Showing papers on "Necking published in 2019"

Achieving high ductility in the 1.7 GPa grade CoCrFeMnNi high-entropy alloy at 77 K

Abstract: CoCrFeMnNi high-entropy alloys (HEAs) with partially recrystallized (PR) structure were fabricated by cold rolling and annealing. The microstructures were characterized and the tensile properties were tested at 77 K and 293 K, respectively. In contrast to the early necking at 293 K, an ultrahigh yield strength of 1692 MPa and a considerable uniform elongation of 10.3% were obtained at 77 K. The notable uniform elongation at 77 K can be attributed to the enhanced strain-hardening capability via introducing multiple deformation mechanisms in the recrystallized grains. This work provides a strategy to design high-strength high-ductility HEAs for applications at cryogenic environments.

Plastic anisotropy and deformation-induced phase transformation of additive manufactured stainless steel

Abstract: Plastic anisotropy and deformation-induced phase transformation of additively manufactured (AM) stainless steels were investigated via in-situ neutron diffraction, electron backscatter diffraction, metallography, and fractography. Two types of tensile specimens were manufactured: (1) One sample was vertically fabricated with its tensile axis parallel to the z-direction (AM-V), (2) The other sample was horizontally fabricated with its tensile axis perpendicular to the z-direction (AM-H). A commercial 15-5PH stainless steel (CA) was used for comparison. AM steel revealed enhanced yield strength, tensile strength, and uniform elongation over CA, which was mainly due to grain refinement and transformation induced plasticity (TRIP). Different onsets of strain nonlinearity between AM-V and AM-H were closely related to martensitic phase transformation. Stresses estimated from lattice strains measured by neutron diffraction matched well with the applied stress-strain curves. After plastic deformation, voids were formed and congregated near the solidified line where fine grains were populated. Higher dislocation density was observed in the fine grain zone, and lower density was shown in the relatively coarse grain zone. AM steels exhibited significant anisotropic fracture behavior in terms of loading direction. In contrast to isotropic failure for CA and AM-V, AM-H revealed anisotropic failure with elliptical formation of the fracture feature. The fracture surface of AM-H possessed many secondary cracks propagating perpendicular to the building direction. The occurrence of secondary cracks in AM-H resulted in rapid load drop during tensile loading after necking.

Effect of Lewis number on premixed laminar lean-limit flames stabilized on a bluff body

Abstract: In this paper, we present a study on the effect of Lewis number, Le, on the stabilization and blow-off of laminar lean limit premixed flames stabilized on a cylindrical bluff body. Numerical simulations and experiments are conducted for propane, methane and two blends of hydrogen with methane as fuel gases, containing 20% and 40% of hydrogen by volume, respectively. It is found that the Le > 1 flame blows-off via convection from the base of the flame (without formation of a neck) when the conditions for flame anchoring are not fulfilled. Le ≤ 1 flames exhibit a necking phenomenon just before lean blow-off. This necking of the flame front is a result of the local reduction in mass burning rates causing flame merging and quenching of the thin flame tube formed. The structure of these flames at the necking location is found to be similar to tubular flames. It is found that extinction stretch rates for tubular flames closely match values at the neck location of bluff-body flames of corresponding mixtures, suggesting that excessive flame stretch is directly responsible for blow-off of the studied Le ≤ 1 flames. After quenching of the neck, the upstream part forms a steady and stable residual flame in the wake of the bluff body while the downstream part is convected away.

Superior tensile properties of Al0.3CoCrFeNi high entropy alloys with B2 precipitated phases at room and cryogenic temperatures

Abstract: Al0.3CoCrFeNi high-entropy alloys (HEAs) with B2 ordered phase mainly precipitated at grain boundaries are obtained by cold rolling, annealing and aging treatment. Tensile experiments show that both CRSA (annealed at 1100 °C for 1 h) and CRSA-600-24 (annealed at 1100 °C for 1 h and subsequently aging treatment at 600 °C for 24 h) samples have excellent combinations of yield strength, tensile strength and tensile elongation at room temperature, and more importantly, simultaneous enhancements in strength and ductility at liquid nitrogen (77 K) are attained for the latter condition. Analyses reveal that after aging treatment, the precipitation of B2 phase with ultrafine-grained or even nanoscale sizes, as well as moderate grain refinement, yields the significant increase in tensile strength and fascinating tensile plasticity. By transmission electron microscopy (TEM) characterization, it is found that the deformation mode dominated by dislocation glide at room temperature is transformed to that combined with dislocation glide plus nanoscale twinning at 77 K. Moreover, the twinning-induced work hardening renders the onset of necking instability to a higher strain/stress value, which therefore improves the strength and ductility simultaneously.

Adiabatic Heating of Austenitic Stainless Steels at Different Strain Rates

Abstract: This work focuses on the effect of strain rate on the mechanical response and adiabatic heating of two austenitic stainless steels. Tensile tests were carried out over a wide range of strain rates from quasi-static to dynamic conditions, using a hydraulic load frame and a device that allowed testing at intermediate strain rates. The full-field strains of the deforming specimens were obtained with digital image correlation, while the full field temperatures were measured with infrared thermography. The image acquisition for the strain and temperature images was synchronized to calculate the Taylor–Quinney coefficient (β). The Taylor–Quinney coefficient of both materials is below 0.9 for all the investigated strain rates. The metastable AISI 301 steel undergoes an exothermic phase transformation from austenite to α’-martensite during the deformation, which results in a higher value of β at any given strain, compared to the value obtained for the more stable AISI 316 steel at the same strain rate. For the metastable 301 steel, the value of β with respect to strain depends strongly on the strain rate. At strain rate of 85 s−1, the β factor increases from 0.69 to 0.82 throughout uniform elongation. At strain rate of 10−1 s−1, however, β increases during uniform deformation from 0.71 to a maximum of 0.95 and then decreases to 0.91 at the start of necking.

Influence of ARB technique on the microstructural, mechanical and fracture properties of the multilayered Al1050/Al5052 composite reinforced by SiC particles

Abstract: In this paper, Al1050/Al5052/SiC composite was manufactured via accumulative roll bonding (ARB) technique, and the fracture toughness for the different passes was experimentally investigated. The microstructure, mechanical properties, and fracture behavior were determined through Optical Microscopy (OM), Scanning Electron Microscopy (SEM), tensile test, microhardness, and plane stress fracture toughness. The results of OM showed that local necking and failure of Al5052 reinforcement layers happened at the 1st pass and after that by raising the exerted strain the shape of the layers varied from the lamellar to particle form, and finally, Al1050/Al5052/SiC composite was provided via perfect dispersion of layers and particles reinforcements. The microhardness variations were ascending for both Al layers in terms of ARB cycles, and this trend was observed for UTS variations except for the third and fourth passes so that in the both passes the strength was downtrend. Finally, the best UTS and elongation values were obtained on the last pass. The results of SEM demonstrated that by raising the exerted strain, the kind of fracture mechanism varied to shear ductile from ductile. From zero pass (primary sandwich) to the second pass, the rate of changing in fracture toughness was ascending and then drastically reduced. Also, maximum and minimum values are achieved at second and third ARB cycles respectively. Due to the higher variation in the strength compared to elongation, as in previous researches, the trend of toughness was similar to that of strength, with this difference that the presence of ceramic particles at the interfaces was causing the brittleness in the composite.

Ag particles for sinter bonding: Flakes or spheres?

Abstract: This paper examines a unique mechanism in Ag micrometer flake sintering which reveals that self-produced Ag nanoparticles, created by the heating of the sintering process, contribute to larger necking and lead to enhanced bonding strength. Ag nanoparticle production, followed by rapid crystal growth, realizes faster mass transfer than spherical Ag submicrometer particles governed by conventional atomic-scale thermal diffusion at the particle surface. Thus, the Ag flakes provide thicker necking, resulting in higher shear strength of the bond layer. The natural properties of the initial particles have obvious differences: Only the flakes have a certain microstrain due to the higher dislocation density of the crystalline structure. The release of microstrain in the flake particles during heating leads to the production of Ag nanoparticles, which is a significant driving force in the sintering process.

Deformation-mechanism-based creep model and damage mechanism of G115 steel over a wide stress range

Abstract: The effects of applied stress (150–250 MPa) on the creep deformation behavior and creep damage (stable and unstable deformation) were studied systematically for a novel tempered martensite ferritic steel G115 at 650 °C. A creep model involving three deformation mechanisms (grain boundary sliding, dislocation glide, and dislocation climb) was applied to fully understand the creep deformation behavior of G115 steel over a wide range of applications. Subsequently, the deformation-mechanism-based creep model was validated for G115 steel over a wide range of stresses (120–250 MPa) and temperatures (625–675 °C). Further, in the stable deformation regions, the martensite laths have no significant change under high-stress conditions, whereas the martensite laths obviously coarsen and micro-cavities formed under low-stress conditions. Four types of precipitates can be characterized after creep. M23C6 carbide and MX carbontride are pre-existing in the initial microstructure. Cu-rich phase was precipitated at 650 °C after very short-term creep (6.87 h), whereas Fe2W Laves phase formed after long-term creep (4404.78 h). The Cu-rich phase particles are cut by a dislocation shearing mechanism and become too small to be stable, leading to the dissolution of the Cu-rich phases after long-term creep. The fine Cu-rich phase particles can significantly retard the recovery of martensite laths in the early stage of creep or during short-term creep. In the unstable deformation (necking) regions, martensite cracking and martensite fracture are the main micro-damage features during creep at high stresses, whereas the growth of micro-cavities and micro-cracks are the main features at low stresses. Ductile fracture is the dominant fracture mode of G115 steel at 650 °C above 150 MPa.

One-dimensional modeling of necking in rate-dependent materials

Abstract: This paper presents an asymptotically rigorous one-dimensional analytical formulation capable of accurately capturing the stress and strain distributions that develop within the evolving neck of bars and sheets of rate-dependent materials stretched in tension. The work is an extension of an earlier study by the authors on necking instabilities in rate-independent materials. The one-dimensional model accounts for the gradients of the stress and strain that develop as the necking instability grows. Material strain-rate dependence has a significant influence on the strain that can be imposed on a bar or sheet before necking becomes pronounced. The formulation in this paper enables a quantitative assessment of the interplay in necking retardation due to rate-dependence and that due to the development of hydrostatic tension in the neck. The connection with a much simpler long-wavelength approximation which does not account for curvature induced hydrostatic tension in the neck is also emphasized and extended.

Necking growth and mechanical properties of sintered Ag particles with different shapes under air and N2 atmosphere

Abstract: Ag sinter joint is gaining greater attention for use with wide-bandgap die attach modules in high-temperature applications. In this study, we have investigated both the necking growth of Ag grain and the mechanical properties of sintered Ag with different Ag particle shapes in a low-temperature, pressure-less sintering process in air and N2. The necking growth ratio of Ag grains plays an important role in the mechanical properties of sintered Ag. Large necking growth corresponds to stronger tensile fracture strength of sintered Ag paste and stronger die shear strength. Self-generated Ag nanoparticles (AgNPs) with of a size less than 10 nm were observed under an air atmosphere, an atmosphere that produces larger necking growth than does that of N2. In addition, self-generated AgNPs were observed in sintered Ag flake particles specimens. Ag flake particles also lead to larger necking growth than that of sintered Ag spherical particle specimens. Self-generated AgNPs accumulated together due to their high surface energy, thus improving the necking growth of Ag grain and leading to a stronger fracture strength. In addition, it was revealed that both the oxygen element and the grain residual strain of initial Ag particles play important roles in the AgNPs self-generation during the sintering process. This study enhances our understanding of the sintering mechanism of Ag particles and the relationship between grain necking growth and its mechanical properties.