Micropillar Compression Tests Research Articles

Dynamic evolution of nanosized NbC precipitates in austenite matrix during deformation and its contribution to strengthening

Abstract An austenitic stainless steel containing high density of nanosized NbC precipitates was subjected to micropillar compression tests. Based on the negligible size effect of micropillars with 1–5 μm diameter, the strengthening effect of NbC precipitates was measured. Further compression tests with various terminal strain were performed to understand the dynamic evolution of NbC precipitates during compressive deformation. The dissolution of nanosized NbC precipitates was observed even in the elastically deformed micropillar, caused by the interaction between the pre-existing dislocations and NbC precipitates. In the plastically deformed micropillar, the re-precipitation of NbC precipitates along the slip band was observed, while the partially dissolved NbC precipitates were observed in the region away from the slip band. In addition, the measured strengthening effect of NbC precipitates was compared with existing prediction models, but large discrepancy was observed between them. Finally, with modification of an existing hardening model by incorporating relaxation at the interface and consideration of dissolution of NbC precipitates before plastic deformation, the measured and predicted strengthening effects became comparable.

Ultra-strong and ductile Ta/Co nanolaminates strengthened via grain-boundary expanding and interfacial sliding

The deformation and strengthening behavior of body-centered cubic/hexagonal closed-pack Ta/Co nanolaminates with individual layer thickness h ranging from 5 nm to 100 nm were studied via nanoindentation and micropillar compression tests. The deformation behavior of the Ta/Co micropillars transitioned from dislocation-dominated co-deformation/deformation at columnar grain boundaries in constituent layers with partial/major shearing of the micropillars at larger h to homogeneous deformation with plastic barreling through expansion of grain boundaries and interfacial sliding of constituent layers at a few nanometers’ length-scale. Also, increased amorphous/intermixed interfaces of the Ta/Co micropillars with a few nanometers’ h may have worked as constraints and sinks for dislocation pileups. The 5 nm h Ta/Co micropillars showed ultrahigh strength with a proof strength (σ0.2%) of ~2.32 GPa, a flow strength (σmax) of ~2.54 GPa and a yield strength (σys) of ~2.67 GPa, respectively, with minimal deformation even at 30% strain. It is worth noting that the 5 nm h Ta/Co micropillar showed an extraordinarily high σys (~2.67 GPa), which is 1.2 times higher than the highest yield strength of nanolaminates with at least one hcp constituent reported in the literature, to date. The dislocation pileup–based Hall–Petch strengthening model operated well with h = 25–100 nm, while the strength at h = 10 nm followed the confined layer slip strengthening mechanism. Surprisingly, the strength of the Ta/Co micropillars at h = 5 nm followed none of the strengthening mechanisms; rather, it became independent of h and mostly followed the plasticity at the grain boundaries and interfaces. The exceptionally high flow strength and ductility of the Ta/Co micropillars at h = 5 nm suggests the development of ultra-strong, yet ductile Ta/Co nanolaminates.

Micropillar compression deformation of single crystals of Fe3Ge with the L12 structure

The plastic deformation behavior of single crystals of Fe3Ge with the L12 structure has been investigated at room temperature as a function of crystal orientation by micropillar compression tests. In addition to slip on (010), slip on (111) is observed to occur in Fe3Ge for the first time. The CRSS (critical resolved shear stress) for (111)[101¯] slip, estimated by extrapolating the size-dependent strength variation to the ‘bulk’ size, is ~240 MPa, which is almost 6 times that (~40 MPa) for (010)[101¯] slip similarly estimated. The dissociation scheme for the superlattice dislocation with b=[101¯] is confirmed to be of the APB (anti-phase boundary)-type both on (010) and on (111), in contrast to the previous prediction for the SISF (superlattice intrinsic stacking fault) scheme on (111) because of the expected APB instability. While superlattice dislocations do not have any preferential directions to align when gliding on (010) (indicative of low frictional stress at room temperature), the alignment of superlattice dislocations along their screw orientation is observed when gliding on (111). This is proved to be due to thermally-activated cross-slip to form Kear-Wilsdorf locks, indicative of the occurrence of yield stress anomaly that is observed in many other L12 compounds such as Ni3Al. Some important deformation characteristics expected to occur in Fe3Ge (such as the absence of SISF-couple dissociation and the occurrence of yield stress anomaly) will be discussed in the light of the experimental results obtained (APB energies on (111) and (010) and CRSS values for slip on (111) and (010)).

Mechanical Behavior of Al–Al2Cu–Si and Al–Al2Cu Eutectic Alloys

In this study, laser rapid solidification technique was used to refine the microstructure of ternary Al–Cu–Si and binary Al–Cu eutectic alloys to nanoscales. Micropillar compression testing was performed to measure the stress–strain response of the samples with characteristic microstructure in the melt pool regions. The laser-remelted Al–Al2Cu–Si ternary alloy was observed to reach the compressive strength of 1.59 GPa before failure at a strain of 28.5%, which is significantly better than the as-cast alloy with a maximum strength of 0.48 GPa at a failure strain of 4.8%. The laser-remelted Al–Cu binary alloy was observed to reach the compressive strength of 2.07 GPa before failure at a strain of 26.5%, which is significantly better than the as-cast alloy with maximum strength of 0.74 GPa at a failure strain of 3.3%. The enhanced compressive strength and improved compressive plasticity were interpreted in terms of microstructural refinement and hierarchical eutectic morphology.

Evaluation of irradiation hardening in ODS-Cu and non ODS-Cu by nanoindentation hardness test and micro-pillar compression test after self-ion irradiation

Abstract Oxide dispersion strengthened (ODS) Cu alloys are expected to have high strength and superior irradiation resistance that are necessary characteristics for the divertor components of fusion reactors. The present study investigated the irradiation hardening after a self-ion irradiation experiment that was carried out at 100 °C with 5.1 MeV Cu2+ ions up to three displacements per atom in ODS-Cu and compared it to that of non-ODS Cu materials such as mechanically alloyed (MAed) Cu and single crystal (SC) Cu. Nanoindentation hardness tests and micro-pillar compression tests on 1 μm cubic pillars were carried out to evaluate the mechanical strength before and after the ion-irradiation. Both MAed-Cu and ODS-Cu showed good resistance against irradiation hardening, while SC-Cu showed a larger increase in the nanoindentation hardness and in the micro-pillar compression yield stress. Relationships between the nanoindentation hardness, Vickers hardness and micro-pillar yield stress were discussed considering the effects of both specimen/indentation size and irradiation hardening.

Modeling of porosity and grain size effects on mechanical behavior of additively manufactured structures

Additive manufacturing (AM) methods such as Aerosol Jet (AJ) printing allow the fabrication of structures via sintering of micro and/or nanoparticles, leading to microstructures that consist of various combinations of pore and grain sizes. It has been reported that AJ printed and sintered silver micropillars show an unusual behavior of high stiffness and high strain-to-failure for structures with high porosity and vice versa (Saleh et al. 2018 [1]). This behavior, however, is accompanied by the stiffer structures having smaller grain sizes and softer structures having larger grain sizes. To explain the physics of this behavior where a trade-off between hardening caused by size effects (grain refinement and gradients) and softening caused by porosity is expected to play a critical role, a multi-scale modeling approach is proposed in this paper. The model formulation consists of a continuum dislocation dynamics (CDD) framework, coupled with continuum plasticity and finite element analysis. The dislocation dynamics formulation is introduced into a user material subroutine and coupled with a finite element commercial solver, in this case, LS-DYNA, to solve the model in three-dimensional scale with the same size as the AM micropillars. The results from the model capture the general trends observed in compression tests of AM micropillars. In particular, it is shown that the grain size and dislocation density have a disproportionately higher influence over the mechanical deformation of metallic structures when compared to the porosity. These results show that the behavior of AM structures in the plastic regime is dominated by grain size effects rather than porosity. Some limitations of the model and possible future refinements are discussed. The paper provides an important analytical framework to model the mechanical behavior of AM structures with internal porosity in the plastic regime.

A Calibration Method of Micropillar Compression Testing for Taper and Eccentricity Induced Errors

Errors induced by pillar taper and eccentric load in micropillar compression experiments have seldom been investigated, although they significantly affect the measurement accuracy, especially in the testing of specimens in a micrometer and nanometer size. A novel calibration method related to stress–strain responses in micropillar compression testing was proposed in this study to eliminate this kind of errors. Axial load and eccentric load were applied to high-entropy alloy (HEA) pillars with taper angles of 1°, 2°, and 3°, respectively, and the measured stress–strain curves were corrected by the proposed calibration method. The results revealed that the difference of the corrected curves with different tapers or eccentricities was smaller than the measured curves. The errors induced by the taper and eccentric load increased with the taper angle and eccentricity. In addition, the difference between measured and corrected flow stresses increased with the taper angle, which was essentially consistent with the results from the calibration method.

Micropillar Compression Deformation of Single Crystals of Fe <sub>3</sub>Ge with the L1 <sub>2</sub> Structure

The plastic deformation behavior of single crystals of Fe3Ge with the L12 structure has been investigated at room temperature as a function of crystal orientation by micropillar compression tests. In addition to slip on (010), slip on (111) is observed to occur in Fe3Ge for the first time. The CRSS (critical resolved shear stress) for (111)[10‾1] slip, estimated by extrapolating the size-dependent strength variation to the ‘bulk’ size, is ~240 MPa, which is almost 6 times that (~40 MPa) for (010)[101] slip similarly estimated. The dissociation scheme for the superlattice dislocation with b=[10‾1] is confirmed to be of the APB (anti-phase boundary)-type both on (010) and on (111), in contrast to the previous prediction for the SISF (superlattice intrinsic stacking fault) scheme on (111) because of the expected APB instability. While superlattice dislocations do not have any preferential directions to align when gliding on (010) (indicative of low frictional stress at room temperature), the alignment of superlattice dislocations along their screw orientation is observed when gliding on (111). This is proved to be due to thermally-activated cross-slip to form Kear-Wilsdorf locks, indicative of the occurrence of yield stress anomaly that is observed in many other L12 compounds such as Ni3Al. Some important deformation characteristics expected to occur in Fe3Ge (such as the absence of SISF-couple dissociation and the occurrence of yield stress anomaly) will be discussed in the light of the experimental results obtained (APB energies on (111) and (010) and CRSS values for slip on (111) and (010)).

Ultra-high strength and plasticity mediated by partial dislocations and defect networks: Part II: Layer thickness effect

Layer thickness dependent mechanical behaviors of metallic nanolaminates have been extensively investigated. In a recent study [1], we show that a particular defect network, consisting of layer interface, stacking faults and twin boundaries, plays an important role in achieving high strength in Cu/Co multilayers. Here, we report a follow-up study on the effect of layer thickness on this unique interplay of defect networks. To this end, we investigate the mechanical behavior of highly textured Cu (111)/Co (0002) multilayers with individual layer thickness of 5, 25 and 100 nm. In situ micropillar compression tests show that the Cu/Co 25 nm multilayers have a much higher strength than 100 nm and 5 nm multilayers. Post-deformation TEM analyses and MD simulations reveal the layer thickness dependent variations of defect density dominating the strengthening effect in multilayers. This study provides new perspectives on optimal defect networks for the design of high strength, deformable metallic materials.

Extreme shear-deformation-induced modification of defect structures and hierarchical microstructure in an Al\u2013Si alloy

Extreme shear deformation is used for several material processing methods and is unavoidable in many engineering applications in which two surfaces are in relative motion against each other while in physical contact. The mechanistic understanding of the microstructural evolution of multi-phase metallic alloys under extreme shear deformation is still in its infancy. Here, we highlight the influence of shear deformation on the microstructural hierarchy and mechanical properties of a binary as-cast Al-4 at.% Si alloy. Shear-deformation-induced grain refinement, multiscale fragmentation of the eutectic Si-lamellae, and metastable solute saturated phases with distinctive defect structures led to a two-fold increase in the flow stresses determined by micropillar compression testing. These results highlight that shear deformation can achieve non-equilibrium microstructures with enhanced mechanical properties in Al–Si alloys. The experimental and computational insights obtained here are especially crucial for developing predictive models for microstructural evolution of metals under extreme shear deformation.

Interstitial carbon induced FCC-Ti exhibiting ultrahigh strength in a Ti37Nb28Mo28-C7 complex concentrated alloy

An interstitial carbon induced-FCC-Ti was observed in a carbon-doped Ti37Nb28Mo28-C7 (atomic percentage) complex concentrated alloys (TNMC alloys). Not only the true crystal structure of this FCC-Ti was directly characterized using Cs-corrected STEM, but also the mechanical properties were tested by nanoindentation and micro-pillar compression for the first time. The FCC-Ti exhibited a high nano hardness of 17.8 GPa and a high elastic modulus of 233.1 GPa. The micropillar compression tests demonstrated the ultrahigh strengths of the interstitial C induced FCC-Ti (4.48 GPa at [1 1 1] direction, 2.67 GPa at [1 0 1] direction, and a CRSS of 1.12 ± 0.07 GPa). The nature of this FCC allotropic transformation was determined to be a rearrangement of lattice structure caused by interstitial C atoms, based on ab-initio studies. Findings in this study provide insights into the characteristics of FCC-Ti as well as the role of interstitial atoms in RCCAs which could open up unlimited possibilities in future material designing.

Plastic deformation of single crystals of the δ1p and δ1k intermetallic compounds in the Fe–Zn system by micropillar compression

The plastic deformation behavior of single crystals of the δ1p and δ1k phase compounds in the Fe–Zn system, which are the major constituent phases in the coating layer of galvannealed (GA) steels, has been investigated by micropillar compression tests at room temperature as a function of crystal orientation and specimen size. (0001)[112‾0] basal slip and (101‾0)[1‾21‾0] prism slip are observed to operate in the δ1p phase compound while (0001)[112‾0] basal slip is observed in the δ1k phase compound. For all cases, a significant strain burst occurs immediately after yielding, forming giant steps on the slip planes along the slip direction and leading in many cases to instantaneous ‘slip plane (shear) failure’. The extent of such strain burst as well as instantaneous ‘slip plane failure’ is reduced by introducing dislocations (pre-straining) prior to micropillar testing, accompanied by the reduction in the stress at which such strain burst occurs as well as that in the extent of strain burst. The critical resolved shear stress (CRSS) for basal and prism slip in the δ1p and δ1k phase compounds all show an inverse power-law scaling against the specimen size with an exponent in the range of 0.02–0.06. The bulk CRSS values are estimated respectively to be approximately 320, 430, and 765 MPa for the basal slip in the δ1p and δ1k compounds and the prism slip in the δ1p compound by taking into account the specimen size effects of CRSS.

Room temperature deformation of single crystals of Ti5Si3 with the hexagonal D88 structure investigated by micropillar compression tests

Micropillar compression tests of Ti5Si3 single crystals were conducted at room temperature as a function of loading axis orientation and specimen size in order to investigate their room temperature plastic deformation behavior. Plastic flow by the operation of three deformation modes, {1{overline{1}}00}[0001], {2{overline{1}}{overline{1}}2} < 2{overline{1}}{overline{1}}{overline{3}} > and {1{overline{1}}01} < 2{overline{1}}{overline{1}}{overline{3}} > slip were observed in [2{overline{2}}05]-, [0001]- and [4{overline{3}}{overline{1}}0]-oriented micropillar specimens deformed at room temperature, respectively. The CRSS values were evaluated to be very high above 2.7 GPa and were confirmed to increase up to about 6 GPa with the decrease in the specimen size. The fracture toughness values are evaluated to be 0.45 MPa m1/2 (notch plane // (0001)) and 0.73 MPa m1/2 (notch plane //(1{overline{1}}00)) based on the results of micro-cantilever bend tests of chevron-notched specimens. The fracture toughness values are considerably lower than those for D8l-Mo5SiB2 and D8l-Nb5Si3 evaluated by the same method, indicating the inherent brittleness of binary Ti5Si3 compared to the other transition-metal silicides of the TM5Si3 type (TM: transition-metal).

Machining-induced thermal damage in cortical bone: Necrosis and micro-mechanical integrity

In bone cutting, the tissue is exposed to necrosis due to temperature elevation, which can significantly influence postoperative results in orthopaedic surgeries. This damage is usually revealed through histological analysis to show the necrotic extent; however, this technique does not capture mechanical damage, which is essential for a full material integrity assessment. Here, with micro-mechanics, it is demonstrated that machining-induced damage in bone extends beyond the necrotic region. Drilling with different conditions was performed on ex-vivo bovine cortical bone, inducing different damage degrees. Micro-pillar compression tests were performed in the machined sub-surface to identify changes in properties and failure modes caused by drilling. It was revealed that at high cutting temperatures, the bone near the machined surface suffers from lower modulus (−42%), strength (−41%) and brittle behaviour, whereas the bulk bone remains undamaged with pristine properties and ductile behaviour. Histology was also performed to evaluate necrosis and, surprisingly, it was found that the brittle and weaker bone layer is more than three times larger when compared to the necrotic layer, clearly showing that the drilling thermo-mechanical effect could affect not only biologically, but also micro-mechanically. Consequently, these results reveal another kind of bone damage that has so far been neglected.

Deformation behavior of nanoscale Al–Al2Cu eutectics studied by in situ micropillar compression

Deformation behavior of nanoscale laser processed Al–Al2Cu eutectics at room temperature is characterized through in situ micro-pillar compression testing in a scanning transmission microscope. Interlamellar spacing of Al–Al2Cu eutectics varies from hundreds of nanometers to 20 nm. Three different sizes of micro-pillars are fabricated in order to study the deformation behaviors of single colony and multiple colonies, corresponding to the single crystal and polycrystal respectively. For single colonies, lamellar orientations parallel, normal or inclined to the loading direction were tested. The main findings are: 1) the plasticity mechanisms strongly depend on loading orientation: buckling and kinking in the parallel-loaded eutectics, planar sliding along Al–Al2Cu lamellar interfaces in the incline-loaded eutectics and localized shearing in the normal-loaded eutectics. 2) the incline-loaded eutectics exhibits the lowest compression flow strength, and the normal-loaded eutectics has the highest compression flow strength. 3) with decreasing inter-lamellar spacing, the strength increases and plasticity is uniformly distributed, as opposed to shear localization. Highest compressive plasticity is observed in polycrystalline eutectics with an ensemble of lamellar orientations with ~20 nm average spacing: 17.9% at flow stress of 1.63 GPa, and degenerate, bimodal morphology: 11.1% at flow stress of 1.36 GPa.

Slip–hydride interactions in Zircaloy-4: Multiscale mechanical testing and characterisation

The interactions between δ-hydrides and plastic slip in a commercial zirconium alloy, Zircaloy-4, under load were studied using in situ secondary electron microscope (SEM) micropillar compression tests of single crystal samples and ex situ digital image correlation (DIC) macroscale tensile tests of polycrystalline samples. The hydrides decorate near basal planes in orientation, and for micropillars orientated for 〈a〉 basal slip localised shear at the hydride–matrix interface is favoured over slip in α-Zr matrix due to a lower shear stress required. In contrast, for pillars oriented for 〈a〉 prismatic slip the shear stress needed to trigger plastic slip within the hydride is slightly higher than the critical resolved shear stress (CRSS) for the 〈a〉 prismatic slip system. In this case, slip in the hydride is likely achieved through 〈110〉-type shear which is parallel to the activated 〈a〉-type shear in the parent matrix. At a longer lengthscale, these results are used to inform polycrystalline samples analysed using high spatial resolution DIC. Here localised interface shear remains to be a significant deformation path which can both cause and be caused by matrix slip on planes closely-oriented to the phase boundaries. Matrix slip on planes nearly perpendicular to the adjacent hydride–matrix interfaces can either result in plastic slip within the hydrides or get arrested at the interfaces, generating local stress concentration. Through these mechanisms, the presence of δ-hydrides leads to enhanced strain localisation in Zircaloy-4 early in the plastic regime.

Micropillar compression deformation of single crystals of the intermetallic compound Γ-Fe4Zn9

The plastic deformation behavior of single crystals of the Γ (Fe4Zn9) phase compound in the Fe−Zn system, which has been believed to be the most detrimental phase causing brittle failure in the coating layer (such as powdering) on galvannealed (GA) steels, has been investigated by micropillar compression tests at room temperature as a function of crystal orientation and specimen size. All micropillar specimens exhibit rich deformability in compression. Two distinct slip systems, {1¯01}<111> and {1¯01}<010>, are observed to operate. The critical resolved shear stress (CRSS) values for {1¯01}<111> and {1¯01}<010> both obey an inverse power-law scaling against the micropillar size with an exponent of -0.245 and -0.046, respectively. The bulk CRSS values for these two slip systems are estimated respectively to be 238 and 649 MPa by considering the specimen size effects of CRSS. The crystal orientation range for the operation of {1¯01}<010> slip is limited to the vicinity of 〈111〉 because of the considerable difference in the CRSS values for these two slip systems. Dislocations with b (Burgers vector) = 〈010〉 dissociate into two partial dislocations, while those with b = 1/2<111> do not show any dissociation, both of which are consistent with the result of theoretical calculation of the generalized stacking fault energy on the {1¯01} slip plane. Possible reasons for brittle failure related to the Γ phase occurring in the coating layer on GA steels are discussed based on the results obtained.

High temperature strength retention of Cu/Nb nanolaminates through dynamic strain ageing

The mechanical properties of Cu/Nb metallic nanolaminates with different layer thickness (7, 16, 34 and 63 nm) were studied by means of micropillar compression tests from room temperature to 400 °C. Both strain-rate jump and constant strain rate tests were carried out and they showed evidence of dynamic strain ageing in the nanolaminates with 7, 16 and 34 nm layer thickness deformed at 200 °C. Dynamic strain ageing was accompanied by a reduction of the strain rate sensitivity to 0, high strength retention at 200 °C and the development of shear localization of the deformation at low strains (5%–6%) that took place along the Nb layers in the nanolaminates. Atom probe tomography of the deformed specimens revealed the presence of O in solid solution in the Nb layers but not in the Cu layers. Thus, diffusion of O atoms to the mobile dislocations in Nb was found to be the origin of the dynamic strain ageing in the Cu/Nb nanolaminates around 200 °C. This mechanism was not found at higher temperature (400 °C) because deformation was mainly controlled by stress-assisted diffusion in the Cu layers. This discovery shows a novel strategy to enhance the strength retention at high temperature of metallic nanolaminates through dynamic strain ageing of one the phases.

Effect of hydrogen on deformation behavior of Alloy 725 revealed by in-situ bi-crystalline micropillar compression test

Nickel-based Alloy 725 bi-crystalline micropillars with different types of grain boundaries (GBs) were compressed in hydrogen-free and in-situ hydrogen-charged conditions to investigate the hydrogen effect on the deformation behavior of the selected GBs. In the presence of hydrogen, the compressive stresses on the micropillars increase regardless of the GB type. It was proposed that this hydrogen-induced hardening behavior is the synergistic effect of hydrogen-enhanced dislocation multiplication and interactions, the pinning effect of hydrogen on dislocation motion, and hydrogen-enhanced lattice friction. Transmission electron backscatter diffraction (t-EBSD) results demonstrate that both low-angle GBs and high-angle GBs can effectively suppress dislocation transmission through the GBs, resulting in dislocations pile up along the GBs in the hydrogen-charged condition. In contrast, this behavior was not observed in the micropillars with twin boundaries.

High strain rate compression of epoxy micropillars

The study of the high strain rate mechanical behaviour of materials using micropillar compression tests has been hindered so far due to the lack of suitable instrumentation. In the present study, a novel high strain rate micropillar compression set-up is introduced. The high speed tests are carried out with the impact configuration of a pendulum-based nanoindentation device that was instrumented with force-sensing capability by means of a piezoelectric load cell. This is necessary due to the considerable inertia effects that make the actual applied force on the sample much higher than the actuated one. The proposed novel technique was successfully applied to study the mechanical behaviour of an epoxy resin over a wide range of strain rates. The results reveal a significant strain rate sensitivity that correlates well with that obtained for the same material at the macroscale, validating the novel set-up.