Micrograin Superplasticity: Characteristics and Utilization

Micrograin Superplasticity refers to the ability of fine-grained materials (1 mu m < d < 10 mu m, where d is the grain size) to exhibit extensive neck-free elongations during deformation at elevated temperatures. Over the past three decades, good progress has been made in rationalizing this phenomenon. The present paper provides a brief review on this progress in several areas that have been related to: (a) the mechanical characteristics of micrograin superplasticity and their origin; (b) the effect of impurity content and type on deformation behavior, boundary sliding, and cavitation during superplastic deformation; (c) the formation of cavity stringers; (d) dislocation activities and role during superplastic flow; and (e) the utilization of superplasticity

Deformation Mechanisms in Nanocrystalline Materials

As a result of recent investigations on nanocrystalline (nc) materials, extensive experimental data on the deformation behavior of these materials have become available. In this article, an analysis of these data was performed to identify the requirements that a viable deformation mechanism should meet in terms of accounting for the mechanical characteristics and trends that are revealed by the data. The results of the analysis show that a viable deformation mechanism is required to account for the following: (1) an activation volume the value of which is in the range 10 to 40 b 3; (2) an activation energy that is close to the activation energy for boundary diffusion but that decreases with increasing applied stress; (3) the magnitudes of deformation rates that cover wide ranges of temperatures, stresses, and grain sizes; (4) inverse Hall–Petch behavior; and (5) limited ductility. The validity of available deformation mechanisms for nc materials is closely examined in the light of these requirements

The transition from dislocation climb to viscous glide in creep of solid solution alloys

There are two distinct and separate classes of creep behavior in metallic solid solution alloys. The mechanism of creep in Class I alloys appears to be some form of dislocation climb process, whereas the mechanism in Class II alloys appears to be a viscous glide process. By making assumptions concerning the nature of the climb and glide processes, and using existing experimental results for an Al-3% Mg alloy, it is shown that, to a, first approximation, the criterion for deformation by viscous glide is given byBσ2k2(1-νϒGb3>T2e2cb6where B ∼ 8 × 10121, σ is the applied stress, k is Boltzmann's constant, v is Poisson's ratio, γ is the stacking fault energy, G is the shear modulus, b is the Burgers vector, T is the absolute temperature, e is the solute-solvent size difference, and c is the concentration of solute atoms. The creep behavior of twenty-eight different solid solution alloys is analyzed, and it is shown that all alloys except one (Au-10% Ni) give results which are consistent with this criterion for viscous glide

Deformation mechanism maps for ceramics

Deformation mechanism maps may be constructed for either a constant grain size or a constant temperature. A simple method is described for constructing maps at constant temperature, and maps are presented for two representative oxides, a carbide, and three alkali halides. A method is also described for superimposing a set of similar deformation mechanism maps

Creep at low stress levels in the superplastic Zn-22% A1 eutectoid

A sigmoidal relationship between strain rate and stress was observed in a superplastic Zn-22% Al eutectoid alloy with grain sizes in the range of 2.1–7.5 μm. The relationship was independent both of the testing technique employed (whether constant stress or constant strain rate) and of the mode of deformation selected (whether tensile or shear).In the superplastic region (strain rates of ~10−5-10−2 sec−1), the stress exponent was ~2.25, the exponent of the inverse grain size was ~2.3, and the activation energy was close to that for grain boundary diffusion. These results are in good agreement with the predictions of a model based on grain boundary sliding accommodated by the climb of dislocations into boundaries. At very low strain rates ≲10−5 sec−1), the stress exponent was ~4.1, the exponent of the inverse grain size was ~2.4, and the activation energy was close to that for volume self-diffusion. These results are not consistent with any of the existing deformation mechanisms, but suggest that the sigmoidal relationship may arise through the sequential operation of two different processe

Method of estimating stacking-fault energies in alkali halide crystals using creep data

A method is presented of estimating the stacking‐fault energies in alkali halide crystals using creep data. Using this procedure, the stacking‐fault energies of NaCl, KCl, and LiF are estimated to be 235, 135, and 390 erg/cm2, respectively, in good agreement with the theoretical predictions for dissociation on {110} planes

Creep behaviour in the superplastic Pb-62% Sn eutectic

The creep behaviour of the superplastic Pb-62% Sn eutectic was investigated for grain sizes from 5·8 to 14·5 μm and at temperatures in the range from 336 to 422 K. The results showed a sigmoidal relationship between strain rate and stress. At intermediate strain rates (∼ 10−5–10−2 sec−1), the stress exponent was ∼1·65, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for grain boundary diffusion. At very low strain rates (≲10−5 sec−1), the stress exponent was ∼3·0, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for lattice self-diffusion. The results are not entirely consistent with either of the two major theories of superplasticity, but suggest instead the sequential operation of two different deformation processes