Al Eutectoid Alloy Research Articles

Investigation of superplasticity by the impression creep technique

Impression creep tests have been carried out on a Zn-22% Al eutectoid alloy in two heattreatment conditions, as a function of applied stress and at temperatures of ∼25 to 270° C. At small grain sizes (∼ 0.9μm) and in the temperature range ∼ 180 to 270° C, the observed deformation response conforms to Regions I, II and III typical of superplastic behaviour. Furthermore, strain-rate index values determined for Region II fall in the range 0.44 to 0.51, consistent with superplasticity. It is concluded that the impression creep technique offers considerable potential for characterization of superplastic alloy systems on small specimen volumes.

An analysis of cavity nucleation in superplasticity

Superplastic alloys possess either a quasi-single phase or a microduplex microstructure: in quasi-single phase alloys, cavities are observed to nucleate predominantly at coarse grain boundary particles whereas in microduplex alloys, cavities tend to form at interphase boundaries and at triple point junctions. A general analysis is presented for cavity nucleation, in both microstructures, under the stress concentrations caused by bursts of grain boundary sliding during superplastic deformation. In quasi-single phase alloys, calculations indicate the cavities nucleate at coarse particles located at grain boundaries because local interphase diffusion creep cannot accommodate the stress concentrations sufficiently rapidly. The analysis demonstrates that it is possible for cavities to nucleate at grain boundary ledges under some limited experimental conditions. It is demonstrated also that the present analysis is in agreement with the available experimental data on a quasi-single phase Cu-based superplastic alloy and a microduplex superplastic Zn-22% Al eutectoid alloy. Calculations show that small pre-existing cavities, if present, are likely to be sintered rapidly prior to superplastic déformation at elevated temperatures.

The influence of rolling direction on the mechanical behavior and formation of cavity stringers in the superplastic Zn-22% Al alloy

The superplastic Zn-22% Al eutectoid alloy was tested over a range of strain rates at 503 K using specimens machined with the rolling direction perpendicular to the tensile axis. It is shown that the mechanical properties of the alloy, including the elongations to failure, are essentially identical to those obtained when the rolling direction is parallel to the stress axis. Inspection after failure showed that cavities are formed in stringers and, as in the same alloy tested with the rolling direction parallel to the tensile axis, these stringers are aligned parallel to the stress axis. The results demonstrate that the cavities are not nucleated primarily at impurity or oxide particles, nor do they grow from pre-existing microvoids which may be introduced during thermomechanical processing. The cavities are located preferentially at the α β interfaces and at the associated triple points, and it is probable that they were nucleated at triple points and grain boundary ledges during bursts of grain boundary sliding.

An investigation of grain boundary sliding in superplasticity at high elongations

Experiments were performed on the superplastic Zn-22% Al eutectoid alloy to determine the contribution of grain boundary sliding at both low (35%) and high (∼235%) elongations. The tests were conducted at two different strain rates in the superplastic Region II, and the results show that, within the accuracy of the measurements, there is a large sliding contribution at both elongations. By taking detailed measurements of both the magnitude of the sliding offset and the type of interface, it is shown that the average offsets are generally a maximum at the Zn-Zn boundaries, there is less sliding at the Zn-Al interfaces, and the offsets are a minimum at the Al-Al boundaries. In addition, the distributions of the magnitudes of the sliding offsets are similar at both the low and high elongations. It is concluded that grain boundary sliding is an important deformation process in the superplastic Region II and that it remains important even when the elongation is very high. The nature of the results indicates also that experimental observations of the deformation behaviour in superplastic materials at low elongations (up to 50%) provide meaningful information on the behaviour at much higher (superplastic) elongations.

A detailed appraisal of steady state flow data for the superplastic Zn-22% Al Alloy

Several sets of experimental results are available for steady state flow in the superplastic Zn-22%Al eutectoid alloy. It is shown that these data are reasonably consistent in the intermediate region II but that there are marked discrepancies in region I at low stress levels. In region II the extensive experimental data indicate an activation energy Q II for grain boundary diffusion, a stress exponent n II of about 2.0, an exponent for the inverse grain size, p II , of about 2.0 and a dimensionless constant in the rate equation, A II , of about 15. However, the precise experimental values of A II may vary by several orders of magnitude because of the interrelationship between A II , n II , p II and Q II : in practice, the value of A II is increased when the experimental values of n II , p II and/or Q II are high. As a result of this interrelationship, it is not possible to interpret the various values reported for A II in Zn-22%Al in terms of substructural variations in the different materials.

A comparison of constant strain rate and creep testing procedures in superplasticity

Tests were conducted on the Zn-22 wt% Al eutectoid alloy to compare experimental conditions of true constant strain rate and true constant stress when there is relatively minor grain growth. The stress-strain curves show the presence of significant strain hardening which precedes steady-state flow. In the steady-state condition, similar stresses are obtained when the strain rate is incrementally cycled on a single specimen. There is an extensive primary stage of creep in both Regions I and II, although in Region II the time involved is often very short. It is re-affirmed that, in the absence of significant grain growth, steady-state flow data may be obtained using either constant strain rate or creep testing procedures.

The mechanical properties of superplastic materials

The relationship between stress and strain rate is often sigmoidal in superplastic materials, with a low strain rate sensitivity at low and high strain rates (regions I and III, respectively) and a high strain rate sensitivity at intermediate strain rates (region II) where the material exhibits optimal superplasticity. This relationship is examined in detail, with reference both to the conflicting results reported for the Zn-22 pct Al eutectoid alloy and to the significance of the three regions of flow.

An evaluation of the roles of intercrystalline and interphase boundary sliding in two-phase superplastic alloys

Experiments were conducted to measure the contribution of boundary sliding in the superplastic Zn-22% Al eutectoid alloy at an elongation of 100%. The results show that the sliding contribution is a maximum in the superplastic region II and decreases in the less superplastic regions I and III. The values obtained for the sliding contribution are very low due to two physical constraints in the experimental procedure at high specimen elongations: 1. (i) a failure to account for the increase in surface area with elongation and 2. (ii) a limitation on the maximum feasible offset in surface marker lines. In Zn-22% Al, maximum sliding tends to occur at the Zn-Zn intercrystalline boundaries, there is slightly less sliding at the Zn-Al interphase boundaries and sliding is a minimum at the Al-Al intercrystalline boundaries. It is demonstrated that the results on Zn-22% Al and earlier experimental data on the Pb-62% Sn eutectic alloy are consistent with the occurrence of maximum sliding on the interfaces having the highest values of δD gb , where δ is the width of the boundary and D gb is the grain boundary diffusion coefficient.

Flow localization and neck formation in a superplastic metal

Experiments were conducted on the superplastic Zn22% Al eutectoid alloy to investigate the extent of flow localization and neck formation in the three regions of plastic flow. The results show that the behavior is qualitatively similar in the low stress region I and the high stress region III, but there are very significant differences in the superplastic region II. Deformation is essentially uniform in region II up to at least ~800% and quasi-uniform thereafter, whereas there are marked deviations from uniform flow in regions I and III at ~200%. Failure occurs by the formation and catastrophic growth of a sharp neck in regions I and III, whereas necking is diffuse at high elongations in region II. The results are consistent with a genuine decrease in the value of the strain rate sensitivity at low strain rates in region I.

Two- and three-dimensional deformation mechanism maps for high temperature creep of Zn-22%Al Eutectoid Alloy

In this paper we analyze the relations between the rate-controlling deformation mechanisms found operative for Zn-22%Al eutectoid alloy in terms of two-dimensional deformation maps that are in current use. Although useful in their simplicity, the two-dimensional deformation mechanism maps have one main drawback, i.e. their lack of generality. Many maps must be constructed, even for limited ranges of stress, temperature and grain size, in order to obtain the overall picture. In order to overcome this limitation a new type of deformation mechanism map, a three-dimensional one, is developed and introduced in this paper. Possible applications of this new type of deformation mechanism map, such as the evaluation and prediction of creep life, are discussed.

Observations on the differences reported in region I for the superplastic Zn-22% Al eutectoid

This paper discusses the different types of creep behavior reported for the steady-state deformation of the superplastic Zn-22% Al eutectoid alloy at low stress levels. All existing theories of creep behavior predict that at sufficiently low stresses the steady-state creep will be diffusion-controlled and will follow a power-law with a strain rate exponent of m = 1.0. The question is whether this diffusional creep mechanism sets the lower bound on superplastic deformation in typical samples of Zn-22% Al, or whether another type of creep behavior intervenes.

High-temperature diffusion-controlled creep behavior of the Zn-22% Al eutectoid alloy tested in torsion

Torsion testing was carried out on Zn-22% Al eutectoid alloy and covered a range of strainrates between 4.4 × 10 −5 S −1 and 4.2 × 10 −1 s −1, temperatures between 413K and 533 K and grain sizes between 0.8 × 10 −4 cm and 3.5 × 10 −4cm. Within the range of strain-rates, temperatures and grain sizes investigated, four rate-controlling deformation mechanisms are found operative and are identified as Coble creep, Nabarro-Herring creep, superplastic creep and dislocation climb creep, respectively. The ranges of manifestation of each mechanism in terms of stress, strain-rate, temperature and grain size, are determined. The four mechanisms are shown to relate to each other and new correlation form based on the newly proposed { AM} parameter is developed. Furthermore, it is shown that for the high-temperature diffusion-controlled deformation mechanism, the value assumed by the { AM} parameter, completely and uniquely identifies the rate-controlling deformation mechanism.

Low stress and superplastic creep behavior of Zn-22 Pct Al eutectoid alloy

The temperature, grain size, and stress dependence of steady-state strain-rates were investigated for Zn-22 pct Al eutectoid alloy using double shear type specimens. Tests were performed on specimens of grain size from 1.3 m to 3.7 μm over a range of temperature from 450 to 525 K. The applied stresses were in the range between 10−1 and 4.0 x 101 MPa with the resulting true strain-rates ranging from 10−7 to 10−2s−1. The alloy exhibited two distinct regions of constant stress dependence with stress exponents 1 and 2.2. The transition stresses between these two regions were between 3 × 10−1 and 7 x 10−1 MPa. The true activation energies associated with these regions were found to be 95.9 ±2.1 and 69.9 ±2.1 kJ/mol, respectively. The grains remained equiaxed following large deformation, although there is evidence of excessive grain growth and “grain clustering.” Specimens deformed in both regions showed few or no dislocations within the grains. There was no evidence of subgrain formation or dislocation pile-up at the grain boundaries. It is possible that dislocations found during deformation were annihilated when stress was withdrawn or lost during thin foil preparation. There was clear evidence of primary stage in both regions. However, it is suggested that whereas the primary stage in Region II is due to dislocation multiplication process, at low stresses the primary stage in Region I may be due to elastic bowing of the existing dislocations in the structure. At low stresses Nabarro-Herring diffusional creep was found to be the rate controlling mechanism. At intermediate stresses, superplastic creep was found to be the dominant mechanism. The transition between these two mechanisms, as well as Coble creep and the ranges of manifestation for superplastic creep, are analyzed and the importance of the preexponential term A, in the dimensionless relation:γκT/DGb = A(b/d) p(Τ/G)n and the role of the concurrent grain growth are emphasized.

Production of whisker crystal by straining ZnAl eutectoid alloy at elevated temperatures

Production of whisker crystal by straining ZnAl eutectoid alloy at elevated temperatures

The activation energies associated with superplastic flow

A sigmoidal relationship was observed between strain rate and stress in the Zn-22% Al eutectoid alloy, dividing the behavior into three distinct regions. At high strain rates (region III), the activation energy was similar to that anticipated for lattice diffusion, and the results support the suggestion that the behavior in this region arises from an intragranular creep process. At intermediate strain rates (region II), the activation energy decreased to a value similar to that anticipated for grain boundary diffusion; whereas at very low strain rates (region I), the activation energy again increased to a value close to that for lattice diffusion. This trend was confirmed using two different modes of deformation (tensile and shear) and three different experimental procedures (strain rate cycling, stress cycling and a change in temperature technique). The results suggest that regions I and II arise through the sequential operation of two different processes.

Creep at low stress levels in the superplastic Zn-22% Al 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 processes

Crystallographic slip during superplastic deformation of the ZnAl eutectoid alloy

Crystallographic slip during superplastic deformation of the ZnAl eutectoid alloy

The kinetics of isothermal and isovelocity pearlite growth in Cu-Al eutectoid alloy

Measurements of pearlite growth rate and inter lamellar spacing have been made on isothermally transformed specimens of Cu-11.8 wt pct Al eutectoid alloy. Growth rates which have been calculated on the assumption that volume diffusion in the parent β phase is the rate-controlling process show excellent agreement with the measured values. Directional control of eutectoid growth by translating specimens through steep temperature gradients resulted in the formation of aligned lamellar structures, and the effect of variations in imposed velocity and temperature gradient on alignment has been examined. A comparison of the kinetics of the two modes of transformation has shown that isothermal and isovelocity pearlite growth in this alloy may be readily interrelated. Analysis of the isovelocity growth data supports the conclusion that growth is controlled by volume diffusion.

The Effect of Processing History on Superplasticity in the Zn–Al Eutectoid Alloy

The Effect of Processing History on Superplasticity in the Zn–Al Eutectoid Alloy