Yield Point Phenomenon Research Articles

Al-Ti合金における降伏点現象

A yield point phenomenon was observed in Al-0.16 wt%Ti alloy when specimens were stretched at room temperature after cold working and annealing under the limited conditions, for example, 1 hr at 280 to 300°C. The phenomenon was most clearly observed in a specimen annealed at 280°C rather than at higher temperatures. Electron microscope observation revealed that the annealed structure of the specimen consisted of very fine subgrains of a few microns, inside of which the dislocation density was low. Therefore, it is concluded that, in addition to locking of dislocations by added titanium, the fine subgrain structure contributes greatly to the phenomenon.

On elimination method of yield point phenomena in aluminum-magnesium alloy sheets

A method to eliminate stretcher-strain marking (random marking), appearing mostly in lightly strained regions, was studied in various sheets of Al-Mg alloys. Further, an aging characteristic and mechanical properties of the alloys sheets which had been subjected to the elimination method were investigated. The results obtained were as follows:(1) Recrystallized fine grained sheets of Al-Mg alloys, which developed random marks during stretching processes, all showed yield points in stress-strain curves at tensile tests.(2) A practical method to eliminate yielding phenomena was found. It consisted of water quenching from 350°C to 400°C.(3) The sheets, subjected to the above treatment, did not develop significant aging.(4) The higher the Mg content, the higher "n" values were obtained in the heat-treated sheets. The addition of Cu in the alloys inhibited grain growth. These results suggest that Al-Mg-Cu alloys with high Mg contents are suitable for forming use.

A note on yield point phenomenon in wave propagation

The propagation of stress pulses in a plastic material which exhibits delayed yielding and upper and lower yield points is considered by the method of expansion behind the wave front. The material model includes an evolutionary equation the presence of which is responsible for the yield phenomena and it is shown that it also leads to a very pronounced negative stress gradient behind the wave front.

Stresses and Strains

Yield Point Phenomena in Metals and Alloys By E. O. Hall. Pp. viii + 296.(Macmillan: London, July 1970.) 76s.

High temperature work softening yield point phenomena in polycrystalline aluminum

Work softening yield points have been studied in high purity polycrystalline aluminum tested in the temperature rangeT2 = 100°to 450°C at constant strain rate subsequent to prestraining in tension at a lower temperature (T1) = room temperature or 350° C) or higher strain rate. The yield drops increase with prestrain as well as with the difference between the flow stresses at the test and prestrain temperature. Reductions in the yield drop caused by annealing atT2 prior to testing indicate that the yield drops are the result of dynamic recovery. It is shown that dynamic recovery is accelerated by concurrent straining.

Plastic Deformation of Corundum Single Crystals

AbstractThe stress‐strain curves and the defect structure of corundum single crystals (sapphire and ruby) were studied. The influence of impurity (Cr) presence, crystallographic orientation, temperature, and deformation rate was investigated. Chromium makes corundum harder and causes a yield point phenomenon. The yield point was also increased by the transition from 60° to 90° orientation of the specimens, by lowering the temperature, and by an increase in the deformation rate. In 60°‐specimens the deformation occurs by means of gliding on basal planes in 〈1120〉 and 〈1010〉 directions. In 90°‐samples beside this one gliding in (1010), (1011), (2021) and (2243) is found.

Yield-point phenomena in substitutional alloys

Yield-point phenomena in substitutional alloys

Yield-point phenomena in substitutional alloys

A material exhibits a yield point when a larger stress is required to initiate deformation by slip than to continue it. In a normal tensile test this is usually observed as a drop in load at the start of gross yielding. Load drops may also be observed that are due to deformation twinning, but this will not be discussed here. Annealed mild steel was once thought to be unique in showing an initial yield point but it is now known that the effect is quite general, and many examples occur in substitutional alloys.

Yield-point phenomena in substitutional alloys

Yield-point phenomena in substitutional alloys

The stress and strain distribution in a propagating Lüders front accompanying the yield-point phenomenon in iron

A model for the stress and the strain distribution across a propagating Lüders front is presented. It is shown that the stress distribution in a Lüders front can be related to the strain distribution by means of a basic stress-strain curve for the material. Equilibrium considerations require a macroscopic stress concentration at the elastic-plastic boundary of the Lüders front. From a consideration of the proposed model various predictions can be made with respect to the distortion accompanying a Lüders front, the width of the Lüders front and the influence of grain size and strain rate on lower yield stress and the Lüders strain. It is shown that all of these predictions are either quatitatively or qualitatively in accord with experimental work performed or reported in the literature. Experimental evidence is also offered that in exceptional cases the yield-point phenomenon in low-carbon steel occurs homogeneously not accompanied by the propagation of a Lüders band.

Elevated temperature yield effect in iron-aluminum

The pronounced stress drop or yield-point phenomenon that is observed in partially ordered Fe-Al alloys which contain approximately 25 at.% Al has been evaluated by means of elevated temperature, constant strain-rate tests. The magnitude of the effect is found to depend on the particular combination of strain rate and test temperature employed. No stress drop is observed when complete Fe 3Al order is present. However, a stress drop is observed whenever FeAl or partial Fe 3Al order is present; this corresponds to a temperature region where the flow stress is unusually sensitive to the strain rate. Transmission electron microscopy revealed that the stress drop at yield is associated with an initially low dislocation density and rapid dislocation multiplication. Consequently, the yield phenomenon is believed to be of the type first discussed by Gilman and Johnston wherein a stress drop is caused by rapid dislocation multiplication in a structure in which the dislocation velocity is relatively insensitive to the stress.

THEORY OF INFINITESIMAL DISLOCATIONS DISTRIBUTED ON A PLANE APPLIED TO DISCONTINUOUS YIELD PHENOMENA

An analysis is made of the behavior of groups of dislocations in a slip band, when the frictional stress that hinders dislocation motion undergoes a static or a dynamic drop. A static stress drop is the lowering of a frictional stress that decreases with increasing displacement across the slipped zone; a dynamic stress drop is associated with a frictional stress that decreases with increasing velocity of displacement. It is shown that the presence of a static stress drop can lead to the upper and lower yield point phenomena, even if the stress drop is very gradual. The dynamic case leads to the Portevin–Le Chatelier effect (serrated stress–strain curves). According to this analysis, the Portevin–Le Chatelier effect has its origin in the inherent instability of a boundary separating dislocations that are acted upon by a high frictional stress from those acted upon by a low frictional stress.

Neutron-acoustic irradiation of aluminium single crystals

Abstract Ultrasonic irradiation of ∼25 w/cm2 at 20 kc may increase the critical resolved shear stress by a factor of 4 in 99·999% pure aluminium single crystals. This acoustic hardening is accompanied by a yield-point phenomenon, which, in its graphical appearance, resembles effects by neutron irradiation with no sound applied. Hardening by ultrasound and by neutron irradiation are compared, and effects of the superposition of fast neutrons (>0·1 MeV; ∼2 × 1016 n/cm2) and intense ultrasound (∼25 w/cm2 at 20 kc) in aluminium single crystals having diameters between 0·6 and 1·5 mm are presented. Increases were found to amount to as much as a factor of 14 in the critical resolved shear stress of these crystals compared to untreated samples. Simultaneously, the electric resistivity increased with combined treatment by 3·2 times over untreated samples. The results from these investigations are discussed in the light of present theories of neutron and acoustic radiation damage.

アルミニウム合金の高温降伏現象の機構

A new type yield point phenomenon observed when some binary aluminium alloys such as Al-Mg or Al-Cu are deformed at high temperatures above about 350°C has been reported in the previous paper. The present paper is devoted to a detailed explanation of the mechanism which controls the high temperature yield point phenomenon. The proposed theory in this report concerns the viscous motion of dislocations dragging the Cottrell atmospheres around them and the state equation of deformation derived theoretically explains quite well the phenomena observed at high temperatures. In the viscous motion of a dislocation, a fairly larger stress is needed to increase the dislocation velocity. This means that a remarkable yield drop must occur due to the dislocation multiplication during deformation. Comparing the theoretical state equations deduced from the analysis of Cottrell and Jaswan with the experimental stress-strain curves, the increasing process of the dislocation density was calculated. The results have been shown to be quite reasonable.The stress-strain curves obtained by a rapid change of the tensile speed agree with the theoretical predictions from the strain rate dependence of the dislocation density. Further the theoretical prediction of the critical temperature above which the viscous motion of dislocations dragging solute atmospheres controls the deformation is in good agreement with the critical temperature for the yield point phenomenon determined experimentally, which shows that the approximation of the theory is highly reasonable.

Mechanism of the High Temperature Yield Point Phenomenon in Some Aluminium Alloys

Mechanism of the High Temperature Yield Point Phenomenon in Some Aluminium Alloys

Stress relaxation prior to the yield point in niobium and molybdenum

Stress relaxation, i.e. the drop in load due to plastic flow at constant strain, has been used as a technique to study the phenomenon of plastic strain in the pre-yield region of the load-elongation curve. Results from experiments on niobium and molybdenum have been interpreted to mean that at room temperature some dislocations are free to move at very low stresses. The amount of plastic strain due to the movement of these free dislocations at any given stress level, below the upper yield stress, was found to be a function of loading-rate. The movement of the free dislocations was inhibited by the presence of interstitial impurity atoms, defects produced by irradiation damage and by debris and dislocation tangles produced by cold work. It was also concluded that the defects produced by irradiation damage, which anneal out in the temperature range 140–200°C, form clusters on annealing rather than condensing onto dislocation lines. It was inferred from these experiments that the yield point phenomenon results from dislocation multiplication rather than unpinning from impurity atmospheres.

Theory of the Thermal Breakaway of a Pinned Dislocation Line with Application to Damping Phenomena

Although the mechanical theory of Granato and Lücke for the strain-amplitude-dependent internal friction and modulus changes of solids containing dislocations gives a fair account of many of the observed effects, simple theoretical considerations show that the effect of thermal fluctuations should be very important. To extend the theory to finite temperatures, a detailed study of the possible static equilibrium configurations of a pinned dislocation line as a function of external stress is required. This is done here for the two specific examples of a dislocation with a single pinning point and a dislocation with a continuous pinning agent. It is shown in a general way how the damping and modulus change can be computed from these results, and a qualitative discussion of expected behavior is given. For high concentrations of pinning points, the theory is also applicable in the field of yield point phenomena, and the results found here are compared with previous calculations.

A model for yielding with special reference to the yield-point phenomena of iron and related bcc metals

A model for yielding based on dislocation multiplication and velocity characteristics specific to iron and related b.c.c. metals is derived on the premise that dislocations anchored by segregates or precipitates remain locked. The abrupt yield drop is a consequence of rapid multiplication and the stress dependence of dislocation velocity. The model describes continuous and discontinuous yielding, the Lüders' band, the delay-time phenomenon, and offers a unified treatment of these phenomena qualitatively in accord with experiments. Macroscopic strain-rate sensitivity and the Lüders' band front velocity are shown to be related to dislocation-velocity characteristics. The model also accounts for yield-point anomalies associated with strain aging. Although the possibility of unlocking is not excluded, these considerations imply that the importance of unlocking may have been overstated.

The hardening of copper single crystals by fatigue

Copper single crystals have been subjected to alternating stresses of such magnitude as would cause complete fatigue failures in 5 x 105 cycles. Slip striations are produced both at 78 and 293 °K and plastic deformation starts in these regions if partially fatigued specimens are extended. In specimens fatigued at 293 °K the material between striations begins to deform at a significantly higher stress than the maximum stress applied during fatigue and yield point phenomena are observed. The flow stress of the yield extension has a strong temperature dependence. Yield point phenomena are less pronounced in specimens fatigued at 78 °K and additional hardening can be produced in such specimens if they are warmed after fatigue to temperatures higher than 200 °K. These observations suggest that point defects make a substantial contribution to the hardening which is observed during fatigue tests at room temperature.

Yield point phenomena in alpha brass and other face-centred cubic metals

Abstract The characteristics of yield points which may be produced in a 70:30 alpha brass have been observed at several temperatures. The yield point phenomena were produced by various techniques which included ageing under stress, unloading and combinations of the two. The effects arising from ageing under stress are compared to experiments on repeated yielding. The particular results in brass are supported by various experiments with a range of face-centred cubic metals. From the evidence presented it seems that the process of ageing is more complex than hitherto supposed.