Room Temperature Mechanical Behavior Research Articles

Hydrogen-enhanced microbanding in an austenitic FeMnAlC low-density steel: Effect on hydrogen embrittlement resistance

We have investigated the influence of 101 mass ppm hydrogen content on the room temperature deformation structure and mechanical behavior of an austenitic Fe30Mn6.5Al0.3C (wt.%) low-density steel by several electron microscopy techniques, such as electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The steel exhibits a high hydrogen embrittlement resistance associated with a moderated increase in strength (yield stress increase of 10%) and ductility (increase in the elongation to fracture of ∼ 8%). Analysis of the deformation structure reveals that hydrogen influences the deformation behavior by promoting deformation mechanisms associated with inhomogeneous plasticity (hydrogen-enhanced deformation banding (HEDB)) and strain localization (hydrogen-enhanced microbanding (HEMB)). These deformation mechanisms are ascribed to hydrogen-induced effects on dislocation plasticity, resulting in macroscopic kink bands, sub-micron localized strain gradients, and localized shear at cell blocks. We find that HEMB plays a relevant role in the deformation behavior of sub-micron localized strain gradients by promoting plastic relaxation and the enhanced storage of geometrically necessary dislocations within them. These effects mitigate the activation of damage mechanisms and enhance the strain-hardening capacity, contributing to the high HE resistance of the steel, comparable to that of high HE-resistant fcc alloys and steels.

Mechanical and high temperature oxidation response of Nb-18.7Si–5Ti–5Zr alloy

In the present work, the influence of addition of Ti and Zr elements (5 at.% each) on the microstructure, room temperature mechanical behavior and cyclic oxidation response (800–1400 °C) of the cast Nb-18.7Si alloy has been studied. The cyclic oxidation behavior of the alloy in both the uncoated and Fe,Cr modified silicide slurry coating conditions have been investigated at the pest oxidation temperature of 800 °C and at high temperatures of 1200 °C, 1400 °C. The microstructure of the Nb-18.7Si–5Ti–5Zr alloy exhibited the desirable α-Nb5Si3, Nbss phases in the cast alloy, which is otherwise needs high temperature heat treatment for longer duration. Further, a minor fraction of γ–Nb5Si3 phase is also present which resulted from the segregation of Ti, Zr elements during the solidification of the alloy. The alloy is measured with significantly high compressive strength (2236 ± 81 MPa) and fracture toughness (10.59 ± 0.35 MPa m1/2). The addition of Ti and Zr has not improved the oxidation resistance and the alloy is dimensionally degraded within 15min/1cycle of oxidation at all the test temperatures. Whereas, the Fe–Cr modified silicide coating provided the oxidation life of 350–425 min (23–28 cycles) during the oxidation test in the temperature range of 800–1400 °C. The formation of multi-layer oxide sale on the surface is attributed to the improvement in the oxidation life of the alloy.

Simultaneous Twinning and Microband-Induced Plasticity of a Compositionally Complex Alloy with Interstitial Carbon at Cryogenic Temperatures

Intermediate to low stacking fault energy (SFE) high entropy alloys (HEA) have shown an excellent combination of strength and ductility as a result of deformation twinning and martensite transformation. However, even in the absence of these mechanisms HEA can show a good strength-ductility combination, as is the case with non-equiatomic (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1. The room temperature mechanical behavior of this alloy has been associated with Taylor lattice and microband formation. The current research focuses on tensile cryogenic deformation of this alloy and investigates if these features and/or alternate mechanisms like deformation twinning are obtained. Surprisingly, it is not one or the other but both deformation twinning and microband formation that are observed during cryogenic deformation. The activation of both deformation mechanisms is a combination that is not often reported as the former is generally associated with intermediate to low SFE alloys and the latter with intermediate to high SFE alloys. The activation of twinning in (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 is attributed to the high yield stress-temperature variation, as a result of solid solution strengthening being far greater than in other commonly researched compositionally complex alloys. A ductility retention down to 4 K was observed, while simultaneously showing a significant increase in flow stress. Despite the intermediate to high SFE deformation behavior, (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 exhibits excellent cryogenic strength-ductility combination.

Room Temperature Mechanical Properties of Additively Manufactured Ni-base Superalloys: A Comparative Study

The unique layer-by-layer manufacturing strategy and near-net shaping capability of Additive Manufacturing (AM) make it a promising technology for Ni-superalloys components, where their typical service conditions demand complex shapes and impose significant production costs due to the alloys’ poor machinability. In this work, the room temperature mechanical behavior of several Ni-superalloys, including Hastelloy X, Inconel 718, and Inconel 625, were investigated and their properties/performances compared. The test specimens were produced using various AM processes such as laser powder bed fusion (L-PBF) and/or laser powder direct energy deposition (LP-DED). Thorough analyses on microstructures as well as mechanical performance under static/cyclic axial loads have been performed on test specimens. This study aimed to provide a better understanding of process-structure-property relationship for the AM processed Ni-superalloys in two steps. In the process-structure step: the microstructural evolution as the result of thermal cycles has been studied by quantitative metallography based on scanning electron microscopy (SEM). In the structure-property step: both measured fatigue and tensile properties were attempted to be correlated with microstructure.

Effect of Cu addition on overaging behaviour, room and high temperature tensile and fatigue properties of A357 alloy

Abstract The aims of the present work are to evaluate the overaging behaviour of the investigated Cu-enriched alloy and to assess its mechanical behaviour, in terms of the tensile and fatigue strength, at room temperature and at 200 °C, and to correlate the mechanical performance with its microstructure, in particular with the secondary dendrite arm spacing (SDAS). The mechanical tests carried out on the overaged alloy at 200 °C indicate that the addition of about 1.3 wt.% Cu to the A357 alloy enables to maintain ultimate tensile strength and yield strength values close to 210 and 200 MPa, respectively, and fatigue strength at about 100 MPa. Compared to the quaternary (Al−Si−Cu−Mg) alloy C355, the A357−Cu alloy has greater mechanical properties at room temperature and comparable mechanical behaviour in the overaged condition at 200 °C. The microstructural analyses highlight that SDAS affects the mechanical behaviour of the peak-aged A357−Cu alloy at room temperature, while its influence is negligible on the tensile and fatigue properties of the overaged alloy at 200 °C.

Room and high temperature mechanical behavior of Ti–Al–Nb–Mo alloy reinforced with Ti2AlN ceramic particles

In order to improve the room and high temperature mechanical properties of Ti46Al4Nb1Mo alloy, Ti46Al4Nb1Mo-xN (x = 0, 0.4%, 0.8%, 1.2%, 1.6%, atomic percent, hereafter in at.%) alloys were prepared by adding TiN powders. Results showed that when the N addition was more than 0.8%, the grain morphology changed from columnar dendrites to equiaxed grains due to the formation of Ti2AlN particles, and the equiaxed grain size decreased with further increasing of the N addition. With 1.6% N addition, the compressive strength increased by 16.8% at room temperature, which was due to the collective strengthening effect of lamellae refinement and Ti2AlN particles. While lamellae refinement and Ti2AlN particles had opposite effects and competed with each other at high temperature, strengthening by Ti2AlN particles had the dominant effect on improving the peak stress of the Ti46Al4Nb1Mo-1.6 N alloy, which increased by 21.8% and 24.1% at 900 °C and 1000 °C, respectively. In addition, compared with the fracture microstructure at room temperature, the deformed microstructure showed that the load transfer effect of Ti2AlN particles was more effective at high temperature, which was due to micro-damages in the Ti2AlN particles and the absence of cracks along irregular Ti2AlN particles.

Significant room-temperature plasticity in a high Zr-containing bulk glassy alloy

Abstract

Microstructure and Mechanical Behavior of CrFeNi2V0.5Wx (x = 0, 0.25) High-Entropy Alloys

CrFeNi2V0.5Wx (x = 0, 0.25) alloys based on these parameters of mixing enthalpy (ΔHmix), mixing entropy (ΔSmix), atomic radius difference (δ), valance electron concentration, and electronegativity difference(Δχ) were designed and prepared. The microstructure and room-temperature mechanical behavior of both alloys were investigated. Compressive test results showed that the CrFeNi2V0.5W0.25 alloy had higher yield strength than that of the W-free CrFeNi2V0.5 alloy, although they all exhibited quite larger compressive plasticity (ε > 70%). Compression fracture surface of CrFeNi2V0.5W0.25 alloy revealed a ductile fracture in the face-centered cubic (FCC) phase and a brittle-like fracture in the σ phase. Moreover, tensile test results indicated that the CrFeNi2V0.5W0.25 alloy exhibited excellent mechanical property with an ultimate tensile strength of 640 MPa and a high tensile elongation of 15.7%. The tensile deformation mode of the FCC phase in the CrFeNi2V0.5W0.25 alloy is dominated by planar glide, relating to dislocation configurations, high-density dislocations, and dislocation wall. Therefore, dislocation slip plays a significant role in tensile deformation of CrFeNi2V0.5W0.25 high-entropy alloy. The higher strength of CrFeNi2V0.5W0.25 alloy is predominantly due to the solid solution strengthening of W element and σ phase precipitation strengthening. Combination of the higher tensile strength and plasticity suggests that the CrFeNi2V0.5W0.25 alloy can be a promising aerospace material.

Structure and mechanical properties of electroplated mossy lithium: Effects of current density and electrolyte

Mechanical suppression, e.g., by applying stack pressures and using functionalized coatings, has been considered a promising approach to inhibit the formation of lithium (Li) dendrites and improve the cycling stability of the Li metal electrode. However, a lack of understanding of the mechanical behavior of electroplated mossy Li, consisting of loosely packed Li dendrites that are covered by solid electrolyte interphase, hinders the development of mechanical suppression strategies and limits the understanding of the electromechanical behavior of mossy Li. In this study, we investigated, using flat punch indentation in an argon-filled glovebox, the room temperature mechanical behavior and its relationship with the microstructure of mossy Li electroplated using different current densities (between 0.25 and 10 ​mA/cm2) in several electrolytes. The dendrite size decreases and the porosity of mossy Li increases with increasing current density. Mossy Li has lower Young’s modulus (E) but significantly higher creep resistance than bulk Li. A scaling relationship between E and porosity is established for mossy Li. The creep behavior of mossy Li exhibits a strong size effect, i.e., the steady-state impression velocity decreases with decreasing dendrite size. These findings provide a comprehensive understanding of the relationship between the microstructure and mechanical behavior of mossy Li.

Effects of Different Wire Drawing Routes on Grain Boundary Character Distribution, Microtexture, δ-Phase Precipitation, Grain Size and Room Temperature Mechanical Behavior of Alloy 718

Over the last decades, alloy 718 usage has expanded and requirements imposed by its industrial applications became more critical. The knowledge about grain boundary character distribution (GBCD) in alloy 718 and its effect on properties improvement is mostly built based on iterative processing through cold rolling steps interspersed with solution annealing. Alloy 718 is found in the industry in many different forms and geometries, and fabricated by multiple thermomechanical processes such as wire drawing, rolling, forging or extrusion. The present study focused on understanding how wires respond to deformation mode related to drawing in regard to GBCD evolution, crystallographic orientation, precipitation of δ-phase and grain size. Lastly, assessing the resulting mechanical properties. The findings show that microstructural evolution is a consequence of competing mechanisms such as strain induced boundary migration, recrystallization, grain growth and phase precipitation. The deformation gradient along wire cross section plays an important role in affecting microstructural features, such as δ precipitation, GBCD and microtexture.

Effects of Ni[sbnd]P coating on mechanical properties of Al0.3CoCrFeNi high-entropy alloys

The Al0.3CoCrFeNi high-entropy alloys were successfully coated with the NiP amorphous films through electroless plating. The room-temperature mechanical behavior under uniaxial tension was systematically investigated. There are good adhesive strengths between the films and substrates. And the surface hardness of the plating for 5 h is around 381HV with dramatically increasing by 121%, as compared to the uncoated substrate of 172HV. As a result, the yield strength and ultimate tensile strength (UTS) of coated samples were significantly increased as the film thickness rises. Specially, the specimen plating for 5 h, the yield strength (σy) is 400 MPa with a 62% improvement, and UTS is 732 MPa, increased by 16% in comparison to the substrate (σy = 246 MPa and UTS = 630 MPa). It can be attributed to the appearance of the repulsive image stress that will counteract the shear applied stress for the generation and propagation of dislocations during the elastic-deformation stage. Owing to the back stress inhibiting local plasticity and promoting the nucleation of cleavage cracks in the substrate, the cracks originated from the brittle film propagate into the substrate, resulting in the ductility losses. A deformation map is proposed to account for the transition of deformation modes from shear banding to cracking. These findings shed light on the fact that the surface coating is an effective means to enhance the mechanical properties of HEAs.

Ductility enhancement of tungsten after plastic deformation

An unusual room temperature mechanical behavior of pure tungsten is investigated by focusing on three specimens prepared with different microstructures: as-received (hot-rolled), recrystallized, and cold-rolled specimens. Contrary to ordinary expectations in metallic materials, only the cold-rolled specimen exhibits significant plastic deformation during tensile testing, with improved strength and ductility, while the recrystallized and the as-received specimens fail in the elastic region. We further provide an explanation of such characteristics by systematically utilizing experimental and theoretical analysis at a small scale: nano-indentation tests clarify the inherent mechanical response inside grains and provide a statistical distribution of the maximum shear stress during pop-in events, corresponding to onset of plastic yielding; atomistic simulations provide information on the overall fracture strength of various grain boundaries. By comparing the maximum shear stress and the grain boundary fracture stress, we could explain that the distinctive plasticity in the cold-rolled specimen is caused by the movement of pre-existing dislocations before grain boundary fracture. This contrasts the recrystallized and as-received specimens, where grain boundary fracture occurs prior to the nucleation of dislocations or activation of dislocation sources.

Room temperature mechanical behaviour of cast in-situ TiAl matrix composite reinforced with carbide particles

Room temperature mechanical behaviour of cast in-situ TiAl matrix composite reinforced with carbide particles was studied using hardness, tensile, compression, and three-point bending tests. The initial microstructure of the in-situ composite consists of TiAl matrix reinforced with coarse primary (HTi2AlC and TiC) and fine secondary (HTi2AlC and PTi3AlC) particles. The in-situ composite shows plastic deformation of TiAl matrix and limited plastic deformation of primary Ti2AlC particles during the compression test. The compressive work hardening behaviour is affected by the plastic deformation of the matrix, plastic deformation of primary carbide particles and formation of cracks. The local equivalent strains in the compression specimens are numerically calculated using finite element analysis (FEA) and related to the size of primary carbide particles. The in-situ composite specimens show brittle fracture behaviour during tensile and three-point bending tests. The numerically calculated critical stress leading to a crack initiation in the notch tip region of three-point bending specimen is comparable with the ultimate tensile strength. The brittle fracture of the in-situ composite includes crack deviation, zone shielding by microcrack toughening, carbide fragmentation, delamination on the matrix-carbide interfaces and pull-out of the carbide particles from the matrix.

Processing, characterization, room temperature mechanical properties and fracture behavior of hot extruded multi-scale B4C reinforced 5083 aluminum alloy based composites

Microstructural characteristics and mechanical behavior of hot extruded Al5083/B4C nanocomposites were studied. Al5083 and Al5083/B4C powders were milled for 50 h under argon atmosphere in attrition mill with rotational speed of 400 r/min. For increasing the elongation, milled powders were mixed with 30% and 50% unmilled aluminum powder (mass fraction) with mean particle size of >100 μm and <100 μm and then consolidated by hot pressing and hot extrusion with 9:1 extrusion ratio. Hot extruded samples were studied by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), tensile and hardness tests. The results showed that mechanical milling process and presence of B4C particles increase the yield strength of Al5083 alloy from 130 to 566 MPa but strongly decrease elongation (from 11.3% to 0.49%). Adding <100 μm unmilled particles enhanced the ductility and reduced tensile strength and hardness, but using the >100 μm unmilled particles reduced the tensile strength and ductility at the same time. By increasing the content of unmilled particles failure mechanism changed from brittle to ductile.

On room-temperature quasi-elastic mechanical behaviour of bulk metallic glasses

Uniaxial compression is used to apply cyclic loading within the elastic range to samples of Zr61Cu27Fe2Al10 bulk metallic glass (BMG). The results are twofold. On one hand the microhardness measured on the end-faces of the samples shows an increase of ∼8% together with a slight gradual increase in the Young's modulus after the 2nd cycle. This appears similar to hardening induced by elastic cycling in nanoindentation. The microhardness measured in the centre of the sample or on the side faces of the BMG samples show little or no change as a result of cyclic loading. These results suggest that the apparent hardening of the end-faces is a surface effect attributed, not to relaxation, but to a build-up of anelastic strain, associated with local anisotropy and stresses arising from uneven loading of the samples. This is confirmed by the decay of the apparent hardening after a several days of natural ageing at room temperature. On the other hand an effect similar to structure rejuvenation takes place in the central part of the samples leading to higher specific heat capacity of the cycled samples, larger crystallization and relaxation enthalpies.

Room Temperature Microstructure and Property Evaluation of a Heat Treated Fully Bainitic 20CrMoVTiB410 Steel

The room temperature mechanical behavior of the fully bainitic steel grade 20CrMoVTiB410 was studied in the as-quenched and tempered conditions. The hardenability response of the steel during heat treatment was assessed. In the as-quenched condition itself, the steel exhibited a good combination of strength, ductility and toughness. Tempering the quenched steel till to 550°C, showed uniform mechanical properties. Tempering at 650°C showed secondary hardening behaviour, where the highest strength and least impact toughness was observed. Tempering at 700°C showed a sharp decrease in strength but with significant enhancement of toughness. The properties obtained were correlated with the microstructure and phase analysis was established using optical, scanning electron microscope, transmission electron microscope and x-ray diffraction techniques.

Co-existence of homogeneous flow and localized plastic deformation in tension of amorphous Ni–P films on ductile substrate

The room temperature mechanical behavior of amorphous Ni–P thin films deposited on Ni substrate under tension was systematically investigated. Due to the effect of substrate confinement, unexpected homogeneous plastic flow occurred in the film simultaneously with cracking or shear banding, which co-contributed to the plastic deformation. The film thickness gradually reduced with increasing tensile strain and the maximum reduction increased up to 69% in 2.6 μm-thick film. Such a severe plastic deformation leads to significant structure change in the film where free volume annihilation dominates, evidently by almost 100% reduction in the total relaxation enthalpy in the deformed film, accompanied by the densification and hardening. The stress required for shear band and crack propagation in Ni–P film was analyzed on the basis of Griffith's criterion. A deformation map is proposed to account for co-existence of homogeneous flow and localized plastic deformation (shear banding or cracking), as well as the transition of deformation modes.

Reverse tension/compression asymmetry of a Mg–Y–Zn alloys containing LPSO phases

Room temperature mechanical behavior of extruded Mg–Y–Zn alloys with varying fractions of LPSO phase was studied in tension and compression along the extrusion direction. The microstructure is characterised by elongated LPSO fibers along the extrusion direction within the magnesium matrix. Moreover, the magnesium matrix presents a bimodal grain structure with dynamically-recrystallized grains and deformed, elongated grains with the basal plane parallel to the extrusion direction. The beginning of plasticity depends on the volume fraction of deformed and DRX grains. Alloys with low volume fraction of LPSO phase (<10vol%), with a high volume fraction of deformed grains, show the typical behavior of extruded magnesium alloys where yield stress in tension is higher than in compression. This effect is, however, reversed as the volume fraction of the LPSO phase increases since DRX grains are majority.

Effect of surface modified silicon carbide particles with Al2O3 and nanocrystalline spinel ZnAl2O4 on mechanical and damping properties of the composite

In our proof-of-concept studies, pure aluminium metal matrix composites reinforced with alumina (Al2O3) and nanocrystalline spinel zinc aluminate (ZnAl2O4) coated silicon carbide (SiC) particles were fabricated by stir casting. Initially, we presented the results of an investigation on Al2O3 and spinel ZnAl2O4 coating layer on SiC particles of sub-micron size via facile sol–gel method. The amount and coverage of different coating crystals on SiC surface were studied through X-ray energy dispersion spectroscopy (EDX) analysis and transmission electron microscopy (TEM) analysis. Further, adequate dispersion, de-agglomeration and stability of coated SiC particles were investigated through Field emission scanning electron microscopy (FE-SEM) and elemental mapping of aluminium metal matrix composites. The room temperature mechanical properties and thermal cyclic damping behaviour of the composites were examined. The obtained results indicate that coating of SiC has a significant influence on the damping and mechanical properties and the resulting composite (ZnAl2O4 coated SiC/Al) exhibited 60% higher stiffness (storage modulus) and 3.6 times increase in tensile strength, and 3.5 times increase in hardness respectively as compared to the pure aluminium. This is attributed to stabilization of the tailored SiC particles and improved interface bonding between coated particles and matrix.

Mechanical Properties of Li7La3Zr2O12 and Li0.33La0.57TiO3

Many new types of high energy Li batteries may require a solid Li-ion conducting electrolyte. The major requirements for the solid Li-ion electrolyte are; high Li-ion conductivity, low electronic conductivity and good chemical/electrochemical stability. However, in many applications adequate mechanical properties are also relevant. For example, an electrolyte with a high shear modulus is required to prevent dendrite penetration when using a Li anode. At present information on the mechanical properties of two new promising solid Li-ion conductors; Li7La3Zr2O12 (LLZO) and Li0.33La0.57TiO3 (LLTO) is lacking. It is therefore the purpose of this talk to present some of the first information on the room temperature mechanical behavior of high density (>98% relative density) LLZO and LLTO. These include: hardness, elastic modulus, fracture strength, fracture toughness and thermal expansion. The results will compared to existing data for other solid Li-ion conductors and theoretical predictions. In addition, the methods to improve the mechanical behavior of LLZO and LLTO while retaining high ionic conductivity will be presented.