electron microscope. The crystal orientations for compression were selected to activate different slip modes. The aim is to probe the different barrier effects that can act on gliding dislocations. The investigations concentrated on copper and α-brass. The latter is a low stacking-fault energy alloy exhibiting a high density of recrystallization twins. Coherent twin boundaries were selected from an EBSD orientation mapping of the sample and oriented by means of a custom sample holder. FIB-milling at these interfaces yielded micropillar samples containing a single twin boundary. Single crystal reference samples were obtained from the bulk of the grain located on both sides of the twin boundary. The microcompression tests enabled the quantification of the influence of the twin boundary barrier on the strength of each sample. The tests evidenced a strong dependency of the strength of the sample on crystal orientation and stacking-fault energy. The activated glide systems were subsequently identified from slip trace analysis and STEM mapping of lamellas obtained by FIB lift-out from the bulk of the tested micropillars. Please click Additional Files below to see the full abstract

Mechanical testing of copper and copper alloy micropillars containing a single twin boundary

Nanotwinned metals are a promising class of modern materials combining a very high strength with a high ductility and excellent electrical properties. This remarkable strength is believed to be connected to the good efficiency of twin boundaries as obstacles to dislocation motion. The present study aims at identifying and characterizing the possible interaction modes between dislocations and coherent twin boundaries. This is achieved by compressing micropillars containing a single twin boundary of a controlled orientation. The influence of the stacking fault energy on the intrinsic strength of the twin boundary is also investigated by varying the investigated material. In detail, the micropillars are fabricated from recrystallized polycrystalline samples of copper and α-brass, which is a low stacking-fault energy alloy exhibiting a high density of recrystallization twins. Coherent twin boundaries are selected from an EBSD orientation mapping of the sample and oriented by means of a custom 3D-printed sample holder. FIB-milling at these interfaces yields micropillar specimens containing a single twin boundary. Single crystalline reference samples are obtained from the bulk of the grains located on both sides of the twin boundary. The microcompression tests allow quantifying the influence of the twin boundary barrier on the strength of the sample as a function of the stacking fault energy of the material. The activated glide systems are subsequently identified from slip trace analysis and STEM mapping of lamellas obtained by lift-off from the bulk of the tested micropillars. This allows identifying the different deformation modes, which will be discussed in the presentation

γ/γ\u27 Co-base superalloys – new high temperature materials beyond Ni-base Superalloys?

In 2006 a new L12 phase, Co3(Al,W), was discovered in the Co-Al-W system which has led to the development of novel Co-base superalloys with g/g¢ microstructures similar to those of the well-established Ni-base superalloys. First investigations on simple ternary alloys could show that these Co-Al-W based alloys exhibit higher solidus temperatures and show less segregations after casting compared to typical Ni-base superalloys. This leads to the question whether this g/g¢ Co-base superalloys can be regarded as new class of high temperature materials that can compete with or even supersede established Ni-base superalloys. In the first part of the talk it will be shown how alloy properties change, when the base element Ni is gradually substituted by Co in a series of Ni-Co-Al-W-Cr alloys with otherwise constant element contents of Al, W and Cr. All alloys form g/g¢ microstructure after a standard aging treatment with a similar g¢ volume content. Liquidus and solidus temperatures are hardly influenced by the Ni/Co content, but the g¢ solvus temperature is strongly decreasing with increasing Co content. This indicates that the potential application temperature of g/g¢ Co-base superalloys will not be beyond the maximum application temperature of advanced single crystal Ni-base superalloys. However, this also shows that g/g¢ Co-base superalloys have a great potential as wrought alloys since the solvus temperature of the intermetallic compound is comparatively low, which gives a large processing window, and because a high volume fraction of the L12 phase at temperatures up to 900°C can be achieved. Please click Additional Files below to see the full abstract

Evaluation of Co-based thermodynamic databases with respect to own and literature experimental data

The development of Ni-based alloys proved the importance of dedicated Gibbs energies databases constructed following the CALPHAD method. Validated databases for Co-based and Ni/Co-based alloys are therefore imperative. These databases are being constructed concurrently with the development of new alloys in an interactive mode: databases anticipate quantities, new measurements are done which validate the database results or demand for changes. In this work we collect several thermodynamic assessments of ternaries and quaternaries systems, relevant for Co-based alloys, published recently in the literature and compare the calculated results with the obtained by using TCNI8 (which can also be used for Co-based alloys). We also compare calculated results to Liquidus and solidus temperatures experimentally determinate for several alloys in development in Erlangen. A comparison between First Principles calculated formation enthalpies of several TCP (topologically close packed) phases with the values calculated from the databases is also presented. As a result of this analysis necessary changes in the databases are pointed out as well as the regions of composition and temperature where more experimental data is required

Anomalous Work Hardening Behavior of a Single Crystalline Co-Base Superalloy

The defect evolution associated with an anomalous work hardening behavior of a single crystalline quaternary Co-Al-W-Ta superalloy at 950 °C was investigated by transmission electron microscopy. As plastic deformation is initially confined to the γ matrix channels, a plateau arises in the stress-strain curve after yielding. At about 1% plastic strain, extensive shearing of the γ′ precipitates under superlattice stacking fault formation occurs leading to extreme work hardening rates up to 12 GPa and a total increase in stress of about 200 MPa. Additional investigations on the temperature and strain-rate dependence of the anomalous work hardening behavior reveal the significance of diffusion and segregation processes on the stress-strain curve and the work hardening behavior