On the multistage nature of deformation of the microcrystalline aluminum-lithium alloy 1420 under superplasticity conditions

The constant-rate tensile deformation and creep of aluminum-lithium alloy 1420 with a grain size of 3 µm (obtained by equal-channel angular extrusion) exhibiting superplasticity at temperatures of 600–670 K and relative-deformation rates of 10^?2–10^?3 s^?1 are considered. It is shown that, upon tension at a constant rate V _m, a steady-state segment appears in the true stress σ_t-true strain ?_t dependence, which is described by the expression $\dot \varepsilon _t \sim \sigma _t^n \exp ( - U/kT)$ with constant coefficients, and that the rate of deformation $\dot \varepsilon _t$ is close to the creep rate at comparable stresses and strains. The conclusion is made that, upon deformation under superplasticity conditions, an equilibrium structure is formed, which remains unaltered in the process of further deformation until the sample goes over (because of geometrical conditions) to a prefracture state.     

Transient and steady state creep of Al-4.5 wt.% Mg during phase transformation

Transient and steady state creep of Al-4.5 wt. % Mg alloy was studied under various constant stresses ranging from 91 MPa to 117 MPa in the temperature range from 473 K to 553 K. The results of creep characteristics have shown two main deformation temperature regions (below 493 K and above 513 K as well as a transient region between these temperatures). Peak values of transient creep parametersB andn were obtained at 493 K. The transient creep parameterB was related to the steady state creep rate _st through the exponent which was found to range from 0.85 to 0.5. The stress exponentm of the steady state creep has been found to be minimum at the steady state strain peaks, which is characteristic of dislocation climb along the grain boundaries. Microstructural analysis confirmed that the above mentioned mechanism took place in the dissolution region of-phase.     

Nonlinear dynamics of deformation bands in aluminum–magnesium alloy in the creep test

Various types of plastic instabilities that emerge in intermittent creep have been studied experimentally for AlMg6 aluminum–magnesium alloy. It has been shown that intermittent creep exhibits threshold dynamics. The deformation step on the creep curve of amplitude is ~1–6% and begins when the rate of the preceding continuous creep attains a certain critical value. In the course of evolution of the step, the strain rate varies in the interval that spans more than two orders of magnitude, and transitions occur between different dynamic regimes of type A and B characterized by different stress drop regularity levels in the force response. Nonlinear aspects of the deformation behavior of the alloy in the intermittent creep conditions are considered.     

Creep behaviour of Zr + 4·5%Sn + 1%Mo in 300 K–750K temperature range

Measurements of the Young modulus normalized yield stress and the stress sensitivity parameter in creep of the Zr + 4·5% Sn + 1% Mo alloy in the temperature interval 300 K –750 K are presented. It is shown that there exists a plateau in the temperature dependence of the Young modulus normalized yield stress in the temperature range 540 K–660 K. The stress sensitivity parameter and the activation area exhibit a maximum at about 550 K. The discontinuous creep deformation is observed. It is suggested that the dynamic strain aging plays a significant role in the creep deformation of the Zr + 4·5% Sn + 1% Mo alloy in the temperature interval 540 K–660 K.This paper has been prepared for the Symposium Recent Problems in the Plasticity of Metals and Alloys, Prague, August 1980.The authors thank Prof. J. adek, DrSc., for valuable discusions.     

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.     

Creep of a Cu-2·5 at. pct. Al alloy at high temperatures

The creep of a Cu-2·5 at.pct. Al alloy has been investigated by the isothermal tests technique. Two dislocation mechanisms operate in parallel in the investigated temperature region. In the region of lower minimum creep rates ( <3×10^–5 sec^–1) — region 1 — the activation energy of creep is close to the expected value of the activation enthalpy of the lattice self-diffusion of copper in the alloy. The stress dependence of the minimum creep rate can be described by a power function of the ⁿ type, where n=n₁=5·8. The results for region 1 can be satisfactorily correlated with the model of nonconservative motion of jogs on screw dislocations dependent on lattice self-diffusion. If a power function of the ^m type is used to describe the minimum creep rate dependence on the stacking fault energy, thenm is equal to –0.66.In the region of higher minimum creep rates — region 2 — the apparent activation energy is higher than the expected value of the activation enthalpy of lattice self-diffusion of any component in the investigated alloy. The dislocation mechanism dominating in region 2 has not been identified.The influence of stacking fault energy on the minimum creep rate in region 2 is weak or even zero.     

铁基块体非晶合金在纳米压痕过程中的蠕变行为研究

利用纳米压痕技术研究了{［（Fe_0.6Co_0.4）_0.75B_0.2Si_0.05］_0.96Nb_0.04}₉₆Cr₄铁基块体非晶合金的室温蠕变行为及不同的加载速率对该块体非晶合金蠕变变形的影响.{［（Fe_0.6Co_0.4）_0.75B_{0.2< 关键词： 块体非晶合金 蠕变 EVEV模型 蠕变速率敏感指数}     

High-temperature creep of nickel under conditions of grain-boundary diffusion of impurities from the surface

Comparative investigations of the effect of diffusion streams of copper atoms (a weakly segregating impurity) and silver atoms (a strongly segregating impurity) from the surface in the high-temperature plastic deformation of nickel have been carried out. It has been established that in the high-temperature plastic deformation of nickel, when there are diffusion streams of copper and silver from the surface, there is a reduction in the creep resistance of nickel due to an increase in the contribution of grain-boundary slip to the overall deformation. Two stages, characterized by different values of the rate sensitivity factor m, are observed on the curve of the stress against the rate of deformation over a certain temperature range for each impurity. In the region of deformation rates of sec⁻¹, m≊0.2, and for sec⁻¹, m becomes less than 0.05. Institute of Physics of the Strength and Study of Materials. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 119–125, July, 1997.     

The effect of Sn segregation at grain boundary on the steady-state creep of Cu-10 wt % Sn alloy

The effect of Sn solutes segregating into grain boundaries on the steady-state creep characteristics was studied for wires of a Cu-10 wt % Sn alloy having various grain diameters,d. The creep tests were performed for samples in the-phase in the temperature range: (0·5–0·58 T_m). The steady state creep rate, _s, was found to vary linearly withd. The variation in the stress sensitivity parameter,m, was found to be much less appreciable for large grains (>50m). Activation energy calculation showed that two competitive mechanisms are operating for creep deformation, namely, dislocation climb and viscous motion of matrix dislocations due to dragging of Sn atmosphere. A value of 6·74 kJ/mol was obtained for the binding energy between a Sn atom and the dislocation, while the binding energy between a vacancy and the Sn atom was found to be 33·71 kJ/mol.     

Dependence of creep characteristics on temperature and applied stress in Zn-1.0 wt.% Cu during transformation

The change in the transient and steady state creep deformation of zinc-1.0 wt.% Cu alloy was studied under various constant stresses ranging from 45.80 MPa to 56.02 MPa in the temperature range from 473 K to 573 K. From the transient creep results, the peak values of transient creep parametersB andn found in this temperature zone can be ascribed to dissolution of-phase (Cu-rich phase). The transient creep parameterB is related to the steady state creep rate through the exponent. This exponent has been found to range from 0.8 to 0.3. At the dissolution temperature (513 K) of-phase, the steady state strain sensitivity parameter has been 0.30±0.01 at the steady state strain peaks which is characteristic of dislocation climb along-grain boundaries. The activation energies of the transient and steady state creep in the phase transformation region have been found to be 42 kJ/mole and 63 kJ/mole characterizing the cross slipping of dislocations and dislocation climb along grain boundaries, respectively.     

Study of diffusion creep accompanying superplastic deformation of the alloy ZhS6KP

The development of diffusion creep (DC) accompanying superplastic deformation (SPD) of the highly doped nickel alloy ZhS6KP is studied based on an investigation of the redistribution of dispersed intragrain deposits of the phase of Ni₃(Al, Ti). Deposit-free zones (DFZ) are formed in the alloy, held at the temperature of SPD and accompanying deformation at the grain boundaries. The contribution of DC to the deformation was determined from a comparative analysis of the width of the DFZ in the deformed and undeformed parts of the samples for different grain sizes and rates of DC taking into account diffusion accommodation. It was established that for the optimum rate of SPD the upper limit of the contribution of DC to deformation for 2-m grains does not exceed 11%. It is concluded, based on the distribution of DFZ, that the effect of DC accompanying SPD is determined by diffusion fluxes, associated with the local concentration of stress accompanying the development of grain-boundary slipping.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 40–45, June, 1986.In conclusion, we thank Yu. M. Mishin for a useful discussion of the results.     

Investigation of creep in Ni3Ge alloy single crystals oriented along axes \left[ {\bar 234} \right] and \left[ {\bar 122} \right]

The creep of Ni₃Ge alloy single crystals is investigated at temperatures of 873, 923, and 973 K, which lie above the temperature of the anomalous peak behavior of mechanical properties with the deformation axis orientations $\left[ {\bar 234} \right]$ and $\left[ {\bar 122} \right]$ for which cubic slip is characteristic. It is shown that the steady state creep rate depends weakly on temperature.     

Experimental Study of High Temperature Fracture Behavior of A286 Superalloy at 650°C

In this paper an experimental work is done for investigation of high temperature fracture properties of A286 superalloy at 650°C. Stress intensity factor K and parameter C* for this superalloy are determined experimentally. For estimation of these parameters, an instrument is developed for investigation of high temperature fracture properties. For estimation of stress intensity factor, compliance method is used. For this purpose four different compact tension specimens are tested and the parameter K is estimated. Creep tests are done for the selected specimens and parameter C* is determined by semiempirical relationships at 650°C. In these tests it is concluded that the specimens are placed near the plane stress condition. Crack growth behavior of this alloy is also studied. High incubation time (600 h) leads to overaging and therefore this alloy after this time showed very ductile creep properties, and fast creep crack growth was the major result of this overaging phenomenon. Finally the obtained results are compared with well-known nonexperimental methods for determination of these parameters. The obtained results showed that the results are in good agreement with each other.     

Interpretation of the stress dependence of creep by a mixed climb mechanism in TiAl

The behaviour of ordinary dislocations in TiAl alloy creep-deformed at 750°C has been investigated. Two alloys processed by the cast and powder metallurgy routes were crept under tensile stresses of 150 and 80?MPa, respectively. Transmission electron microscopy was performed on the crept samples to determine the characteristics of the dislocations. The stress dependence of the dislocation mechanisms was determined from stress jumps performed during creep deformation. Complementary in-situ heating experiments performed on previously crept samples are also presented. From these experimental investigations, it is shown that some ordinary dislocations move by a mixed climb mechanism for which the elementary process is the nucleation and the lateral propagation of a jog pair. The coherency between this mechanism and the determined activation parameters, as well as the driving force at its origin, are discussed.     

Plastic deformation at high strain rates

In the present paper a new method, an original testing device and techniques for the study of plastic deformation in materials at high strain rates up to 2×10⁴ s^–1 achieved with the use of very short stress pulses (length within 10–20 s) are described. The suggested method of yieldpoint determination respects the effects of delayed plastic deformation in the yield point at loading by very short intensive stress pulses. The investigations were carried out in polycrystalline ARMCO-iron and low-alloyed steel, and besides on the specimens irradiated by neutrons with integral dose 1·35×10¹⁹ ncm^–2. The experimental results obtained are interpreted from the point of view of the present knowledge of the process of plastic deformation in bcc metals.     

Structural changes in TiC-TiNi alloys on deformation

The deformation of a composition material with a binding phase of shearing-unstable alloy, namely, titanium nickelide, is considered. X-ray structural analysis and electron—positron annihilation methods are used to investigate the structural changes in the composite material as a function of the degree of deformation. It is shown that the deformation produces the structural phase transition B2 B19, which is confirmed by x-ray structural investigations and electron positron annihilation.Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Uchebnykh Zavednii, Fizika, No. 4, pp. 100–103, April, 1994.     

The creep of alpha iron in the temperature range 220–900°C

The creep of alpha iron of about 99·9% purity has been studied by means of the isothermal test technique in the temperature interval of 500–1170°K. At temperatures lower than 800°K the apparent activation energy of creep depends primarily on the stress, at temperatures higher than 800°K but lower than the Curie temperatureT _c=1042°K it depends mainly on the temperature. At the Curie temperature the apparent activation energy reaches a value of about 175 kcal mol^–1. The stress dependence of the minimum creep rate cannot be generally described either by a simple power function of the ⁿ type, or by a simple function of the exp () type.An analysis of the results obtained leads to the conclusion that the minimum creep rate is controlled by the gliding movement of dislocations dependent on lattice self-diffusion. The mechanism controlling the creep rate at temperatures above the Curie point needs further study.     

Experimental and computational creep characterization of Al–Mg solid-solution alloy through instrumented indentation

Carefully designed indentation creep experiments and detailed finite-element computations were carried out in order to establish a robust and systematic method to extract creep properties accurately during indentation creep tests. Samples made from an Al–5.3?mol%?Mg solid-solution alloy were tested at temperatures ranging from 573 to 773?K. Finite-element simulations confirmed that, for a power-law creep material, the indentation creep strain field is indeed self-similar in a constant-load indentation creep test, except during short transient periods at the initial loading stage and when there is a deformation mechanism change. Self-similar indentation creep leads to a constitutive equation from which the power-law creep exponent n, the activation energy Q _c for creep, the back or internal stress and so on can be evaluated robustly. The creep stress exponent n was found to change distinctively from 4.8 to 3.2 below a critical stress level, while this critical stress decreases rapidly with increasing temperature. The activation energy for creep in the stress range of n = 3.2 was evaluated to be 123?kJ?mol^?1, close to the activation energy for mutual diffusion of this alloy, 130?kJ?mol^?1. Experimental results suggest that, within the n = 3.2 regime, the creep is rate controlled by viscous glide of dislocations which drag solute atmosphere and the back or internal stress is proportional to the average applied stress. These results are in good agreement with those obtained from conventional uniaxial creep tests in the dislocation creep regime. It is thus confirmed that indentation creep tests of Al–5.3?mol%?Mg solid-solution alloy at temperatures ranging from 573 to 773?K can be effectively used to extract material parameters equivalent to those obtained from conventional uniaxial creep tests in the dislocation creep regime.     

Untersuchungen im Anfangsbereich der Verformungskurve von Eisen-Einkristallen

The stress-strain curves of iron single crystals at room temperature and at –78°C (strain rate =5·5.10^–5 sec^–1) are characterized by a dynamical yield point followed by a strong work-hardening. The plastic deformation starts by the movement ofa/2111 dislocations from several places of the specimen. Macroscopically, the dislocations move approximately along the planes with maximum resolved shear stress. Slip bands are formed by multiplication of the moving dislocations. They broaden and fill the whole specimen at a strain smaller than 0·1% or 0·2% for the deformation at room temperature and –78°C respectively. The corresponding dislocation densities are <4.10⁶ cm^–2 and 2.10⁷ cm^–2 respectively. At these strains a strong work-hardening begins. The mosaic structure causes an increase of the yield stress and of the difference between the upper and lower yield stress. Differences between iron and iron-3% Si alloy single crystals at the beginning of the plastic deformation have only a quantitative character and can be explained by the fact that the lattice friction stress in iron is approximately 15 times smaller than in iron-3% Si alloy.

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Vorgetragen auf der Diskussionstagung der Arbeitsgemeinschaft Metallphysik über Die Plastizität und Strahlenschädigung von Metallen und Legierungen, 10.–13. Oktober 1967; (diese Tagung wurde mit Unterstützung des Wirtschaftsministeriums Baden-Würtemberg durchgeführt).

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Herrn Dr. F. Kroupa möchten wir für die wertvollen Ratschläge zur Diskussion der experimentellen Ergebnisse und Frau Dipl. Phys. N. Zárubová für ihre Bemerkungen zum Manuskript danken. Frau I. Velartová sei für ihre Hilfe bei den Berg-Barrett-Aufnahmen gedankt.     

Rotational properties of the odd-Z transfermium nucleus 255Lr by a particle-number-conserving method in the cranked shell model

Experimentally observed ground state band based on the 1/2-[521] Nilsson state and the first exited band based on the 7/2-[514]Nilsson state of the odd-Z nucleus ~(255)Lr are studied by the cranked shell model(CSM) with the paring correlations treated by the particle-number-conserving(PNC) method. This is the first time the detailed theoretical investigations are performed on these rotational bands. Both experimental kinematic and dynamic moments of inertia(J~(1)and J~(2)) versus rotational frequency are reproduced quite well by the PNC-CSM calculations. By comparing the theoretical kinematic moment of inertia J~(1) with the experimental ones extracted from different spin assignments, the spin 17/2~-→13/2~- is assigned to the lowest-lying 196.6(5) ke V transition of the 1/2~-[521] band, and 15/2~-→11/2~- to the 189(1) ke V transition of the 7/2~-[514] band, respectively. The proton N = 7 major shell is included in the calculations. The intruder of the high- j low-? 1 j_((15)/2)(1/2~-[770]) orbital at the high spin leads to band-crossings at ω≈0.20( ω≈0.25) Me V for the 7/2~-[514] α =-1/2(α = +1/2) band, and at ω≈0.175 Me V for the1/2~-[521] α =-1/2 band, respectively. Further investigations show that the band-crossing frequencies are quadrupole deformation dependent.