Sharp Yield Point Research Articles

Yield-Point Phenomena of Ti-20V-4Al-1Sn at 1073 K and Its Constitutive Modelling

The deformation behaviour of β titanium alloy Ti-20V-4A1-1Sn sheet at 1073K was investigated by performing uniaxial tension experiments. The stress-strain curves show a sharp yield point and the subsequent abrupt yield drop followed by the strain softening. From the EBSD analysis, a crystallographic misorientation was found in grains even at an early stage of yielding, even though the dynamic recovery had not yet taken place. To describe such a characteristic stress-strain response, a viscoplastic constitutive model is proposed that is built on the premise that the yield point phenomena are associated with the rapid dislocation multiplication at an early stage of yielding.

A plasticity model describing yield-point phenomena of steels and its application to FE simulation of temper rolling

To describe the yield-point phenomena of steels, an extended version of the first author’s model (Yoshida, F., 2000. A constitutive model of cyclic plasticity. International Journal of Plasticity 16, 359–380) is proposed on the premise that the material behavior of sharp yield point and the subsequent abrupt yield drop result from a rapid dislocation multiplication and the stress-dependence of dislocation velocity. A specific feature of this model is that it describes well a high upper yield point, the rate-dependent Lüders strain at the yield plateau and the subsequent workhardening, as well as cyclic plasticity characteristics, such as the Bauschinger effect and rate-dependent ratcheting. Using this model, an FE simulation of temper rolling process is conducted in order to clarify its role for the elimination of the yield point of steel sheets. Particularly, the effect of upper yield point on the deformation characteristics in the process is discussed.

Directionality of yield point in strain-aged steels: The role of polar dislocations

The directionality of the sharp yield point in strain-aged steels has been investigated by modeling tension/compression and forward/reverse torsion tests separated by accelerated aging. The occurrence of a Bauschinger effect and the absence of a yield point after a forward straining–aging–reverse straining sequence are interpreted within the framework of a field dislocation theory coupling the evolution of statistical and polar dislocation densities with that of point defects due to strain aging. The polar dislocation density reflects lattice incompatibility and long-range internal stresses. By assisting yielding in reverse straining, the associated back-stress is seen as the origin of the Bauschinger effect. By also promoting dislocation unlocking, the back-stress is found to be responsible for the absence of a yield point in reverse straining. Polarized dislocation structures formed in forward straining in association with back-stress build up may annihilate and inverse polarization occur in reverse straining. This microstructure evolution translates into an inflexion of strain hardening after strain path reversal.

Sub-structure strengthening and work hardening of an ultra-fine grained aluminium–magnesium alloy

The mechanical properties of an Al–Mg1 alloy, processed to obtain grain sizes in the range from less than a micrometer to relatively coarse sizes, have been investigated by tensile and compression testing. A characteristic feature of the stress–strain curves of fine-grained Al–Mg-alloys is a sharp yield point followed by Lüders-band elongation. The flow stress at each given strain level followed a Hall–Petch type grain size dependence. However, the slope of the Hall–Petch curve is larger at the yield point than at strains beyond the yield-point elongation. The compression testing revealed a new phenomenon, namely that the range of stage-III deformation decreased with decreasing grain size, and disappeared completely for a grain size of about 0.5 μm.

Segregation and wetting transition at dislocations

We investigate solute segregation and wetting transition at dislocations and the corresponding drag effect on dislocation glide using a continuum model developed previously for grain boundary and based on gradient thermodynamics. The dislocation core structure and stress field are described by the newly developed phase field model. This study differs from much previous work because it takes into account not only the long-range elastic interactions but the short-range chemical interactions between solute atoms and dislocation core as well as among solute atoms themselves. The latter leads to the prediction of a wetting transition at the dislocation core with respect to varying temperature, solute concentration, or dislocation velocity. The transition temperatures obtained during heating and cooling are different from each other, leading to a hysteresis loop in the solute concentration-temperature plot and the solute concentration-velocity plot. These predictions could provide new insights into the phenomena of sharp yield point drop and strain aging observed in metal alloys.

Modelling grain boundary strengthening in ultra-fine grained aluminum alloys

The mechanical properties of aluminum alloys with grain sizes in the range from less than a micron (ultra-fine) to hundreds of microns have been modelled within the framework of the multi-parameter microstructural work hardening model developed by Nes and co-workers [E. Nes, Prog. Mater. Sci. 41 (1998) 129–194; K. Marthinsen, E. Nes, Mater. Sci. Technol. 17 (2001) 376–388; E. Nes, K. Marthinsen, Mater. Sci. Eng. A322 (2002) 176–193; E. Nes, K. Marthinsen, B. Holmedal, Mater. Sci. Technol. B 20 (2004) 1377–1382]. The effect of grain size on the flow stress and work hardening, including a deviation from the Hall–Petch grain size dependency for ultra-fine grain sizes, is well accounted for by the model. A characteristic feature of the stress–strain behaviour of Al–Mg-alloys and ultra-fine grained variants is a sharp yield point followed by Lüders-band elongation. A mechanism for such an elastic–plastic transition is suggested.

Methodological concerns using intra-cortical pins to measure tibiofemoral kinematics.

The complexity of human tibiofemoral joint motion is now better understood with the advancement of new methodologies to measure tibiofemoral kinematics in vivo. Marker clusters anchored to stainless steel bone pins inserted directly into the femur and tibia provide the most sensitive and accurate means for directly measuring skeletal tibiofemoral joint motion. Despite its invasiveness, this technique has been successful, although complications have been reported with the femoral pin and its insertion site. The purpose of this technical report is twofold: to review the difficulties with the femoral pin and its insertion site from a historical perspective, and to identify the load force required from biological tissue to permanently deform the pin. In addition, proposals in the advancement of this method are discussed in the context of reducing impingement with the femoral pin and the Iliotibial band. Because stainless steel exhibits plastic behaviour with no sharp yield point, Apex self-drilling/self-tapping bone pins underwent incremental loading on an Instron materials testing machine. Loads were transmitted perpendicular to the pin with the threads partially exposed and fully secured in vice. Since the accuracy of our combined stereophotogrammetry and Optoelectric motion analysis was less than 0.4 mm, it was decided that plastic deformation occurred after deflections of 0.4 mm. With exposed threads, deflections larger than 0.4 mm were observed at 150 N and 100 N when loads were applied at 15 mm and 20 mm from the vice (representative of where the tissue came in contact with the pin). Loads greater than 200 N produced deflections less than 0.2 mm when threads were fully inserted. The 90 Hz resonant frequency for the marker cluster-bone pin complex is beyond the spectrum of human movement and can be lowpass filtered. To reduce impingement and pin bending, one solution may be to implant pins with a shorter threaded section. By completely penetrating the bone, only the smooth surface of the pin is exposed which is more resistant to bending. Otherwise pins with larger diameters and longer longitudinal incisions about the femoral insertion site are an alternative. Lengthening the longitudinal incisions about the insertion site, and correctly aligning and inserting the femoral pin between the Iliotibial band and quadriceps tendon may diminish impingement. Performing dynamic open chain flexion and extension movements while on the operating table may aid in aligning the pin at the incision site. This may stretch the IT band and quadriceps tendon and may guide the femoral pin into a more optimal position prior to it being inserted into the cortex of the bone.

Influence of austenitisation temperature on the structure and properties of weather resistant steels

The influence of austenitisation temperature on the structure and properties of three experimental weather resistant steels has been studied. All these steels contain 1% Mn, 0.3% Ni, 0.47% Cr and 0.47% Cu. In addition, steel #1 has 0.1% C, 0.1% P, steel #2 has 0.1% C, 0.05% P and 0.024% Nb while steel 3 has 0.2% C, 0.054% Nb and 0.046% V. It has been found that the hardness, yield strength and tensile strength do not change significantly with austenitisation temperature over the range 900–1200 °C for steel #1 but they increase considerably when austenitised above 1000 °C for steels 2 and 3. Similarly, the ductility decreases with increasing temperature of austenitisation. All the steels austenitised up to 1000 °C exhibit sharp yield points. None of these steels shows sharp yield point after 1200 °C. At 1100 °C, however, sharp yield points were observed in steels 1 and 2. There has been a noticeable change in optical microstructure. In steels 2 and 3 the pearlite is gradually replaced by granular bainite when austenitised above 1000 °C. The transmission electron microscopy study reveals that the granular bainite consists of acicular ferrite and martensite/austenite constituent.

A theory of sharp yield point in low-dislocation crystals

The condition for yield drop existence in stress-strain curves for crystals with an initially low dislocation density is derived. A new theory that radically differs from the well-known Alexander-Haasen model is proposed to describe the stressed state of the crystal near the lower yield point.

Aspects of Grain Size Strengthening in Polycrystals

ABSTRACTThe strengthening effect of grain boundaries is well established and observed experimentally as the Hall-Petch relationship. In this paper different mechanisms proposed in the literature to explain the observed Hall-Petch effect are reviewed critically. The fundamental implications of the different approaches are discussed with reference to experimental data for two different classes of materials;-Materials with locked dislocations, i.e. with a sharp yield point behaviour.-Materials without locked dislocations, i.e. with a smooth yielding behaviour.It is shown that a simple model (Bergström) can be used to understand the grain size strengthening in the latter class of materials while more work is needed to quantitatively understand the behaviour of materials showing a sharp yield point.

Investigation of boron-enriched Cottrell atmospheres in FeAl on an atomic scale by three-dimensional atom-probe field-ion microscopy

In 1949, on the basis of theoretical considerations, Cottrell proposed the concept of 'atmospheres' (called later by his name) to explain some specific behaviour of materials during plastic deformation, such as sharp yield-point formation or the Portevin-LeChatelier effect. In this letter, atomic-scale observations and three-dimensional analyses of a Cottrell atmosphere are reported. They have been performed by three-dimensional atom-probe field-ion microscopy techniques. The ability of this new experimental method to provide atomic-resolution images, both structural and chemical, was confirmed; the basic stacking structure of (001) planes in FeAl could be visualized with success. Moreover the presence of a <001> edge dislocation was also detected in the analysed zone. Further, B enrichment was measured in the vicinity of this defect; the B-rich region appeared as a pipe 5 nm in diameter, parallel to the dislocation line. The concentration of B in the core reached 3 at.%; this local enrichment in boron was accompanied by an Al depletion of more than 10 at.%. Boron in FeAl has a well known tendency to segregate to internal interfaces. In this letter, we show experimental evidence of the solute segregation to dislocation lines. The observed effects of this segregation on mechanical properties of FeAl, both at room temperature and high temperatures, are also discussed.

Cyclic Plasticity Behavior of Mild Steel with Special Emphasis on Yield-Point Phenomena.

Annealed steels exhibit a sharp yield point and the subsequent abrupt yield drop followed by the yield plateau (Luders deformation) in their stress-strain curve of uniaxial tension. This paper provides some typical examples of experimental data of cyclic plasticity with special emphasis on the yield-point phenomena for an annealed mild steel, i.e., uniaxial tension at several crosshead speeds;cyclic straining;stress- and strain-controlled ratchetting. In order to describe the stress-strain response characteristics such as the yield-point phenomena, cyclic hardening/softening, the Baus-chinger effect and rate-dependent ratchetting, constitutive modelling of cyclic elasto-viscoplasticity is discussed on the premise that the sharp yield point and the subsequent abrupt yield drop result from the rapid dislocation multiplication and the stress-dependence of dislocation velocity.

A constitutive model of cyclic plasticity

Abstract This paper addresses a constitutive model of cyclic plasticity with special emphasis on the yield-point phenomena. In order to point out the deformation characteristics of a mild steel, four types of experiments were conducted, i.e. uniaxial tension at several crosshead speeds, cyclic straining, and stress- and strain-controlled ratchetting. A viscoplastic constitutive model of cyclic plasticity is proposed on the premise that the phenomena of sharp yield point and the subsequent abrupt yield drop result from rapid dislocation multiplication and the stress-dependence of dislocation velocity. Besides, cyclic plasticity behavior, such as the Bauschinger effect, cyclic hardening/softening characteristics and ratchet-strain accumulation, is described by some kinematic and isotropic hardening rules. The cyclic stress–strain responses predicted by this model agree well with the corresponding experimental results.

The influence of C and Si on the flow behavior of NiAl single crystals

Alloys based on the intermetallic compound NiAl are considered potential replacements for Ni and Co-based superalloys in high temperature structural applications due to their excellent oxidation resistance, low densities, high thermal conductivities, and increased melting points. Unfortunately, NiAl exhibits low tensile ductility at room temperature and low strengths at elevated temperatures which have combined to hinder its development. Recent efforts, have revealed that NiAl in the presence of sufficient solute levels, is subject to the phenomenon of strain aging which manifests itself as: sharp yield points, abnormally low strain rate sensitivities (SRS), plateaus or peaks in yield stress and work hardening rate as a function of temperature, flow stress transients upon an upward change in strain rate, reduced tensile elongations at elevated temperatures, and serrated stress-strain curves. Though recent efforts via either alloying or the removal of interstitial impurities, have resulted in consistent room-temperature tensile elongations exceeding 5% and the elimination of serrated flow, the effects of particular substitutional and interstitial elements and the mechanisms by which they might enhance or hinder the mechanical properties remain unknown. Consequently, the purpose of the present paper is to provide a preliminary assessment of the influence of common substitutional and interstitial impurities on the deformation behavior of NiAl. To accomplish this goal a series of NiAl single crystal alloys containing various interstitial solutes were prepared and their mechanical properties were evaluated between 77 and 1100 K. Because Si is a common impurity in conventional purity single crystals grown by the Bridgman method, Si concentrations were also varied in order to determine the influence of this element.

Strain aging and breakaway strain amplitude of damping in NiAl and NiAlZr

Extruded NiAl and NiAlZr alloys often show discontinuous yielding on strain aging in compression at room temperature. Two sets of experiments were conducted to understand the reasons for this yield-point behavior. First, strain-aging experiments were carried out on NiAl alloys containing O to 0.1 at. % Zr. The specimens were all deformed in compression at room temperature at a nominal initial strain rate of 1.1 × 10−4S−1, and the effect of annealing at 700 and 1200 K on the stress-strain curves and the yield strength was studied after an initial prestrain. While annealing at 700 and 1200 K consistently reduced the yield strength of both NiAl and NiAlZr, the effects were quite different. In the case of NiAl, annealing at 1200 K did not result in discontinuous yielding, whereas it generally resulted in a sharp yield point for the Zr containing alloys. Second, the PUCOT (piezoelectric ultrasonic composite oscillator technique) was used to measure the dynamic Young modulus, breakaway strain amplitude, and damping for the alloys. Only small differences were observed in the values of Young's modulus, but the breakaway strain was at least a factor of 2 to 3 lower for NiAl than for NiAlZr. The experimentally determined values of damping were used in the Granato-Lücke model to estimate the binding energy for NiAl. While the binding energy values were found to be in agreement with the calculated values of dislocation kink nucleation and migration energies in this material, to within an order of magnitude, other effects, such as dislocation pinning by quenched-in vacancies, cannot be ruled out. The observations made in this study suggest that the yield-point behavior in NiAl may be due to several factors, such as difficulties in double kink nucleation, and single kink migration, as well as dislocation-vacancy interactions; whereas, the yield-point behavior in the Zr-alloyed material is due at least in part to dislocation-solute interaction.

Flow curves of an adsorbed protein layer at the saliva-air interface

At the air-liquid interface of human saliva a protein layer is adsorbed. An apparatus is described with which a flow curve of this layer was measured. In the majority of samples the viscosity of the surface layer changed gradually and could be described by a power-law dependence on the shear rate. The zero-shear viscosity was 1–100 MPa·s. In some saliva samples a sharp yield point was observed.

The yield and flow stress of dilute CuCd and CuMg alloys

The results of tensile tests on dilute high conductivity CuCd alloys with a range of cadmium contents are presented. It is shown that, above about 0.6 wt.% Cd, dislocation pinning is strongly developed, giving apparently linear elastic deformation up to the sharp yield point. The strength of dislocation pinning is also reflected in a pronounced grain size dependence of the yield and flow stress which is maintained up to 30% strain. At lower cadmium contents the stress-strain behaviour is more like that observed in copper. The effect of the cadmium is associated with the degree of saturation of dislocation cores. Dislocation pinning also gives rise to strain-aging effects like those observed in iron alloys but, in contrast with iron, pinning has to compete with recovery at the high homologous aging temperature, thereby causing a peak in the aging curves. The strain hardening of CuCd is also marked at large strains in contrast with copper and, again, the effect is due to a combination of dislocation pinning and dynamic strain aging. For comparison, the results of tests on CuMg alloys are reported. Like cadmium, magnesium has a valency of 2, which is favourable to electrical conductivity, but it is shown that the dislocation pinning is not strongly developed in CuMg alloys.

The Physical Properties of the Pedal Mucus of the Terrestrial Slug, Ariolimax Columbianus

ABSTRACT The pedal mucus of gastropods functions in locomotion by coupling the movements of the foot to the substratum. The pedal mucus of the terrestrial slug, Ariolimax columbianus, is suited to this role by the following unusual physical properties. At small deformations the mucus is a viscoelastic solid with a shear modulus of 100–300 Pa. The mucus shows a sharp yield point at a strain of 5–6, the yield stress increasing with increasing strain rate. At strains greater than 6 the mucus is a viscous liquid (η = 30–50 poise). The mucus recovers its solidity if allowed to ‘heal’ for a period of time, the amount of solidity recovered increasing with increasing time.

The yield and flow stress of Cu1% Cd alloy

The tensile properties of a commercial copper-1% cadmium alloy have been investigated at 77, 300 and 420 K. The presence of cadmium produces a sharp yield point similar to that observed in mild steel and in aluminium-magnesium alloys. The yield and flow stresses of the material were found to obey the Hall-Petch relation and it is concluded that cadmium strengthens copper mainly by increasing the grain-size-dependent component of the strength. The alloy exhibits strain ageing. The interaction between a cadmium atom and dislocations in the alloy is considered and it is shown that the atomic size misfit interaction is dominant. It is concluded that cadmium atoms can strongly pin both dissociated edge and screw dislocations in copper. The value of the grain-size-dependent component of the yield strength at low temperatures and its temperature dependence suggest that the critical event at yield is the unpinning of dislocation sources within the grain boundaries. It also appears that this process is still important in the region of homogeneous deformation at higher strains, in contrast to what occurs in copper, where the grain-size-dependent component of the flow stress diminishes markedly with increasing plastic strain. A possible mechanism for the persistence of a strong grain-size effect in the alloy is suggested.

The effect of hydrogen on the mechanical properties of high purity iron II. Effect of quenched-in hydrogen below room temperature

High purity iron specimens prestrained to 1–2% were hydrogen charged at room temperature and quenched to testing temperatures below room temperature. The yield stress of specimens with a residual resistivity ratio (RRR H) of 4600 or larger was decreased at and above 200 K and increased below 190 K. At 200 K a sharp yield point was observed in specimens with RRR H = 5500. After about 0.5% yield strain a rapid work hardening took place and the flow stress soon exceeded that of uncharged specimens. In low purity specimens (RRR H ≈ 3500) the yield stress was always increased by hydrogen. The effect of hydrogen disappeared on aging for 10–20 min at room temperature after charging. The above observations for the specimens with RRR H = 4600 or larger are satisfactorily explained by two hypotheses: (1) hydrogen atoms trapped at the screw dislocation core increase the mobility of screw dislocations; (2) edge dislocations introduced by prestraining are pinned or heavily dragged by hydrogen atoms in the core below 190 K.