Micropillar Compression Tests Research Articles

Development of the Concurrent Multiscale Discrete-Continuum Model and Its Application in Plasticity Size Effect

A concurrent multiscale model coupling discrete dislocation dynamics to the finite element method is developed to investigate the plastic mechanism of materials at micron/submicron length scales. In this model, the plastic strain is computed in discrete dislocation dynamics (DDD) and transferred to the finite element method (FEM) to participate in the constitutive law calculation, while the FEM solves the complex boundary problem for DDD simulation. The implementation of the whole coupling scheme takes advantage of user subroutines in the software ABAQUS. The data structures used for information transferring are introduced in detail. Moreover, a FE mesh-based regularization method is proposed to localize the discrete plastic strain to continuum material points. Uniaxial compression tests of single crystal micropillars are performed to validate the developed model. The results indicate the apparent dependence of yield stress on sample size, and its underlying mechanisms are also analyzed.

The effect of hydrogen on the multiscale mechanical behaviour of a La(Fe,Mn,Si)13-based magnetocaloric material

Magnetocaloric cooling offers the potential to improve the efficiency of refrigeration devices and hence cut the significant CO2 emissions associated with cooling processes. A critical issue in deployment of this technology is the mechanical degradation of the magnetocaloric material during processing and operation, leading to limited service-life. The mechanical properties of hydrogenated La(Fe,Mn,Si)13-based magnetocaloric material are studied using macroscale bending tests of polycrystalline specimens and in situ micropillar compression tests of single crystal specimens. The impact of hydrogenation on the mechanical properties are quantified. Understanding of the deformation/failure mechanisms is aided by characterization with transmission electron microscopy and atom probe tomography to reveal the arrangement of hydrogen atoms in the crystal lattice. Results indicate that the intrinsic strength of this material is ~3–6 GPa and is dependent on the crystal orientation. Single crystals under compressive load exhibit shearing along specific crystallographic planes. Hydrogen deteriorates the strength of La(Fe,Mn,Si)13 through promotion of transgranular fracture. The weakening effect of hydrogen on single crystals is anisotropic; it is significant upon shearing parallel to the {111} crystallographic planes but is negligible when the shear plane is {001}-oriented. Atom probe tomography analysis suggests that this is associated with the close arrangement of hydrogen atoms on {222} planes.

Reveal Hydrogen Behavior at Grain Boundaries in Fe–22Mn–0.6C TWIP Steel via In Situ Micropillar Compression Test

In this study, the effect of hydrogen on dislocation and twinning behavior along various grain boundaries in a high-manganese twinning-induced plasticity steel was investigated using an in situ micropillar compression test. The compressive stress in both elastic and plastic regimes was increased with the presence of hydrogen. Further investigation by transmission electron backscatter diffraction and scanning transmission electron microscope demonstrated that hydrogen promoted both dislocation multiplication and twin formation, which resulted in higher stress concentration at twin–twin and twin–grain boundary intersections.

Determination of Constitutive Parameters of Crystal Plasticity using Micro-pillar Testing with Pure Aluminum

The uniaxial compression test using the micrometer-sized specimen is employed to evaluate elastic-plastic deformation behavior of the selective single crystal from polycrystalline materials. Through reflecting these properties into the crystal plasticity finite element method (CPFEM), elastic-plastic analyses of the polycrystalline materials can be performed based on the single crystalline properties of real materials. In this study, the micro-pillar compression tests were conducted in the some single crystals of recrystallized polycrystalline pure aluminum. A method identifying CRSS, work hardening parameters and anisotropic elastic constants was proposed. The predicted stress-strain curves of single crystals obtained by CPFEM agree well with the original experimental data.

Effects of Nb Addition on Microstructures and Mechanical Properties of Nbx-CoCrFeMnNi High Entropy Alloy Films

In the present work, Nbx-CoCrFeMnNi high entropy alloy films (HEAFs, 0 to 7.2 at.% Nb) were fabricated by radio frequency (RF) magnetron co-sputtering of CoCrFeMnNi alloy and Nb targets. The effects of Nb addition on the microstructures and mechanical properties of HEAFs were systematically investigated. For Nb-free film (0 at.% Nb), the face-centered cubic (FCC) peaks were identified in the X-ray diffraction (XRD) pattern. The addition of Nb resulted in a broadening of diffraction peaks, a decrease in peak intensity, and the vanishment of high-angle peaks. Transmission electron microscope (TEM) images indicated the formation of nanotwins at low Nb concentrations, and a transition from a single phase FCC solid solution to an amorphous phase was observed with the increasing Nb concentration. The films were strengthened with an increase in Nb concentration. Specifically, the hardness characterized by nanoindentation increased from 6.5 to 8.1 GPa. The compressive yield strength and fracture strength measured from micropillar compression tests were improved from 1.08 GPs and 2.56 GPa to 2.70 GPa and 5.76 GPa, respectively, whereas the fracture strain decreased from >29.4% (no fracture) to 15.8%. Additionally, shear banding was observed in the presence of amorphous phase.

Dissolution of nanosized NbC precipitates in austenite matrix during elastic deformation - Deleterious effect of high number density

An austenitic stainless steel with large number density of nanosized NbC precipitates distributed in matrix was developed with the addition of small amount of Mn and application of proper thermo-mechanical process. Micro-pillar compression tests were performed to evaluate the strengthening effect of nanosized NbC. Despite much higher number density of NbC precipitates in the Mn-added alloy, the strengthening effect was measured to be significantly less than expected. Then, the compressed micro-pillars were then subjected to post-mortem microstructure analyses focusing on the dynamic evolution of NbC precipitates and austenite matrix. Extensive dissolution and re-precipitation of NbC precipitates were observed in the elastically and plastically deformed micro-pillars. In addition, stacking faults and deformation twin were also observed near the precipitates. From the relationship between the number density of the NbC precipitates, microstructure evolution, and dissolution of NbC, it was found that higher number density of nanosized NbC precipitates associated with dislocations in austenitic stainless steel enhanced dissolution of them by nano-scale yielding during elastic deformation, which limited the strengthening effect of nanosized NbC precipitates.

Laser aided additive manufacturing of spatially heterostructured steels

Current heterostructured materials demonstrate the high potential to circumvent strength-ductility tradeoff but face challenges with unconfigurable hetero-zone distribution and mechanical anisotropy. This study explored laser aided additive manufacturing (LAAM) of non-laminar spatially heterostructured materials (SHM) with configurable architectures to combine superior properties from the constitutive AISI 420 stainless steel and C300 maraging steel to enhance the overall properties. It is proven that the hatch spacing (h) has a significant effect on the microstructural evolutions and mechanical properties of the SHMs as it affects the layer thickness and inter-dilution regions. The mechanical properties of the SHMs were evaluated at multi-scales. The sample with h of 1.5 mm possesses a high tensile strength of about 1.6 GPa along with a reasonable break elongation of 8.1%, showing a good strength-ductility combination. Micropillar compression tests were conducted to measure localised mechanical properties, which is critical to understand the strengthening mechanism. Additionally, the SHM substantiates a much higher strength than many lamellar and linear functionally graded materials reported in the literature. This can be explained by the rule-of-mixture effect and hetero-deformation induced strengthening (HDIS). Furthermore, in-situ deformation observation reveals multiple deformation bands in SHM, which delays necking and contributed to ductility in tandem with the transformation induced plasticity (TRIP) effect. The findings highlight a novel approach for the development of SHM with a tunable performance by using LAAM of multiple materials following the configurable architectures.

Excellent work-hardening in ZrCu/NiNb amorphous/amorphous nanolaminates

Work-hardening behavior could be rarely observed in amorphous alloys during plastic deformation because the dilation and softening effect often occur in shear bands. Here, we proposed a novel mechanism to realize an excellent work-hardening rate (R=dσdε) in ZrCu/NiNb amorphous/amorphous nanolaminates (A/ANLs), which is investigated by in-situ uniaxial micro-pillar compression tests. The work-hardening rate value (25.24 GPa) obtained in ZrCu/NiNb A/ANLs with single layer thickness (h) 5 nm and volume ration 1:1, is relatively high, when comparing with the values in other results that considered the effect of crystalline phases and structural heterogeneity on work-hardening rate in composites. Such a high work-hardening effect is attributed to serious suppression of shear transformation zones (STZs) percolation by amorphous/amorphous interfaces. This work-hardening effect increased sharply as h decreases to 5 nm.

Fiber/matrix debonding evaluation of SiCf/SiC composites using micropillar compression technique

Third-generation silicon carbide (SiC) composites reinforced by SiC fibers (Hi-Nicalon S [HNS] and Tyranno SA3 [SA3]) are attractive for use in next generation reactors owing to their high strength and chemical inertness at high temperatures, as well as enhanced radiation tolerance under neutron irradiation environments. To optimize composite performance, the interfacial mechanical properties of chemical vapor–infiltrated (CVI) SiCf/SiC composites are investigated in this effort by using a slant interface micropillar compression testing procedure. The micropillar test specimens, containing an inclined pyrolytic carbon (PyC) interphase, are prepared using a focused ion beam. The novel microcompression testing successfully quantifies the debond shear strength and internal friction coefficient of micropillar test samples by using the Mohr-Coulomb formulation. According to four types of SiCf/SiC composite microcompression test results, interfacial properties and debond mechanisms are significantly affected by the PyC layer thickness, the local bonding mechanism of the PyC interphase on the SiC fiber surface, and the surface roughness of fibers. Regardless of PyC thicknesses, SA3-reinforced CVI SiCf/SiC composites are found to have much higher debond shear strengths than HNS-reinforced SiCf/SiC CVI composites. By using this micropillar compression technique alongside analytical methods, we uncover new understandings of PyC interface properties. Additionally, the micropillar test results obtained are correlated with macroscopic mechanical properties of neutron-irradiated CVI SiCf/SiC composites.

Effects of cerium addition on microstructures and mechanical properties of CoCrNi medium entropy alloy films

In this work, the effects of rare-earth element Ce addition on the microstructures and mechanical properties of (CoCrNi)100-xCex (x = 0, 1.05, 2.61, 4.2, 5.96) medium entropy alloy films (MEAFs) were investigated. A series of (CoCrNi)100-xCex MEAFs was fabricated by radio frequency magnetron co-sputtering. The XRD and TEM results showed that the structure transformed from FCC to FCC + HCP at x = 2.61. Also, as Ni precipitated with Ce to form CeNi5 HCP phase, the localized depletion of Ni would encourage CoCrNi to transform from FCC to HCP phase. With the further increase in Ce content, Ce4.2 film became mixed nanocrystalline and amorphous structures and Ce5.96 film was fully amorphous. The nanoindentation and micro-pillar compression tests were conducted to study the mechanical properties of films. Both the hardness and Young's modulus showed an initial increase, reached the maxima of ~10 GPa and ~200 GPa, respectively, at x = 1.05, and then decreased with the increasing Ce content. The trends of yield strength and ultimate strength versus Ce content were similar to hardness. These results demonstrated that small amounts of Ce addition could strengthen CoCrNi MEAFs due to solid solution strengthening. However, excessive Ce addition would reduce the grain size to below ~10 nm and weaken the CoCrNiCe system due to the inverse Hall-Petch effect.

Microstructural origin of the anisotropic flow stress of laser powder bed fused AlSi10Mg

The microstructural origin of anisotropy in the yield stress and ultimate tensile strength of AlSi10Mg tensile specimens produced by laser powder bed fusion (LPBF) is revealed using a combination of micropillar compression tests and microstructure-based numerical simulations. Uniaxial tensile tests demonstrate that specimens with tensile axis parallel to the build direction (vertical specimen) exhibit a higher yield stress and ultimate tensile strength than those perpendicular to the build direction (horizontal specimen). Micropillar compression experiments and a microstructure-based crystal plasticity model together confirm that the anisotropy has its origin in an elongated Al-Si cellular network (nominally 0.7 µm × 1.4 µm with long axis aligned with the build direction) within individual grains. A multiscale microstructure-based model predicts the mechanical properties of LPBF AlSi10Mg tensile specimens by accounting for the hierarchical microstructural features across length scales, which include the Al-Si network, the crystallographic grain structure, and the differences in microstructure between melt pool interiors and melt pool boundaries. The multiscale model over-estimates the yield stress, but correctly predicts the ultimate tensile strength and the anisotropy in the flow strength of tensile specimens. The model is used to computationally predict new microstructures driven by composition and processing parameters for LPBF AlSi10Mg alloys with improved mechanical properties.

Mechanical behavior of the HfNbZrTi high entropy alloy after ion irradiation based on micro-pillar compression tests

High entropy alloys (HEAs) have exhibited excellent irradiation resistance regarding structure stability. However, limited knowledge has been available on the tendency of mechanical properties of HEAs with body-centered cubic structures after ion irradiation. In this study, the evolution of mechanical properties of the HfNbZrTi irradiated with 3 MeV C ions is studied based on micropillar compression tests. The yield strength increases almost linearly from 974 to 2068 MPa with increasing irradiation doses up to 0.98 dpa. The correlation between the mechanical and microstructural changes agrees well with the prediction of Δδy=αMGb(ND)12, indicating a similar strengthening mechanism with conventional alloys. Nonetheless, the irradiation strengthening effect in this alloy is less pronounced in the low dose regime, and the saturation is postponed, compared with the stainless steel and the 3d metal high entropy alloys with face-centered cubic structures, exhibiting promising irradiation resistance regarding mechanical properties.

Role of oxygen on the precipitation and deformation behavior of an aged β Ti-15Mo alloy

The mechanical behavior of aged metastable β titanium alloys is directly influenced by the formation and stability of metastable ω and stable α phases during thermomechanical processing. Interstitial oxygen, which is known to cause embrittlement in Ti alloys, affects phase stability in these alloys and also impacts their structural properties. The present study investigated the role of interstitial oxygen on ω and α precipitation in an aged metastable β Ti-15Mo alloy and the subsequent effect of these microstructural changes on the alloy’s compressive mechanical properties. High oxygen levels reduced the ω growth rate and induced a shape change for ω precipitates with oxygen partitioning to ω precipitates during ageing. Oxygen-containing specimens displayed higher compressive yield strengths than in oxygen-free specimens during micropillar compression testing. However, in both cases, ω precipitates were ultimately sheared leading to slip band formation and plastic flow localization. Elevated oxygen also refined the α precipitate distribution whether α nucleation happened homogeneously or heterogeneously. The finer α precipitates with oxygen led to increased compressive yield strength during micropillar compression compared to specimens without oxygen. Oxygen plays a key role in the evolution of ω and α precipitate distributions and associated mechanical properties, which is another example of how oxygen may be a useful alloying element in β titanium alloys.

Bending stress relaxation of microscale single-crystal copper at room temperature: An in situ SEM study

The bending stress relaxation of microscale single-crystal copper was studied by in situ SEM bending experiments to elucidate the time-dependent plasticity involving the effect of the strain gradient and the sample size. Three specimens with varied heights of square cross sections were fabricated using a focused ion beam. Interestingly, an obvious bending plateau was observed in the load-displacement curves. The calculated flow stress on the cross section indicated an inverse relationship between the specimen size and strength. In addition, the proportion of decay stress is approximately 2.7% for the beam with a height of h = 1.82 μm and 13.9% for that of h = 0.94 μm in the bending plateau. Moreover, during the dwell period, a sudden stress drop occurred, which is ascribed to the strain burst or stress jerky during the deformation of single-crystal metals. Microbending is usually related to a significant increase in geometrically necessary dislocations (GNDs), which is supposed to improve the resistance for further deformation. However, the bending plateau implied perfect plastic flow and an evolving balance in the microstructures. On the other hand, thermal activation theory was employed to interpret the time-dependent plasticity. The apparent activation volume of the dwell periods in the bending plateau suggested that cross slip dominates the inelastic strain rate evolution. Finally, the inelastic strain rate for stress relaxation was derived from the curve of decay stress, and it was inversely proportional to the height of the microbeam, which is in complete contrast with the trend observed in the previous compression tests of micropillars. In current microscale bending specimens, the thinner beam containing fewer initial dislocation sources exhibits a larger creep ductility with the assistance of GNDs.

Deformation mechanisms of Mg-Ca-Zn alloys studied by means of micropillar compression tests

The effect of Ca and Zn in solid solution on the critical resolved shear stress (CRSS) of <a> basal slip, tensile twinning and <c+a> pyramidal slip in Mg alloys has been measured through compression tests on single crystal micropillars with different orientations. The solute atoms increased the CRSS for basal slip to ~ 13.5 MPa, while the CRSS for pyramidal slip was lower than 85 MPa, reducing significantly the plastic anisotropy in comparison with pure Mg. Moreover, the CRSSs for twin nucleation and growth were very similar (~ 37 MPa) and the large value of the CRSS for twin growth hindered the growth of twins during thermo-mechanical processing. Finally, evidence of <a> prismatic slip and cross-slip between basal and prismatic dislocations was found. It is concluded that the reduction of plastic anisotropy, the activation of different slip systems and cross-slip and the weak basal texture promoted by the large CRSS for twin growth are responsible for the improved ductility and formability of Mg-Ca-Zn alloys.

Room-temperature deformation of single crystals of ZrB2 and TiB2 with the hexagonal AlB2 structure investigated by micropillar compression

The plastic deformation behavior of single crystals of two transition-metal diborides, ZrB2 and TiB2 with the AlB2 structure has been investigated at room temperature as a function of crystal orientation and specimen size by micropillar compression tests. Although plastic flow is not observed at all for their bulk single crystals at room temperature, plastic flow is successfully observed at room temperature by the operation of slip on {1{bar{1}}00}<11{bar{2}}3> in ZrB2 and by the operation of slip on {1{bar{1}}00}<0001> and {1{bar{1}}00}<11{bar{2}}0> in TiB2. Critical resolve shear stress values at room temperature are very high, exceeding 1 GPa for all observed slip systems; 3.01 GPa for {1{bar{1}}00}<11{bar{2}}3> slip in ZrB2 and 1.72 GPa and 5.17 GPa, respectively for {1{bar{1}}00}<0001> and {1{bar{1}}00}<11{bar{2}}0> slip in TiB2. The identified operative slip systems and their CRSS values are discussed in comparison with those identified in the corresponding bulk single crystals at high temperatures and those inferred from micro-hardness anisotropy in the early studies.

3D Periodic and Interpenetrating Tungsten-Silicon Oxycarbide Nanocomposites Designed for Mechanical Robustness.

Metal-ceramic nanocomposites exhibit exceptional mechanical properties with a combination of high strength, toughness, and hardness that are not achievable in monolithic metals or ceramics, which make them valuable for applications in fields such as the aerospace and automotive industries. In this study, interpenetrating nanocomposites of three-dimensionally ordered macroporous (3DOM) tungsten-silicon oxycarbide (W-SiOC) were prepared, and their mechanical properties were investigated. In these nanocomposites, the crystalline tungsten and amorphous silicon oxycarbide phases both form continuous and interpenetrating networks, with some discrete free carbon nanodomains. The W-SiOC material inherits the periodic structure from its 3DOM W matrix, and this periodic structure can be maintained up to 1000 °C. In situ SEM micropillar compression tests demonstrated that the 3DOM W-SiOC material could sustain a maximum average stress of 1.1 GPa, a factor of 22 greater than that of the 3DOM W matrix, resulting in a specific strength of 640 MPa/(Mg/m3) at 30 °C. Deformation behavior of the developed 3DOM nanocomposite in a wide temperature range (30-575 °C) was investigated. The deformation mode of 3DOM W-SiOC exhibited a transition from fracture-dominated deformation at low temperatures to plastic deformation above 425 °C.

Cold dwell fatigue analyses integrating crystal-level strain rate sensitivity and microstructural heterogeneity

Cold dwell fatigue remains an important life-limiting factor in aircraft engine titanium alloys. Microstructure-level creep and stress accumulation during each loading cycle are controlled by strain rate sensitivity. Here, an integrated experimental and computational framework is used to link crystal-level slip properties to microstructure-sensitive cold dwell debit of a forged rotor graded Ti-6Al-4V alloy. Slip strengths and anisotropic strain rate sensitivities are extracted from micro-pillar compression tests for different slip systems, incorporated within α+β microstructurally-faithful polycrystal representations. Dwell and non-dwell cyclic loading in alloy Ti-6Al-4V are investigated for two differing microstructures, and the cycles to failure predicted based solely on the crystal c-axis tensile strength, and the dwell debit quantified. The dwell effect is predicted to diminish to zero below a peak applied stress of about 790 MPa in the alloy studied.

Phase Transformation and Incompatibility at Grain Boundaries in Zirconia-Based Shape Memory Ceramics: A Micromechanics-Based Simulation Study

Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite-martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is developed to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress-strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars, prior to being used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bicrystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries.

Magnetron sputtering of carbon supersaturated tungsten films – A chemical approach to increase strength

Tungsten (W)-based materials attract significant attention due to their superior mechanical properties. Here, we present a chemical approach based on the addition of carbon (C) for increased strength via the combination of three strengthening mechanisms in W thin films. W:C thin films with C concentrations up to ~4 at.% were deposited by magnetron sputtering. All films exhibit a body-centred-cubic structure with strong <hh0> texture and columnar growth behaviour. X-ray and electron diffraction measurements suggest the formation of supersaturated W:C solid solution phases. The addition of C reduced the average column width from ~133 nm for W to ~20 nm for the film containing ~4 at.% C. The column refinement is explained by a mechanism where C acts as re-nucleation sites. The W film is ~13 GPa hard, while the W:C films achieve a peak hardness of ~24 GPa. The W:C films are ~11 GPa harder than the W film, which is explained by a combination of grain refinement strengthening, solid solution strengthening and increased dislocation density. Additional micropillar compression tests showed that the flow stress increased upon C addition, from ~3.8 to ~8.3 GPa and no brittle fracture was observed.