Micropillar Compression Tests Research Articles

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.

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.

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.

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.

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.

Additive manufacturing of an ultrastrong, deformable Al alloy with nanoscale intermetallics

Light-weight, high-strength, aluminum (Al) alloys have widespread industrial applications. However, most commercially available high-strength Al alloys, like AA 7075, are not suitable for additive manufacturing due to their high susceptibility to solidification cracking. In this work, a custom Al alloy Al92Ti2Fe2Co2Ni2 is fabricated by selective laser melting. Heterogeneous nanoscale medium-entropy intermetallic lamella form in the as-printed Al alloy. Macroscale compression tests reveal a combination of high strength, over 700 MPa, and prominent plastic deformability. Micropillar compression tests display significant back stress in all regions, and certain regions have flow stresses exceeding 900 MPa. Post-deformation analyses reveal that, in addition to abundant dislocation activities in Al matrix, complex dislocation structures and stacking faults form in monoclinic Al9Co2 type brittle intermetallics. This study shows that proper introduction of heterogeneous microstructures and nanoscale medium entropy intermetallics offer an alternative solution to the design of ultrastrong, deformable Al alloys via additive manufacturing.

The effect of grain boundaries and precipitates on the mechanical behavior of the refractory compositionally complex alloy NbMoCrTiAl

Refractory compositionally complex alloys (CCAs) have been found to exhibit promising high-temperature properties. However, the brittleness of most CCAs at room temperature limits their application. To understand the influence of microstructure on the deformation behavior of these alloys, we conducted micromechanical experiments, including nanoindentation and micropillar compression tests, on a representative NbMoCrTiAl alloy at room temperature. The indentation at the grain boundaries showed increased shear stress and hardness compared to the matrix (ordered B2 crystal structure) due to the presence of local intermetallic precipitates (C14 and A15 phases). Although the NbMoCrTiAl alloy exhibits no ductility at the millimeter scale as shown in previous studies, the micropillars at the grain boundaries, which were decorated with precipitates, demonstrated higher yield strength and significant plastic strain &amp;gt;30% at room temperature. Numerous slip lines were observed, while cracks and fracture occurred at higher levels of deformation. The grain boundaries with precipitates increased the probability of crack initiation and propagation, but they did not necessarily lead to catastrophic failure of the pillars.

Basal and prismatic slip in hafnium

Metals with hexagonal crystal structure are found in many safety-critical applications, from titanium alloy fan blades in jet engines to magnesium alloys in the automotive industry. They also exist in nuclear reactors, for example hafnium alloy control rods whose structural integrity is critical to nuclear safety. However, mechanistic understanding of the micromechanical properties and deformation behaviour of Hf remains elusive. To aid in this understanding, we performed in situ scanning electron microscopy single crystal micropillar compression tests of a commercial Hf alloy, where the samples were aligned to activate 〈a〉 prismatic and 〈a〉 basal slip systems. We then employed transmission electron microscopy to examine the dislocation structure in the deformed pillars. These two slip systems exhibited distinctly different behaviour, where prismatic slip is planar and basal slip is wavy. Prismatic slip is the easiest deformation mode, while basal slip is at least twice as difficult. Generally, the characters of prismatic and basal slip in Hf show similarities to those in some other hexagonal metals such as Ti and Zr, except for the higher relative difficulty of basal slip. Our results provide fundamental knowledge and parameters that can be used for modelling degradation and predicting performance of Hf, and shed light on the general behaviour of basal and prismatic slip systems in low-c/a-ratio hexagonal metals.

Hydrogen-induced softening and embrittlement in 316L stainless steel fabricated using laser-powder bed fusion

To investigate the effects of hydrogen (H) on the mechanical properties of 316L austenitic stainless steel (316L) additively manufactured using laser-powder bed fusion (L-PBF), tensile, nanoindentation, and micropillar compression tests were performed on samples in both the as-built (AB) and H-charged (HC) conditions. Microstructural characterization revealed that the electrochemical H charging exerted marginal effects on grains, phases, and overall dislocation densities of L-PBF 316L. However, the dislocation networks inherent to L-PBF 316L appear to disentangle upon H charging and are distributed more homogeneously through the matrix. Mechanical test results showed that H charging results in a marginal reduction in hardness and strength, indicating H-induced softening, possibly due to the elastic shielding effects of H that weaken the interactions between dislocations. The elastic shielding effect and the enhanced slip planarity, facilitate the H accumulation along slip bands and twins, promote H-enhanced localized plasticity, resulting cracking when a critical H concentration is reached. This, in turn, aids in H embrittlement mechanism that results in a significant loss in ductility upon H charging. A detailed discussion of all these micromechanisms is presented.

From 2D to 3D electrochemical microfabrication of nickel architectures at room temperature: Synthesis and characterization of microstructure and mechanical properties

Recent development in direct electrochemical 3D microprinting allows the fabrication of 3D metallic micro-components of predominantly noble metals with the positive standard reduction potentials (e.g., Cu, Au, or Pt). These pure metals (e.g., Cu, Au, or Pt) have a small shear modulus up to 60 GPa, which limits one of the important strengthening mechanisms, Taylor hardening. Hence, their mechanical strength is relatively low and limits the application in a structural component. 3D printing of mechanically high-strength metals (e.g., Ni), specifically, printing of microscale parts by direct electrodeposition using a voxel-by-voxel printing technique, is so far very challenging due to the negative standard reduction potential, which leads to hydrogen evolution and unstable printing. In this paper, we demonstrate the stable and successful room temperature (RT) electrochemical printing of Ni micro-architectures by using a newly formulated Ni ink and supporting electrolyte, which were developed through 2D voltammetry and deposition analysis. The micro manufacturing of Ni succeeds at a high deposition speed up to ∼61 nm/s, which is the fastest speed reported so far for microprinting of Ni. 3D printed micropillars feature a heterogeneous nanostructure from ∼13 nm at the center to ∼27 nm at the perimeter. By performing micropillar compression tests, the printed pillars exhibit a high compressive yield-strength of 2.3 ± 0.1 GPa, only ∼15 % lower than 2D processed samples (2.7 ± 0.1 GPa) featuring a homogeneous nanostructure of 27 ± 16 nm. Thus, by deposition of stronger, but less noble metals we show a new pathway to enhance the mechanical robustness of 3D-printed structures, opening up new applications where mechanical rigidity is essential.

Achieving improved compression plasticity/strength in plasma sprayed Al2O3–GAP amorphous ceramic coatings by regulating spray powder phase compositions

In this work, Al2O3–GdAlO3 (GAP) amorphous ceramic coatings were prepared via plasma spraying. A novel method of regulating spray powder phase compositions was provided to obtain the different amorphous phase contents in Al2O3–GAP coatings. The α-Al2O3/c-Gd2O3 and α-Al2O3/GAP sprayable powders were individually applied to deposit the coatings, namely Coating 1# and Coating 2#. Their amorphous phase contents were 91.8 wt% and 55.4 wt%, respectively. X-ray diffraction, scanning electron microscope, electron backscattered diffraction, X-computed tomography, atom probe tomography and micropillar compression tests combined with finite element method were applied to analyze microstructural features and mechanical performance of the coatings. The results showed that Coating 2# possessed higher compressive strength (3.42 GPa) and greater compression plasticity (21.8 %) than Coating 1#. Compared with nearly 40 amorphous materials, Coating 2# had an excellent compression plasticity, which was better than almost all amorphous alloy materials, showing extremely attractive engineering application potential.

Effects of the strain rate in compression of electrodeposited gold micro-pillars toward the design of MEMS components

In this study, the strain rate dependence in yield strengths of electrodeposited gold micro-pillars is evaluated for the design of movable components in MEMS devices. The micro-pillars are fabricated from electrodeposited gold by focused ion beam system. The strain rate dependence is quantified by the strain rate sensitivity, and the strain rate sensitivity is calculated from the yield strength obtained from compression tests of the gold micro-pillars having different sizes at different strain rates. An increase in the yield strength following a reduction in the pillar size is observed, which is the sample size effect. Also, weakening of the yield strength is observed following a decrease in the strain rate, which is the strain rate dependence, and the strain rate sensitivity of the gold micro-pillars is found be at roughly 0.03.

Excellent strength-ductility synergy of Cu-Al alloy with a gradient nanograined-nanotwinned surface layer

Gradient nanostructured (GNS) metallic materials have shown significant potential in achieving excellent mechanical properties. However, the underlying strengthening mechanisms resulting from plastically inhomogeneous deformation in GNS materials, particularly those involving nanotwinned structures, remain inadequately understood. In this study, a gradient nanograined-nanotwinned (GNNT) surface layer was generated on a Cu-Al alloy using the severe plastic deformation technique known as single-point diamond turning. Uniaxial tensile and localized micropillar compression tests were conducted to compare the mechanical properties of GNNT samples with conventional gradient nanograined (GNG) and uniform coarse-grained (CG) Cu/Cu-Al counterparts. The results revealed that the GNNT samples exhibit excellent strength-ductility synergy, with a yield strength of approximately 329 MPa, tensile strength of around 477 MPa, and uniform ductility of about 48 %, thereby demonstrating remarkable strain hardening capacity and mechanical stability. These characteristics are further supported by the observed strong back stress effect, well-preserved mobile dislocation density, and effective suppression of grain coarsening in the GNNT surface layer. In contrast to the evident mechanically induced grain coarsening through grain boundary (GB) migration in the GNG surface layer, grain coarsening through twin boundary (TB) migration in GNNT surface layers is effectively inhibited by Lomer-Cottrell (L-C) locks formed by TB-stacking fault and TB-TB intersections, resulting in significantly enhanced structural stability. Furthermore, the L-C locks also extensively contribute to the high density of mobile dislocations observed in the GNNT samples. These findings provide valuable insights into the optimization of GNS structures for achieving excellent strength-ductility synergy in metallic materials.

Micro-mechanics investigation of heterogeneous deformation fields and crack initiation driven by the local stored energy density in austenitic stainless steel welded joints

This work investigates the heterogeneous deformation and failure of HR3C austenitic stainless steel welded joints at room and typical service temperatures. Such types of welded joints are widely used in the new generation of fossil fuel power stations and are known to suffer from premature high temperature failure. Observation of in-service failures revealed that cracks may nucleate either in the heat affected zone or in the weld metal. The main objective of this work is to identify the local microstructural conditions and stress–strain fields responsible for both ductile failure and micro-crack nucleation within the weldment at low and high temperatures, respectively. To that purpose, a novel multi-scale micromechanics-based modelling framework is proposed. It relies on representative weld microstructure models digitally reconstructed from electron backscatter diffraction measurements, and dislocation density-based crystal plasticity models to describe the behaviour of individual grains in each weld region. A novel thermo-dynamically consistent relation for the configuration stored energy per unit volume is derived from the crystal plasticity framework.The single crystal models are calibrated and validated from a combination of uniaxial tensile tests on cross-weld specimens using high resolution digital image correlation techniques, representative volume elements of the polycrystals at the macro-scale, as well as micropillar compression tests at the scale of the individual grains. Predictions of heterogeneous inelastic deformation fields within the weldment at both 22 °C and 665 °C, and of micropillar compression behaviour of the individual weld single crystal phases are consistent with experimental data. The experimentally observed ductile failure of the weld metal at room temperature is successfully predicted using a Gurson-type void nucleation, growth and coalescence constitutive model. The suitability of the stored strain energy density and the accumulated plastic strain as crack initiation/creep failure indicators is investigated. It was found that predicted creep lifetimes with either indicator were very similar, and that they were relatively accurate for low stress levels.

Effect of Cr content on thermally activated deformation in single-crystal micropillars of Fe–Cr binary alloys

This study identifies the role of solute Cr in the thermal activation process in the plastic deformation of α-(Fe, Cr) ferrite (BCC solid solutions) using micropillar compression tests for micron-sized single-crystals fabricated on various Fe–Cr binary alloys that were solution-treated at 1573 K. Fe–18%Cr single-crystal micropillars with a mean diameter d of ∼2 μm exhibited a relatively higher strain rate sensitivity (m) of 0.12 for the stress required for slip initiation. Larger Fe–18%Cr micropillars (d = 5 μm) exhibited a lower m value of 0.04. The corresponding activation volume of 50 b3 was equivalent to the millimeter-sized pure Fe and Fe–18%Cr alloy specimens (60–70 b3), indicating the prevalence of a general mechanism involving dislocation motion (kink-pair nucleation of screw dislocations). This mechanism controls the thermal activation process of plastic deformation in the Fe–18%Cr alloy, which has sufficient sources for dislocation multiplication. In the Fe–25%Cr and Fe–40%Cr single crystal micropillars, a less pronounced size-dependent m value (0.01–0.03) was determined, almost independent of specimen size. The corresponding activation volumes of large micropillars (74–77 b3) were similar to those of millimeter-sized specimens, which indicates a mild effect of solute Cr atoms on the thermal activation process of dislocation motion in Fe–Cr binary alloys at ambient temperature. Initial dislocations, rather than Cr content in Fe–Cr binary alloys, led to the low changes in size-dependent activation volume of Fe–Cr alloys with higher Cr content. Planar faults (Cr-enriched regions) were observed in Fe–40%Cr alloys, which may become sufficient sources for dislocation nucleation. However, the influence of Cr-enriched regions on dislocation motion remains unclear.

Mechanical and helium swelling responses of K-RAFM steels to Fe ion irradiation and helium implantation

This study explores the effects of Fe ion irradiation and helium implantation on the mechanical and microstructural behaviors of K-RAFM steels. Through micropillar compression tests, we observed irradiation hardening, which manifested as an increase in strength and a decrease in strain bursts in the irradiated specimens. This change in deformation behavior indicates the impact of radiation-induced defects on dislocation dynamics. Helium implantation resulted in the formation of surface steps and, as shown by nanoindentation mapping, led to a localized increase in hardness, highlighting the significant influence of helium on mechanical properties. Microstructural examination revealed the steel's tempered martensitic structure and pointed to the importance of M23C6 and MX precipitates in modulating the effects of irradiation. Notably, K-RAFM steels showed a diminished extent of irradiation hardening, which can be attributed to the higher fraction of MX precipitates serving as effective trapping sites for point defects. Understanding how microstructural features influence mechanical performance in K-RAFM steels assists in steering the creation of materials tailored to improve radiation resistance in nuclear environments.

Mechanical characterizations of η′-Cu6(Sn, In)5 intermetallic compound solder joint: Getting prepared for future nanobumps

As advanced silicon manufacturing processes slow down in scaling, three-dimensional stacking of chips has become a future trend in microelectronic industry. High-density bonding technology is one of the core technologies used to achieve the stacking. In this work, we explored a new bonding technology that is possible to manufacture future high-density interconnects. We fabricated a series of sandwich structure solder joints with different thicknesses using SnBiIn-based nanoparticles under a low bonding temperature of 100 °C. The intermetallic compound (IMC) at the interface is proven to be η′-Cu6(Sn, In)5 nanocrystals with random grain orientations through X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and atomic probe tomography (APT). The hardness and elastic modulus of η′-Cu6(Sn, In)5 were measured to be 4.093 GPa and 79.586 GPa by micropillar compression tests, respectively, decreasing by 35% and 29.1% compared to Cu6Sn5. Therefore, doping In atoms reduces the brittleness of IMCs, providing a new approach for mitigating the mechanical failures of full IMC bump. First-principles calculation also verified the change in mechanical properties. Furthermore, a full-Cux(Sn, In)y IMCs solder joint and an array of 3 μm solder joints were fabricated experimentally using the bonding technology. This technology successfully demonstrates a pioneer interconnection bonding methodology for future ultra-high-density chip stacking using low melting point solder nanoparticles.