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.     

Microstructure dependence of yielding and fracture in Cd-Zn alloys

The coefficient of logarithmic work-hardening, the yield stress and the fracture stress of Cd-2 wt. %Zn alloy of different grain diameters and of Cd-17·4 wt. %Zn alloy decrease with increasing working temperature. Two relaxation temperature regions have been found, the low-temperature region of relaxation (below 483 K) and the high-temperature region (above 483 K). The fracture surface energy for Cd-2 wt. % Zn alloy has been calculated and found to be 1·2 J/m² at the two temperature regions of relaxation. X-ray investigations show that the residual internal strains in the deformed samples increase with increasing working temperature and exhibit a peak value at 483 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.     

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.     

Anomalous behaviour of transient creep in commercially pure aluminium

Transient creep of pure and commercial aluminium was studied under a constant stress of 18·6 MPa at different temperatures ranging from 613 to 698 K. The anomalous values of transient creep parameters, n ranging from 0·003–0·055 (±0·001) and 0·5–1·0 (±0·05) respectively, which were found in this temperature rangs, might bs ascribed to the superplastic behaviour of pure and commercial aluminium. The activation energy of transient creep for pure Al was found to be about (2·4 +0·07) × 10^–22) kJ/atom. characterizing a dislocation glide-cross slipping mechanism.     

Microstructure dependence of steady state creep in Cd–Sn eutectic alloy

The steady state creep of Sn–33 wt.% Cd alloy was studied under various constant stresses ranging from 25.56 to 30.85 MPa in the temperature range from 353 to 433 K. The stress exponent n was found to change from 6.25 to 4.55 in the above temperature range. The energy activating the steady state creep amounted to 59.3kJ/mol in the temperature range from 353 K to 393 K and to 37 kJ/mol in the temperature range from 413 K to 433 K characterizing the grain boundary diffusion in Cd and in Sn, respectively. Microstructure analysis confirmed that the above mentioned mechanisms took place during steady state creep.     

Creep Behaviour of Fe-Mn Binary Alloys

Tensile creep behaviour of fine-grained Fe-Mn binary alloys containing 0.42-1.21 wt. % Mn has been investigated in the temperature range from room temperature to 475K under 10-50 MPa. Tensile tests are carried out with a constant cross-head speed under uniaxial load at a strain rate 10^-4s^-1. Stress exponent and activation energy are determined to clarify deformation mechanism. The obtained variation of steady state creep rate with respect to the applied stress for Fe-Mn binary alloys exhibits two distinct regimes at about 20 MPa, indicating a possible change in creep mechanism. The average stress exponent is approximately 2.2, which is a characteristic of grain boundary sliding in the alloys. The activation energy for plastic flow varies from 135 to 92k J/mol, depending on the Mn content.     

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 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.     

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

In the paper the results of the deformation behaviour study in a Zn-1·1 wt. % Al alloy are presented. The influence of temperature ranging from 223 K (0·34T_m) to 645 K (0·98 T_m) on the flow stress, strain rate sensitivity parameterm and activation energy was investigated at various strain rates. The superplastic behaviour was observed in a very broad temperature interval the limits of which were dependent on the strain rate. The best superplastic properties—ductility A>1000 % and parameterm0·7—were established at strain rates at temperatureT=520 K. Superplastic characteristics were found in coarser-grained samples with grain sizes up to 50 m. An intensive grain growth occurring at temperaturesT370 K was found to be probably responsible for the origin of anomalies in temperature dependences of the flow stress and parameterm. The activation energy established in the region of superplastic behaviour was much lower than the activation energy measured in the non-superplastic region. The valueQ90 kJ/mol obtained in the non-superplastic region at very high temperatures is comparable with the activation energy of lattice selfdiffusion in pure Zn or heterodiffusion of Al in Zn along the hexagonalc-axis. This result suggests that the deformation mechanism in the non-superplastic region has a nature similar to the high-temperature dislocation creep. The activation energyQ 20;40 kJ/mol obtained in the superplastic region cannot be compared with activation energies of any known diffusion types in Zn. The explanation of this discrepancy could be find in the presence of other temperature dependent terms the contributions of which to the temperature dependence of the flow stress were neglected. The structure study confirmed the decrease in the contribution of grain boundary sliding when passing from the superplastic to the non-superplastic region.     

Phase relations and transformations in the system PbTe-GeTe

Results of a study of the pseudobinary system PbTe-GeTe are reported and discussed. A new phase diagram, the dependence of the lattice constants on alloy composition, and measurements of a phase transformation in Pb_1−xGe_xTe are presented. Complete solid solubility is found above 570°C. An exsolution dome extends from a maximum at 570°C (near 60 mole % GeTe) to about 5 and 96 mole % GeTe at 300°C. For alloys with compositions near GeTe the unit cell parameters depend markedly on the concentration of cation vacancies. The temperature for the cubic to trigonal phase transformation depends on alloy composition, decreasing from about 670°K for x = 1 to 0°K for x ≈ 0·01. The variation of lattice parameters at the transition temperature is continuous within experimental precision.     

Mechanical and electrical properties of supersaturated Mg based alloys

Isothermal annealing of creep resistant Mg-6 wt. % Y-3 wt. % Nd and Mg-6 wt. % Y-3 wt. % Nd-0·4 wt. % Zr in the temperature range 150–350 °C was investigated by hardness testing. The observed hardness changes are ascribed to a rearrangement of solutes (most probably of yttrium atoms). An attempt was made to study this rearrangement of solute atoms by electrical resistance and differential scanning calorimetry measurements.     

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.     

Resistivity changes in cold-worked Cd-Ag and Cd-Zn

Polycrystalline wires of Cd-0.3 at. %Ag, Cd-1.0 at. % Ag, Cd-3.3 at. % Ag and Cd-0.1 at. % Zn, Cd-0.5 at. % Zn, Cd-2.6 at. % Zn were deformed by torsion in liquid nitrogen. Up to 200 K their isochronous annealing spectrum of electrical resistivity reveals the same main features (i.e. annealing stages C1, C2 and C3) as that of pure Cd and of dilute alloys Cd-Mg and Cd-Sn. The position of the annealing stages is insensitive to the type and amount of impurity and the degree of deformation. The gradual diminution of stage C3 with increasing Ag content is ascribed either to a dislocation mechanism or a disappearance of point defects by short range migration.We wish to express our thanks to Dr. P.Luká and Dr. I.Stulíková (Charles University, Prague) for stimulating discussions and valuable comments.     

Resolved shear stress of Zn single crystals deformed in (0001)\left\langle {11\bar 20} \right\rangle system

Resolved shear stress (RSS) in basal slip system in Zn, ZnAg 0·2 at. % and ZnGa 0·2 at. % crystals was investigated in the temperature range from 4·2 K to 515 K. The TEM observations of the dislocation structure were also performed. At low temperatures an anomaly was observed, i.e. decrease and subsequent increase of RSS in the range of 4·2 K to 77 K. This effect may result from other properties of the dislocations at low temperatures like those suggested by Wille and Schwink (1985) and based on the zero-point vibrations of dislocations theory.     

Grain boundary deformation in Cd-Zn and Zn-Al alloys

Activation energies for creep have been measured on fine-grained and coarse-grained specimens of pure cadmium, zinc, and Cd-Zn and Zn-Al alloys in the temperature range (0·4–0·8)T _m. It is found that in the case of fine-grained specimens the activation energies for creep are equal to the activation energies for grain boundary diffusion in cadmium and zinc, and in the case of coarse-grained specimens — to that of volume self-diffusion.     

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%).     

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.     

Deformation of Cd-based alloys single crystals at very low temperatures

The temperature dependence of the critical resolved shear stress — CRSS on Cd-Ag andCd-Zn single crystals was studied at very low temperatures 1·5–80 K. The deformation experi-ments were made by a creep technique. The CRSS for Cd-Ag alloys was determined from re-constructed shear stress — shear strain curves, while the method of one sample was applied tothe determination of ₀-T dependence for Cd-Zn alloys. The difference in the temperaturedependences of ₀ for both Cd-based alloys can be caused by different methods for determiningthe CRSS.