Superplasticity in Ultrafine-Grained Materials

4R6ev. Adv. Mater. Sci. 54 (2018) 46-55 M. Kawasakiand T.G. Langdon SUPERPLASTICITY IN ULTRAFINE-GRAINED MATERIALS Megumi Kawasaki1 and Terence G. Langdon2 1School of Mechanical, Industrial and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331-6001, U.S.A. 2Materials Research Group, Department of Mechanical Engineering, University of Southampton, Southampton SO17 1BJ, U.K. Received: June 18, 2018 Abstract. Superplasticity refers to the ability of a polycrystalline solid to exhibit a high elongation, of at least 400% or more, when testing in tension. The basic characteristics of superplastic flow are now understood and a theoretical model is available to describe the flow process both in conventional superplastic materials where the grain sizes are a few micrometers and in ultrafine- grained materials processed by severe plastic deformation where the grain sizes are in the submicrometer range. This report describes the basic characteristics of superplastic metals, gives examples of flow in ultrafine-grained materials, demonstrates the use of deformation mechanism mapping for providing a visual display of the flow processes and provides a direct comparison with the conventional model for superplastic flow. The report also describes the potential for using nanoindentation to obtain detailed information on the flow properties using only exceptionally small samples. 1. INTRODUCTION by Underwood [3], drew world-wide attention to the unusual properties of superplastic materials. A few When polycrystalline solids are pulled in tension, years later it became clear that superplastic alloys they generally exhibit necking and then they fail at provided a potential for forming smooth and com- relatively low elongations. Under some special cir- plex shapes from sheet metals [4] and this was the cumstances, however, it is possible to achieve re- significant step that initiated the process of markably high elongations without the occurrence superplastic forming as a viable manufacturing tool. of any significant necking within the gauge lengths. As of today, superplastic forming is now established This process is known as “superplasticity” where as a valuable processing procedure that is used this is a direct translation of the Russian word extensively to fabricate complex parts in a range of sverkhplastichnost meaning “ultrahigh plasticity” [1]. industrial applications in the aerospace, automo- The first direct demonstration of true superplastic tive and architectural sectors as well as making sim- flow was in research conducted by Pearson in Eng- ple parts, such as boxes and panels, for a range of land in 1934 [2]. Using a bismuth-tin eutectic alloy, household products [5]. Pearson obtained an exceptional elongation of In the early stages of superplasticity, research 1950% which was far in excess of any other was undertaken to determine the dependence of elongations reported in any metals up to that time. superplastic flow on a range of fundamental testing Thereafter, significant research was conducted in parameters such as temperature and grain size. This the Soviet Union to more fully investigate this phe- work led to a basic understanding of the flow char- nomenon and in 1962 a comprehensive review of acteristics and especially the ranges of testing con- this Russian work, published in the United States ditions associated with the ability to achieve high Corresponding author: M. Kawasaki, e-mail: [email protected] © 2018 Advanced Study Center Co. Ltd. Superplasticity in ultrafine-grained materials 47 Fig. 1. Elongation to failure (upper) and flow stress (lower) versus initial strain rate for a Zn-22% Al eutectoid alloy having an average spatial grain size of 2.5 m pulled in tension at a testing temperatures of 473K [6]. superplastic elongations. For example, an initial first behavior is representative of a wide range, if not all, step entailed testing a Zn-22%Al eutectoid alloy, superplastic metals. It is important to note also that selected as a representative two-phase superplastic the strain rate sensitivity, m, defined as ln/ln , material with a grain size, d, of 2.5 m, and deter- is high and ~0.5 in region II whereas the values of m mining the variation of elongation over a wide range are much lower and only ~0.2 in regions I and III. of strain rates when testing all of the samples at a The elongations for the Zn-22% Al alloy in Fig. 1 temperature of 473K. The results are shown in are high and reach maximum values in region II of Fig. 1 where the upper plot gives the various >2000%. Even higher superplastic elongations may elongations recorded for each sample as a function be recorded in other superplastic alloys: for exam- of the strain rate, , and the lower plot shows the ple, a maximum elongation of 7550% was recorded maximum recorded flow stress, , for each testing in a Pb-62% Sn eutectic alloy tested at a tempera- condition, where the elongation is denoted as L/ ture of 413K [7]. In order to provide a clear definition Lo% where L is the change in length and Lo is the of the elongation required for true superplastic flow, initial gauge length, respectively [6]. These results and especially to avoid confusion with the relatively show that the flow behavior divides into three dis- high elongations of >100% that may occur when tinct regions, marked regions I, II and III, where dislocation glide is the rate-controlling flow process superplasticity occurs in region II at intermediate with m 0.3, the advent of superplasticity is now strain rates over a range of about two orders of mag- defined as elongations to failure of at least 400% nitude in strain rate and the elongations are much [8]. lower, and outside of the superplastic range, at both The basic characteristics and the requirements low strain rates in region I and high strain rates in for superplastic flow are now well established [9]. region III. It is now known from many more recent Specifically, superplasticity requires a very small experiments that this type of three-stage flow grain size, typically smaller than ~10 m, and a 48 M. Kawasakiand T.G. Langdon subgrain boundary at B where they climb into the boundary. Conversely, superplasticity requires a small grain size and it has been shown that it needs a grain size which is no larger than the average subgrain size so that d < [14] as in Fig. 2b. The stress concentration at triple point C due to GBS is then accommodated by intragranular slip in the adjacent grain and now, in the absence of any subgrain boundaries, the dislocations move across the grain and climb into the opposing grain boundary at D. Both of these processes may be expressed by a rate equation of the following form: p n ADGb b , (1) kT d G where D is the appropriate diffusion coefficient which is given by Do exp (-Q/RT) where Do is a frequency factor, Q is the activation energy, R is the gas con- Fig. 2. Schematic illustration of a unified model for stant and T is the absolute temperature, G is the grain boundary sliding in (a) conventional creep when shear modulus, b is the Burgers vector, k is d > and (b) superplasticity when d < [15]. Boltzmann’s constant, p and n are the exponents of the inverse grain size and the stress, respectively, where n = 1/m and A is a dimensionless con- high testing temperature, typically higher than ~0.5 stant. It can be shown that the rate of GBS in conventional creep with d > is given by Eq. (1) with Tm where Tm is the absolute melting temperature of the material. Under true superplastic conditions, the Q Ql where Ql is the activation energy for lattice samples elongate by several hundreds or even thou- diffusion, n = 3, p = 1, and A 103 whereas the rate sands of percent but the grains within the materials of GBS in superplasticity with d < is also given by remain reasonably equiaxed. This microstructural Eq. (1) but with Q = Qgb where Qgb is the activation observation provides a clear demonstration that grain energy for grain boundary diffusion, n = 2, p = 2, boundary sliding (GBS) is the dominant flow mecha- and A 10 [15]. The validity of Eq. (1) in describing nism in superplasticity [10]. Furthermore, several superplasticity in ultrafine-grained metals is exam- sets of detailed experiments have shown that the ined later in Section 3. sliding in superplasticity is accommodated concur- rently by the occurrence of intragranular dislocation 2. SUPERPLASTIC FLOW IN slip within the grains but this slip makes no signifi- REPRESENTATIVE cant contribution to the overall strain [11-13]. ULTRAFINE-GRAINED Using this information, it is possible to develop (UFG) METALS a theoretical model for superplasticity and also to provide a clear distinction between the GBS occur- As noted in Section 1, the dominant flow mecha- ring in superplastic flow and the GBS which is an nism for superplasticity by GBS requires a grain inherent feature of conventional high temperature size that is smaller than the equilibrium subgrain creep in coarse-grained materials. These two proc- size, [14]. The processing of metals through the esses are illustrated in Fig. 2. In high temperature application of severe plastic deformation (SPD) pro- creep with coarse-grained samples, the grains be- vides an opportunity for achieving excellent come divided into arrays of subgrains having an av- superplastic properties in ultrafine-grained (UFG) erage size of so that Fig. 2a corresponds to the metals having high fractions of high-angle bounda- condition where d > . Thus, in high temperature ries. Numerous experimental reports are available creep GBS occurs through the movement of dislo- to date and it has been shown that high fractions of cations along the boundaries and this produces high-angle grain boundaries may be achieved after stress concentrations as at the triple point A, ac- the accumulation of a large strain of at least ~6-8 in commodating slip is initiated in the adjacent grain materials processed by equal-channel angular and these dislocations then move to the first pressing (ECAP) through large numbers of passes Superplasticity in ultrafine-grained materials 49 (a) (b) Fig.

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