John E. Dorn

Viscous creep of aluminum near its melting temperature

Abstract The creep of pure polycrystalline aluminum was studied at low stresses, and temperatures near the melting point. It was found that at stresses between 3 and 13 lb/in2 the secondary creep rate varied linearly with the stress, whereas at slightly higher stresses, the creep rate increased with the fourth power of the stress. The activation energy for creep was determined to be about 35,500 calories per mole over the entire stress range. The low stress creep results in the range of “viscous” creep were analysed from the viewpoint of the Nabarro-Herring model for stress directed self-diffusion of vacancies, and it was found that the experimental creep rates were about one thousandfold greater than those predicted theoretically. This factor plus additional experimental observations on the displacements of markers at grain boundaries, and on creep recovery upon removal of the stress led to the conclusion that the Nabarro-Herring model does not apply to the case of aluminum, and that creep occurs by a dislocation climb mechanism at all stresses considered.

Some fundamental experiments on high temperature creep

At high temperatures, the creep strain, ϵ, appears to be a function of a temperature (T), compensated time (t), namely te−ΔHRT, and the stress. X-ray analyses and plastic properties reveal that the same structures are developed at the same values of te−ΔHRT following creep at the same stress. Thus ϵ = ƒ(te−ΔHRT), for the same stress. When the creep rate, ϵ, is evaluated as a function of stress, σ, for the same structure ϵ = Se−ΔHRT φ(σ) where S depends on the structure and Although B and n appear to be insensitive to structural changes attending creep of annealed alloys, they decrease with increasing solute additions and cold working. Transients attending loading and unloading and the coincidence of the activation energy for creep, ΔH, with that for self-diffusion suggest that high temperature creep might be ascribed to a dislocation climb process.

Viscous glide, dislocation climb and newtonian viscous deformation mechanisms of high temperature creep in Al-3Mg

Abstract Studied in Al-3Mg solid solution alloy in the stress range of 10−6–10−3 G using double-shear type specimens in a range of temperatures to near the melting point. A composite plot of dimensionless parameters λ kT DGb vs.τ/G indicated three distinct regions: region I extending from stresses of about 6 × 10−5 G and above, region II from 10−5 G to 6 × 10−5 G and region III below 10−5 G. Plots of logarithm of modulus-compensated strain-rate vs. reciprocal of the absolute temperature in all the three regions yielded an average value of 33 kcal/mole for the activation energy for creep. This value is in essential agreement with the activation energy for self-diffusion. Values for the stress exponents were found to be 3.2, 4.1 and 0.9, respectively, in regions I, II and III. Viscous glide and dislocation climb mechanisms were found to be operative in regions I and II, respectively. Present experimental results in region III agreed very closely with the earlier data on pure Al in this normalized stress range. Modified versions of models based on jogged screw dislocation and/or climb of jogged edge dislocation were found to explain the present experimental results on Al-3Mg as well as the earlier data on Al. Experimental results were compared, in addition, with various theories of creep, and Nabarro, Coble, Nabarro-Herring in subgrains as well as Nabarro-Bardeen-Herring creep mechanisms were found to make at most a minute contribution to the overall creep-rate.

Activation energies for creep of high-purity aluminum

Abstract Activation energies for creep of high-purity aluminum were obtained over the temperature range from 77°K to 880°K by rapidly changing the temperature during creep at constant stress. The experimentally obtained activation energy was shown to be insensitive to stress and strain, and this fact questions the validity of those theories for creep that postulate stress and strain dependent activation energies. From 500°K to 880°K the activation energy for creep was found to be independent of temperature and equal to 35,500 calories per mole which is the same as that for self-diffusion of aluminum. Between 0.25 and 0.40 Tm (250 to 375°K) the activation energy for creep was found to be equal to about 27,500 calories per mole. Below 0.25 Tm the activation energy for creep was found to decrease rapidly with decreasing temperature; it was shown that the presence of at least four discrete activation energies could account for these low temperature results.

Creep of aluminum under extremely small stresses

Abstract At high temperatures Al exhibits two laws for creep, dependent on the stress. At stresses below 13.5 lb/in2 the creep rate increases linearly with the stress, and at intermediate stresses between 33.5 and 1000 lb/in2 the creep rate increases with a power of the stress. Creep in both regions is believed to take place by dislocation mechanisms. Over the low stress range the contribution of grain boundary shearing to the total creep strain increases linearly with the stress. But over the intermediate range of stresses, the grain boundary shearing contribution to creep rate decreases as the stress increases. The two distinctly different laws for grain boundary shearing reflect the fact that high temperature creep is controlled by two distinctly different crystallographic mechanisms, the relative proportions of each being dependent on the stress. A dislocation model involving motion of jogged screw dislocations appears to account well for the low stress creep behavior of Al. Intermediate stress creep behavior was found to agree with Weertmans dislocation climb creep hypothesis.

Anelastic creep of polymethyl methacrylate

Abstract The deformation of polymethyl methacrylate over the temperature range 263° K to at least 410° K is entirely anelastic. That is, removal of the stress causes the material to return completely to its original length, provided sufficient time is allowed to permit complete recovery to take place. In the temperature range 263° K to 320° K it is shown that the activation energy for creep is independent of the strain but decreases linearly with an increase in the applied creep stress. These results are strikingly dissimilar from the results obtained for metals where the activation energy for creep is known to be insensitive to the applied creep stress. Above 320° K, the activation energy for creep increases rapidly with increasing temperature. In this range, transients are observed in the creep curves when sudden changes in temperature are made. These transients are best explained by a second order phase change occurring over a range of temperature.

Viscous drag on dislocations in aluminum at high strain rates

The dislocation drag coefficient of aluminum single crystals was measured by the strain rate method at 10°K, 77°K, 300°K and 500°K. A maximum shear strain rate of 2.6 × 104sec−1 was obtained by the Kolsky thin wafer technique in pure shear. The stresses at one percent and at 20 percent shear strain were a linear function of strain rate above a back stress τB. The back stress was attributed to the over-coming of internal stresses, both long range and short range, associated with forest dislocations without the aid of thermal fluctuations. The temperature dependence and the effect of strain on the dislocation drag coefficient are discussed in terms of the dislocation damping theories.

Dislocation Damping in Aluminum at High Strain Rates

Impact shear tests of the Kolsky thin‐wafer type were used to determine the effect of temperature and strain rate on the critical resolved shear stress for slip in aluminum single crystals at strain rates of 104 sec−1 and in the temperature range 20° to 500°K. The aluminum deformed in a viscous manner in that the flow stress was proportional to the plastic strain rate. The behavior was found to be temperature‐dependent. The results are discussed in terms of dislocation damping models where the friction force acting on a dislocation results from, at cryogenic temperatures, electronic viscosity, and at higher temperatures, phonon viscosity. The theories predicted general agreement as to the magnitude of the observed damping but some discrepancy was found to exist between the observed and theoretical temperature dependence of the damping.

High-temperature deformation mechanisms in superplastig ′n-22Al eutectoid

Abstract The temperature and the stress dependences of steady-state strain-rates in ′n-22Al eutectoid ′ere studied by tensile and creep testing using double shear type specimens in a normali′ed stress ( τ G ) range of ~5 × 10 −7 to ~5 × 10 −3 . The stress dependence of the strain-rate revealed three distinct regions ′ith stress exponents of 0.87 ± 0.12, 1.99 ± 0.15 and 3.73 ± 0.17. The activation energy for deformation obtained in the regions I and II ′as identified as that for grain-boundary diffusion ′hile that found in the region III ′as approximately e′ual to the self-diffusion value. The experimental results in the employed stress range obeyed the follo′ing phenomenological e′uation, γ ggkT D b Gb = A( b d ) m ( τ G ) n ( D D 1 ) α . From the experimental values of the creep parameters n , m , α and A , the mechanisms of deformation are identified. Dislocation climb and Coble creep ′ere found to be the controlling mechanisms in the regions III and I respectively. The present results in the region II are in close agreement ′ith the predictions based on Ball-Hutchison model for superplasticity, and the models based on grain boundary sliding yield strain-rates about 250 times the experimental values.

A universal law for high-temperature diffusion controlled transient creep

It is suggested that transient creep at high temperatures arises principally as a result of the dispersal of entanglements by the climb mechanism. The dispersal of the entanglements is assumed to follow a unimolecular reaction kinetics with a rate constant that depends on stress and temperature in the same way as does the secondary creep rate. The analysis shows that the strain (e) versus time (t) relation can be represented by e=e0+e.3t+β−1K[1-exp(−Ke3t)] , where e0 is the instantaneous strain on loading, e3, the secondary creep rate, Ke3 the rate constant, and β the ratio of initial to secondary creep rate. The experimental creep data on several b.c.c. and f.c.c. metals and alloys correlate quite well with the proposed mechanism. The constants β and K were found to be independent of temperature and stress. The proposed formulation becomes inapplicable for correlating creep data in polycrystals at low stresses because of the significant contribution of grain-boundary sliding to the total creep at these stress levels.