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

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

Plasticity of an atomically layered crystal: A combined nanomechanical and ab initio study on Mo2BC

Plasticity in atomically layered crystals, such as X2BC or MAX phases, is not yet fully understood. Particularly plasticity on non-basal planes is rarely considered. The reason for this lies both in the prevalence of basal deformation observed (MAX) or predicted (X2BC) and the difficulties in performing single crystal experiments on anisotropic and brittle materials challenging to produce in bulk form. We therefore employed a combined approach using microcompression, TEM including conventional and LACBED dislocation analysis and ab initio calculations to elucidate the active deformation mechanisms in Mo2BC. We show that appreciable ductility in Mo2BC is indeed achieved due to the activation of previously unexpected non-basal slip. Please click Additional Files below to see the full abstract

Orientation relationship of FeNiC and FeNiCSi from variant detection in EBSD data

The determination of orientation relationships in dual or multi-phase materials is very important in the field of interface engineering for the design of materials with tailored properties. In this work, a code is developed for the automated and statistical analysis of the orientation relationship of electron backscatter diffraction data. On the example of Fe-Ni-(Si)-C alloys containing lenticular martensite and retained austenite, the code is applied and it is shown that the orientation relationship (OR) corresponds to the Greninger-Troiano OR and that a statistically reliable investigation of the OR between the retained austenite and the related martensite variants is feasible using the code developed in this study.Comment: in revision currently in Crystal

(Nano-)Mechanical properties and deformation mechanisms of the topologically closed packed Fe-55Mo µ-phase at room temperature

Topologically close-packed (TCP) intermetallic phase precipitates in nickel-base superalloys are assumed to cause a deterioration of the mechanical properties of the γ - γ‘matrix. Although these intermetallic phases are well studied in terms of their structure, their mechanical properties have not yet been investigated in detail due to their large and complex crystal structures and pronounced brittleness. In this study we have chosen the Fe-Mo system as a model system in order to investigate the plastic deformation behavior of these phases. A special focus is placed on the hexagonal μ-phase. To this aim we apply nano-mechanical testing methods: nano-indentation and micropillar-compression to enable plastic deformation of these brittle phases. This is due to the confining pressure in nano-indentation and the reduction in specimen size in micro-compression experiments. Indentation experiments at room temperature show a hardness of ~11 GPa and a Young’s modulus of ~270 GPa. Electron backscatter diffraction (EBSD) assisted slip trace analysis reveals dominant dislocation activity on basal planes at room temperature. Micro-compression experiments on well-oriented single-crystalline micro-pillars reveal the structure related anisotropy of the critical shear stresses (CRSS) of different slip systems. Finally, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) investigations of specimens target-prepared from nano-indents and deformed micro-pillars reveal the dislocation and defect structures of the µ-phase

Plastic deformation and anisotropy of long-period-stacking-ordered structures in Mg-Zn-Y alloys

Wider application of magnesium alloys as light-weight structural materials requires improvement of strength and toughness. Recently, Mg-Y-Zn and Mg-RE-Zn alloys containing long period stacking ordered (LPSO) structures have received considerable attention, due to their potential to possess excellent mechanical performance at ambient and elevated temperatures. Sharing the same basal plane of α-Mg, LPSO structures are periodically stacked along the c-axis of the hexagonal crystal structure forming so-called 10H, 14H, 18R and 24R structures. LPSO structures are also chemically ordered where Y/RE and Zn atoms replace the positions of Mg atoms in neighboring (0001) planes. The underlying deformation mechanisms of LPSO structures and their co-deformation with α-Mg leading to a concomitant increase of strength and ductility with respect to pure Mg and most commercial Mg alloys are not understood yet. Therefore, we performed micro-pillar compression experiments on 7°(0001), 46°(0001) and 90°(0001) oriented α-Mg and 18R LPSO micro-pillars to investigate the deformation and co-deformation mechanisms of Mg-LPSO alloys. Electron backscatter diffraction-assisted slip trace analysis and post-mortem transmission electron microscopy analysis showed predominant deformation by basal dislocation slip in 46°(0001) and 7°(0001) oriented micro-pillars in both phases, LPSO and α-Mg. In 90°(0001) oriented micro-pillars (1-100)[11-20] prismatic slip was predominantly activated during the early deformation stages. With increasing strain, the formation of kink bands, shear bands and (-211-4)[-4223] deformation twins was observed. The activation energies of basal and prismatic slip are higher for 18R LPSO than for α-Mg. These results shed light on how LPSO structures deform plastically and might be used to purposely design microstructure and texture of Mg-LPSO alloys in the future. Please click Additional Files below to see the full abstract

(Nano‐)Mechanical properties and deformation mechanisms of the topologically closed packed Fe‐Mo55 µ‐Phase at room temperature

Topologically close-packed (TCP) intermetallic phase precipitates in nickel-base superalloys are assumed to cause a deterioration of the mechanical properties of the γ - γ‘ matrix. Although these intermetallic phases are well-studied in terms of their structure, their mechanical properties and intrinsic deformation mechanisms are largely unknown, due to their large and complex crystal structures and pronounced brittleness. Here, we present a first detailed investigation of the mechanical properties and deformation behaviour of the Fe7Mo6 µ-phase acting as a model system. Utilising room temperature nanoindentation and varying load and loading rates, the average hardness and indentation modulus are measured to be 11.7 GPa and 250 GPa, respectively. EBSD-assisted slip-trace analysis and TEM reveal that deformation occurs predominantly by basal and prismatic slip, where the highest hardness results from prismatic slip and intersecting slip planes and the lowest hardness values occur where only basal slip is activated. Micropillar compression experiments are used to calculate the CRSS for the dominant glide planes. Further, more detailed investigations of plastic deformation and dislocation movement is carried out by HR-TEM, showing clear evidence of mechanically induced synchro-shear on basal planes of stacked C14- Laves-subcells. Please click Additional Files below to see the full abstract

Room temperature deformation mechanisms of the C14 Laves Phase in the Mg‐Al‐Ca system

In order to improve the creep resistance of magnesium alloys and thereby increase their operating temperature, hard intermetallic phases can be incorporated in the microstructure. In particular the addition of Al or Ca to Mg results in the formation of a skeleton-like intermetallic structure at the grain boundaries. This structure consists predominately of Laves phases, which reduces the minimum creep rate by a few orders of magnitude. In bulk, these Laves phases are extremely brittle at low temperatures, limiting our understanding of the underlying mechanisms of plasticity. Additionally, the small size of the microstructural features in technical alloys make bulk-scale tests unsuitable for studying these phases. Using nanomechanical testing (nanoindentation and microcompression) in individual grains, cracking can be suppressed and plastic deformation can be observed [1]. Micropillars were milled using FIB in individual grains of a polycrystalline specimen, and orientations determined by EBSD to activate and interrogate slip systems. These data have then been combined with slip line analysis around indents. Such an approach reveals the presence of pyramidal, prismatic and basal slip at ambient conditions, with pyramidal 1st order being the predominant slip plane. Critical resolved shear stresses for these slip systems have been calculated, and TEM analysis of the deformation microstructure performed. This work therefore exemplifies how nanomechanical testing in conjunction with electron microscopy can extend the current knowledge of plasticity in macroscopically brittle crystals. [1] S. Korte, W.J. Clegg, Studying Plasticity in Hard and Soft Nb–Co Intermetallics, Advanced Engineering Materials, 14, No. 11 (2012), 991-99

Room temperature deformation in the Fe7_7Mo6_6 μ\mu-Phase

The role of TCP phases in deformation of superalloys and steels is still not fully resolved. In particular, the intrinsic deformation mechanisms of these phases are largely unknown including the active slip systems in most of these complex crystal structures. Here, we present a first detailed investigation of the mechanical properties of the Fe7Mo6 {\mu}-phase at room temperature using microcompression and nanoindentation with statistical EBSD-assisted slip trace analysis and TEM imaging. Slip occurs predominantly on the basal and prismatic planes, resulting also in decohesion on prismatic planes with high defect density. The correlation of the deformation structures and measured hardness reveals pronounced hardening where interaction of slip planes occurs and prevalent deformation at pre-existing defects.Comment: Accepted manuscript in International Journal of Plasticit

Exploring the transfer of plasticity across Laves phase interfaces in a dual phase magnesium alloy

The mechanical behaviour of Mg-Al alloys can be largely improved by the formation of an intermetallic Laves phase skeleton, in particular the creep strength. Recent nanomechanical studies revealed plasticity by dislocation glide in the (Mg,Al)$_2$Ca Laves phase, even at room temperature. As strengthening skeleton, this phase remains, however, brittle at low temperature. In this work, we present experimental evidence of slip transfer from the Mg matrix to the (Mg,Al)$_2$Ca skeleton at room temperature and explore associated mechanisms by means of atomistic simulations. We identify two possible mechanisms for transferring Mg basal slip into Laves phases depending on the crystallographic orientation: a direct and an indirect slip transfer triggered by full and partial dislocations, respectively. Our experimental and numerical observations also highlight the importance of interfacial sliding that can prevent the transfer of the plasticity from one phase to the other.Comment: 23 pages, 8 figures, 1 tabl