Plastic Elongation Research Articles

Effect of nickel content on the processing and microstructure evolution of Co–Cr–Mo–Ni alloys

Two type cobalt-chromium-molybdenum-nickel (Co–Cr–Mo–Ni) alloys were prepared by the powder metallurgy technology. The effect of the element powder nickel content on the milling process and then the microstructure evolution and mechanical properties of the sintered alloys was examined. The increase of the element powder Ni promotes the work harden of the product powders and then decreases the porous defects of the sintered samples. With the increase of the nickel content, the morphology of the sintered alloys changes from coarsely nonuniform grains to finer equiaxial grains with fault striations and the phase changes from single hcp phase to a dual phase of hcp and fcc structure. Obvious improvements in the tensile strength and uniform plastic elongation are achieved in the high nickel alloy. The elongation of the low nickel alloy decreases while that of the high nickel alloy increases with the raw powder milling time.

Effect of Underloads on Plasticity-Induced Crack Closure: A Numerical Analysis

The effect of underloads is mostly quantified by the averaged effect on the fatigue crack growth rate, and the transient behavior is rarely investigated. The objective of this paper is to study the mechanisms behind the effect of underloads, periodic underloads, and underloads combined with overloads. A single underload smashes the material around the crack tip, producing a depression on crack flank and a local reduction of contact forces at the minimum load. The reduction of plastic elongation behind the crack tip has an immediate effect on crack opening level, which rapidly disappears with crack propagation. The smashing associated with the compressive force occurs mainly behind the crack tip position where the underload was applied. The effect of the underload is intimately linked to reversed plastic deformation, which explains its enhanced effect for kinematic hardening. The decrease of load below the minimum baseline load is the main loading parameter. The application of periodic underloads extends the effect of a single underload. The effect of the underload is enhanced by the presence of obstacles in the form of residual plastic deformation, which explains the great effect of underloads applied after overloads.

Inhomogeneous Warm Deformation Behavior and Its Effect on the Deformability of an Fe-6.5PctSi Alloy with Ce Doping

The inhomogeneous deformation behavior generated by deformability differences between soft and hard phases is a critical factor that influences the deformation and processability of materials. For Fe-6.5pctSi alloys with coexisting disordered and ordered phases, this work studied warm (300 °C to 500 °C) tensile deformation property differences and mechanisms for non-Ce-doped and 0.02 wt pct Ce-doped alloys. The warm deformation behavior was analyzed according to the stress and strain partitioning of the two-phase alloy by establishing a two-phase (A2 disordered phase + B2/D03 ordered phase) system model based on the structural characteristics of the alloy. The results show that the plastic deformation is mainly attributed to the A2 disordered phase, and the existence of a hard and ordered phase leads to local and inhomogeneous deformation behavior, which decreases the uniform plastic elongation and deformability of the alloy. Ce doping improves the warm deformation homogeneity and deformability by reducing the size and amount of the B2 ordered phase, which improves the warm rolling deformation and increases subsequent cold rolling deformability. Cold rolling with a total thickness reduction greater than 63 pct was achieved after the Ce-doped hot-rolled specimen with a 2 mm thickness was warm rolled to an 82.5 pct reduction, and the warm-rolled and cold-rolled sheets had good surface integrity and no obvious cracking.

In situ X-ray tomography investigation of the crack formation in an intermetallic beta-stabilized TiAl-alloy during a stepwise tensile loading

The impact of high temperature exposure on the mechanical damage behavior of the intermetallic TiAl alloy TNM has been investigated by means of high resolution laboratory computer tomography (CT). An in situ testing device was adapted allowing the study of cracking behavior during tensile testing of unnotched samples by CT. Accordingly, failure of exposed and unexposed samples is controlled by the fracture toughness. Unexposed samples reveal crack formation in the interior at only about 1/3 of their plastic elongation to failure. These internal cracks grow gradually with increasing nominal stress and are subjected to significant crack deflection by the microstructure until the crack growth accelerates and the samples fail. After exposure for 1 h at 800 °C cracks originate in the surface region of the samples at almost the same nominal stresses at which internal cracks first appear. However, due to higher fracture mechanical geometry factor, surface cracks become critical almost instantly after their formation resulting in a reduced ductility, which is known as environmental embrittlement in literature. It is supposed that crack formation at the surface after exposure is the result of stress concentrations which are built-up during plastic deformation due to interstitials in the surface layer.

Grain structure and precipitate variations in 7003-T6 aluminum alloys associated with high strain rate deformation

The variation of the grain and precipitate structure plays a rather important role on the response of the 7003-T6 Al-Zn-Mg-Cu alloys to high speed deformation but related reports are limited. This paper represents a detailed investigation about the flow behavior and microstructure evolution of these alloys under high strain rate impacting. Methods such as split Hopkinson pressure bar testing, optical and TEM microscopy, Vickers hardness testing, etc. were employed. Features such as plastic compression, tilting, distortion, elongation and refinement manners of the grains as deformation proceeds were studied. The changes of the precipitates in phase type, size and density, and their contribution to the flow stress during this thermo-mechanical process was investigated. The results show that the motion of dislocations and temperature rising induced by the impact work were responsible for these microstructure changes. Deformed shear bands with elongated grains formed in the diagonal direction of the sample when total strain reached 0.17. Prototype of the refined grain structure in these bands was constructed when the dislocation walls accumulate in the triangle grain boundary and form closed cells. With the assist of localized temperature rising and further increase of the strain to above 0.19, the deformed shear band was finally changed into transformed shear band with fine recrystallized grain structure in the center. Dynamic re-precipitation including phase transformation from GPII zones to ƞ′ and then to ƞ, coarsening in size and then dissolving was promoted. Elevated temperature and multiplied dislocation facilitated these processes. Precipitation coarsening in the matrix and dissolving in the ASB played a significant role of softening in the flow behavior as a counterbalance of the deformation and geometric hardening. A schematic model considering both of the grain structure and precipitate evolutions was established and discussed.

Reduction of impurity contents in aluminum plates electrodeposited from a dimethylsulfone-aluminum chloride bath

High productivity and bath stability of electrodeposition for Al and its alloys from a dimethylsulfone (DMSO2)-aluminum chloride bath are highly desirable characteristics for a wide range of critical applications. However, a long-standing problem for such electrodeposits is S and Cl contamination, which results in limited ductility. In this study, we explored a reduction method for S and Cl in Al electrodeposits from a DMSO2 bath to facilitate plastic elongation of the electrodeposited Al. For 2–5:10 M ratios of aluminum chloride to DMSO2, tailoring the conditions of electrodeposition decreased the S and Cl contents to ∼0.2 at%. The addition of various metal salts revealed that gallium chloride further reduced the S and Cl contents to less than 0.1 at%. This reduction permitted the electrodeposited Al–Ga alloys to exhibit plastic elongation in tensile tests. Based on the results of experiments and first-principles calculations, we discuss the role of Ga in the growth mode of electrodeposits and its effect on grain boundary cohesion. Thus, we elucidate the mechanisms for realizing high strength and ductility in electrodeposited Al alloys.

Preparation and Characterization of Bacterial Cellulose-Carbon Dot Hybrid Nanopaper for Potential Sensing Applications

Green and facile approaches aiming at the manufacture of biocompatible paper-based optical sensors reporting the presence of photoluminescence (PL) modulating compounds is an emerging field of research. This study investigates the preparation of bacterial cellulose nanopaper containing covalently immobilized carbon dots for potential biosensing applications. Preliminary work of this feasibility study included TEMPO-mediated ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl-mediated) oxidation and nanofibrillation of bacterial cellulose (TOBC) on the one hand as well as synthesis and comparative analysis of different types of carbon dots (CDs) on the other hand. The two source materials of the targeted functional nanopaper were finally linked to each other by two different N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride/ N-hydroxysuccinimide (EDC/NHS) coupling approaches to clarify whether grafting of CDs prior to or after TOBC paper formation would be the method of choice. Synthesis of the carbon nanodots was accomplished by microwave-assisted co-hydrothermolysis of appropriate precursor compounds. After isolation and purification by dialysis particles in the single-digit nanometer-range were obtained and characterized with regard to their photoluminescence properties in terms of emission wavelength, pH stability, and quantum yield. All types of synthesized CDs reached their PL maxima (450–480 nm; light blue) in a narrow excitation wavelength range of 340–360 nm. Variation of molar (C/N) ratio of the CD precursors and substitution of the nitrogen donor EDEA by urea increased PL and quantum yield (QY), respectively. The highest relative QY of nearly 32% was obtained for CDs synthesized from citric acid and urea. PL of all CDs was virtually insensitive to pH changes in the range of 4–10. Tensile testing of hybrid nanopaper prepared after EDC/NHS-mediated grafting of GEA-type CDs onto TOBC (0.52 mmol·g−1 COOH) in dispersion state revealed that both stiffness and strength are not compromised by incorporation of carbon dots, while plastic deformation and elongation at break increased slightly compared to nanopaper formed prior to decoration with CDs. Water contact angle of the nanopaper is unaffected by introduction of carbon dots which is supposedly due to the presence of surface amino- and amide groups compensating for the loss of carboxyl groups by grafting.

Elasto-plastic behaviour of a columnar structure of nanocrystalline iron with sharp 〈011〉 fibre texture

We explore, using molecular dynamics (MD), the strength and mechanical stability at 298 K of pure bcc Fe with an oriented columnar nanostructure of long grains randomly distributed about 〈011〉 and average size ranging from 20 to 5 nm in the cross-section. According to the simulations, the tensile flow stress of such aligned nanostructure spans from 4 GPa to 6 GPa as grain size decreases from 20 nm to 5 nm. The axially oriented tilt boundaries are very stable to axial plastic elongations as large as 0.5 true total (elastoplastic) tensile strain. The structure shows an important Bauschinger effect in compression after tensile elongation, but remains stable for relatively large compressive deformations. However, moderate plastic transverse shear deformations, which promote shear-coupled tilt boundary migrations, destabilize it, by provoking grain growth and softening. Our simulations connect with real world materials: very fine wires obtained by large-strain drawing bcc metals or alloys develop a similar columnar structure of 〈110〉 axially oriented grains of nanometric cross section and extraordinary mechanical strength.

Simplified evaluation of pipe strains crossing a normal fault through the dissipated energy method

A dissipated energy method is proposed in this paper to calculate the maximum and minimum pipe strains crossing a normal fault perpendicularly. Two plastic hinges are assumed to form on either side of the fault plane. The curved pipe segment between plastic hinges has different lengths on the footwall and hanging wall sides due to the difference in bearing and uplift soil resistances. A curved pipe length ratio is determined for use in deriving simplified geometric relations. Four energy components are then computed considering rotations at plastic hinges, plastic elongations of the curved pipe segment, and yielding of axial, uplift and bearing soil springs. Minimization of dissipated energy is carried out to find the optimal position of the curved pipe segment for different fault offset. The pipe strains evaluated from the proposed technique are compared with calculations using the existing three-beam analytical method and centrifuge experiments. Upon the successful calibration of the calculation model, a sensitivity analysis is performed to assess the influence of backfill material, dip angle, burial depth and pipe strength. In the end, an illustrative example is presented to employ the developed analysis framework to run fragility analysis for three pipelines with different diameter to thickness ratios.

Correlation of selective laser melting-melt pool events with the tensile properties of Ti-6Al-4V ELI processed by laser powder bed fusion

In this study, we present the first results of a newly developed melt pool monitoring tool for selective laser melting, called DMP-meltpool. A manual data analysis method is given, and the events indicated by the analysis (DMP-meltpool events) are shown to correlate to the static tensile properties of the samples built. These events indicate the probability of material discontinuities (defects) in the metal additive manufacturing (AM) parts. In order to do so, cylindrical bars of Ti-6Al-4V ELI were built and monitored using DMP-meltpool. The tensile properties of the printed cylinders were correlated with the events detected by DMP-meltpool. An inverse relation between plastic elongation and the DMP-meltpool event density was observed. These results show that DMP-meltpool can be used to predict the quality of AM parts by detecting variations in the signals and tagging these events throughout the build as defects. Thus the technique can be employed for first stage in-line quality control of AM parts and for sorting out parts with potential defects non-destructively. The DMP-meltpool events could have significant correlations with other mechanical properties (like fatigue, hardness, fracture toughness, and crack propagation) since such properties are influenced by defects originating from the process instabilities.

Control of the microstructure and mechanical properties of electrodeposited graphene/Ni composite

Nickel composites reinforced with reduced graphene oxide (rGO) nanosheets were fabricated by a direct current electrodeposition technique. The low volume fraction of graphene can promote the cathodic polarized potential, facilitate the transport of ions and electrons on electrodes, and provide a large number of nucleation sites, consequently accelerate the formation of heterogeneous microstructure features. The graphene/Ni composite with 2 mL graphene dispersions demonstrates a tensile strength of 864 MPa and a plastic elongation of 20.6%, which are 25% and 36% higher than that of the pure bulk Ni. The enhancements in strength and ductility of the composite can be ascribed to the bimodal microstructure, for which the fine grain population provides for enhanced strength, whereas the coarse grain population enhances ductility by enabling strain hardening. On the contrary, the positive role of rGO in microstructure control will be weakened due to the agglomeration of rGO sheets in a high-volume plating bath. The low adsorption quantity of rGO is unfavourable for the nucleation of Ni matrix, accordingly produces a uniform fine-grained microstructure. The tensile strength of the composite with 5 mL rGO is 750 MPa, whereas the fracture elongation is only 7.5%. It is believed that a proper addition of rGO dispersion makes a promising microstructure for advanced graphene/Ni composite.

Metal Injection Moulding of High Nb-Containing TiAl Alloy and Its Oxidation Behaviour at 900 °C

High Nb-containing TiAl alloy with a nominal composition of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y (at %) was fabricated by metal injection moulding (MIM) technology with an improved wax-based binder. The critical powder loading and feedstock rheological behaviour were determined. The influence of sintering temperature on microstructures and mechanical properties of the sintered samples and their oxidation behaviour were also investigated. Results showed that a feedstock, with a powder loading of 68 vol % and good flowability, could be obtained by using the improved binder, and oxygen pick-up was lower than that of the sample prepared by using a traditional binder. The ultimate tensile strength (UTS) and plastic elongation of the sample sintered at 1480 °C for 2 h were 412 MPa and 0.33%, at room temperature, respectively. The 1480 °C-sintered sample consisted of γ/α2 lamellar microstructure with the average colony size of about 70 µm, and its porosity was about 4%. The sintered alloy showed better oxidation resistance than that of the cast alloy counterpart.

Effect of Nickel on the Hydrogen Stress Cracking Resistance of Ferritic/Pearlitic Low Alloy Steels

Nickel additions to low alloy steels improve mechanical and technological properties. However, Part 2 of ISO Standard 15156 limits the nickel content to a maximum of 1 wt% in oil and gas environments containing H2S because of controversial concerns regarding sulfide stress cracking. The objective of this work was to investigate the effect of nickel in solid solution in the ferrite phase on hydrogen stress cracking resistance. Ferritic/pearlitic research-grade low alloy steels with nominal nickel contents of 0, 1, 2, and 3 wt% were tested by the slow strain rate test method with cathodic hydrogen charging to −1.05 VAg/AgCl and −2 VAg/AgCl. No difference in fracture mode or morphology was found between the alloys. However, the plastic elongation ratios and reduction in area ratios decreased with increasing nickel content when tested at −2 VAg/AgCl. The direct and indirect effects of nickel, such as the influence of an increasing fraction of pearlite with increasing nickel content, are discussed.

Hydrogen Embrittlement of Ultrafine-Grained Austenitic Stainless Steels

Abstract The effect of electrochemical hydrogen-charging on tensile properties, mechanisms of plastic deformation and fracture micromechanisms was studied using two ultrafine-grained (UFG) Cr-Ni austenitic stainless steels. UFG austenitic structures with an average subgrain size of 200 nm for CrNiMo (316L-type) and 520 nm for CrNiTi (321-type) steel were produced using hot-to-warm ABC-pressing. Hydrogen-charging up to 100 hours weakly influences stages of plastic flow, strength properties and elongation of the UFG steels. TEM analysis testifies to hydrogen-assisted partial annihilation and rearrangement of dislocations into dislocation tangles, and to hydrogen-induced variation in ratio of low- and high-angle misorientations in UFG structure of both steels. Hydrogen-alloying promotes mechanical twinning and deformation-induced γ ® e martensitic transformation in the UFG steels under tension. Ultrafine-grained CrNiTi steel with lower stacking fault energy (SFE) is more susceptible to mechanical twinning and deformation-induced γ ® e martensitic transformation in comparison with CrNiMo steel with higher SFE. The micromechanism of the fracture in hydrogen-assisted surface layers of the steels is compositional, grain-size and hydrogen content dependent characteristic. The present results demonstrate that the steels with UFG structure possess higher resistance to hydrogen embrittlement compared to coarse-grained analogues.

Tensile properties of Zr70Ni16Cu6Al8 BMG at room and cryogenic temperatures

The mechanical behaviour in tension of a hypoeutectic Zr70Ni16Cu6Al8 Bulk Metallic Glass (BMG) was studied at room (295 K) and cryogenic temperatures (150 K and 77 K) using various strain rates between 10−4 and 10−1 s−1. The yield strength was found to increase at lower temperatures with average values increasing by 16%, from 1503 MPa at 295 K to 1746 MPa at 77 K. The Zr-based BMG was found to exhibit tensile plastic elongation of about 0.4% before fracture at room temperature and high strain rates (10−1 s−1). Even higher tensile plasticity was recorded at low temperatures; plastic deformation was found highest at the intermediate temperature (150 K) reaching remarkable plastic strains in the order of 3.9%, while values up to 1.5% were recorded at 77 K. The lateral surface of the tensile specimens was observed in-situ during deformation using a high frame rate camera offering interesting insights with regard to the deformation mechanisms. Room temperature plasticity occurred through the formation and interaction of several nucleated shear bands before critical failure, while at intermediate and liquid nitrogen temperatures, most of the plastic deformation was accommodated through stable flow within a single shear band.

Effect of Mechanical Loading on Osteogenesis of Human Embryonic Stem Cell-Derived Mesenchymal Progenitors within Collagen Microspheres

Mechanical forces and 3D topological environment can be used to control differentiation of mesenchymal stem cells. However, mesenchymal stem cell fate determined by the effect of physical and mechanical cues is not yet fully understood. Understanding how mechanical cues in the microenvironment orchestrate stem cell differentiation provides valuable insight that can be used to improve current techniques in cell therapy. This study investigates the osteogenic effect of mechanical stimulations on soft cellular microspheres loaded with human embryonic stem cell-derived mesenchymal progenitors (hES-MPs) when subjected to dynamic loading and in the absence of chemical stimulation. Microspheres were produced by gelation of bovine collagen type I with 1000 to 2000 hES-MP cells seeded per droplet. Four loading conditions were studied: (1) 10% constant strain was applied by a Bose biodynamic bioreactor for 15 min/day or 40 min/day for 5 or 10 days respectively; (2) 10% adjusted strain was applied (subtraction of polydimethylsiloxane (PDMS) plastic elongation from global strain) using Bose biodynamic bioreactor for the same 4 duration/conditions as in the constant strain protocol. The results indicate that applying mechanical stimulation to hES-MPs/collagen microspheres induced osteogenic differentiation of cells when the loading protocol was adjusted. Alkaline phosphatase activity of samples in the adjusted loading protocol increased significantly on day 14 whilst, the deposited minerals, matrix reorganisation and alignment of collagen fibres enhanced from day 21 post encapsulation onward. Application of cyclic loading to 3D culture of hES-MP cells can be used as a model to regulate mechano-stimulation and linage differentiation in vitro.

Effect of Mechanical Loading on Osteogenesis of Human Embryonic Stem Cell-Derived Mesenchymal Progenitors within Collagen Microspheres

Mechanical forces and 3D topological environment can be used to control differentiation of mesenchymal stem cells. However, mesenchymal stem cell fate determined by the effect of physical and mechanical cues is not yet fully understood. Understanding how mechanical cues in the microenvironment orchestrate stem cell differentiation provides valuable insight that can be used to improve current techniques in cell therapy. This study investigates the osteogenic effect of mechanical stimulations on soft cellular microspheres loaded with human embryonic stem cell-derived mesenchymal progenitors (hES-MPs) when subjected to dynamic loading and in the absence of chemical stimulation. Microspheres were produced by gelation of bovine collagen type I with 1000 to 2000 hES-MP cells seeded per droplet. Four loading conditions were studied: (1) 10% constant strain was applied by a Bose biodynamic bioreactor for 15 min/day or 40 min/day for 5 or 10 days respectively; (2) 10% adjusted strain was applied (subtraction of polydimethylsiloxane (PDMS) plastic elongation from global strain) using Bose biodynamic bioreactor for the same 4 duration/conditions as in the constant strain protocol. The results indicate that applying mechanical stimulation to hES-MPs/collagen microspheres induced osteogenic differentiation of cells when the loading protocol was adjusted. Alkaline phosphatase activity of samples in the adjusted loading protocol increased significantly on day 14 whilst, the deposited minerals, matrix reorganisation and alignment of collagen fibres enhanced from day 21 post encapsulation onward. Application of cyclic loading to 3D culture of hES-MP cells can be used as a model to regulate mechano-stimulation and linage differentiation in vitro.

A Large Plastic Elongation followed by Brittle Fracture : a Case of Tensile Deformation at –180 °C in a DP Steel

A Large Plastic Elongation followed by Brittle Fracture : a Case of Tensile Deformation at –180 °C in a DP Steel

Differentially Expressed microRNAs and Target Genes Associated with Plastic Internode Elongation in Alternanthera philoxeroides in Contrasting Hydrological Habitats.

Phenotypic plasticity is crucial for plants to survive in changing environments. Discovering microRNAs, identifying their targets and further inferring microRNA functions in mediating plastic developmental responses to environmental changes have been a critical strategy for understanding the underlying molecular mechanisms of phenotypic plasticity. In this study, the dynamic expression patterns of microRNAs under contrasting hydrological habitats in the amphibious species Alternanthera philoxeroides were identified by time course expression profiling using high-throughput sequencing technology. A total of 128 known and 18 novel microRNAs were found to be differentially expressed under contrasting hydrological habitats. The microRNA:mRNA pairs potentially associated with plastic internode elongation were identified by integrative analysis of microRNA and mRNA expression profiles, and were validated by qRT-PCR and 5′ RLM-RACE. The results showed that both the universal microRNAs conserved across different plants and the unique microRNAs novelly identified in A. philoxeroides were involved in the responses to varied water regimes. The results also showed that most of the differentially expressed microRNAs were transiently up-/down-regulated at certain time points during the treatments. The fine-scale temporal changes in microRNA expression highlighted the importance of time-series sampling in identifying stress-responsive microRNAs and analyzing their role in stress response/tolerance.

Metal passivation effect on focused beam-induced nonuniform structure changes of amorphous SiOx nanowire

In this paper, the passivation effect of Au nanoparticles (AuNPs) on the focused electron beam (e-beam) irradiation-induced nonuniform structure changes of amorphous SiOx nanowire (a-SiOx NW) is in-situ investigated at room temperature in a transmission electron microscope. It is found that the pristine a-SiOx NW demonstrates an accelerated necking at the nanoscale along with a fast, plastic elongation and a local S-type deformation in the axial direction. However, when the surface of a-SiOx NW is modified with uniform AuNPs, it becomes remarkably passivated and the processing transfers from a diffusion (or plastic flow)-dominated status to an evaporation (or ablation)-dominated status. As a result, the necking of the AuNPs-modified a-SiOx NW is greatly retarded with a much lower shrinking rate and there is no visible elongation and bending deformation observed. Further, the combined effects of nanocurvature and beam-induced soft mode and instability of atomic vibration are applied to elucidate the observed new phenomena.