Micropillar Compression Research Articles

Effect of sub-micron deformations at opposing strain rates on the micromagnetic behaviour of non-oriented electrical steel

We are entering an era of re-electrification, seeking high-power density electrical machines with minimal resource use. Significant performance gains in electrical machines have been achieved through precise manufacturing processes, including the shaping/cutting of soft magnetic materials. However, most studies have evaluated magnetic performance at a macro level, focusing on components, while the fundamental mechanisms, e.g., how the micromagnetic behaviour is affected by mechanical interference, remain unclear. In this study, we examine the impact of sub-micron deformations at opposing strain rates (10−2 to 101 s−1) on the micromagnetic behaviour of soft magnetic non-oriented electrical steel. Using a diamond probe to indent within a single grain of polycrystalline material at different velocities, we induce quasi-static and dynamic mechanical loading. Our analysis, employing magnetic force microscopy, transmission Kikuchi diffraction, and scanning transmission electron microscopy with a pixelated detector, reveals that magnetic texture disturbances rely on the time-dependent dislocation dynamics of the Fe-BCC material. Additionally, we compress micro-pillars to further investigate these effects under bulk-isolated deformation. These findings highlight the importance of considering even ultra-small loads, such as nano-indentations and micro-pillar compressions, in the manufacturing of next-generation electric machines, as they can affect magnetic texture and performance.

On the Cover: Accounting for Localized Deformation: A Simple Computation of True Stress in Micropillar Compression Experiments

On the Cover: Accounting for Localized Deformation: A Simple Computation of True Stress in Micropillar Compression Experiments

Deformation Mechanisms of (100) and (110) Single-Crystal BCC Gum Metal Studied by Nanoindentation and Micropillar Compression

AbstractIn this paper, small-scale testing techniques—nanoindentation and micropillar compression—were used to investigate the deformation mechanisms, size effects, and strain rate sensitivity of (100) and (110) single-crystal Gum Metal at the micro/nanoscale. It was observed that the (100) orientation exhibits a significant size effect, resulting in hardness values ranging from 1 to 5 GPa. Conversely, for the (110) orientation, this effect was weaker. Furthermore, the yield strength obtained from the micropillar compression tests was approximately 740 MPa for the (100) orientation and 650 MPa for the (110) orientation. The observed deformations were consistent with the established features of the deformation behavior of body-centered cubic (bcc) alloys: significant strain rate sensitivity with no depth dependence, pile-up patterns comparable to those reported in the literature, and shear along the {112}<111> slip directions. However, the investigated material also exhibited Gum Metal-like high ductility, a relatively low modulus of elasticity, and high yield strength, which distinguishes it from classic bcc alloys.

Investigation of failure analysis on fatigue crack initiation influenced by critical resolved shear stress in X10CrMoVNb9-1 steel

This study investigates the influences of the critical resolved shear stress (CRSS) values on calculating the number of cycles for the fatigue crack initiation cycle of X10CrMoVNb9-1 (P91) under cyclic loading conditions at room temperature while considering varied surface roughness on microstructure models. The micropillar compression test was utilized to calculate the CRSS values, incorporating the Schmid factor, the Frank–Read source together with the Hall–Petch effect, measuring the diameter of the micropillar after compression, and the Mlikota and Schmauder equation. Two primary microstructure surface roughness – plane surface roughness (PS) and surface roughness (SR) with an average of 1 µm – were generated using the Voronoi Tessellation (VT) method. Finite Element Method (FEM) simulations were conducted to identify different stress distributions across these artificial microstructures. These stress distributions were subsequently analyzed using the physics-based Tanaka-Mura model (TMM) to estimate the number of cycles for fatigue crack initiation at several stress amplitudes and six types of microstructure. The number of cycles varies significantly at lower stress amplitudes and convergence at higher stress amplitudes. The study of the CRSS of steel P91 offers a significant advancement in fatigue research, particularly with implications for power plants.

Stress-strain curve estimation from micropillar compression with transverse contraction effect

The Focused Ion Beam (FIB) technique and the associated compression testing capabilities provide versatile access to understanding the microstructural behaviour of materials. Among many objectives, determining the stress-strain relationship from experimental or numerical force-displacement data remains a fundamental challenge. However, for micropillers under compression, the inside deformation constraint is complex like in components and the transformation of a measured force-displacement curve into an accurate uniaxial stress-strain curve is not straightforward. In this context, in a previous numerical based analysis for perfect volume constancy or incompressibility the determination of reliable and accurate stress-strain curves from nonlinear load-deformation curves of micropillers was already verified. In the case of transverse contractions, however, numerical simulations of micropillers provide unexpected force-displacement behaviour, i.e. their force-displacement curves are almost superimposing and are very close to the force-displacements for incompressibility. This effect implies, that transverse contraction would have no contributions in micropillar compression. In contrast, independent simulations using a single finite element accurately explore the true uniaxial stress-strain behaviour for the whole range of transverse contractions. These are considered as the reference behaviour and are also treated by an analytical expression. With the presented new stress-strain estimation approach, including transverse contraction, any force-displacement curve from a micropillar under compression can be stepwise corrected. The first step of the new stress-strain curve estimation method, including transverse contraction, is carried out in the same way as in the preceding analysis for incompressibility, followed by further steps to obtain the final true stress-strain curve. Although the presented numerical analysis is exemplarily performed for the behaviour of fused silica, but the stress-strain approach is in the same manner generally applicable for a material behaviour with other stress-strain curves. The assumption behind is isotropic and nonlinear material behaviour.

On the microscale deformation of the oxide layer formed on a Zr-2.5 Nb alloy: A micropillar compression study

Oxides are prone to brittle failure, leading to passivity loss and exposing the underlying metal during service. Thus, understanding the mechanical response of oxides is crucial for evaluating metal passivity. The present study examines the oxide layer on a Zr-2.5 Nb alloy used in CANDU® reactors after different exposure times. Micropillar compression tests on the oxide layer revealed that the mean failure stress and strain remain unchanged (are within the error) for both samples. Post-deformation characterization confirms that the failure of the oxide layer is brittle and could be pseudo-shear, which is consistent with the critical failure mode for brittle materials.

Accounting for Localized Deformation: A Simple Computation of True Stress in Micropillar Compression Experiments

Accounting for Localized Deformation: A Simple Computation of True Stress in Micropillar Compression Experiments

Controllable stress-adsorbed layers in Ti/TiN/TiAlN coatings: Mechanical performance and micropillar compression

Solid particle erosion often compromises the durability of turbine engine blades, particularly those used in offshore and desert environments. Surface protective coatings have emerged as a promising solution to significantly enhance the erosion resistance of compressor blades. In this study, TiAlN multilayer coatings were deposited onto Ti-6Al-4V substrates using a vacuum arc deposition technique. The article explored the strategic incorporation of TiN-Ti-TiN stress-adsorbed layers (SALs) within TiAlN-based coatings. It also investigated the microscale mechanisms leading to coating failure under conditions of micropillar compression. The findings revealed that the presence of metal/ceramic interfaces significantly improved the adhesion strength, crack resistance, erosion resistance and fracture toughness of the coatings. The multilayer deformation behavior of the coatings was primarily determined by the plastic flow of the softer metal layers. The SALs played a crucial role in reducing the elastic strain energy by generating nanotwins, coherent interfaces, and high-density dislocations during deformation, thereby improving the coatings' plastic deformation capability. Moreover, the implementation of dual-cycle SALs successfully reduced radial crack propagation, facilitating a transition from brittle fracture to a combined brittle-ductile fracture mechanism. This study highlights the critical role of metal/ceramic interfaces and SALs in improving the erosion resistance and co-deformability of TiAlN-based hard coatings.

Compressive mechanical properties of thermal sprayed AlCoCrFeNi high entropy alloy coating

Atmospheric plasma spraying (APS) was used to deposit an AlCoCrFeNi high entropy alloy (HEA) coating on stainless steel substrate. The as-deposited coating was about 300 μm thick and the microstructure consisted of a nickel solid-solution matrix, together with a number of secondary phases/intermetallics. In addition, phase and splat boundaries were also prevailing. However, the extent of intermetallics and secondary phases were supressed, compared to other processing techniques (e.g., melting) of HEA, due to fast solidification in the APS process. In-situ micro-pillar compression was employed to evaluate the mechanical properties of the coating. The experimental results show that, mechanical properties of the coating in the cross-sectional direction (963.14 ± 28.58 MPa of yield strength and 1005.58 ± 22.08 MPa of compressive strength) is marginally higher than that of planar direction (802.33 ± 43.76 MPa of yield strength and 817.73 ± 43.84 MPa of compressive strength). The presence of secondary phases and/or intermetallics in the coating microstructure acts as a reinforcement medium to allow effective load bearing capacities of the coating. On the hindsight, phase and splat boundary areas are the ‘weakest link’, that acts as a slip/shear plan initiation site. The orientation of these phase/splat boundaries to that of direction of loading plays a significant role on the coating failure mode.

AlCrTaTiZr-based high entropy alloy nitride coatings on stainless steel substrates: Characterization and microscale tension testing

High entropy alloy nitride coatings promise improved mechanical properties and are under current investigation for a range of applications. In this work, we report deposition of high entropy alloy nitride coatings on 316 stainless steel substrates by inductively coupled plasma assisted dc magnetron reactive sputtering of nominally equiatomic AlCrTaTiZr alloy targets. Characterization of coating composition and structure was carried out through X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, X-ray diffraction, and scanning/transmission electron microscopy. Mechanical testing was conducted through instrumented nanoindentation, interfacial shear loading through micropillar compression, and tension loading of coated microscale tensile bars in a scanning electron microscope. Microscale tension testing offers a potential avenue to assess the limiting shear strength of the coating/substrate interface, as well as the level of cohesion within the coating. The present study indicates that microscale tension tests yield results consistent with macroscale testing, while avoiding the need for multiple macroscale specimens. The present results suggest that cohesion of the high entropy alloy nitride coating and limiting shear strength of the coating/steel interface both increase with increasing substrate bias voltage during deposition, and that these coatings may be more adaptive to increased levels of ion bombardment during deposition without inducing excessive residual stresses.

Micro-pillar compression of proton-irradiated chromium examined using cross-sectional site selection, electron microscopy, and molecular dynamics simulation

Deformation of proton-irradiated chromium was investigated using micro-pillar compression and electron microscopy. After 2 MeV proton irradiation at 350 °C, four micro-pillars were prepared from a single grain on the polished specimen cross section. Depending on the distance away from the irradiated surface, hardness as a function of local damage level was studied. All pillars developed a narrow deformation band on one set of near-adjacent {110} planes, arising from closely-positioned parallel gliding. The critical resolved shear stress for gliding along 〈111〉/{110} was measured to be 59.6 MPa in unirradiated material beyond the proton range. The critical stress increased by 20 % after 0.5 dpa, and by 58 % after 1 dpa, with saturation of hardening occurring by 0.7 dpa. Post-compression characterization using transmission electron microscopy showed extensive formation of nanometer size voids in a matrix dominated by tangled dislocations. No twinning was observed. The experimental observations are in good agreement with molecular dynamics simulation of pillar compression of chromium, showing dislocation gliding along 〈111〉/{110} and 〈111〉/{112}. The continued stability of chromium for LWR application requires extension of the exposure level from 1 dpa to ∼15 dpa expected for typical fuel pin exposure.

A dual-phase Fe-Co-Ni-Cr-Mn high entropy alloy thin film with superior strength and corrosion-resistance

Improving the mechanical properties of high-entropy alloys (HEAs) with a face-centered cubic (fcc) structure is critical for their potential applications. In this work, a Fe-Co-Ni-Cr-Mn thin film was prepared and deposited with a power of 360 W. This alloy thin film exhibits a dual-phase microstructure consisting of a fcc matrix and sigma phase (σ phase) precipitates. The 360 W-film shows ultra-high strength-ductility synergy, with a hardness of 10.4 GPa, yield strength of 6.2 GPa, and a fracture strain of ∼25 % (micro-pillar compression tests). The excellent mechanical properties can be mainly attributed to the grain size effect, the hardening effect of the σ precipitates, and deformation twinning. Moreover, this alloy thin film exhibits excellent tribological performances and corrosion resistance. Our results demonstrate high potential of the dual-phase HEA thin film for industrial applications as high-performance protective coating materials.

Effect of Microstructure on Oxidation and Micro-mechanical Behavior of Arc Consolidated Mo-Ti-Si-(B) Alloys

AbstractThe present study deals with the development and characterization of Mo-35Ti-10Si and Mo-35Ti-10Si-2B (wt.%) alloy for ultra-high temperature applications beyond the temperature limit of existing super alloys. The microstructural characterization using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), electron back scattered diffraction (EBSD), x-ray diffraction (XRD) revealed that the Mo-35Ti-10Si-2B alloy was consisted of three phases, namely, (Mo, Ti)ss, (Mo, Ti)5SiB2 and (Ti, Mo)5Si3; whereas, Mo-35Ti-10Si alloy was found to be consisting of (Mo, Ti)ss, and (Mo,Ti)3Si phases. Since quantification of boron is difficult by EDS, Particle Induced Gamma-ray Emission (PIGE), a nuclear reaction analysis technique was used for chemical composition analysis of boron. The oxidation behavior of the Mo-35Ti-10Si-2B alloy in the temperature regime of 825-1250 °C was studied in detail and compared with boron-free Mo-35Ti-10Si alloy. Mo-35Ti-10Si-2B alloy exhibited superior oxidation behavior at intermediate temperatures of 825 °C, and excellent oxidation resistance at higher temperatures between 1000 and 1250 °C due to the formation of the protective borosilica and double oxide layers (TiO2 and duplex borosilica-TiO2), respectively. High-temperature oxidation mechanisms were discussed using detailed microstructural cross section analysis of the oxidized alloy samples. The micro-mechanical behavior of constitutive phases of the Mo-35Ti-10Si-2B alloy were studied by microhardness, nano-indentation and micropillar compression testing. The micropillar compression of (Mo, Ti)ss phase showed fairly ductile behavior with the evidence of activation of dislocation in the form of slip lines revealed through the post-deformation fractography. Deformation studies of (Mo, Ti)5SiB2 and (Ti, Mo)5Si3 phases were also carried out which showed large strain bursts indicating possibility of activation of dislocation activities even at room temperatures imparting low level of ductility.

Operative slip systems and their critical resolved shear stresses in η-Fe2Al5 investigated by micropillar compression at room temperature

The plastic deformation behavior of single crystals of orthorhombic η-Fe2Al5 has been investigated by micropillar compression at room temperature as a function of crystal orientation and specimen size. Plastic flow is observed even at room temperature by the operation of six slip systems; (001)<010>, (001)<110>, (001)<130>, {22‾3}<110>, {311}<1‾03> and {301}<1‾03>. The CRSS values for the six identified slip systems are very high all in the range of 1.1∼1.5 GPa and do not vary much with specimen size. In the middle of the stereographic projection, the (001)<010>, (001)<110>, (001)<130> and {22‾3}[110] slip systems operate according to the relative Schmid factors with the similar CRSS values in the range of 1.08∼1.23 GPa. In orientations close to [001], the {311}<1‾03> slip system as well as the {301}<1‾03> slip system operate with a much higher CRSS values around 1.5 GPa, producing wavy slip traces due to the occurrence of frequent cross-slip among these slip planes. In orientations close to the [100]-[110]-[010] symmetry line, on the other hand, premature failure occurs without the operation of any slip systems, although, the Schmidt factor-wise, the {311}<1‾03> and {301}<1‾03> slip systems could operate. The selection of slip systems, their CRSS values and the possible dislocation dissociation modes are discussed based on the overlapped atomic volume that occurs during shear along the slip direction on the slip plane, taking into account the partial occupancies of Al atoms in the linear atomic chain along the orthorhombic c-axis direction.

The effect of silicon on the critical resolved shear stress of solid solution strengthened ferritic ductile iron

Solid solution strengthened ferritic ductile iron (SSFDI) exhibits improved mechanical properties compared to conventional ductile cast iron (DCI) grades, however, its potential widespread application is hindered by unpredictable brittle fracture which might be attributed to microstructural silicon segregation and associated superstructure formation. The aim of the present study is therefore to deepen the understanding on the effect of local silicon segregation on the mechanical properties of SSFDI, which is crucial especially for the common applications of DCI in cyclically loaded structures. Micropillar compression tests were carried out on three different casts to investigate the solution strengthening effect of silicon in the ferritic matrix. An almost perfect linear relationship between critical resolved shear stress (CRSS) and global silicon content was found. It was also found that the variation of CRSS with silicon content corresponds well to the variation of the macroscopic yield limits (under tension and compression) with global silicon content of different SSFDI alloys. This indicates that the ferritic matrix dominates the yield limit of the DCI alloys investigated in this study, while the morphology of the graphite nodules plays a minor role under monotonic loading conditions.

Investigation on the incipient plasticity of 〈001〉-oriented CoCrFeNi micropillar

Micro-level mechanical tests are of extreme sensitivity to the initial internal state and early response of materials. The stability of elastic property and yield behavior of macro-level polycrystal specimens fade significantly under micro-level uniaxial loading state. In this study, the incipient plasticity and underlying mechanisms of medium-entropy alloy CoCrFeNi were investigated by in-situ micropillar compression. Stress fluctuations at elastic stage of 〈001〉-oriented pillar were observed in the stress-strain curves and then evidenced by TEM characterization as the activation of leading partial dislocations. The discreteness of yield strength was thus attributed to the impediment of pre-formed stacking faults. Furthermore, a model illustrating the incipient plasticity and its consequences on micropillar compression tests was proposed after an evaluation on twinning stress of alloys with low stacking fault energy.

Synthesis, microstructure and micro-mechanical characterization of metal (Nb, Ti) – MAX phase (Ti2AlC) nanolaminates

We utilize elevated temperature physical vapor deposition (PVD) techniques to design metal/MAX multilayered nanocomposite thin films with alternating nanoscale metallic (Nb, Ti) and MAX phase (Ti2AlC) layer thicknesses. These metal/MAX nanolaminate architectures attempt to exploit a unique hierarchical topology – as interfaces between the layers are expected to be in direct competition with the internal interfaces within the MAX layers, to drive their tunable macroscopic mechanical behavior. Two metal/MAX nanolaminates – Nb/Ti2AlC and Ti/Ti2AlC – were deposited. The Nb/Ti2AlC metal/MAX system showed highly diffused layer interfaces with distinct Ti – rich and Nb–Al – rich layers, with the presence of MAX phase alongside TiC and other Ti–Al and Nb–Al intermetallic phases. The Nb/Ti2AlC system possessed a layered architecture, though the MAX phases were not found to be continuously present in each alternating layer. The second Ti/Ti2AlC system showed a non-lamellar nanocomposite microstructure and the formation of mixed Tin+1AlCn phases (a mix of n = 1, 2), and no indication of layering. Diffusion occurring between the metal/MAX layers in both cases, likely due to the elevated temperatures during the deposition process, is speculated as the likely cause of these resultant microstructures. The mechanical properties of both systems were evaluated using micromechanical (nanoindentation and micro-pillar compression) techniques, which demonstrated high strengths for both systems (Nb system: yield and instability strengths of 4.88 ± 0.1 GPa and 5.57 ± 0.03 GPa, Ti system: yield and instability strength of 5.61 ± 0.28 GPa and 6.21 ± 0.25 GPa). This work highlights the promising mechanical properties of metal/MAX multilayered depositions and summarizes the challenges in PVD synthesis of metal/MAX multilayered nanolaminates.

The dependence of strength and strain hardening on dislocation-interface interaction in dual-phase titanium alloys

The slip transmission across an interface is essential for the mechanical properties of dual-phase alloys like Ti-6Al-4 V. However, the correlation between the dislocation-interface interaction and the strength and strain hardening anisotropy remains unclear due to the lack of direct experimental evidence. Via in situ scanning electron microscopy micropillar compression, prismatic plane dislocations were preferentially activated and interacted with an individual α/β interface at different angles. Based on transmission electron microscopy characterization, this study suggests that α/β interface shows a more pronounced strengthening effect when the coordinated slip system is more difficult to be activated and the slip deflection angle is larger. Differently, its higher strain hardening rate is initially determined by the larger Burger vector magnitude of interfacial residual dislocation after slip transmission. These results provide a unique basis for understanding the contribution of the interface to the mechanical properties of dual-phase alloys.

Characterization of the strain rate sensitivity of basal, prismatic and pyramidal slip in Zircaloy-4 using micropillar compression

The slip strength of individual slip systems at different strain rates will control the mechanical response and strongly influence the anisotropy of plastic deformation. In this work, the slip activity and strain rate sensitivity of the <a> basal, <a> prismatic, and <c+a> pyramidal slip systems are explored by testing at variable strain rates (from 10−4 s−1 to 125 s−1) using single crystal micropillar compression tests. These systematic experiments enable the direct fitting of the strain rate sensitivities of the different slips using a simple analytical model and this model reveals that deformation in polycrystals will be accommodated using different slip systems depending on the strain rate of deformation in addition to the stress state (i.e. Schmid's law). It was found that the engineering yield stress increases with strain rate, and this varied by slip systems. Activation of the prismatic slip system results in a high density of parallel, clearly discrete slip planes, while the activation of the <c+a> pyramidal slip leads to the plastic collapse of the pillar, leading to a ‘mushroom’ morphology of the deformed pillar. This characterization and model provide insight that helps inform metal forming and understanding of the mechanical performance of these engineering alloys in the extremes of service conditions.

Study of Serrated Boundary Micromechanics During Micropillar Compression in Nickel-Based Superalloy

Study of Serrated Boundary Micromechanics During Micropillar Compression in Nickel-Based Superalloy