Automatic measurement of the creep behavior of linear polymers in the course of homogeneous drawing

An automatic device was built, enabling the creep behavior of fibers of linear semicrystalline polymers to be studied during various stages of homogeneous drawing. The device first deforms the sample at constant speed up to a certain draw ratio; then, without interrupting the stress, the character of deformation is changed from deformation at constant speed to that caused by constant load (creep). It was verified that the method described supplies more detailed information on the changes of the mechanical behavior of polymers at various stages of orientation than does the more conventional stress-strain curve.     

Modeling creep in L12-superstructure single crystals

Creep curves for compression and extension under constant loads and constant external stress are calculated using a model of the creep of L1₂-superstructure single crystals. It is shown that features of the creep in alloys with L1₂ superstructure can be explained via the superpositioning of normal and anomalous deformation mechanisms. It is established that during creep, the localization of deformation, which can lead to inverse creep, is important.     

In situ synchrotron radiation topography of NaCI during high temperature creep

High temperature plastic deformation is associated with large changes in the microstructure of single crystals. To observe this microstructure during the creep test, we have performed X-ray reflection topography, taking advantage of the high intensity of the synchrotron radiation. A special creep machine was designed which permits in situ observation.

Creep tests and microstructural observations were performed on NaCl single crystals compressed along <100> at about 600°C. As soon as the deformation started, subgrains appeared within the crystal, independent of the initial microstructure. Migration of the subboundaries during transient creep is followed by stabilization during steady state creep where a well developed subgrain structure keeps constant while new appearing subboundaries migrate. Misorientation between sub-grains increases progressively although more slowly in the steady state creep. A correlation between the microstructure evolution and the changes in the creep curves has been attempted.     

Plastic behaviour of Fe-3% Si alloy single crystals

Fe-3% Si alloy single crystals of single slip and double slip orientation have been tested in tension at temperatures between 113 K and 473 K. The stress-strain curves exhibit a yield drop followed by inhomogeneous yielding (yield propagation stage), a parabolic and an approximately linear parts. The slip line observations carried out in various stages of deformation show that only at the beginning of the parabolic part of stress-strain curve the specimen is completely filled out with slip bands and the deformation becomes relatively homogeneous. With both single slip and double slip orientations deformation takes place predominantly in one slip system only. At higher temperatures ageing occurs during deformation.Na Slovance 2, Praha 8, Czechoslovakia.The authors would like to thank Dr. B. esták for suggestion of the problem and for his interest in discussion of the results. We are also grateful to Mr. J. Poucha for help with some experimental work and to Miss G. Výborná for help with specimen preparation. To Dr. S. Libovický and Dr. F. Kroupa we thank for helpful discussions.     

Pseudoelastic deformation and generation of reactive stresses in a Cu-Al-Ni shape-memory alloy in the temperature range 4.2–293 K

Pseudoelastic deformation and the magnitude of reactive stresses in Cu-14.2% Al-4.5% Ni shape-memory alloy single crystals were studied experimentally in the temperature range 4.2–293 K. It is established that pseudoelasticity and the shape-memory effect are observed in this alloy over the entire temperature range indicated above. It is found that, as the constrained samples are heated at a constant rate from liquid-helium temperature, the reactive stresses increase continuously at temperatures of up to 100 K and then remain constant. When the temperature of preliminary deformation is 77 K, the generation of reactive stresses with an increase in temperature occurs by two stages, which agrees with the multistage behavior of the pseudoelastic-deformation curves of this alloy above the liquid-nitrogen boiling temperature. Using the theory of diffuse martensitic transitions, a quantitative calculation is performed of pseudoelastic-deformation curves and reactive-stress curves over the temperature range 4.2–293 K under conditions of two-stage behavior of the martensitic transformation.     

Discontinuous creep curves of Cd+0·44 at.% Zn alloy single crystals

The creep behaviour of Cd+0·44 at.% Zn alloy single crystals is described. Contrary to the Cd single crystals discontinuous creep curves have been observed, which do not occur at 78 K. The low- temperature limit of this effect is 130 K approximately. The characteristics of these discontinuous creep curves are discussed on the assumption that the Portevin-Le Chatelier effect is acting in the Cd+0·44 at.% Zn alloy.     

Effect of the intermediate action of hydrostatic pressure on the high-temperature creep and durability of copper

The intermediate action of hydrostatic pressure on the high-temperature creep of copper is studied at various creep stages. Tests performed at a constant tensile stress of 12.5 MPa at 773 K show that the application of a pressure at the creep third stage decreases the steady-state creep rate and extends the time to failure. At the steady-state stage of creep, the effect of the pressure may be ignored. At pressures of up to 1 GPa, this effect is found to be only related to healing of grain-boundary porosity. At higher pressures, the steady-state creep rate is governed by porosity healing and structural changes.     

Creep deformation of single crystals of new Co–Al–W-based alloys with fcc/L12 two-phase microstructures

The creep deformation behaviour of single crystals of Co–Al–W-based alloys with γ?+?γ′ two-phase microstructures has been investigated in tension under a constant stress of 137?MPa in air at 1000°C as a function of the γ′ solvus temperature and the volume fraction of the γ′ phase. When described by the creep strain rate versus time curve, the creep deformation of Co–Al–W-based alloys consists of transition and accelerating regions without a steady-state region, as observed in many modern nickel-based alloys. However, the creep strength of the present Co–Al–W-based alloys is comparable with nickel-based superalloys of the first generation but is much weaker than those of the second and higher generations. Unlike in nickel-based superalloys, the so-called p (parallel)-type raft structure, in which the γ′ phase is elongated along the tensile axis direction, is formed during creep in Co–Al–W-based alloys, being consistent with what is expected from the positive values of lattice misfit between the γ and γ′ phases. As a result, of the alloys investigated, the best creep properties are obtained with the alloy possessing the highest volume fraction (85%) of the γ′ phase, which is far larger than usual for nickel-based superalloys (55–60%).     

Localized strain autowaves at the initial stage of plastic flow in single crystals

Plastic strain localization in single crystals of pure metals and alloys is studied on the yield plateau and at the easy glide stage with a zero or small strain hardening coefficient. The difference between localization patterns in the two cases is explained, and strain localization mechanisms are suggested. At these stages of plastic deformation, various types of autowaves are observed.     

Analysis of the factors that cause unstable deformation and loss of plasticity in neutron-irradiated copper

Copper is used as an example to analyze the effect of radiation on the stress-strain curves and deformation stability of radiation-hardened metals. The analysis is based on an equation that describes the evolution of the dislocation density with deformation in a plastically deformed material. Deformation instability in the initial stage of the stress-strain curve is caused by strong deformation localization at the microscopic level as a result of the transformation of immobile radiation defects (vacancy and interstitial loops) into mobile dislocations. The channeling of a large number of dislocations along slip planes causes the appearance of a yield drop and a yield plateau in the stress-strain curves. The critical conditions for their appearance, as well as the theoretical irradiation-dose dependences of the yield-plateau length and the uniform strain to necking, are found.     

Localization of stretching strain in doped carbon γ-Fe single crystals

The evolution of local strain during stretching of high-manganese carbon austenite was studied. The ordered patterns of strain localization proved to be closely related to the stages in the stress-strain curve. The results of this study are compared with analogous data for chromium-nickel nitrogen austenite single crystals. The velocity of self-consistent motion of the sites where plastic strain during stretching of γ-Fe single crystals is nonuniform was determined as a function of the strain hardening coefficient and deformation mechanism.     

Orientation dependence of creep in Ni<Subscript>3</Subscript>Ge single crystals

The creep of Ni₃Ge alloy single crystals oriented along [001], \([\bar 139]\), \([\bar 234]\) and \([\bar 122]\) strain axes is investigated. It is established that the transition from an octahedral to a cubic slip raises the alloy’s resistance to creep deformation. The slip in cube planes demonstrates the high stability of the rate of steady-state creep with respect to changes in the testing temperature.     

Special features of the localized plastic deformation and fracture of high-chromium steel of the martensitic class

The deformation behavior of high-chromium steel (0.4%C–0.6%Si–0.55%Mn–12.5%Cr) of martensitic structure upon quenching and of sorbitic structure upon high-temperature tempering has been investigated. Each of the states is shown to be represented by a particular stress-strain curve. The stress-strain curve for the steel in the martensitic state consists of a single linear-hardening stage, whereas in the sorbitic state, it exhibits a three-stage deformation pattern. The plastic flow of the examined material in the two states has been found to be of a localized character. The evolution of localized-strain center distributions follows the law of plastic flow, i.e., it depends on the deformation stages in the stress-strain curve. The fracture process is determined by the kinetics of the localized-strain centers in the final (prefracture) deformation stage in the stress-strain curve.     

Study of creep and features of the dislocation structure in single crystals of Ni3Ge alloy

Data from investigating the dislocation structure of Ni₃Ge alloy single crystals formed during creep are presented. The creep of the Ni₃Ge single crystals with the [001] orientation of the deformation axis was studied. It was found that a fragmented substructure with varying degrees of disorientation occurs in the areas of macrolocalized deformation. A polycrystalline substructure consisting of fragments with a low dislocation density is formed in the local areas.     

惯性对多孔金属材料动态力学行为的影响

 对泡沫金属材料的力学性能已经进行了十分广泛的研究,但在对泡沫金属的应变率效应和惯性效应的研究中,尚存在一些矛盾的结论。为进一步认清惯性在多孔金属动态响应中的作用,用有限元计算方法模拟了二维Voronoi蜂窝的动态压缩行为,得到了不同速度下Voronoi蜂窝的3种变形模式。通过改变基体材料的密度和冲击速度进行“数值实验”,得到了相应“试件”的由冲击面和支撑面得到的宏观平均应力应变曲线和平台应力。根据数值模拟的结果,着重分析了惯性效应的影响。研究发现,惯性并不影响蜂窝的应力应变曲线,但它导致试件中宏观变形不均匀,是平台应力提高的主要原因。     

Measurement of stored energy of KCl after 4·6°K reactor irradiation

Five KCl single crystals were irradiated at 4·6°K in the core of the Munich Nuclear Reactor for periods of 100 sec, 10 min, 1 hr or 10 hr, respectively. After irradiation the stored energy of the samples was measured in a differential-heat-flow calorimeter at two different heating rates: 0·29 and 1·1°K/min. At 0·29°K/min peaks of stored energy release were resolved at 21°K, near 27·5°K, at 32·5°K, at 42·5°K and near 50°K. An attempt was made to evaluate the corresponding activation energies using two different methods (see Table 1 in the text). The annealing stages at 21 and 32·5°K correspond to first order kinetics. The annealing stages at 42·5 and near 50°K are not of first order. A similar experiment was performed on KBr single crystals and is reported in the preceding paper. The results are compared with annealing studies on low temperature X-irradiated KCl and KBr crystals. From both our experiments it follows that also during reactor irradiation at 4·6°K ionization mechanisms of defect production should be responsible for the four observed low temperature recovery peaks; only defect recovery at higher temperatures may be at least partially explained by recombination of collision produced defects, i.e. the typical neutron irradiation effect as it takes place in metals.     

Creep of heat-resistant steel under a bending load caused by the Gorskii effect

We have studied the creep and subsequent recovery in three-point bending of the heat-resistant steel 12X1MF over the temperature range from 500 to 625°C. It is shown that in this temperature range the Gorskii effect is present, in which carbon determines the creep rate in its initial stages. By subtracting the recovery curves from the creep curves we are able to separate the diffusion creep associated with the Gorskii effect from the dislocation creep. Institute of Strength Physics and Materials Production. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 3–6, October, 1998.     

The inhomogeneity of plastic deformation

Considerable local strain differences occur in single crystals as a consequence of slip bands. A significant contribution to this phenomenon is the effect of the deformation rate on the inhomogeneity of its distribution. In polycrystalline materials the plastic deformation is affected by grain boundaries and their vicinity as well as by various orientations of single grains in onephase metals and alloys, and by various component properties in multiphase materials. In some cases it has been possible to describe these phenomena by means of micromechanics. Valuable information has also been acquired by means of three-dimensional stereology.     

Acoustoplastic effect and internal friction of aluminum single crystals in various deformation stages

The dependences of the acoustoplastic effect and the internal friction on the oscillatory strain amplitude are measured in various deformation stages of low-purity aluminum single crystals. It is discovered that the acoustoplastic effect is observed not only in the macroscopic plastic region of the stress-strain diagram, but also for microplastic deformation in the “elastic” loading and unloading stages. The sign of the effect reverses during unloading. An increase in the strain rate leads to enhancement of the acoustoplastic effect and the absorption of the energy of ultrasonic vibrations causing this effect with a frequency of about 100 kHz. It is concluded that the acoustoplastic effect observed during both macro-and microplastic deformation is caused by the irreversible high-speed motion of dislocations through the long-range stress field of the other dislocations after breaking through the Cottrell atmospheres. Fiz. Tverd. Tela (St. Petersburg) 39, 1794–1800 (October 1997)