Steady state creep during transformation in Cd-Zn alloys

Apparent steady-state creep of Cd-2 wt. % Zn and Cd-17·4 wt. %Zn alloys has been studied under different constant stresses ranging from 6·4 MPa to 12·7 MPa, near the transformation temperature of 483 K. The strain rate of the steady-state creep for both compositions has shown two temperature regions of deformation, the low-temperature region (below 483 K) and the high-temperature region above this temperature. The stress exponent m' was found to change from 4·7 to 2 for Cd-2 wt. % Zn and from 3·1 to 2·4 for Cd-17·4 wt. % Zn alloys. The activation energies in the temperature region below the transformation temperature have been found to be 84 kJ/mole for Cd-2 wt. % Zn and 70 kJ/mole for Cd-17·4 wt. % Zn alloys characterizing the mechanism of volume self-diffusion in Cd.     

Transient creep during transformation in Cd-Zn alloys

Transient creep of Cd-2 wt. % Zn and Cd-17·4 wt. % Zn alloys has been studied under different constant stresses ranging from 6·4 MPa to 12·7 MPa near the transformation temperature. The results of both compositions showed two transient deformation regions, the low temperature region (below 483 K) and the high temperature region (above 483 K). From the transient creep described by the equation _tr=Bt ⁿ, where _tr andt are the transient creep strain and time. The parametersB andn were calculated. The parameterB was found to change with the applied stress from 0·3×10^–4 to 3×10^–4 and from 0·6×10^–4 to 18×10^–4 for Cd-2 wt. % Zn and Cd-17·4 wt. % Zn, respectively. The exponentn was found to change from 0·8 to 0·95 for both alloys. The parameterB was related to the steady state creep rate through the equation , the exponent was found to be 0·5 for Cd-2 wt. % Zn and 0·6 for the eutectic composition. The activation energies of transient creep in the vicinity of the transformation regions (above 483 K) were found to be 50·2 kJ/mole for Cd-2 wt. % Zn and 104·7 kJ/mole for the eutectic composition characterizing the mechanisms of grain boundary diffusion and volume diffusion in Cd, respectively.     

Microstructure changes and steady state creep characteristics in Pb-Sn alloys during transformation

The steady state creep of Pb-10 wt.% Sn and Pb-61·9 wt.% Sn alloys have been investigated under different constant stresses near the transformation temperature. The temperature dependence of steady creep rate has shown two different transition points; at 423 K for Pb-10 wt.% Sn alloy and at 403 K for Pb-61·9 wt.% Sn (the eutectic composition). The strain rate sensitivity parameter (m) has been found to increase by raising the working temperature and to reach 0·45 and 0·85 for the first and second alloy, respectively. The activation energies of steady state creep of Pb-10 wt. % Sn have been found to be 46·2 kJ/mole and 88·2 kJ/mole in the low and high temperature regions (below and above 423 K) referring to dislocation and self diffusion mechanisms. While activation energies of steady creep in Pb-61·9 wt.% Sn have been found to be 42 kJ/mole and 63 kJ/mole in the low and high temperature region (below and above 403 K), characterizing grain boundary diffusion in Sn and Pb respectively. X-ray analysis and microscopic investigations of the test alloys have confirmed the above mentioned mechanisms.     

Effect of phase transformation on the work hardening characteristics of Al-4.5 wt.% Mg alloy

Stress-strain curves of slowly cooled and/or quenched Al-4.5 wt.% Mg alloy samples were studied in the temperature range from 473 K to 553 K. Two main temperature regions of relaxation have been found, a low temperature region (below 493 K) and a high temperature region (above 513 K); a transient region lies between these temperatures. The parabolic work hardening coefficient, the fracture time, the yield stress and the fracture stress of annealed and quenched samples have been found to decrease with increasing deformation temperature and to exhibit minimum values at 493 K. The X-ray analysis of the slowly cooled and quenched samples has shown that the lattice parameter a of the aluminium matrix reaches a minimum value at 493 K.     

Stress enhanced diffusion process in Al-10 wt.% Zn alloy

The change in the steady state creep of Al-10 wt. % Zn alloy was studied under various constant stresses ranging from 77 MPa to 88·3 MPa and at different constant temperatures ranging from 423 K to 483 K. The strain rate sensitivity parameter (m) varied between 0·15 to (0·4 ± 0·05) in the above temperature range. The energy activating the steady state creep amounted to 70·3 kJ/mole in the temperature range from 423 K to 443 K and to 124·3 kJ/mole in the temperature range from 453 K to 483 K characterizing the grain boundary diffusion of Zn in B-phase and Al in-phase, respectively. This was affected by increasing the applied stress. The decrease in the activation energy was attributed to the stress enhanced diffusion processes. Microstructural analysis confirmed that the above mentioned mechanisms took place during steady state creep.     

Effect of dissolution of Θ-phase on characteristic parameters of work-hardening in AL-2·5wt. % Cu alloy

Stress-strain curves of slowly cooled and quenched Al-2·5 wt. % Cu alloy were studied in the temperature range 693 K to 773 K. The linear work-hardening coefficient, the fracture time, the yield stress and the fracture stress of annealed and quenched samples decreased with increasing deformation temperature and exhibited a minimum at 733 K. The X-ray analysis of the slowly cooled and quenched samples showed that the lattice parametera of Al-matrix and the ratioc/a of the tetragonal -phase reached a minimum and a maximum value, respectively, at the dissolution temperature.     

Thermally (non-)activated deformation of Zr-Sn polycrystals

A non-monotonic temperature dependence of the activation volume of alpha zirconium is observed in the temperature range between 500 and 750 K. In this temperature range polycrystals of Zr-Sn alloys (with concentrations of 0·8, 3·0, 4·5 and 6·0 wt. % of Sn) have been deformed in order to investigate the influence of tin on the thermally activated deformation. The activation volume has been determined using the stress relaxation test.     

The influence of temperature on superplasticity of Zn-Al-Cd alloy

Experimental results of the study of superplastic behaviour in Zn-0·35 wt. % Al-0·25 wt. % Cd alloy are presented. The attention has been paid to the influence of temperature on stress-strain curves, ductility, strain rate sensitivy parameterm and activation energy of superplastic flow. The best superplastic properties have been established at temperatures T370 K (0·53 T_m): ductilityA=600% and parameterm=0·56. A fast grain growth observed at temperaturesT 380 K has been suggested to be responsible both for the decrease in ductility and parameterm values and for the origin of an anomaly in the decreasing temperature dependence of flow stress. The measurements of activation energy have shown an expected decrease in activation energy at the transition from the non-superplastic region at lower temperatures to the superplastic region at higher temperatures. Significant transient effects after strain rate changes have been observed. The experimental results obtained in the Zn-Al-Cd alloy have been compared with those obtained in binary Zn-1·1 wt. % Al alloy and discussed from the point of view of possible models of structural superplasticity.     

Superplasticity in A Zn-1·1 wt. % Al alloy

In the paper the results of the deformation behaviour study in a fine-grained Zn-1·1 wt. % Al alloy are presented. The influence of strain rate ranging from 4·2×10^–5 to 4·2×10^–2 s^–1 on the true stress — true strain curves, ductility and strain rate sensitivity was investigated at temperatures 295 K and 375 K. At both temperatures the superplastic behaviour was observed. The increase in temperature improved the superplastic properties and shifted the region of the superplastic behaviour to higher strain rates. The maximum values of ductilityA=700% and strain rate sensitivity parameter m=0·48 were established at 375 K at strain rates . The true stress — true strain curves were found to be influenced by a grain growth taking place during the deformation. The grain growth was also found to be responsible for significant differences in the parameterm values obtained from the log plots and from the strain rat-changes.     

Effect of Heat Treatment on the Stress-Strain Characteristics of Cd-Sn Eutectic Alloy

Stress-strain curves of slowly cooled and quenched Sn-33wt. alloy were studied in the temperature range from 353 to 433 K. The coefficient of logarithmic work/hardening, the fracture time, the yield stress and the fracture stress of annealed and quenched samples decreased with increasinig deformation temperature and exhibited a minimum at 393 K. The activation energy of the fracture mechanism is found to be 5.4 and 12.8 KJ/mol characterizing a dislocation mechanism for annealed and quenched samples. The X-ray analysis of the slowly cooled and quenched samples showed that the residual internal strains increase in the low temperature relaxation stage and decrease in the second stage.     

The influence of Cd on the superplasticity of Zn-Al alloys

In the paper the results of the superplastic deformation study in the fine grained Zn-1·1 wt.% Al and Zn-0·35 wt.% Al — 0·25 wt.% Cd alloy are presented. The influence of the long-termed ageing at room temperature on the deformation characteristics is investigated and their changes are explained on the basis of the grain growth. The presence of Cd is found to increase the stability of the fine grained structure. The influence of strain rate is studied at 293 and 373 K. Both alloys exhibit superplastic properties with maximum ductilitiesA600% and maximum values of the parameterm0·5. The region of the best superplastic properties is shifted to slower strain rates as a consequence of the Cd atoms presence. The flow stress corresponding to a given strain rate is found to be much higher in the Zn-Al-Cd alloy. The grain boundary segregation of Cd atoms is suggested as a possible reason for better stability of the fine grained structure in the Zn-Al-Cd alloy as well as for the differences observed in the deformation behaviour of both alleys studied.     

Recovery during stress relaxation in Cd-Ag alloy single crystals

Stress relaxations in a series of cadmium-silver alloy single crystals containing up to 0·25 at% Ag were carried out at temperatures of 77 K, 145 K and 200 K. The samples were deformed in tension at a strain rate of 1×10^–4 s^–1. When the specimens deformed at 145 K and 200 K were reloaded after stress relaxation, the deformation started with the flow stress which was lower than that before stress relaxation. The observed differences in the stresses could be due to a variation in the internal stress. The activation volume should be estimated from the part of stress relaxation in which the internal stress remains practically constant.     

Determination of internal stresses in cyclically deformed metals

Single crystals of Fe and Fe-0·5 wt. % Si alloy were cyclically deformed to saturation. Internal stresses have been determined by stress relaxation and stress dip methods as a function of prior strain rate. The values of internal stresses are generally strain rate dependent and are systematically higher for the stress relaxation method. The physical meaning of strain rate independent internal stress obtained by extrapolation to zero strain rate is discussed.     

Superplasticity in A Zn-1·1 wt.% Al alloy III. The influence of grain size

The paper represents the third instalment of the series dealing with the superplastic deformation in a Zn-1·1 wt. % Al alloy and is devoted to the influence of grain size on the deformation behaviour of this alloy. Deformation characteristics were measured at two temperatures — 293 and 500 K. The grain size dependence of the flow stress observed was found opposite to that predicted by the Hall-Petch relation. Such a behaviour was explained under the assumption that grain boundaries might act as sites of rapid recovery of lattice dislocations. The results obtained at temperature 293 K proved that the transition between the regions of abnormal behaviour (with the flow stress increasing with increasing grain size) and normal behaviour (with the flow stress decreasing with increasing grain size) was not directly connected with the transition from the superplastic to the nonsuperplastic state. The results obtained at 500 K revealed dynamic recrystallization and a grain refinement in samples with initially coarser structures. Such a change in grain structure was accompanied with a development of superplastic characteristics.     

Superplasticity in single-phase Zn-0·25 wt. % Cd alloy

A single-phase Zn-0·25 wt. % Cd alloy was prepared with the mean grain sized¯ 1m. Superplastic characteristics (A _max=320% andm _max>0·4) were established at room temperature at the strain rates.10^–4s^–1. Because of the absence of any second phase particles the fine-grained structure was not stable and an intense grain coarsening was observed both during the long-term ageing and during straining at room temperature. The increasing temperature accelerated this grain coarsening and suppressed the superplastic behaviour. Nevertheless, the stress relaxation tests suggested that the superplastic behaviour might be expected at higher temperatures in coarser grained specimens, too, but at substantially lower strain rates.     

Thermoactivated creep and stress relaxation in Cd + 0·08 at% Sn single crystals

The activation area of plastically deformed Cd + 0·08 at% Sn single crystals was determined from differential creep test and stress relaxation at temperatures 78 K, 200 K and 295 K. The activation areaA depends on the resolved shear stress, ,A ^{* –r} wherer is dependent on temperature and the testing method used. For temperatures above 0·3T _m recovery during stress relaxation should be 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.     

Diffusion coefficient of selenium in indium antimonide

The diffusion of selenium in indium antimonide has been studied in the temperature range 400–490°C, by the method of removing layers. Two regions have been distinguished in the donor distribution profiles. The first has a low diffusion coefficient and a high surface concentration, near to the limit of solubility of selenium (8·10¹⁸ cm^–3). In the second region, a much larger diffusion coefficient and a surface concentration lower by two orders of magnitude (8·10¹⁶ cm^–3) with weak temperature dependence are found. The temperature dependences of the diffusion coefficients of the first and second regions can be described by the expressions: D=4.8·10¹³ exp(–4.1 eV/kT) cm²/sec, D₂=1.9·10¹³ exp(–3.9 eV/kT) cm²/sec.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 40–43, November, 1991.     

Study of stress-strain characteristics of SBR and blended NR/SBR rubber

The effect of temperature and carbon black (CB) on the mechanical characteristics of styrene-butadine rubber (SBR) and natural rubber (NR) was studied at various temperatures. The relation obtained between true stress and true strain for both types of rubber showed three regions at room temperature and two regions at high temperature. The optimum CB concentration was found to be 95 phr for the unblended samples as it increases the stiffness of the SBR rubber materials at a maximum value. It was also found that the addition of NR to SBR increases the elasticity in the plastic region. The activation energy at the fracture of SBR samples decreased from about 2.7×10^–20 to 1.8×10^–20 J while for the blended samples NR/SBR it increased from 8×10^–20 to 10.1×10^–20 J with increasing CB concentration.     

Investigation of the behaviour of the impurity atoms in Si by μ− SR-method

The dependence of the residual polarization of negative muons in p-type Si on temperature in the 4.2–270 K range has been investigated. Measurements were carried out in external magnetic field of 0.08 T transverse to the muon spin. The impurity concentration in the sample was 2 · 10¹³ cm^–3. Muon spin relaxation was observed at temperatures below 30 K. The relaxation rate atT=30 K is equal to 0.18±0.08s^–1. The relaxation rate grows with the decrease of temperature and at 4.2 K exceeds 30s^–1. The value of the residual polarization at zero timeP(t=0) is constant within the investigated temperature range.In the rangeT<30 K data on the relaxation rate are well described by the dependence =B·T^–q, whereq=2.75. Power dependence of may evidence the essential role of the phonon mechanism in the relaxation of the electron momentum of the acceptor center.The authors express their gratitude to V.B. Brudanin and I.A. Yutlandov for providing the sample, and to Yu.B. Gurov for advices.