Threshold Stress For Superplastic Flow Research Articles

Influence of Temperature and Grain Size on Threshold Stress for Superplastic Flow in a Fine-Grained Magnesium Alloy

The parametric dependencies of threshold stress for superplastic flow were characterized over a wide range of temperatures using a fine-grained magnesium alloy with various grain sizes. The threshold stress was proportional to the reciprocal of the square root of the grain size at 473 K, whereas it was independent of the grain size at 598 K. The threshold stress decreased with increasing deformation temperature more quickly than the shear modulus did. It was suggested that the temperature dependence of the threshold stress is associated with the interaction between the solute atoms and dislocations. The origin of the threshold stress probably resulted from the dislocations breaking away from the solute atmospheres formed in the grain boundaries and in the lattice at low temperatures and at high temperatures, respectively.

Microstructure and deformation in the spray deposited Zn–22% Al alloy

Microstructure and deformation in Zn–22% Al that was prepared by spray atomization and deposition were investigated. The results show that the as-deposited alloy exhibits a non-uniform microstructure composed of coarse and fine lamellar eutectoid domains, and that elongations to failure in tensile samples having such a microstructure are less than 100%. When the lamellar structure is transformed into a fine equiaxed grain structure that is free of porosity as a result of heat treatment and extrusion, the alloy exhibits the characteristics of superplastic flow including: the attainment of elongations to failure in excess of 1600%, the observation of a sigmoidal relationship between stress and strain rate, and the presence of a threshold stress for superplastic flow that is sensitive to the presence of impurities. Microstructure characterization of the samples after creep deformation reveals a limited dislocation activity that is manifested in the presence of dislocation-particle interaction in some of the grains. A comparison in creep behavior between Zn–22% Al and 7 vol.% SiC p–Zn–22% Al composite that, like the alloy, was prepared by spray atomization and deposition shows the presence of a strengthening effect. This effect is explained in terms of the details of a deformation mechanism for superplastic flow that is based on dislocation accommodated boundary sliding.

Determination of the value of the threshold stress for superplastic flow

A robust method to determine threshold stresses from experimental superplasticity data is described in this paper. The method suggested does not require any special investigations to be carried out (e.g. low strain rate tests or the load relaxation). It is based upon the results of standard tensile tests. Experimental implementation of the method suggested is carried out for a number of aluminum-based alloys. It is shown that the method involved enables one to obtain stable results with various choices of data points used in the process of extracting the threshold stress value. The fact that no assumptions are needed to extract a meaningful threshold stress from data obtained in a convenient manner is the main advantage of the method suggested.

The Effect of Grain Size on the Threshold Stress for Superplastic Flow in Aluminum Alloys

Effect of grain size on the threshold stress for high strain-rate superplastic aluminum alloys was investigated. By taking account of the grain-size effect on threshold stress with the solidus temperature, it was demonstrated that the threshold stress data for many superplastic aluminum alloys with different grain size and chemistry are excellently represented by a single relation.

Influence of Cr Additive on the Threshold Stress for Superplastic Flow in Al-33%Cu Alloy

The effect of Cr additive on the threshold stress in region I superplastic behavior of Al-Cu eutectic alloy is investigated using high-purity alloys with various Cr contents ranging from 0.002 to 0.26 mass%. The threshold stress increased largely with increasing Cr content up to 0.05 mass% and exhibited a tendency toward a possible saturation. It decreased with increasing temperature and grain size. The temperature dependence was similar to that of grain boundary segregation of impurity atoms. Transmission electron microscopy did not reveal the presence of fine precipitates at the grain and phase boundaries. Therefore, it was proposed that segregating Cr atoms are pinning grain boundaries, inhibiting grain boundary migration that accompanies grain boundary sliding and bringing about the threshold stress.

The dependence of the threshold stress for superplastic flow on structure

The model of a threshold stress for superplastic flow is tested in Al-Zn-Mg-Cu based alloys prepared both using ingot (IM) and powder metallurgy (PM) routes. The threshold stress is found to increase with increasing grain size in the IM alloy. The true value of the parameter p t characterizing the grain size dependence of the strain rate is close to two at low and medium strain rates and decreases to zero at high strain rates. A significantly higher threshold stress in the PM alloy may result from the prevailing small angle character of grain boundaries.

On the characteristics of the threshold stress for superplastic flow in Zn-22 Pct Al

On the characteristics of the threshold stress for superplastic flow in Zn-22 Pct Al

Microstructural evolution and threshold stress for superplastic flow in the Zn-Al eutectoid

The microstructural evolution and the stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy were investigated by prestraining specimens at two strain rates corresponding to Regions I and II. Even though the scale of microstructure was similar, the stress-strain rate curves of differently prestrained specimens were distinctly different in the lower strain-rate regime. While Region I of low rate sensitivity was more prominent when prestrained at a lower strain rate of Region I, it was less distinct because of prestrain in Region II. The threshold stress for superplastic flow, as assessed by an extrapolation procedure, varied with the nature of prestrain. The interphase boundaries were more rounded (higher mean curvature) on prestraining on Region II, compared to Region I. The correlation between the changes in the mean curvature of phase boundaries and the threshold stress arising from the nature of prestrain was consistent with the boundary-migration controlled sliding mechanism to interpret the threshold stress for superplastic flow.

Threshold Stress for Superplastic Flow in the Zn–Al Eutectoid Alloy

The high temperature deformation of a commercial Zn-Al eutectoid alloy was investigated over the grain size range of 0.8 to 2.6 μm and the temperature range of 423 to 473 K, with a view to assessing the interpretation of region I in terms of a threshold stress for superplastic flow. The threshold stress evaluated by an extrapolation procedure is observed to decrease with increasing temperature and grain size. The activation energy and grain size exponent for the threshold process are 34 kJ/mol and 0.85, respectively. Boundary sliding controlled by boundary migration is the likely origin of the threshold stress

Microtensile superplasticity in ceramic fibers

Superplastic experiments were carried out in tension on small specimens having a cross section of approx. 50 × 1.1 μm, and a gage length of 4 mm. The specimens, in the form of fibers, were prepared by lithography from thin films that were obtained by physical vapor deposition. Experiments with ZrO 2-2.6 mol% Y 2O 3 exhibited a threshold stress for superplastic flow. While the measurements of the average flow stress were in agreement with data from bulk materials, the stress-strain curves obtained from the microspecimens were serrated, which we attribute to discrete grain sliding events. The serrations are belived to occur because the gage section contained relatively few grains. A new approach for conceptualizing superplastic flow in ultrafine grained ceramics is proposed.

Rate sensitivity in the high temperature deformation of dispersion strengthened Al-Fe-V-Si alloys

The rate sensitive flow characteristics in the elevated temperature deformation of Al-Fe-V-Si alloys processed by rapid solidification/powder metallurgy route were assessed by the strain rate change tests in compression. With an ultrafine grain size, stabilized by fine dispersoids, a peak rate sensitivity index of ∼0.15 and normal ductility were observed in alloys containing dispersoids up to a volume fraction of 0.37. The lack of superplastic response is interpreted in terms of a high threshold stress for superplastic flow. The threshold stress assessed by an extrapolation procedure is observed to be grain size and temperature dependent. Its origin is suggested to be Zener drag limited boundary migration, which is an essential part of the superplastic flow mechanism.

Superplastic behavior of an Al-4wt.T% Ti alloy processed by the powder metallurgy route

The superplastic behavior of an Al-4wt.% Ti alloy processed using rapid solidification powder metallurgy was investigated. The peak value of the strain rate sensitivity index increased with temperature and grain size. The drop in flow stress with increasing grain size and anomalously high activation energy for flow at low strain rates are explained in terms of a grain size and temperature dependent threshold stress for superplastic flow. The origin of the threshold process is interpreted to be the inhibition of grain boundary migration by the particles during superplastic flow. An increase in the activation energy for flow was observed at temperatures close to the melting point of aluminum.

On the threshold stress for superplastic flow

Analyse des donnees experimentales obtenues sur l'alliage Zn-22% Al soumis a des essais de traction. Mise en evidence d'une contrainte seuil dependant fortement de la temperature. Comparaison avec les resultats obtenus sur les alliages Zn-22% Al et Pb-62% Sn soumis a des essais de fluage. Generalisation du comportement superplastique et du processus de deformation

Investigation of region I of a superplastic AlZnMgCuMn alloy

The high temperature deformation of a fine-grained aluminum alloy processed by rapid solidification powder metallurgy was investigated. Region I of low rate sensitivity with a high activation energy and a negative grain size exponent (in contrast with the positive exponent of the superplastic region) was observed. These observations support the existence of a threshold stress for superplastic flow which varies inversely with grain size and depends strongly on temperature. The restricted mobility of the grain boundaries pinned by the dispersed particles during the superplastic flow is suggested to be the origin of the threshold stress.

Grain boundary sliding model for superplastic deformation

Comparison of the data for superplastic Pb–Sn eutectic with the existing theories on diffusion accommoda ted grain boundary sliding shows mixed agreement. In the superplastic region the observed deformation rates are about 102–103 times faster than Coble creep. In the present study, this discrepancy is resolved by a physically realistic three-dimensional grain rearrangement model for superplastic deformation. This model assumes that the ‘macroaccommodation’ during superplastic flow results in grain rearrangement due to grain boundary sliding and ‘microaccommodation’ at triple points is diffusional. The proposed model carries all the structural features of superplastic deformation and the constitutive equation predicts faster strain rates than the existing theories on diffusion accommodated grain boundary sliding. The model gives an activation energy equal to that for grain boundary diffusion. The model predicts the observed grain size dependence of the strain rate and also a threshold stress for superplastic flow.

Theory of Superplastic Flow in Two-Phase Materials: Roles of Interphase-Boundary Dislocations, Ledges, and Diffusion

A new theory is developed to explain superplastic flow in two-phase materials. It is postulated that boundary-dislocations, piled up in dislocation-Interphase-Boundaries (IPBs) climb away into disordered regions of the IPB. Sliding then occurs at an IPB as dislocations glide toward the head of the pile up to replace those which have climbed into disordered regions of the boundary. An energy barrier which would otherwise render sliding virtually impossible on dislocation-IPBs can, it is shown, be largely eliminated if the dislocations glide in pairs. The disorder (actually an antiphase domain boundary) which is created by the passage of the leading dislocation is then repaired by passage of its successor. The threshold stress for superplastic flow is provisionally identified with the stress which pins IPB dislocations to boundary ledges. The activation energy is theoretically that for IPB diffusion. Good agreement is obtained between the theoretical equation for superplastic flow and the results of published experiments.