On the structure of defects in the Fe7Mo6 μ\mu-Phase

Topologically close packed phases, among them the $\mu$-phase studied here, are commonly considered as being hard and brittle due to their close packed and complex structure. Nanoindentation enables plastic deformation and therefore investigation of the structure of mobile defects in the $\mu$-phase, which, in contrast to grown-in defects, has not been examined yet. High resolution transmission electron microscopy (HR-TEM) performed on samples deformed by nanoindentation revealed stacking faults which are likely induced by plastic deformation. These defects were compared to theoretically possible stacking faults within the $\mu$-phase building blocks, and in particular Laves phase layers. The experimentally observed stacking faults were found resulting from synchroshear assumed to be associated with deformation in the Laves-phase building blocks

Three-dimensional characterization of damage in dual phase steels with deep learning

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Atomistic Simulations of Basal Dislocations Interacting with Mg17_{17}Al12_{12} Precipitates in Mg

The mechanical properties of Mg-Al alloys are greatly influenced by the complex intermetallic phase Mg$_{17}$Al$_{12}$, which is the most dominant precipitate found in this alloy system. The interaction of basal edge and 30$^\text{o}$ dislocations with Mg$_{17}$Al$_{12}$ precipitates is studied by molecular dynamics and statics simulations, varying the inter-precipitate spacing ($L$), and size ($D$), shape and orientation of the precipitates. The critical resolved shear stress $\tau_c$ to pass an array of precipitates follows the usual $\ln((1/D + 1/L)^{-1})$ proportionality. In all cases but the smallest precipitate, the dislocations pass the obstacles by depositing dislocation segments in the disordered interphase boundary rather than shearing the precipitate or leaving Orowan loops in the matrix around the precipitate. An absorbed dislocation increases the stress necessary for a second dislocation to pass the precipitate also by absorbing dislocation segments into the boundary. Replacing the precipitate with a void of identical size and shape decreases the critical passing stress and work hardening contribution while an artificially impenetrable Mg$_{17}$Al$_{12}$ precipitate increases both. These insights will help improve mesoscale models of hardening by incoherent particles.Comment: 13 pages with 9 figures and 2 tables. Supplementary materia

Electroplastic deformation studies of an Al-Cu eutectic alloy using nanoindentation

A promising approach to deform various groups of materials with poor deformability, such as metallic-intermetallic composite materials, is the exploitation of the electroplastic effect, which lowers the yield strength and enhances the elongation to fracture. However, its underlying metal physical phaenomena are not well understood yet. Since any experimental attempts to further understand the effect have been limited to the macroscopic scale so far, we developed an in-situ electro-nanomechanical testing setup which enables us to apply electric current pulses during indentation. This allows us to electroplastically deform single crystalline phases of defined orientation. Additionally, due to the microscopic contact area, high current densities can be achieved with this setup. Here, we present our experimental setup as well as recent results on the deformation of the eutectic Al-Al2Cu system as well as on the single crystalline Al2Cu phase. These results reveal displacement shifts upon pulsing, with a larger displacement shift following on the first current pulse, indicating that depinning of dislocations from obstacles is the underlying mechanism. Furthermore, a change in shift direction during unloading was observed which is assumed to be caused by long-range internal stress fields present in the deformed microstructure

Investigation of a high angle grain boundary in Fe2.4wt.%Si BCC micropillars

Iron-silicon sheet steel is the most widely used material for the iron cores of electrical machines like generators, motors or transformers. Although already ubiquitous, the demand will nevertheless rise in the future since electro-mobility is spreading rapidly. For this reason, even small improvements of efficiency have a huge energy saving potential. Currently, hysteresis losses are one of the main limiting factors for efficiency, resulting from the movement of domain walls, which may be pinned by dislocations. Even though electrical sheet steel is generally used in a fully recrystallized state, it is the final stages of production involving cutting that introduce large plastic strains, and hence high local dislocation densities. These have been shown to cause significant loss in performance. The aim of this work is to understand the evolution of deformation structures on a fundamental basis taking grain boundaries, size effects and different strain-rates into account. To this end, single- and bi-crystalline-micropillars of 1, 2 und 4 µm in diameter were investigated. 158 micropillars were deformed in order to provide a statistically-relevant dataset. In addition, macroscopic single- and bi-crystal-samples with a diameter of 2.5 mm were deformed as a reference for the size effect. The considered grain boundary has an angle of about 50° and a very high geometrical transmission factor (m’=0.89). Regarding the strain-rate-sensitivity three different strain rates were used for the deformation of the micro-/macroscopic single- and bi-crystals, with strain rate jump tests additionally conducted for the single-crystals. To visualize the deformation structure, selected micropillars were lifted out of the sample, thinned to the middle and analyzed utilizing EBSD. For most micropillars a clear slip system could be determined. Regarding one orientation the active slip system changed from the single- to the bi-crystal, likely because the newly-activated slip system was better aligned relative to the slip system of the other half-crystal. The bi-crystal-micropillars showed a higher resolved shear stress despite direct slip transmission across the grain boundary. Furthermore, a pronounced strain-rate sensitivity and size effect was found

On Extracting Mechanical Properties from Nanoindentation at Temperatures up to 1000∘^{\circ}C

Alloyed MCrAlY bond coats, where M is usually cobalt and/or nickel, are essential parts of modern turbine blades, imparting environmental resistance while mediating thermal expansivity differences. Nanoindentation allows the determination of their properties without the complexities of traditional mechanical tests, but was not previously possible near turbine operating temperatures. Here, we determine the hardness and modulus of CMSX-4 and an Amdry-386 bond coat by nanoindentation up to 1000$^{\circ}$C. Both materials exhibit a constant hardness until 400$^{\circ}$C followed by considerable softening, which in CMSX-4 is attributed to the multiple slip systems operating underneath a Berkovich indenter. The creep behaviour has been investigated via the nanoindentation hold segments. Above 700$^{\circ}$C, the observed creep exponents match the temperature-dependence of literature values in CMSX-4. In Amdry-386, nanoindentation produces creep exponents very close to literature data, implying high-temperature nanoindentation may be powerful in characterising these coatings and providing inputs for material, model and process optimisations

Deep-learning assisted damage observations on the microscale – A new viewpoint on microstructural deformation, fracture and decohesion processes

In recent years, state-of-the-art micromechanical systems have given researchers the ability to observe deformation processes in-situ. While this technology enables a site-specific observation, this very achievement can turn into a major limitation: To deduct conclusions about the relevance of specific processes for the bulk material, a larger field of view than typically possible in microscale observations is often required. Please click Additional Files below to see the full abstract

Towards nanoindentation at application-relevant temperatures – A study on CMSX-4 alloy and amdry-386 bond coat

With nickel-based superalloys reaching their fundamental limit in high-temperature applications, new alloys are required with improved mechanical properties. Small-scale mechanical testing – particularly nanoindentation – is of great benefit to alloy development, allowing hardness and modulus to be measured on small volumes of newly-developed materials. We show that it is now possible to carry out such tests in vacuum up to 1000˚C, paving the way for candidate alloys and coatings to be tested at operation-relevant conditions. In this work, a \u3c001\u3e oriented single-crystal CMSX4 sample and a 200 µm Amdry-386 bond coat were tested using a modified MicroMaterials NanoTest indenter. 1 µm indents were placed at 50 µm spacings from the bulk into the coating, allowing local mechanical properties to be determined. The data show a room-temperature hardness of CMSX4 of 4 GPa and modulus of 110 GPa, close to that found in the literature. The Amdry-386 at this temperature has a hardness and modulus of 4 GPa and 95 GPa, respectively. The CMSX4 shows a hardness peak at 400˚C and 5.5 GPa, after which the hardness rapidly decreases to around 2 GPa at the highest temperatures. The bond coat matches this behaviour closely. At both room and elevated temperatures, almost 100% of the indents show a thermal drift of \u3c0.3 nm/s, corresponding to a depth uncertainty of \u3c5%. This unparalleled drift performance allows future investigations of creep behaviour that were not possible until now