Micropillar Compression Tests Research Articles

A combined experimental-analytical modeling study of the artificial aging response of Al–Mg–Si alloys

The isothermal aging response at 170 °C of three extruded Al–Mg–Si alloys that had been solution-treated and quenched was experimentally tracked to understand the influence of Mg + Si content and the Mg/Si ratio on microstructure and mechanical behavior. Transmission electron microscopy (TEM) was employed to quantitatively characterize the needle-shaped β'' precipitates in the underaged, peak-aged and overaged conditions; while the alloy with a high Mg + Si content raised the number density of β'' precipitates, a higher Mg/Si ratio increased the number density but decreased the size of β'' precipitates in the peak-aged condition. In the overaged condition, coarsening of β'' precipitates occurred primarily along its length. Single crystal micropillar compression tests were conducted in the naturally-aged, under-aged, peak-aged and over-aged conditions to correlate the microstructure to the critical resolved shear stress (CRSS). In the peak-aged condition, the alloy with the high Mg + Si content and a Mg/Si ratio corresponding to the β'' stoichiometry (Mg5Si6) exhibited the highest CRSS. A precipitation kinetics model that assumed a cylindrical morphology for the β'' phase with a fixed aspect ratio was developed to describe β'' phase microstructure evolution as a function of alloy chemistry, solution treatment temperature and artificial aging parameters. The results from this model served as input to a dislocation-precipitate interaction model to calculate single crystal yield strength. The predictions from these analytical models were validated with experimental results from this study and from the literature and fundamental insights were obtained on the role of microstructural parameters in affecting yield strength for non-spherical precipitates.

Ultra-fine-grained and gradient FeCrAl alloys with outstanding work hardening capability

FeCrAl alloy is a promising accident tolerant fuel cladding material for next generation nuclear reactors. Grain refinement may lead to FeCrAl alloys with improved mechanical properties. However, the addition of Al retains the ferrite phase and makes the grain refinement difficult through conventional thermomechanical treatments. In this study, a model FeCrAl alloy, C35M, was processed by surface mechanical grinding treatment. Microscopy studies show the formation of gradient microstructures, consisting of surface nanolaminate layer and ultra-fine-grained sublayer. In situ micropillar compression tests reveal the gradient FeCrAl alloy has a high flow stress, exceeding 1.4 GPa, and excellent work hardening capability. A modified Kocks-Mecking model was utilized to explain the grain size-dependent work hardening behavior. This study sheds lights on the design of gradient ferritic steels with high-strength and good work hardening ability for various industrial applications.

A discrete slip plane model for simulating heterogeneous plastic deformation in single crystals

In small-scale mechanical tests, such as micropillar compression tests, plastic deformation is often localized in narrow slip traces. These slip traces result from a few dislocation sources with relatively low nucleation stresses that are present in the material. In order to accurately simulate such small-scale experiments, the stochastics of the underlying dislocation network must be taken into account, which is usually done by performing discrete dislocation dynamics simulations. However, their high computational cost generally restricts these simulations to small and simple geometries and small applied displacements. Furthermore, effects of geometrical changes are usually neglected in the small strain formulation adopted. In this study, a discrete slip plane model for simulating small-scale experiments on single crystals is proposed, which takes the most important characteristics of dislocation plasticity for geometries in the micrometer range into account, i.e.the stochastics and physics of dislocation sources. In the model, the properties of all lattice planes are sampled from a probability density function. This results in a heterogeneous flow stress within a single crystal, unlike the uniform properties assumed in conventional crystal plasticity formulations. Moreover, the slip planes can be grouped together in bands via a weakest-link principle. The resulting equations are implemented in a standard crystal plasticity finite element model, using a finite deformation formulation. Within this setting, only the collective dislocation motion on glide planes is modeled, resulting in a significantly lower computational cost compared to frameworks in which the dynamics of individual dislocations are considered. This allows for simulating multiple realizations in 3D, up to large deformations. A small case study on micropillar compression tests is presented to illustrate the capabilities of the model.

Thermal activation process of plastic deformation in Fe–18Cr single-crystal micropillars with high-density dislocations

Micropillar compression testing was performed on bcc solid-solution single-crystal micropillars with two diameter (d) ranges (2–3 and 5–6 μm) prepared on the surfaces of fully annealed and subsequently 10% cold-rolled 18Cr ferritic steel specimens. This was done to investigate the effect of initial dislocations on the plastic deformation of micron-sized crystals. The 10% cold rolling process significantly increased the stress level (provided by an activated single slip system) required to initiate slip in the annealed micropillars with different diameter ranges. In the case of the annealed micropillars, the strain-rate sensitivity (m) of the yield stress becomes lower for larger specimens (the m value decreased from 0.12 to 0.04). However, the 10% cold-rolled micropillars exhibited a much lower strain-rate sensitivity regardless of the specimen size. The activation volume (v*) of the annealed micropillars showed a significant specimen size dependency (v*∝d1.47), consistent with published data on the micropillar compression experiments of single-crystals of the bcc metals and alloys. For the 10% cold-rolled micropillars, the high v* values (88b3 and 112b3) showed a slight dependency on the specimen size (v*∝d0.33). The v* values appeared somewhat higher but were equivalent to those of bulk-size specimens, suggesting that the initial high-density dislocations would provide sufficient sources for dislocation multiplication in the micron-sized specimens under compression. The observed high v* values might be associated with the strong dislocation interactions in the α-(Fe, Cr) solid-solution matrix.

Microstructures and mechanical properties of (CoCrFeMnNi)100-xMox high entropy alloy films

The effects of Mo addition on the microstructures and mechanical properties of CoCrFeMnNi high entropy alloy films (HEAFs) were studied in this work. To achieve this, a series of (CoCrFeMnNi)100-xMox HEAFs was prepared by magnetron co-sputtering. The films were FCC without Mo addition and composed of FCC and HCP structures with minor Mo addition. With the further increase in Mo content, the films transformed to an amorphous structure. Nanoindentation revealed an initial decrease in hardness and then increase with the increasing Mo content. While the initial decrease in hardness could be attributed to the detwinning-induced softening for films with small twin spacing as observed by TEM after micropillar compression, the subsequent increase could be attributed to solid solution strengthening and formation of nanotwins with larger twin spacing and no detwinning observed after compression. Using scratch tests, the Mo-doped HEAFs exhibited a high hardness, good wear resistance and low coefficient of friction. The results of micropillar compression tests showed the increasing compressive yield strength and the decreasing fracture strain with the increasing Mo content. When x = 4.9, the film revealed the optimum mechanical properties with fracture strength of 6.51 GPa and little reduction in ductility.

Effects of radiation damage and precipitate distribution on micro-pillar compression testing of irradiated CuCrZr

The results of small-scale mechanical tests are convoluted by the so-called size effect, whereby materials appear stronger when the scale of the test is reduced to the order of microns or less. The dimensional range over which this occurs has been shown to be linked to a change in sample microstructure, such as the addition of defects induced by irradiation. To investigate this response, a CuCrZr alloy was subjected to proton irradiation and mechanically tested using micro compression of pillars with a range in size. It was found that irradiation defects dominate over the extrinsic size effect and the sensitivity to differences in precipitate microstructure was also somewhat reduced, suggesting that size-independent results could be obtained from much smaller test volumes in irradiated material compared to their non-irradiated counterparts. Finally, comparison was made between the increase in yield strength predicted by models and the experimentally measured values to establish the key parameters driving the strengthening behaviour.

Design of Comb Crack Resistant Milling Inserts: A Comparison of Stresses, Crack Propagation, and Deformation Behavior between Ti(C,N)/α-Al2O3 and Zr(C,N)/α-Al2O3 CVD Coatings

Investigations on comb crack resistance of milling inserts coated with chemical vapor deposition (CVD) Ti(C,N)/α-Al2O3 and Zr(C,N)/α-Al2O3 showed a distinct wear evolution in both systems. Wear studies revealed that the appearance of comb cracks is connected to the initial CVD cooling crack network. Micropillar compression tests indicated a brittle intergranular fracture mechanism for the Ti(C,N) layer and a transgranular fracture accompanied with signs of plastic deformation for the Zr(C,N) coating. Additionally, for the Zr(C,N) based system, a compressive stress condition in the temperature range of interest (200–600 °C) was determined by in-situ synchrotron X-ray diffraction. The set of residual compressive stresses together with the ability of the Zr(C,N) layer to deform plastically are key features that explain the enhanced resistance to comb crack wear of the Zr(C,N) based system in milling of cast iron.

Ultrastrong nanocrystalline binary alloys discovered via high-throughput screening of the CoCr system

Nanocrystalline (nc) alloys are stronger than their coarse-grained versions. Here, we report ultrastrong alloys, discovered via high-throughput screening of the nc CoCr33-69 materials library. The nc materials library was fabricated using magnetron co-sputtering. The alloys consist of textured, columnar structures with grain size in the nanometric regime. We found that the texture and phase composition can be tailored by changing the Cr concentration. In the investigated region of the nc CoCr system, a relatively broad spectrum of yield strength, determined via micropillar compression tests, was found ranging from 1.41 GPa up to 3.64 GPa. The remarkable strength increment was caused by a chemically- and thermally-driven phase and microstructure evolution of the system. The strongest alloys were found in the regions containing the δCoCr phase, which was considered previously as metastable. Density functional calculations revealed that the δCoCr phase is more energetically favourable in Cr-rich regions compared to single-phase simple solid solutions (HCP, BCC). Experimental results showed that the range of its occurrence is wider than previously thought, i.e. after annealing the δCoCr phase was found above 44 at. % of Cr. We demonstrate that systematic screening of materials libraries can boost the discovery of new materials with outstanding properties.

Simultaneously enhancing strength and ductility in graphene nanoplatelets reinforced titanium (GNPs/Ti) composites through a novel three-dimensional interface design

The interface debonding caused by mismatched thermal expansion is a key issue in graphene nanoplatelets/Ti matrix composites (GNPs/TiMCs), which significantly decreases the composites' mechanical properties. In this study, GNPs/Ti composites were fabricated by spark plasma sintering (SPS) followed by heat treatment (HT) routes. A novel strategy that strengthening the GNPs-Ti interface bonding via tailored three-dimensional (3D) interface configuration was proposed for the first time. Characters of interface evolution (before and after HT processes) were discussed detailedly in conjunction with the in-situ formed TiB whiskers (TiBw). The interfacial microstructure displayed that GNPs was used as template for the heteroepitaxial growth of TiBw, then TiBw connected the TiC layer and the adjacent Ti matrix. Tensile tests showed that GNPs-(TiBw)/Ti composites exhibited excellent strength-plasticity compatibility as compared to GNPs/Ti. Ex-situ compression tests of micro-pillars fabricated from the bulk composites revealed that the 3D interface configuration in GNPs-(TiBw)/Ti possessed significantly higher strain accommodation and strain-hardening ability during deformation. This study highlights the importance of interface optimization, which is helpful for developing the interfacial engineering and achieving good properties in GNPs/TiMCs.

Critical resolved shear stress of activated slips measured by micropillar compression tests for single-crystals of Cr-based Laves phases

Abstract Single-crystal micropillar compression tests were performed in this study to investigate the mechanical properties and deformation mechanisms of Cr-based Laves phases. The results showed that, the activated slip system in the C15–Cr2Nb was exclusively the basal slip {111}⟨110⟩, transiting from the pyramidal slip {11 2 ¯ 2}⟨ 1 ¯ 1 ¯ 23⟩ to the prismatic slips {10 1 ¯ 0}⟨0001⟩ and {11 2 ¯ 0}⟨1 1 ¯ 00⟩ in the C14–Cr2Ta with the increased inclination angle. The average compressive strength of the C15–Cr2Nb changed between 10.0 GPa and 11.7 GPa, and that of the C14–Cr2Ta ranged within 11.6–12.4 GPa. The compressive strength changed due to the evolutions of activated slip systems in both Laves phases. The critical resolved shear stress (CRSS) of the C15–Cr2Nb was estimated as 4.4–4.6 GPa, which was lower than 5.1–6.1 GPa in the C14–Cr2Ta, and the higher CRSS of the C14–Cr2Ta resulted from higher shear modulus. The mechanical properties and deformation mechanisms obtained were helpful for better understanding and designing the Cr-based Laves phase alloys.

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.