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

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

Nanomechanical testing study of the elementary deformation mechanisms in the Ti2AlN and Cr2AlC MAX phases

Abstract: Deformation mechanisms in MAX phases are still not well understood. The complex mechanical behavior of these materials, including mechanical hysteresis, arises both from their crystallography, with a nanolayered structure alternating nitride or carbide layers with metal atoms layers, and from their macroscopic polycrystalline structure, composed of platelets-like grains. In order to distinguish from these two contributions, we focused our study at the sub-micrometer scale, in order to probe the mechanical response of individual grains. Please click Additional Files below to see the full abstract

(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

Microcompression experiments on glasses ‐ strain rate sensitive cracking behavior

Figure 11 – microcompression experiments on glasses showing stable crack growth (a) and reversible deformation (b) It is well known that the mechanical properties of glasses are closely related to their atomic structure. The exact structure-property-relationship, however, is only poorly understood even for fundamental mechanisms like shear and densification. Nanomechanical test methods like micropillar compression and nano indentation can help fill this gap. In this study a sodium-boro-silicate glass is quenched from different temperatures to induce changes in the atomic structure. Micropillar compression was used to introduce plastic deformation into these glasses at room temperature under a uniaxial stress state. By changing the strain rate it is shown that deformation shifts from completely reversible deformation, to stable crack growth, and finally brittle failure. It is shown that by changing the glass structure, the strain rates corresponding to these deformation regimes are shifted. Finally, the occurrence of shear and densification is discussed. These findings are analysed against the background of the glass structure. 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

Using impact‐nanoindentation to test glasses at high strain rates and room temperature

In many daily applications glasses are indispensable, and novel applications demanding improved strength and crack resistance are appearing continuously. Up to now, the fundamental mechanical processes in glasses subjected to high strain rates at room temperature are largely unknown and thus guidelines for one of the major failure conditions of glass components are non-existent. Here, we elucidate this important regime for the first time using glasses ranging from a dense metallic glass to open fused silica by impact as well as quasi-static nano-indentation. We show that towards high strain rates, shear deformation becomes the dominant mechanism in all glasses accompanied by Non-Newtonian behavior evident in a drop of viscosity with increasing rate covering eight orders of magnitude. All glasses converge to the same limit stress determined by the theoretical hardness, thus giving the first experimental and quantitative evidence that Non-Newtonian shear flow occurs at the theoretical strength at room temperature

Dislocations in Laves phases – Phantastical beasts and how to understand them

How macroscopically hard and brittle materials deform is not well understood in many cases with not even the dominant slip systems known and no critical stresses or dislocation mechanisms available. This is true even for the most abundant type of intermetallic phase, the Laves phases. However, this knowledge is essential to improve many metallic-intermetallic composite alloys, such as Mg with an interconnected Laves network in Mg-Al-Ca alloys preventing creep. Please click Additional Files below to see the full abstract

Investigation of Mechanical Properties of Fibre Metal Laminates based on a Thermoplastic Resin

Fibre Metal Laminates (FML), consisting of alternatively stacked steel foils and glass bre reinforced PEEK layers, where investigated. The metal surface was treated with a cleaning laser and an appropriate set of parameters was chosen, the GFPEEK surface was degreased with acetone. The laminates were fabricated afterwards by means of a hot press and several specimens were cut out of the specimen plate via water jet cutting. Specimen dimensions and testing parameters for tensile, shear tensile, 4-point-bending and 3-point-bending tests were developed. The aim of this thesis was to investigate the application of several testing methods on FML on the one hand and the mechanical properties of a FML based on a thermoplastic resin on the other hand. The specimens though delaminated during water jet cutting, so that no mechanics of materials testing could be performed. Further analysis exhibited diverse reasons for this total failure: The process parameters have to be revised. Especially the dimensions of the laminate have to be adjusted to the dimensions of the tool and pressure and temperature have to be coordinated. It is furthermore important to improve the laser treatment so that a uniform structure can be achieved. Extensive sulphur contamination could be detected on the metal surface after delamination, which states the most grave reason for delamination. Possible modications of the pretreatment and production processes were presented