Increase In Flow Stress Research Articles

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A micro-tension testing technique was used to investigate the deformation behaviour of lath martensite structures with several types of boundaries in carbon steel. The martensite structures exhibited sufficient necking strains and ductile fractures, whereas the uniform strain was limited owing to a lack of strain-hardening ability despite the increased flow stress. The yield stress of the lath martensite structures strongly depended on the in-lath-plane orientation. The critical resolved shear stress of the in-lath-plane slip systems was considerably lower than that of the out-of-lath-plane slip systems. This finding suggests that the block boundaries are an effective grain boundary for impeding dislocation gliding. Plastic deformation transfer was restricted by the packet boundaries, which greatly rotated the crystallographic orientation of the in-lath-plane slip systems between neighbouring martensite variants.

High temperature tensile behavior of ultra-fine grained Al–3.3Mg–0.2Sc–0.2Zr alloy by equal channel angular pressing

Abstract High temperature tensile tests at temperatures of 293–773 K and initial strain rates of 5×10 −4 to 1×10 −1 s −1 were carried out on Al–3.3Mg–0.2Sc–0.2Zr alloy prepared by equal channel angular pressing (ECAP) at 473 K. The microstructure of the alloy before and after ECAP was investigated by TEM and XRD. The results showed that the intensities of (1 1 1)/(2 2 2) and (2 0 0)/(4 0 0) peaks for the ECAPed alloy decrease significantly and the peak widths at half height become broadening in comparison to the corresponding values for the alloy before ECAP. The equiaxed ultra-fine grains of ∼0.2 μm were obtained in the alloy after eight pass ECAP. The distribution of Sc and Zr elements and the second phase particles was homogeneous in the specimen after ECAP. The ultra-fine grained microstructure was stable below 773 K because the alloy retained extremely fine grain size of ∼1 μm after static annealing at 773 K for 1 h. When tested at 293 K and strain rates varying from 5×10 −4 to 1×10 −1 s −1 , the alloy showed strain rate insensitive plasticity. When tested at 523, 623 or 773 K, there were monotonous increases in maximum flow stress and decreases in ductility with the increasing of strain rates. The maximum elongation to failure of 365% was obtained at 773 K and 5×10 −4 s −1 .

On the Mechanics of Crack Initiation and Propagation in Elasto-Plastic Materials in Impact Fretting Wear

Normal and oblique impact wear processes are characterized by unique features, which include the development of some residual stress components that vanish in unidirectional sliding. Parametric finite element analyses were conducted to estimate the likelihood locations for crack initiation, and the subsequent direction and rate of crack propagation in an elasto-plastic material with bi-linear isotropic hardening properties. The results showed that the increase in contact pressure can cause a significant increase in the size of the plastically deformed crack initiation zone and allows it to reach the surface. Such behavior is not predicted under continuous sliding conditions. The presence of surface friction forces in oblique impact, can also result in the development of a secondary region of high tensile stresses at the contact area. Using the crack tip slip displacement CTSD method, the rate of crack growth was found to be linearly proportional to the crack length, and significantly dependent on the contact pressure and the coefficient of friction at the crack surface. The small effect of the coefficient of friction at the micro-contact area on wear suggests that the effect of shear traction is mainly due to the increase in the depth of the crack nucleation zone. As expected, the increase of the material flow stress with strain-hardening has a wear reducing effect.

Deformation characteristics of Mg 60Cu 30Y 10 alloy at temperatures near Tg

The mechanical behaviour of nanocrystallized samples with a crystalline volume fraction of 15%, 50%, 75% and 100% was investigated by constant compression rate tests at different strain rates at 430 K and shows an increase in yield or flow stress provided that their crystalline volume fraction were less than 50%.

Notch Effects on Impact Tensile Properties in A6061-T6 Aluminum Alloy

The strain rate sensitivity of a A6061-T6 aluminum is examined as a function of triaxial stress. The triaxial stress state is changed by changing the notch acuity of notched specimens. Under the high strain rate conditions, increasing triaxiality increases flow stress. Plastic constraint factor decreases in notched specimens under high strain rate conditions. Reduction-of-area in notched specimen increases with increasing strain rate. The relaxation of plastic constraint takes place under high strain rate in the notched specimens. The effects of triaxial stress and strain rate on the fracture surfaces are discussed.

Phenomenological behaviour of long and short chain polypropylenes under tension

Long and short chain polypropylenes, designated PP-L (long chains with lower melt flow index) and PP-R (short chains with higher melt flow index), were subjected to tensile deformation at strain rates in the range 10-5-10-1 s-1 to examine microstructural evolution and tensile flow behaviour as a function of strain and strain rate. Scanning electron microscopy was used to characterise surface deformation processes and fracture surfaces of the polypropylenes. The surface of tensile deformed PP-L was characterised by closely spaced crazes. The array of crazes multiplied with an increase in strain and strain rate and grew inwards. The final separation occurred by tearing along the individual bundles of crazes growing from opposite faces, resulting in crazing–tearing at the edge and a brittle mode of fracture in the centre. In PP-R, the predominant mode of deformation at all strain rates was wedge cracking. Additionally, ductile ploughing was observed at low strain rates and less pronounced crazing at all strain rates investigated. Final fracture at various strain rates occurred through a combination of wedge cracking and brittle fracture, connected by plastic flow, around the wedge cracks. The modes of deformation are summarised and depicted in terms of rate of mechanical deformation–strain diagrams, providing a broad perspective of deformation processes operating in different deformation rate–strain regimes. The modes and domains of deformation processes varied depending on the physical and mechanical characteristics of the material (long chain and short chain). Regarding the tensile flow behaviour, the yield stress increased linearly with an increase in strain rate, and followed the thermal activation concept, with an activation volume consistent with the thickness of the lamellar crystallite. True stress–true strain plots indicated an increase in flow stress with increase in strain rate. Depending on the melt flow conditions, the flow behaviour of the two polypropylenes exhibited differences in strain rate sensitivity.

Dynamic simulations of the interaction between dislocations and dilute particle concentrations in metal–matrix composites (MMCs)

3D Dislocation Dynamics (DD) simulations are presented which capture the interaction between gliding dislocations in a ductile metallic matrix and nearby dispersed spherical particles. Here the strengthening effect arises from the stress field caused by the particle-matrix mismatch, and no image stress caused by the particles is considered. A radial misfit strain of 1%, simulating thermal processing induced elastic field in aluminum matrix-silicon carbide particulate composite system, is used. Attention is limited to composites with reinforcement concentrations smaller than about 3.5% and with particle sizes smaller than about 0.8 μm. The simulations show dislocation storage in the matrix, in the form a pileup of glide loops near the particle and as part of stress relief, which in turn exerts a back stress on upcoming gliding dislocations. This manifests itself in the simulations in increased flow stress as demonstrated by the stress–strain curve. The results show increased flow stress with increased particle volume fraction as would be expected. However, flow stress is also sensitive to other microstructural features such as particle radius and the distance between the dislocations and particles, and to a lesser extent to the spatial arrangement between dislocations and particles. Overall, the calculated strengthening effect is comparable with that found in experiments.

Structure adaptation and blood flow control in the uterine arterial system after hemochorial placentation

In the course of hemochorial placentation, trophoblast cells build a most conductive placental stream bed for maternal blood with intramural foetal capillaries. The stream bed is adapted to the maternal vascular system, causing local increase in flow and shear stress. Subsequently, local maternal microvascular system vanishes. The remaining upstream arteries, up to the aorta, undergo “physiological changes”, i.e. increase in circumference and length, predominantly by growth and remodeling. Peripheral uterine arteries, possibly collapsed due to low intraluminal pressure, show over-dilatation associated with endothelium destruction and trophoblast cell invasion. Overall arterial conductance increases to controlled extent; placental blood flow rate rises moderately. Intraplacental blood pressures are kept low allowing for free perfusion of placental foetal capillaries. According to the generally accepted concept, destructing impact of invading trophoblast cells causes the physiological changes. However, trophoblast invasion does not explain physiological changes occurring before and outside trophoblast invasion. On the other hand, physiological changes may well be induced by shear stress, which operates in extrauterine body regions as signal for vascular adaptation to growth. Thus, it is hypothesized that trophoblast cells control vascular adaptation and placental blood flow rate simply by constructing the hemochorial steam bed, thereby building up shear stress and triggering the general physiological automatism for controlled vascular adaptation to growth.

The Role of Hydrostatic Pressure in Severe Plastic Deformation

AbstractThe contribution presents several experimental examples which show that the presence of an enhanced hydrostatic pressure—as compared to conventional large deformation modes—is one of the main features of severe plastic deformation (SPD). At the example of systematic high pressure torsion experiments with Cu at room temperature, strength measurements after deformation showed that the onset strains of deformation stages III, IV, and V are not affected by the pressure applied; however, the related onset flow stresses increase by at least 10 % of the values of low pressure torsion, per GPa of pressure increase. During deformation, increases of flow stresses by at least 40 % of the values of low pressure torsion, per GPa of pressure increase, have been found. From comparisons with tests on Ni, the increases appear to grow with the materials melting temperature. For a theoretical explanation of flow stress increases the pressure induced changes of i) the elastic moduli, and ii) the formation energy of lattice defects. While contribution i) is almost negligible, contribution ii) accounts for an increase of flow stress during deformation by about 15 % per GPa of pressure increase. The difference left to experiment has to be attributed to a third contribution, i.e., the pressure specific evolution of the structure. For this contribution, a modification of the model of Zehetbauer and Les[1–3] is introduced which is based on the pressure caused decrease of lattice diffusion. The latter is thought to restrict the diffusion controlled annihilation of dislocations, thus leading to a higher density of vacancies, dislocations and/or grain boundaries causing the higher stress level observed.

On the mechanism of dislocation creep of calcite at high temperature: Inferences from experimentally measured pressure sensitivity and strain rate sensitivity of flow stress

Previous laboratory experiments on coarse grained calcite materials at intermediate conditions of stress and temperature have shown low strain rate sensitivity of flow stress, expressed by standard power law stress exponents n ≥ 7. This conflicts with conventional models for creep controlled by dislocation climb (n = 3–4.5), a recovery mechanism widely used to explain steady‐state dislocation creep. This paper addresses the question whether dislocation cross slip rather than climb is the mechanism controlling creep of calcite at intermediate conditions. New uniaxial compression tests were performed on calcite single crystals and Carrara marble at T = 800–1000°C, = 10−4–2 × 10−7 s−1, and P = 100–600 MPa. Emphasis was on the pressure (P) and strain rate () sensitivity of flow stress, because these form potential ways for discriminating between climb and cross slip models. Both single crystals and Carrara marble demonstrated a small increase in flow stress with increasing pressure (∼1.6% per 100 MPa) at constant strain rate and temperature. The low strain rate sensitivity of flow stress was confirmed, though n gradually changes with stress or temperature rather than taking a constant value. Evaluation of the experimental data against microphysical models indicated that cross slip controlled by dissociation of dislocations offers the best explanation for the observed mechanical behavior. If the associated creep equation is used to estimate flow stresses under natural conditions, the resulting values at T < 500°C are substantially less than those following from previous flow laws. The influence of pressure on the behavior of marble, however, can be safely ignored for most practical purposes.

Hot and warm forming of 2618 aluminium alloy

The hot and warm formability of 2618 aluminium alloy, in the as-solutioned condition, was investigated in extended ranges of temperature and strain rate by means of torsion testing. Precipitation was found to occur during deformation. The effect of the precipitation of second phase particles, occurring during deformation, on the flow curve shape and on the stress level was evaluated. At the lowest temperatures, precipitation, during deformation, resulted in a continuous increase of flow stress; by contrast, when testing temperature exceeded 250 °C, precipitation and subsequent coarsening of precipitates, during deformation, resulted in a peak of flow curves, followed by softening. The constitutive equations of the material were calculated and the ductility of the alloy was evaluated as strain to fracture values in all the tested condition.

An analysis of thermo-mechanical treatments of a 2618 aluminium alloy: study of optimum conditions for warm forging

The warm formability of a solution-treated 2618 aluminium alloy was investigated by means of torsion tests at temperatures between 423 and 573 K. Dynamic precipitation took place during deformation raising the flow stress and lowering ductility. The flow stresses were substantially higher than those observed by testing the same alloy in the as-extruded state. At the lowest temperatures dynamic precipitation resulted in a continuous increase of flow stress, while above 500 K dynamic precipitation and subsequent coarsening of precipitates resulted in a peak of equivalent-stress versus equivalent-strain curves, followed by softening. These differences in deformational behaviour were attributed to the nature of precipitates, as well as to different kinetics of the ageing processes at each testing temperature.

Lattice defects and their influence on the deformation behavior of single crystals of TiAl

The precipitation behavior of the Al 5Ti 3 phase in the TiAl matrix as well as its influence on the mechanical properties of TiAl has been investigated for single crystals with compositions of Ti–54 and –56 at.% Al. The precipitation tendency for the Al 5Ti 3 phase is largest at approximately 40 °C for both Ti–54 and –56 at.% Al. The precipitation behavior of the Al 5Ti 3 phase is unique since the Al 5Ti 3 phase once precipitated dissolves in the TiAl matrix upon prolonged annealing, resulting in the initial and final phases being the same TiAl phase. Al 5Ti 3 precipitates and short-range-ordering of the Al 5Ti 3-type increase the flow stress of TiAl with the extent of increase in flow stress being larger for 〈110〉 ordinary slip than for 〈011〉 superlattice slip. Both the flow stress and the deformation structure are reversible with respect to changes in deformation temperature, if Cottrell–Stokes type experiments are conducted very carefully so as to minimize the time duration for which the specimen is exposed to the temperature of 400–500 °C where the tendency for Al 5Ti 3 precipitation is largest.

Damage and size effect during superplastic deformation

Abstract Superplastic forming is a valuable metal working technique because of the extreme ductility that can be achieved. However, it is limited in application due to the presence of small voids that grow and coalesce during the forming process, often causing premature failure. In order to understand and control this phenomenon accurate constitutive models must be developed which account for void parameters that affect the macroscopic behavior of the material. This paper looks specifically at the effect of void size and spacing on the ductility and flow stress of viscoplastic materials. Based on the gradient-dependent theory of plasticity, a model is proposed that accounts for size effects by incorporating strain gradient terms into a continuum based constitutive equation. Both experimental testing and finite element (FE) modeling were performed on Pb–Sn, tensile specimens with small holes drilled in them in random patterns. The experimental tests indicate that a decrease in void size results in an increase in ductility. The FE results demonstrate that the gradient terms strengthen the material by diffusing the strain in areas of high strain concentration and delay failure by slowing void growth. In addition, the model predicted an increase in ductility and flow stress with decreasing void size.

Quasistatic and dynamic compression of aluminum-oxide particle reinforced pure aluminum

Pure aluminum matrix composites reinforced with 40–55 vol.% Al 2O 3 particles of various sizes (5, 10, 29, and 58 μm) are produced by gas-pressure infiltration. Comparison of compressive flow stresses of these composites at quasistatic and dynamic strain rates shows that, in accord with the literature, the increase in flow stress of dynamically compressed composites results from the sensitivity of the matrix to strain rate. The accumulation of damage in the composites is quantified through high-precision density measurements. Damage accumulates primarily as a function of strain due to particle cracking followed by separation of the broken-particle segments and, to a lesser extent, by matrix cavitation. Composites reinforced by smaller particles have a higher flow stress, lower strain-rate sensitivity, and accumulate less damage, while a greater concentration of reinforcement increases the flow stress, strain-rate sensitivity, and the rate of damage accumulation.

Effect of reaction on the tensile behavior of infiltrated boron carbide–aluminum composites

Gas pressure assisted liquid metal infiltration was employed in the fabrication of various high volume fraction B 4C–Al composites. Two reaction products were observed and identified in the composites, namely AlB 2 and Al 3BC. An increase in total volume fraction of these phases results in an increase in flow stress of the composites, concomitant to a decrease in ductility. The latter was shown to be related to an accelerated accumulation of damage, which we quantified by monitoring the stiffness of the composites as a function of strain. A simple relationship is used to quantitatively link internal damage accumulation and tensile ductility in the present composites.

Deformation behaviour of a prestrained duplex stainless steel

The anisotropic flow behaviour due to a change in strain path during uniaxial deformation of a duplex stainless steel has been investigated. The anisotropic flow behaviour of the as-received material could be predicted from the crystallographic texture. However, it was found that prestraining introduces a transient work hardening behaviour during the second stage deformation, which causes an anisotropic flow behaviour immediately after yielding that cannot be described by the crystallographic texture. Samples subjected to a coaxial second stage deformation show a small increase in flow stress and similar work hardening rate as in the monotonic loading. Samples subjected to noncoaxial second stage deformation show a pronounced transient stage characterised by early yielding and a high work hardening rate in the first part of the transient stage. Beyond the earliest stage of deformation a higher flow stress and a lower work hardening rate compared to monotonic loading is observed for the noncoaxial samples. The transient phenomena are discussed based on the measured internal stresses and the differences in dislocation structures in the two different phases.

Microstructural gradients in the superplastic forming of Ti-6Al-4V

The effect of microstructural gradients, introduced by local induction heating, on the behavior of a Ti-6Al-4V sheet in superplastic forming has been investigated. Heat treatment led to a change in the morphology of the α phase present at the start of superplastic deformation, which caused an increase in initial flow stress. This has a significant effect on the strain distribution. Trials using two axially symmetric shapes showed that the effect of microstructural gradients can be adequately predicted—via numerical modeling—and controlled. The technique has the potential to control the thickness distribution in formed parts.

Strain Hardening in Superplastic Codoped Yttria-Stabilized Tetragonal-Zirconia Polycrystals

The superplastic characteristics of 3-mol%-yttria-stabilized tetragonal-zirconia polycrystals (3Y-TZP) were drastically improved by the combined addition of germanium oxide and other oxides (titanium, magnesium, and calcium), resulting in up to 1000% elongation under optimum conditions. Extensive work hardening of the codoped compositions for temperatures >1400°C was observed. Dynamic grain growth was not a sufficient mechanism to explain this anomalous high strain hardening. The appearance of a cubic phase that caused abnormal grain growth during testing may have resulted in increased flow stress, which resulted in exaggerated work hardening.

Length scale and time scale effects on the plastic flow of fcc metals

We examine size scale and strain rate effects on single-crystal face-centered cubic (fcc) metals. To study yield and work hardening, we perform simple shear molecular dynamics simulations using the embedded atom method (EAM) on single-crystal nickel ranging from 100 atoms to 100 million atoms and at strain rates ranging from 10 7 to 10 12 s −1. We compare our atomistic simulation results with experimental data obtained from interfacial force microscopy (IFM), nano-indentation, micro-indentation and small-scale torsion. The data are found to scale with a geometric length scale parameter defined by the ratio of volume to surface area of the samples. The atomistic simulations reveal that dislocations nucleating at free surfaces are critical to causing micro-yield and macro-yield in pristine material. The increase of flow stress at increasing strain rates results from phonon drag, and a simple model is developed to demonstrate this effect. Another important aspect of this study reveals that plasticity as reflected by the global averaged stress–strain behavior is characterized by four different length scales: (1) below 10 4 atoms, (2) between 10 4 and 10 6 atoms (2 μm), (3) between 2 μm and 300 μm, and (4) above 300 μm.