Contribution Of Plastic Deformation Research Articles

The effect of secondary dendrite orientation on thickness debit effect of nickel-based single-crystal superalloy with tubular samples

Secondary dendrite orientation and wall thickness considerably affect the stress rupture life of thin-walled samples. However, the effect of the secondary dendrite orientation on the thickness debit effect of nickel-based single-crystal superalloys has not been thoroughly investigated until now. Owing to geometrical constraints, typical sheet samples cannot reveal the mechanism responsible for the thickness debit effect in turbine blades. This study examined the effect of secondary dendrite orientation on the thickness debit effect of nickel-based single-crystal superalloys at 1100°C/137 MPa in tubular samples. As the wall thickness decreased from 1.5 mm to 0.3 mm, the stress rupture life decreased from approximately 170 h to 64 h, demonstrating a noticeable thickness debit effect. Among the different secondary dendrite orientation areas, the variation in plastic deformation increased from 7% (1.5 mm) to 45% (0.5 mm) and subsequently decreased to 4% (0.3 mm). In thinner samples, the thickness contraction and microstructure evolution were more pronounced in the [100] areas than that in the [110] and [210] areas. The theoretical calculation quantitatively indicated that as the effective stress increased, the contribution of plastic deformation (45%) was slightly lower than that of oxidation (55%) in 0.3 mm samples; nevertheless, plastic deformation played a prominent role in 0.5, 0.8, 1, and 1.5 mm samples and increased from 61% (0.5 mm samples) to 85% (1.5 mm samples). In thinner samples, increased plastic deformation in the secondary dendrite orientation of the [100] areas and oxidation increased the effective stress, resulting in a shorter rupture life. These findings are conducive to the structural optimization and performance improvement of turbine blades.

The plastic strain effect on cleavage microcracks propagation

The effect of plastic strain by tension on microcracks propagation is investigated in probabilistic and deterministic statements on the basis of the experimental results for the critical brittle fracture stress and the calculations of brittle fracture probability with local approach model. Investigation is performed for two materials: 2Cr-Ni-Mo-V steel used for WWER-1000 RPV in the thermally-embrittled state and low-alloyed low-strength steel of St.3 grade taken as model material ruptured by cleavage up to plastic strain >50 %. Experimental study includes uniaxial tension of smooth round bars over wide temperature range, mainly over brittle fracture range, and SEM examination of the fracture surfaces. Brittle fracture probability is calculated with the Prometey model. It has been found that the plastic strain effect on the microcrack propagation probability is caused by two reasons: (1) the critical brittle fracture stress increase due to formation of new barriers for microcrack under plastic deformation, and (2) the working volume decrease due to the neck formation in tensile round bar. A procedure has been proposed to separate the above different contributions of plastic deformation in the brittle fracture probability.

Serrated Plastic Flow of Various Metallic Glasses during Nanoindentation

Nanoindentation experiments were executed on amorphous metallic ribbons made of Fe40Ni40B20, Cu47Ti35Zr11Ni6Si1 and Zr65Cu17.5Ni10Al7.5 that differ in microhardness and glass forming ability. The individual serrated plastic flow events were analyzed in a wide range of the loading rates. In the individual pop-in events of the load-displacement (P-h) curve the contributions of plastic deformation (Δhpl) were calculated depending on the loading rate and the alloy composition. It is concluded that the contribution of the serrated plastic deformation flow varies with the composition of the alloy. The highest plastic deformation for the individual pop-ins was observed for Zr-based metallic glasses.

Void shrinking process and mechanisms of the diffusion bonded Ti–6Al–4V alloy with different surface roughness

The diffusion bonding of Ti–6Al–4V alloy with different surface roughness was performed at 5 and 10 MPa. The influence of surface roughness on the void shrinking process and mechanisms was investigated. The average void size increases as the Ra increases from 0.33 to 0.44 μm, while it decreases as the Ra increases to 0.46 μm because of the decreasing of Rλq. The void shrinking mechanisms were analyzed by using the dynamic model of void shrinking. Power-law creep is a dominant mechanism on void shrinking, of which the contribution decreases as the Ra increases from 0.33 to 0.44 μm, while it increases as the Ra increases to 0.46 μm. The influence of surface roughness on the contribution of plastic deformation and surface source mechanism on void shrinking is not significant while that on the contribution of interface source mechanism is dependent on the imposing pressure. The optimizing surface roughness is with a Ra of 0.33 μm and Rλq of 5.38 μm in this study.

Bonding interface characteristic and shear strength of diffusion bonded Ti-17 titanium alloy

The bonding interface characteristic and shear strength of diffusion bonded Ti-17 titanium alloy at different bonding time were investigated. The results show that the average size of voids decreases while the amount of voids decreases after increasing to the maximum value with the increasing bonding time. The irregular void with a scraggly edge tends to an ellipse void with smooth surface and then changes to a tiny void with round shape. The grains across bonding interface occur at bonding time of 60 min. The shear strength of bond increases with increasing bonding time, and the highest shear strength of bond is 887.4 MPa at 60 min. The contribution of plastic deformation on the void closure and the increase of shear strength is significant even though the action time of plastic deformation is short.

Frictional Asymmetry and Wear Pattern Formation by Slip and Cleavage Detected at Directional <I>r</I>{1 0 -1 4} Face of Calcite (CaCO<sub>3</sub>)

Frictional force was measured on directional calcite (1 0 -1 4) surface, where triangular carbonate ions are all tilted to [4 2 -1] direction. With the normal load -[4 2 -1], was the same as that measured in nm-scale. In the higher normal load range, a new component was added to the friction in -[4 2 -1] direction, and the asymmetry was reversed. Only in the direction of stronger friction, characteristic triangular deformation patterns were formed. Microscopic observation after indentation experiments revealed that the patterns were formed by slip on c (0 0 0 1) faces on one side of the stylus, followed by cleavage on r{1 0 -1 4} faces. A mechanical model was proposed to explain why the deformation occurs only in one quadrant of the surface. The result of scratch perpendicularly in [0 1 0] direction, also supports the mechanism of deformation. The results show when and how the contribution of plastic deformation is added to intrinsic surface friction and affects the frictional asymmetry. The frictional force in the -[4 2 -1] scratch increased linearly with the total lengths of the traces of c-slip.

Practical Work of Adhesion of Polymer Coatings Studied by Laser Induced Delamination

AbstractLaser Induced Delamination is a novel technique aimed at measuring the practical work of adhesion of thin polymer coatings on metal substrates. In this technique a laser pulse is used to create initial blisters which initiate further delamination of the film under the blister pressure. A simple elastic model is developed to describe the formation of the blisters. The model predicts the values for the blister height and pressure, which are in fair agreement with the experimental results. In order to account for possible plastic deformations, simulations using a finite element model with a mixed mode cohesive zone were carried out. The calculated stress fields are in agreement with those predicted by the elastic model suggesting that the contribution of plastic deformation to the measured work of fracture is rather limited.Measurements are carried out on a number of samples, presenting ECCS steel substrate covered with 35 μm thick polyethylene terephthalate (PET) film. The tensile stresses created in the film at the interface required for delamination are estimated at 7-8 MPa, which corresponds to the practical work of adhesion G = (0.6± 0.1) J/m2.

Micromechanical Analysis of Residual Stress Effect in CVD-Processed Diamond Wafer

AbstractDiamond wafer, considered as promising material for electronic substrate, thermal spreader, etc., has been made using deposition technology such as microwave plasma assisted CVD (MPACVD). However, high residual stress of diamond film induces bowing and throughthickness cracking in thick diamond wafer, and so hampers the economic fabrication of the wafer. Thus, to investigate the causes of bowing and cracking, the measurement of residual stress is very important. Of the residual stress, in particular, growth stress is one of the most important research topics because its effect on wafer-related problems is strong though the magnitude is relatively small. Quantification of growth stress is limited because of hightemperature plastic deformation of Si substrate. In this study, the initial growth stress without contribution of plastic deformation of substrate can be predicted through proposed beam bending stress model and numerical calculation in that plastic deformation of Si is considered. Finally, growth stress of film can be calculated from the apparent curvature of substrate measured just after deposition and the plastic curvature after diamond film removal.

Understanding mechanical infusion of nitrogen into iron powders

Nitrogen alloying of iron-based metals greatly improves mechanical and corrosion properties. However, nitrogen solubility is less than 0.1 weight percent (wt%) in b.c.c iron. In this study nitrogen concentrations in excess of 1 wt% were obtained by high-energy milling of pure iron powders in a nitrogen gas environment. The nitrogen concentration in the powder, the grain size, and internal strain in the mechanically processed iron powder were determined as functions of processing time in both nitrogen and argon gas environments. The contributions of plastic deformation and interstitial nitrogen to changes in lattice d-spacing, and the distribution of nitrogen between interstitial sites and defect sites were estimated. Approximately 25% of the incorporated nitrogen was contained in interstitial sites, with the remainder associated with defect structures and surfaces.

Analysis of the pullout of single fibers from low‐density polyethylene

AbstractIn the present work, a single‐fiber pullout test was used to study the interface/interphase between various fibers and low‐density polyethylene (LDPE) and between glass fibers and a range of other thermoplastic matrices. For well‐bonded fibers, experimental evidence suggests the involvement of plastic deformation and strain‐hardening prior to debonding and pullout. The interfacial shear strength was determined to be the ultimate shear strength of the matrix and was found to be insensitive to the fiber surface structure. A new theoretical model was developed to predict the relationship between the debonding force and the embedded length. The contribution of friction to the debonding force was found to be insignificant when compared with the contribution of plastic deformation. © 1994 John Wiley & Sons, Inc.

A new model for the pull-out of single fibres from low density polyethylene

Based on the observation and interpretation of single-fibre pull-out results of E-glass fibre from low density polyethylene, a new theoretical model, incorporating the concepts of critical transition length, ultimate shear strength and a deduced stress distribution, was developed to predict the relationship between the debonding force and embedded fibre length for a well-bonded interface. With reasonable values of model parameters, better consistency between the new model and experimental results was achieved, compared with other existing models. The complete contribution of friction to debonding force was conclusively demonstrated to be insignificant compared with the contribution of plastic deformation.

Fracture Surface Energy of Highly Oriented Polymers

Abstract A Monte Carlo technique is used to estimate the fracture surface energy of highly oriented polymers ruptured under conditions at which the contribution of plastic deformation in front of the propagating crack is small compared with the energy required to form new surfaces at the tip of the crack. The effects of molecular weight, cohesive energy, and breaking energy of the polymer chain are discussed for the cases where: (1) all molecules are fully extended and (2) chain folding occurs at random. With the extended chain model the fracture surface energy increases monotonically with the molecular weight and approaches asymptotically its maximum value NQb . (N = number of polymer chains per unit area and Qb = energy required to break the polymer chain.) The cohesive energy, while not contributing substantially to the fracture surface energy at high values of the molecular weight, strongly affects the width of the molecular weight range in which fracture surface energy shows a pronounced molecular weight dependence. The lower the cohesive energy of a polymer, the higher must be the molecular weight to utilize a given fraction of the potential offered by an isolated polymer chain. For the systems where chain folding occurs at random, the model predicts a maximum in fracture surface energy at a value of number average molecular weight (Mn ) corresponding approximately to the number average molecular weight between the folds. For higher values of Mn the fracture surface energy first decreases and then assumes a near constant value. This decrease is attributed to the weakening of the structure through chain folding while the effective length of the molecules remains unchanged.