Elongation In Tension Research Articles

Mechanism of solute hardening and dislocation debris-mediated ductilization in Nb-Si alloy

Niobium (Nb) is sensitive to even minute quantities of silicon (Si) solutes, which are known to induce pronounced hardening. However, the underlying mechanism for hardening remains elusive since the effect of Si solutes on dislocation behavior is unclear. Here, using tensile testing, in-situ microscopy and nanomechanical testing, the behavior of dislocations in dilute Nb-Si alloys, containing from 0 at.% to 0.8 at.% Si, is investigated. We show that the hardness, strength and strain hardening rate increase from two to four times, while the uniform elongation in tension only reduces 50 % as the Si content increases. Dislocations evolve from complex entangled patterns in Nb to parallel long-straight screw dislocation-dominated structures in Nb-Si alloys. In-situ indentation reveals that the origins of the marked hardening in Nb-Si alloy are the reduction of dislocation mobility and cross-slip propensity. Large densities of dislocation debris-superjogs and loops introduced throughout the sample during warm rolling and annealing are found to provide active internal dislocation sources, which explain the minimal ductility loss seen in these Nb-Si alloys. These findings can help guide the alloy design of high-performance refractory materials for extreme temperature applications.

Spark plasma sintered Mg-4Y-3Nd with exceptional tensile performance

This work utilizes the spark plasma sintering (SPS) technique and exploits the physical mechanisms leading to the preparation of a fully compacted Mg-4wt.%Y-3wt.%Nd (WN43) material to exhibit superb mechanical properties, including outstanding plastic elongation in tension of more than 10 % among the SPS-ed Mg-based materials. To this end, the detrimental effect of oxide shells present on the Mg-Y-Nd powder particles is purposefully suppressed. The essential principle underlying the strong interconnection of the former powder particles is based on the combination of sufficiently high pressure and temperature approaching the eutectic points of Mg-Y/Nd systems. Thus-prepared WN43 material also retains the initial fine powder microstructure and its microstructural features are translated into good ductility and strength of the final product comparable with the conventionally-prepared (e.g. wrought) Mg-RE alloys. At the same time, the advantages of SPS – the production of a relatively large volume of near net-shape material with weak crystallographic texture – are retained. The deformation mechanisms are investigated by means of complementary in-situ techniques (acoustic emission, digital image correlations) and semi in-situ electron backscatter diffraction. A relatively low tension–compression asymmetry is revealed owing to the weak texture and the activity of deformation mechanisms (dislocation slip and twinning) follows the conventions of bulk Mg. In support of potential practical applications of the material, no significant cracking of the residual oxide phases is observed although they seem to play some role in the initiation of critical crack leading to the fracture. The SPS technique offers undemanding fine-tuning of processing parameters leading, in turn, to expeditious modification of material properties and, given the understanding of microstructure evolution unveiled in this work, it can be effectively utilized for the preparation of different Mg-based (or other) materials.

ВЛИЯНИЕ КРИСТАЛЛИЧЕСКОЙ СТРУКТУРЫ SiO2 НА ТЕРМИЧЕСКОЕ ЦИКЛИРОВАНИЕ ПОЛИМЕРНЫХ КОМПОЗИТОВ

polymer composites are widely used in the space industry for the manufacture of spacecraft, satellite panels, antennas, thermostatically controlled coatings, etc. In space, they are subjected to harsh environmental conditions, such as ultraviolet, deep vacuum, atomic oxygen, charged particles, anthropogenic debris, micrometeoids, electromagnetic radiation and thermal cycles that cause severe degradation of the material. One of the most important environmental effects of materials based on polymers is the thermal cycle, in which the composite undergoes a large temperature difference from -170˚C to + 200˚C. The paper presents an assessment of the use of composites based on a polyalkane-rich matrix and a filler in the form of an SiO2 amorphous and crystalline structure under thermal cycling conditions. The data on the change in tensile strength, modulus of elasticity in tension and relative elongation in tension of materials after several cycles of a sharp differential temperature (from -190 to +200°C) are presented. The thermal cycle was repeated 5, 10 and 20 times. It is shown that the sample polyalkanimide has a large value of tensile strength and elastic modulus compared with highly filled composites.However, during thermal cycling there is a significant decrease in these parameters.For a highly filled composite sample with 65% crystalline SiO2 content, the decrease in tensile strength and elastic modulus after thermal cycling is insignificant and is within the measurement error. A composite with amorphous SiO2 is more susceptible to a change in mechanical properties after thermal cycling in comparison with a composite containing crystalline SiO2.

Multilayer coatings based on polyimide track membranes and nanodispersed lead

Multilayer coatings are obtained by “bonding” several layers of polyimide track membranes filled with metallic lead. The bonding of the layers is carried out by an imidization reaction, specifically, the production of polyimide from polyamic acid at 250–270 °C and a pressure of 200 MPa for 30 min. To fill the pores with nanodispersed lead, we use galvanic deposition of metallic lead through the etched channels of the polyimide track membranes. This study presents data on the electrodeposition of nanodispersed lead, including the formation of a cathode coating, the electrolyte used and the optimal electrolysis conditions. Potentiostatic curves obtained during electrodeposition of lead into pores with a diameter of 200 nm at various fixed potentials (−200, −400 and −600 mV) are presented. It is established that as a result of electrodeposition, the channels are completely filled, as evidenced by the formation of lead nanowires. According to the results of X-ray diffraction, the nanowires are metallic lead with a pronounced cubic crystal structure of Fm3m, with no additional peaks detected. The diffraction peaks of lead nanowires are slightly wider compared to bulk lead. The resulting multilayer coating with a thickness of 300 μm has high physicomechanical and dielectric properties with a tensile strength of 76 MPa, an elongation in tension of 10.2% and a specific volume resistivity of 103 Ω m.

Evolution of anelastic behaviour and twinning in cyclic loading of extruded magnesium alloy ZM21

In this study, the cyclic response of the wrought magnesium alloy ZM21 is investigated. The microstructure of the original extruded bar consists of equiaxed grains with a grain size range of 15–18 μm. The anelasticity was measured using a method in which the elastic modulus in every loop is obtained by calculating the slope of lines fitted to the loading and unloading parts of the curves. The cyclic test for ZM21 shows that the anelastic strain increases with the strain, reaching its maximum between 2-4% applied strain and then levels off. The anelastic strain reaches a maximum between 2-4% at which point the fraction of extension twins also reaches a maximum. Twins were separated from their parent grains in order to quantify twin evolution during cyclic loading. Overall, it was found that most of the twins observed are extension twins up to 8% accumulative strain. Beyond 8% strain, exhaustion of 90% of extension twins occurred and contraction twins appeared in the microstructure. Moreover, double twins were identified after a strain of 8%. These twins were composed of extension twins and contraction twins which can contribute to softening. Such softening can result in the reduction of uniform elongation in tension for the cyclic loading in comparison with the monotonic loading. It was possible for the material to sustain continued straining without failure to over double this value, that is, 15%.

Effect of hot working and aging heat treatments on monotonic, cyclic, and fatigue behavior of WE43 magnesium alloy

This paper examines monotonic and cyclic behavior of WE43 alloy in several conditions including direct-chill as-cast and rolled heat-treated with an emphasis on the comparison of microstructural features governing the difference in their behavior. To facilitate the evaluation of the effect of hot working conditions on the material behavior, the as-cast-WE43 was converted into two rolled plates with one (P2) being rolled at a temperature for 16.5 °C lower than the other plate (P1). The plates were further processed into T5 and T6 conditions. The plate P2 in the T5 condition was found to have the highest strength and elongation to fracture in tension along both the rolling direction (RD) and the transverse direction (TD). The other plate, P1, showed a higher ductility in compression, while exhibiting slightly lower strength and elongation in tension. The trend of lower strength and elongation in tension with higher ductility in compression continued to the material in the T6 condition and finally to the as-cast material. Strain to fracture in compression was the largest for the as-cast material. In addition to quasi-static strength, P2 was found to exhibit superior low cyclic fatigue (LCF) and high cyclic fatigue (HCF) behavior. Consistent with the quasi-static strength, the rolled plates in the T5 conditions exhibited slightly longer lives in LCF and HCF along TD than along RD. The interesting deformation behavior characteristics of the alloy in different conditions are rationalized in terms of their microstructures. The results reveal that it is possible to further optimize the hot working conditions to obtain the alloy WE43 exhibiting better performance characteristics.

Casting of Iron Aluminides

Iron aluminides form an interesting class of materials which combine excellent corrosion and oxidation resistance with good mechanical properties at moderate to high temperature (up to 500 °C). These materials, however, suffer from low room-temperature ductility (under 5% elongation in tension), which is mostly due to environmental effects. Casting is a processing route traditionally applied to brittle alloys (e.g., gray cast irons), but to cast a part without defects, several thermochemical properties are needed, as well as information on the tendency of the alloy to form foundry defects (e.g., shrinkage voids, pores). The present work aims to provide this information using parts produced on laboratory scale. In particular, the solidification contraction and the efficiency of TiB2 as inoculant are investigated. Three alloys with nominal composition (in at.%) Fe28Al, Fe28Al6Cr, and Fe28Al6Cr1Ti (about 1.5 kg for each melt) were melted in an induction furnace under argon flux protection using conventional raw materials (carbon steel, commercial aluminum, metallic chromium, and commercial ferrotitanium). The resulting melts were treated by adding Al-TiB2 master alloy used in the aluminum industry and poured into “staircase” molds, designed to investigate feeding distance effects in complex parts. Characterization of the microstructure of the alloys revealed that alloys Fe28Al and Fe28Al6Cr showed κ-carbide precipitation, while alloy Fe28Al6Cr additionally showed chromium carbides at dendritic boundaries. Addition of 1 wt.% Ti in alloy Fe28Al6Cr1Ti changed the solidification microstructure, refining the dendrite morphology and forming TiC-containing eutectic in interdendritic spaces.

Roll-formability of cryo-rolled ultrafine aluminium sheet

Ultrafine-grain aluminium sheet was produced by rolling at cryogenic (CR) and at room temperature (RTR). Commercial purity aluminium plate was reduced in 30 passes from an initial material thickness of 10mm to a final thickness of 2mm (80% reduction). Tensile stress and strength were significantly increased while total elongation was drastically reduced. It was found that despite the low tensile elongation both materials are able to accommodate high localised strains in the neck leading to a high reduction in area. The formability of the material was further investigated in bending operations. A minimum bending radius of 6mm (CR) and 5mm (RTR) was found and pure bending tests showed homogeneous forming behaviour for both materials. In V-die bending the cryo-rolled material showed strain localisations across the final radius and kinking of the sample. It has been found that even if the total elongation in tension is close to zero leading to early failure in V-die bending, ultra-fine grained and low ductile sheet metals can be roll formed to simple section shapes with small radii using commercial roll forming equipment.

Characterization of torsion behavior and fracture morphology of single carbon fiber

A novel approach for evaluating the torsional performance of single carbon fiber filament was established based on a self-designed apparatus. The number of torsion turns and twist angle at fracture from the direct torsion failure test were defined to characterize the torsion-bearing capacity of single carbon fiber. The fiber fracture strain under torsion stress was calculated based on a single fiber torsional deformation model and was compared with the fiber elongation in tension, which showed that the toughness of carbon fiber under torsion shear stress was inferior to its resistance to tensile stress. In addition, the resistance of carbon fiber to torsion shear stress was changed after changing the fiber surface structure by removing the sizing agent or coating the fiber surface with epoxy resin. The established method will enrich the existing research work on mechanical properties of carbon fiber as well as those for different types of fibers.

Toward a New Interpretation of the Mechanical Behaviour of As-quenched Low Alloyed Martensitic Steels

Though as-quenched martensite exhibits a low uniform elongation in tension, it is highlighted that this phase has a very high strain-hardening which increases with carbon content and a large Bauschinger effect. Because usual dislocation storage can not explain reasonably this particular behaviour, an approach based on a continuum composite view of martensite (CCA) is developed suitable to capture all the experimental features.

Biomechanical Comparison of Four Soft Tissue Replacement Materials: An In Vitro Evaluation of Single and Multilaminate Porcine Small Intestinal Submucosa, Canine Fascia Lata, and Polypropylene Mesh

To compare mechanical performance of 4 soft tissue replacement materials. Experimental. Polypropylene mesh (PM), single-layer porcine small intestinal submucosa (SIS), multilaminate (4-layer) porcine small intestinal submucosa (MLSIS), and canine fascia lata (FL). The mechanical properties of each material were determined by testing to failure on a materials testing machine. Samples of each material (n=10) were tested in 3 different modes: resistance to suture pullout, tensile testing, and push-through testing. PM was tested both parallel (PMa) to and perpendicular (PMb) to its longitudinal cord orientation. SIS and FL were similarly tested in 2 orthogonal directions. With some exceptions, the following generalizations can be made regarding the mechanical performance of the materials tested: Suture pullout-FL>PMa=PMb>MLSIS>SIS (P< or =.04). Tensile testing-FL>PMa>PMb>MLSIS>SIS (P< or =.02). Push-through testing-FL>PM>MLSIS>SIS (P< or =.003). PM accommodated a significantly higher load and energy to yield when its longitudinal cords were oriented parallel with the tension axis (PMa). FL performed similarly to the PM, with the exception of limited elongation in tension. MLSIS had biomechanical characteristics that were inferior to FL and PM but superior to SIS. PM's orientation may need to be considered when used clinically. FL is a biomechanically suitable soft tissue replacement material but its use may be limited by currently available sizes. SIS cannot be recommended in high-strain environments.

Grain boundary dynamics in ceramics superplasticity

Superplasticity refers to an ability of polycrystalline solids to exhibit exceptionally large elongation in tension. The application of superplasticity makes it possible to fabricate ceramic components by superplastic forming (SPF), concurrent with diffusion bonding, and superplastic sinter-forging just like superplastic metals. Furthermore the superplastic deformation plays an important role in stress-assisted densification processes such as hot isostatic pressing (HIP) and hot pressing (HP). The ceramics superplasticity has been one of intensive research fields in the last decade. Although most of reports are still limited to those of zirconia [1] , new developments have been achieved in superplasticity of Si 3 N 4 and SiC in recent years. It is clearly demonstrated that the superplasticity is one of the common natures of fine-grained ceramics and nanocrystalline ceramics at elevated temperatures.

Effect of epitaxial ferrite on yielding and plastic flow in dual phase steel in tension and compression

A low carbon, microalloyed steel was heat treated to obtain dual phase microstructures containing constant levels of 18 and 25 vol.-% martensite at two levels of microstructural refinement and with varying epitaxial ferrite content. Tensile and compression tests were conducted at a strain sensitivity of 2 × 10-5. Elastic limits in tension and compression were indistinguishable and very low, suggesting that mobile dislocations were present in the ferrite as a consequence of stress relaxation processes. These mobile dislocations accommodated the volume increase accompanying the austenite to martensite transformation during heat treatment. Epitaxial ferrite had little effect on the 0·2% proof stress, but average proof stresses were generally higher in compression than in tension owing to residual stresses in the martensite and ferrite following heat treatment. The residual stresses calculated from this asymmetry in the proof stresses were small because of stress relaxation in the ferrite at the temperature at which the martensite formed. Epitaxial ferrite significantly increased uniform elongation in tension with a small decrease in tensile strength for both levels of martensite in the finer microstructure but only at the 18 vol.-% martensite level in the coarser microstructure. The cause of the increased ductility was the effect of epitaxial ferrite on the work hardening rate between approximately 0·5 and 3% strain; epitaxial ferrite reduced the work hardening rate in this range of strain.

TENSILE AND LCF PROPERTIES OF AISI 316LN SS AT 300 AND 77 K

Tensile and low‐cycle fatigue tests were performed on a 316LN austenitic stainless steel at 300 and 77 K. The tensile and low‐cycle fatigue properties were obtained and analysed in terms of the influence of temperature on the plastic deformation process and the formation of strain‐induced martensite. The martensite content was evaluated using measurements of magnetic saturation. No α′‐martensite was detected at 300 K under either monotonic or cyclic straining. On the contrary, at 77 K, strain‐induced martensitic transformation is responsible for the higher elongation in tension and the secondary hardening observed on hardening/softening curves in low‐cycle fatigue. The induced martensite content in tensile tests is a function of strain which deviates from Angel’s model. In low‐cycle fatigue, it is a function of the strain level and the accumulated plastic strain. At a given total strain amplitude, the decrease of temperature from 300 to 77 K results in the decrease of plastic strain amplitude and homogenization of plastic strain distribution, and thus in the prolongation of fatigue life. The cyclic over‐stress at 77 K, due to an intermediate ageing at 300 K, is related to pinning of initially free dislocations resulting from nitrogen diffusion during isothermal holding at room temperature. This results in a reduced fatigue life.

Strain softening and instability of plastic flow in Cu-Al single crystals

The work brings the results of the study of mechanisms of deformation in Cu-6 wt% Al, low stacking fault energy single crystals. It was observed that deformation up to 180% of elongation in tension proceeds in three distinct stages, all having a linear representation in the stress-strain curve. Particular attention was paid to an inhomogeneous transient from the second to third stage and to the mechanisms of deformation in both stages. It has been found that a sudden decrease of the strain hardening rate in the third stage is associated with activation of a new slip system. Based upon the experimental results the model for a strain softening process is suggested.

Tear Strength and Tensile Strength of Model Filled Elastomers

Abstract Measurements have been made of the tear strength, tensile strength, and energy dissipated during stretching for model filled elastomers consisting of polybutadiene with glass beads incorporated. The glass beads were pretreated with various silanes, some of which could, in principle, form covalent bonds with the polybutadiene matrix during free-radical crosslinking of the latter and some of which could not. The tear strength of the elastomer was increased by the addition of glass beads, by about 25% for the largest beads, having a mean diameter of 150 µm. This effect is attributed to increased roughness of the tear path. The breaking elongation in tension was reduced by the addition of glass beads but the breaking stress was only seriously reduced for the least-well-adhering beads. The stored strain energy density at break was reduced in all cases. This is attributed to large glass beads acting as fracture nuclei in tension. Calculated sizes of a Griffith crack, 150–300 µm, are consistent with this hypothesis. Strain energy dissipated due to dewetting was found to be in the range 4–13% of the input energy, depending upon the degree of interfacial adhesion, in addition to about 10% dissipated in the unfilled material. The maximum value observed is in reasonable agreement with theoretical predictions.

Tear strength and tensile strength of model filled elastomers

AbstractMeasurements have been made of the tear strength, tensile strength, and energy dissipated during stretching for model filled elastomers consisting of polybutadiene with glass beads incorporated. The glass beads were pretreated with various silanes, some of which could, in principle, form covalent bonds with the polybutadiene matrix during free‐radical crosslinking of the latter and some of which could not. The tear strength of the elastomer was increased by the addition of glass beads, by about 25% for the largest beads, having a mean diameter of 150μm. This effect is attributed to increased roughness of the tear path. The breaking elongation in tension was reduced by the addition of glass beads but the breaking stress was only seriously reduced for the least‐well‐adhering beads. The stored strain energy density at break was reduced in all cases. This is attributed to large glass beads acting as fracture nuclei in tension. Calculated sizes of a Griffith crack, 150–300 μm, are consistent with this hypothesis. Strain energy dissipated due to dewetting was found to be in the range 4–13% of the input energy, depending upon the degree of interfacial adhesion, in addition to about 10% dissipated in the unfilled material. The maximum value observed is in reasonable agreement with theoretical predictions.