Strain Dip Tests Research Articles

Comparison of Internal and Residual Stresses Measured by Strain-Dip Test and XRD during High Temperature Deformation of Al-Mg Solid Solutions

Creep behavior of solid solution alloys are reasonably explained by concepts of the “internal and effective stress of high temperature deformation”. The internal stress is considered to be brought by formation of dislocation substructures, and the dislocation structures should have caused long range stress filed in interior of materials. Thus, residual stresses should also be brought by the same origin. In this paper, measurements of the residual stresses after creep deformation by 2D-Xray method are attempt, and the stresses are compared with so-called the “internal stress of high temperature deformation” measured by strain-dip stress-transient test. Although, the stress tensor depends on the deformation condition, the relation with the applied stress show complex manner at a glance. The maximum principal stresses, however, show relatively smaller than the applied stress, and fairly agree with that measured by strain-dip stress-transient technique. Importance of further considerations of the origin of so-called internal stresses is suggested.

Dislocation densities and internal stress in Ni3Al

Dislocation densities have been measured in Ni3Al(Hf) single crystals after deformation at various temperatures and strains. In addition, effective and athermal stresses have been determined by strain dip tests. A model is proposed for the strength anomaly domain which correlates the athermal part of the stress with the operation of dislocation sources, to compensate for the strong dislocation exhaustion. Dislocation density data, which evidence an anomaly in the density with temperature over the stress anomaly domain, are shown to support this model predictions between 373 K and 683 K for plastic strains larger than 3 × 10−2 but not at yield. In addition, the presence of Hf atoms (3.3 at. %) induces a friction stress close to 63 MPa.

Stress Exponent of Minimum Creep Rate and Activation Energy of Creep for Oxide Dispersion-strengthened Nickel-based Superalloy MA754

The stress exponent of the minimum creep rate, n, and the activation energy of creep, Qc, were obtained for the oxide dispersion-strengthened nickel-based superalloy MA754 by conducting creep tests at 1223–1273 K in the stress range of 130–190 MPa. The values of n and Qc in MA754 were determined to be 26 and 962 kJ/mol, respectively. The causes of these high values were determined by measuring the internal stress, σi, using the strain dip test. The ratio of σi to applied stress, σa, was very high at lower stresses, while at higher stresses, σi reached its saturated value with increasing stress. In this stress range, the ratio of σi to σa decreased drastically with increasing stress, which reflected the large increase in creep rate. Such a large increase in creep rate with increasing applied stress led to an unpredictably higher value of n. With increasing temperature, the saturated σi decreased, which resulted in a relatively large creep rate. The larger creep rate at high temperatures led to a large value of Qc. In addition, the dislocation density, ρ, of the interrupted creep specimens at the time of the minimum creep rate increased with increasing stress and reached the saturated value. This change in ρ with stress must be reflected in a large change in n and Qc through a change in σi.

The investigation of internal stress fields by stress reduction experiments

Abstract The decomposition of stress applied on a material into two distinct parts called effective and internal stresses is often assumed but rarely experimentally measured. The strain-dip test technique, used occasionally in creep testing, is employed here for the investigation of the two stress components under conditions of constant strain-rate compression tests for four different single crystalline materials. The Cottrell–Stokes law, i.e. the linear dependence between applied and effective stresses, is verified in the case of metallic specimens while the effective stress depends only slightly on the applied stress in the case of germanium.

Stress reduction experiments during constant-strain-rate tests in Cu and Ge

The technique of rapid reductions in applied stress (dip test) is occasionally performed in creep testing. In the work reported here, the technique was applied during a constant-strain-rate test. Both variants, the stress dip test and the strain dip test, give equivalent results. The specimen strain has to be measured in both cases; however, the strain dip technique is preferred since anelastic effects do not interfere. It is shown that the response of Cu and Ge single crystals to fast stress reductions, on the level of the internal stress and below it, is strongly material dependent, reflecting differences in microstructural processes involved in the plastic deformation. These processes are discussed.

Dislocation multiplication, exhaustion and mechanical behaviour for Ge singlecrystals

Dislocation multiplication and exhaustion processes in germanium arerevisited in the light of new data from mechanical test transients. They aredocumented by transmission electron microscope (TEM) observations anddislocation velocity measurements by the etch pit technique. The transient testsconsist of stress relaxations, creep tests and stress dip tests performedalong the monotonic curve. The relaxation and creep curves exhibit anon-logarithmic dependence on time, unlike in other classes of materials.Dislocation multiplication is evidenced by these transient curves before the upperyield point. In addition, after the lower yield point, dislocation exhaustion takesplace at the end of the transient. Some evidence of strain localization isalso provided by strain dip tests and transmission electron micrographs.

Creep characteristics and internal stress in γ-TiAl

The creep deformation of γ-TiAl single phase alloy with ordered L1 0 structure has been characterized in terms of activation energy for creep Q c, stress exponent n and internal stress σ i. The stress exponent of minimum creep rate n becomes 3 at stresses below 70 MPa in the temperatures between 1123 and 1223 K, whereas it increases to 8 above 150 MPa. The activation energy for creep Q c obtained at 68.6 MPa increases from 250 to 700 kJ mol −1 with increasing the temperature. The large variation of Q c is attributed to the temperature dependence of dislocation substructure. The internal stress σ i measured by strain dip tests decreases with increasing temperature. Based on these findings, it is found that the activation energy Q* c obtained at a constant effective stress σ e (= σ a− σ i) becomes 245 kJ mol −1, independent of the temperature.

２０２４アルミニウム合金４２３Ｋ時効材における初期急速硬化の起源

In order to reveal the origin of the initial rapid hardening of Al–Cu–Mg alloys, stress-strain response under various tensile loading conditions was investigated in a 2024 aluminum alloy aged at 423 K (15O°C). Cyclic tensile loading at 77 K resulted in no significant change in stress-strain response. On the other hand, yield-point phenomena evidently appeared in the specimen tested at room temperature. In addition, reduced flow stress was obtained at a higher strain rate in the room-temperature strain-dip test. These experimental findings supported the idea proposed by Ringer et al. that the interaction between dislocations and solute atoms (or co-clusters) was responsible for the initial rapid hardening. However, the idea cannot explain the large stress gain during the initial hardening. Microstructural change at the early stage of aging was examined using TEM. A uniform dispersion of dislocation loops was evident from the very early stage of aging and it remained during the initial hardening stage. Simple calculation based on Orowan mechanism indicated that the existence of dislocation loops resulting from quenched-in excess vacancies was the predominant reason for the initial hardening.

Unified Inelastic Constitutive Model for Modified 9Cr-lMo Steel Incorporating Dynamic Strain Aging Effect.

A unified constitutive model considering dynamic strain aging effect was developed in order to describe inelastic deformation behavior of modified 9Cr-1Mo steel precisely. The inelastic behavior of the material was summarized as follows. A rate-dependent deformation was observed above 500°C, and there was no rate-dependency under 400°C. However, stress relaxation behavior under constant strain was observed even at rate independent temperature region. Further, the stress after relaxation depended on prior loading strain-rate, and it showed a higher value as the strain-rate was decreased. A feature of the proposed constitutive model was that an applied stress consisted of three components:a back stress, an overstress and an aging stress which corresponded to dynamic strain aging effect and showed a negative strain-rate-dependency. The aging stress was measured by stepwise strain-rate change tests, and it showed a larger value at smaller strain-rate and lower temperature. The back stress and the overstress were measured by strain dip tests. The back stress was approximately rate-independent below 400°C, however it showed rate-dependency above 500°C. The overstress showed a larger value as the strain-rate or the temperature was increased. Material constants involved in the constitutive model were determined systematically based on the measured values of these stress components. In order to evaluate the validity of the constitutive model, numerical simulations were conducted for various inelastic deformation behavior of modified 9Cr-1Mo steel. The simulations agreed with experimental results very well in all cases.

Unified Inelastic Constitutive Equations Incorporating Dynamic Strain Aging for Mod. 9Cr-1Mo Steel.

A unified constitutive model considering dynamic strain aging effect was developed in order to describe inelastic deformation behavior of the Modfied 9Cr-1Mo steel precisely. The inelastic behavior of the steel was summarized as follows. A rate dependent deformation was observed above 500°C, and there was no rate dependency under 400°C. However, stress relaxation behavior was observed even at rate independent temperature region. Further, a stress after relaxation depended on prior loading strain rate, and it showed a higher value as the strain rate was slow. A feature of the proposed constitutive model was that an applied stress consists of three stress components : a back stress, an overstress and an aging stress which corresponds to dynamic strain aging and shows a negative strain rate dependency. The aging stress was measured by strain rate change tests, and it showed larger values as the strain rates were slow and the temperatures were low. The backstress and the overstress were measured by strain dip tests. The backstress was approximately rate independent under 400°C, however it showed rate dependency above 500°C. The overstress showed larger values as the strain rates were fast and the temperatures were high. The material constants were determined systematically based on the measured values of each internal variable. In order to evaluate the validity of the constitutive model. numerical simulations were done for various inelastic deformation behavior of Mod. 9Cr-1Mo steel. The simulations agreed with experimental results very well in all cases.

Creep mechanisms in 6246 titanium alloy monitored by stress change testing

The transient strain dip test was applied to a commercial titanium alloy. Incubation times were recorded while a positive creeprate was re-established. The incubation times were much longer than those recorded for pure metals. Negative creep responses were also observed and associated with reverse yielding during the stress reduction process. It is proposed that the length of theincubation period is determined by a thermally activated recovery mechanism. An activation energy which is broadly comparable to that observed elsewhere during hot compression testing of the same alloy was monitored.

Creep mechanisms in 6246 titanium alloy monitored by stress change testing

The transient strain dip test was applied to a commercial titanium alloy. Incubation times were recorded while a positive creeprate was re-established. The incubation times were much longer than those recorded for pure metals. Negative creep responses were also observed and associated with reverse yielding during the stress reduction process. It is proposed that the length of theincubation period is determined by a thermally activated recovery mechanism. An activation energy which is broadly comparable to that observed elsewhere during hot compression testing of the same alloy was monitored.

Effects of nitride distribution on internal stress during high temperature deformation of 12%Cr–15%Mn austenitic steels

Many investigations on internal stress of various materials have been performed as the internal stress is considered to be an important factor to control high temperature deformation. The present authors have researched the effect of very fine and dense vanadium nitride (VN) precipitates on internal stress and effective stress which were measured by a strain dip test during high temperature creep of 12%Cr-15%Mn austenitic steels. In the present study, the authors have investigated the difference of the response to a strain dip test among the 12%Cr-15%Mn materials which are alloyed with small amounts of V, Ti and Ta and have different distributions of nitrides formed by an aging treatment, and have discussed the effect of nitride distribution on the interaction of dislocations with the precipitates.

12%Cr-15%Mn鋼の高温クリープ変形中の内部応力に及ぼす微細析出物(VN)の影響

The objective of the present study is to investigate the effects of fine precipitates of vanadium nitride on the creep deformation behaviors of 12%Cr-15%Mn austenitic steels through the measurement of internal stress and effective stress. These stresses were obtained by strain dip tests. The internal stress of VN free materials was determined as a unique function of applied stress. On the other hand, that of VN containing materials was not obtained uniquely as a function of applied stress. This indicates that fine precipitates of VN with high number density are strong barriers for the dislocation motion in spite of wide range variation of stress applied on the dislocations. At lower applied stress, the change of deformation rate during the strain dip test of the VN containing materials was very small and could not be detected with the increase of deloaded stress. Consequently this means that the forward or backward motion of almost all dislocations were inhibited by the precipitates. At higher applied stress, the VN containing materials showed the decrease of deformation rate with the slight deloading during strain dip test. This means that there exist movable dislocations which can glide without being blocked by the precipitates. The difference of stress dependence of minimum creep rate between at lower applied stress and at higher one was considered to be caused by such interaction of the fine precipitates with dislocation motion.

Effect of Fine Vanadium Nitride Precipitates on Internal Stress during High-Temperature Creep of 12%Cr-15%Mn Steels

The objective of the present study is to investigate the effects of fine precipitates of vanadium nitride (VN) on the creep deformation behavior of 12%Cr-15%Mn austenitic steels through the measurement of internal stress and effective stress. These stresses are obtained by strain dip test. The internal stress of VN free materials was unequivocally determined as a function of applied stress. On the other hand, the change of deformation rate during the strain dip test of the VN containing materials was very small and could not be detected in a range of reduced applied stress. Consequently, the internal stress and effective stress of VN containing material showed a band range of values as a function of applied stress. This range indicates that fine and dense precipitates of VN become strong barriers to the dislocation motion in a wide variation of stresses worked on the dislocations. The temperature dependence of internal and effective stresses of the VN containing material was also clarified. The difference between the stress dependence of minimum creep rate at the lower applied stress and that at the higher one was also discussed to be caused by the behavior of internal and effective stresses

多量のM<SUB>23</SUB>C<SUB>6</SUB>型炭化物を含むオーステナイト耐熱鋼の高温クリープ中の内部応力

Creep test has been performed at temperatures of 873 K and 973 K on an austenitic 21-4N steel with a large amount of M23C6 carbides as well as on a SUS 304 steel without any precipitates. Discussion was then made on the relation between internal stress in the steady-state creep obtained by the strain dip test with application of the extrapolation method and precipitate particles or the dislocation structure.It was found that internal stress increased with an increase of applied stress in both steels, internal stress reached a constant value of 280 MPa at 873 K in the 21-4N steel. This value is close to the Orowan stress at 873 K (σo\fallingdotseq2Gb⁄L0=312 MPa, L0: free spacing between particles). By means of transmission electron microscopy to thin films, unlike in the SUS 304 steel, the cell structure was not formed during the steady-state creep in the 21-4N steel, but the dislocation density around precipitates increased with an increase of applied stress. It was considered from calculations in this study that internal stress in both steels is a long-range force acting on mobile dislocations when they surmount particles by the Orowan mechanism or pass through regions of the high-dislocation density around particles or cell boundaries.The activation energy of creep was 291±8 kJ/mol for the 21-4N steel and 297±19 kJ/mol for the SUS 304 steel, independent of applied stress and temperature. These values were almost equal to that of self-diffusion in gamma iron. An activation area which was of the same order as lb (l: dislocation spacing in a region of the high-dislocation density around particles or at cell boundaries) decreased with an increase of applied stress in both the steels.