Materials Science and Engineering A 410–411 (2005) 447–450 The significance of grain boundary sliding in the superplastic Zn–22% Al alloy after processing by ECAP Praveen Kumar ∗ , Cheng Xu, Terence G. Langdon Departments of Aerospace and Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA Received in revised form 9 March 2005 Abstract Specimens of the superplastic Zn–22% Al eutectoid alloy were processed by equal-channel angular pressing (ECAP) through a total of eight passes at 473 K. Tensile specimens were cut from the as-pressed billets and tested at two different strain rates at 473 K under conditions where there is superplastic flow. Marker lines were placed on the polished surfaces of the specimens prior to testing and these lines were used to take measurements of the grain boundary sliding offsets at elongations of ∼30%. The measurements are used to estimate the contribution of sliding to the total strain and they show that grain boundary sliding is the dominant deformation mechanism under these conditions. © 2005 Elsevier B.V. All rights reserved. Keywords: Deformation mechanisms; Equal-channel angular pressing; Grain boundary sliding; Superplasticity 1. Introduction Superplastic flow refers to the ability to achieve high neck- free tensile elongations without premature failure. Two basic requirements must be fulfilled in order to achieve superplas- tic elongations: (i) the grain size of the material must be very small, typically <10 m, and (ii) the testing temperature must be high for reasonably rapid diffusion, typically at or above ∼0.5T m where T m is the absolute melting point of the mate- rial [1]. Since these two requirements tend to be incompatible because of the occurrence of grain growth, superplastic alloys are generally either two-phase or they have precipitates within the matrix. Numerous attempts have been made to estimate the contri- bution of grain boundary sliding to the total strain, ξ, in super- plasticity, where ξ is defined as ε gbs /ε t where ε gbs and ε t are the strain due to sliding and the total strain, respectively. The results from these investigations are remarkably consistent: the mea- surements show that, for superplastic alloys having grain sizes in the range of ∼1–10 m, the values of ξ are in the range of ∼50–70% [2]. These values suggest there may be some addi- tional “missing strain” but it was shown subsequently that the ∗ Corresponding author. Tel.: +1 213 740 4342; fax: +1 213 740 8071. E-mail address: praveenk@usc.edu (P. Kumar). values of ξ are a natural consequence of the procedure used to estimate the sliding contributions and the results are consistent with a situation where sliding accounts for all of the deformation under optimum superplastic conditions [3]. It was demonstrated over a decade ago that the application of severe plastic deformation (SPD) to polycrystalline materi- als provides the opportunity of producing bulk solids having grain sizes within the submicrometer or nanometer ranges [4,5]. Thus, processing by SPD has now become established as a tech- nique for achieving ultrafine grains in a range of metallic alloys [6]. Since the grain sizes introduced by SPD are typically in the submicrometer range, it is reasonable to anticipate these materials will exhibit excellent superplastic properties: this is supported by several recent reports of superplastic flow after processing by ECAP [7–9]. Despite the numerous demonstra- tions of superplasticity after ECAP, only one investigation has been conducted to date to determine the value of ξ in a material processed by ECAP. This investigation was conducted on an Al- 1421 (Al–Mg–Li–Zr–Sc) alloy where precipitates are present and the measurements gave a very high value of ξ ≈ 80% [10]. The present investigation was conducted to provide the first measurements of ξ in a two-phase alloy where the Zn–22% Al eutectoid alloy was selected as a typical and widely used super- plastic material. The following section describes the procedure adopted to measure ξ and the following sections describe the experimental procedure and the results. 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.092