Developing superplasticity and a deformation mechanism map for the Zn–Al eutectoid alloy processed by high-pressure torsion
Highlights
► A Zn–Al alloy was processed by HPT to reduce the grain size to 350 nm. ► Excellent superplastic properties were achieved when testing at 473 K. ► The results are consistent with a deformation mechanism map for 473 K.
Introduction
Superplastic flow is associated with the ability of a polycrystalline solid to pull out uniformly, without necking, to a very high strain prior to failure. In a recent review, superplasticity was defined formally as elongations in tension of at least 400% and with measured strain rate sensitivities close to ∼0.5 [1]. This elongation and strain rate sensitivity were selected specifically to avoid confusion with the solute drag flow mechanism where the strain rate sensitivity is generally close to ∼0.33 and it is possible to achieve high tensile elongations [2]: for example, there is a recent report of elongations up to 325% under conditions of solute drag creep in Al–Mg alloys [3].
There are two fundamental requirements for superplasticity [4]. First, the grain size must be very small and typically less than ∼10 μm. Second, the testing temperature must be within the diffusion-controlled regime which means in practice a temperature at or above ∼0.5 Tm, where Tm is the absolute melting temperature.
Over the last two decades, processing through the application of severe plastic deformation (SPD) has provided an excellent opportunity for achieving significant grain refinement in bulk solids. Although several techniques for SPD processing are now available, attention has centered primarily on the two procedures of equal-channel angular pressing (ECAP) [5] and high-pressure torsion (HPT) [6]. As tabulated in a recent review, there are now more than fifty reports describing the occurrence of superplastic flow in metals processed by ECAP [7]. By contrast, and primarily because the processing operation is generally conducted using very thin disks, there are only a small number of reports of superplasticity in materials processed by HPT. These results are summarized in Table 1 where it is apparent that the highest elongation achieved to date is ∼1600% when processing an Al–3% Mg–0.2% Sc alloy by HPT using a sample in the form of a small bulk cylinder [9].
It is well-known that two-phase eutectic and eutectoid alloys are especially attractive materials for achieving superplastic flow because grain growth is then limited by the presence of the two separate phases. There are several reports describing the superplastic properties achieved in the Zn–22% Al eutectoid alloy after processing by ECAP [20], [21], [22], [23], [24], [25] or by using the similar process of cross-channel extrusion [26] and there are also some limited reports of superplasticity in the Pb–62% Sn eutectic alloy processed by ECAP [27], [28]. Nevertheless, no reports are at present available describing superplastic properties achieved in any eutectic or eutectoid alloy after processing by HPT.
Recent experiments on the Zn–22% Al eutectoid alloy have revealed unusual physical characteristics when processing by HPT. First, in the early stages of deformation agglomerates of Zn-rich and Al-rich grains are produced near the edges of the disks lying in bands delineating the direction of torsional straining [29]. Second, the high pressures imposed in HPT lead to a significant reduction in the distribution of rod-shaped precipitates of stable hexagonal close-packed Zn within the Al-rich grains [20], [30] and hardness measurements show this leads to a weakening, rather than a strengthening, by comparison with the annealed and unprocessed alloy [29], [31].
Accordingly, the present investigation was initiated with two specific objectives. First, to determine the feasibility of achieving good superplastic properties in the Zn–Al eutectoid alloy after processing by HPT and especially to compare results obtained on the Zn–Al alloy with the experimental data listed in Table 1. Second, to evaluate the potential for presenting the flow data in the form of a deformation mechanism map that provides an accurate representation of the mechanical properties of the alloy.
Section snippets
Experimental material and procedures
The alloy was supplied in the form of a plate having a thickness of 25 mm and it was machined into a rod with a diameter of 10 mm and then cut into billets having lengths of ∼60 mm. These billets were annealed in air at 473 K for 1 h to remove any residual stresses. As described in earlier reports [29], [31], the material contained a binary microstructure of Al-rich α and Zn-rich β phases with an average equiaxed linear intercept grain size of ∼1.4 μm.
The processing by HPT was conducted at room
Experimental results
Fig. 1 shows the general appearance of the Zn–Al specimens after processing by HPT through 1, 3 and 5 turns and pulling to failure at 473 K using an initial strain rate of 1.0 × 10−1 s−1: the upper specimen is untested. It is apparent that all specimens pulled out to exceptionally high superplastic elongations of >900% and the elongations increase with increasing numbers of HPT turns. A comparison with Table 1 shows that the maximum elongation of ∼1800% after 5 turns represents the highest
Comparison with data obtained when processing by ECAP
The results from these experiments show that it is feasible to use HPT in order to achieve excellent superplastic elongations in the Zn–22% Al eutectoid alloy. The optimum elongation in the present experiments is ∼1800% which occurs at an imposed strain rate of 1.0 × 10−1 s−1 within the region of high strain rate superplasticity. As shown in Table 1, this elongation is significantly higher than the elongations reported for all other disks processed by HPT. It is appropriate, therefore, to compare
Summary and conclusions
- (1)
A Zn–22% Al eutectoid alloy was processed by HPT to produce an ultrafine grain size of ∼350 nm.
- (2)
Excellent superplastic elongations were recorded in this alloy in tensile testing at 473 K. The highest elongation to failure was ∼1800% at an initial imposed strain rate of 1.0 × 10−1 s−1 which is within the range of high strain rate superplasticity. This elongation is the highest recorded to date for tensile specimens processed by HPT.
- (3)
The experimental results are in excellent agreement with a deformation
Acknowledgement
This work was supported by the National Science Foundation of the United States under Grant No. DMR-0855009.
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