Activation Parameters Of Deformation Research Articles

Effect of ultrasound on the characteristics of superplastic deformation

The deformation of aluminum–lithium alloy 1420 is investigated in the region of temperatures of its superplasticity T = 320–395°C under the simultaneous action of stretching load and axial ultrasonic vibrations. The estimates of the activation parameters of deformation show that the activation energy of deformation during stretching with ultrasonic vibrations and without them are close both at the stage of hardening and at the stage of softening. It is concluded that the ultrasonic vibrations facilitate intragranular deformation at the hardening stage and promote an increase in its contribution to the total deformation without changing the deformation mechanisms.

Grain size dependence of the activation parameters for plastic deformation: Influence of crystal structure, slip system, and rate-controlling dislocation mechanism

A critical review of available results on the dependence of grain size on the activation parameters for deformation, specifically, the activation volume, V*, and the thermal component of flow stress, σ*, has been carried out with a view to verifying the Armstrong prediction that identifies the Hall-Petch (H-P) intercept with the easy slip system and the H-P slope with the most difficult system in polycrystals. The influence of slip system choice is demonstrated using results on Cd and Zr. The Armstrong prediction is valid for basal slip hcp metals, such as Cd and Zn, with V* and σ* determined by the difficult pyramidal slip. For the prism slip metals such as Zr and Ti, V* and σ* are controlled by interstitial solutes and are independent of grain size. The results on Zr are used to highlight the influence of dynamic strain aging on the H-P parameters. In bcc metals, in which the Peierls-Nabarro barrier is the rate-controlling obstacle, V* and σ* are again independent of grain size. For fcc metals, correlation of the H-P slope with the cross-slip stress, predicted by the Armstrong model, has been demonstrated for a few cases. The variation of V* with grain size in Ni as reported by Narutani and Takamura (Acta Metall. Mater., 1991, vol. 227, pp. 2037–49) is newly interpreted in terms of the Armstrong model that associates the H-P intercept in fcc metals with dislocation intersections and the H-P slope with cross-slip, and provides realistic results for the activation volumes for the two processes.

Plastic deformation of yttria stabilized cubic zirconia single crystals I. Activation parameters of deformation

Cubic zirconia containing 10 mol% yttria was deformed along the hard 〈100〉 direction and materials with 15 and 20 mol% yttria along the soft 〈112〉 orientation between 1400 °C and the lowest possible temperatures without brittle failure. The 10% material shows along 〈100〉 a strong increase in the flow stress at decreasing temperature, while the materials with 15 and 20 mol% yttria along 〈112〉 exhibit a higher flow stress with a weak temperature dependence. The activation parameters of deformation were measured by stress relaxation and temperature change tests. The microstructures are studied by high-voltage transmission electron microscopy. The results are interpreted by long-range interactions between the dislocations, the Peierls mechanism at the lowest temperatures, precipitation hardening between 500 °C and 800 °C, and by recovery-controlled deformation at high temperatures. The occurrence of plastic instabilities are described in Part II [phys. stat. sol. (a) 201, No. 1 (2004)] of this paper. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal activation parameters of deformation and the critical stress intensity factor of low- and medium-strength steels

Thermal activation parameters of deformation and the critical stress intensity factor of low- and medium-strength steels

Structural aspects of deformation of solid cross-linked polymers

1. The role of the structure of cross-linked polymers in the highly elastic state in the formation of the structure and properties of the glassy state was demonstrated. 2. The activation parameters of deformation characterize the structure of cross-linked polymers and are used for the quantitative estimation of the deformational longevity.

Features of the kinetics of deformation or oriented polyethylene

IR spectroscopy was used to determine the concentration of molecular rupture for different levels of deformation in uniaxially oriented polyethylene. It appeared that molecular chain rupture occurs from the initial elongation and the number of chain rupture increases continuously, reaching saturation in the range of “steady” creep. A study was made of kinetics of deformation of oriented polyethylene in a wide range of stress and temperature. Activation parameters of deformation were determined according to the relative elongation of the sample. Comparison of kinetics of macrodeformation with processes taking place on the molecular level suggests that the conformation mechanism which controls kinetics of deformation, be replaced by a breakdown mechanism on transition to the range of “steady” creep.

Relevance of cage recombination in the plastic deformation of polymers

AbstractFor a polymer in which permanent rupture of individual molecules is the rate‐limiting process for plastic deformation, the kinetics of chain‐end diffusion and secondary radical reactions should be compared with the kinetics of caged radical recombination in the calculation of activation parameters for plastic deformation. If mechanisms of cage escape are slower than those for cage recombination, the activation parameters for plastic deformation will differ from those for the initial bond‐breaking process. For the case of polyethylene deformed in the vicinity of 250°K, the critical thermally activated event appears to involve scission of the polymer molecule near the site of an abstracted hydrogen atom. For this system the dominant cage‐escape mechanism is diffusion, which is faster than either hydrogen abstraction or unzipping to the monomer. However, at low stresses the rate of cage recombination is expected to be higher than the rate of cage escape, so that the activation parameters for deformation should be the sum of those for chain scission and diffusion. The contribution of diffusion (ca. 15 kcal/mole) to the activation energy for deformation (E*, extrapolated to zero stress conditions) is relatively modest. However, the calculated molar activation volume for deformation V* increases by almost an order of magnitude, i.e., from ca. 10 to ca. 76 cm3/mole when diffusion is required. Consideration of experimental values of E* and V* for high molecular weight polyethylene indicates that, in the regime examined, chain scission plus chain‐end diffusion is required to effect plastic deformation.