Evidence for Solute Drag during Recrystallization of Aluminum Alloys

Transcription

1 Mater. Res. Soc. Symp. Proc. Vol. 882E 2005 Materials Research Society EE6.5.1/BB6.5.1 Evidence for Solute Drag during Recrystallization of Aluminum Alloys Mitra L. Taheri 1, Jason Sebastian 2, David Seidman 2 and Anthony Rollett 1 1 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15232, USA 2 Department of Materials Science and Engineering, Northwestern University, Evanston, IL, USA ABSTRACT Evidence of both solute drag as well as differences in migration mechanisms of certain boundary types has been found for an Al-Zr alloy. In-situ Transmission Electron Microscopy (TEM) annealing experiments coupled with Scanning Transmission Electron Microscopy (STEM) showed a stark contrast between Zr segregation at small and large scales. Specifically, Zr was found to segregate to, as well as precipitate at boundaries of grains smaller than 2 microns, whereas less segregation and no precipitation was found at large grains sizes, or long annealing times. Motivated by these results and those previously obtained by Orientation Imaging Microscopy (OIM), Local Electrode Atom Probe Microscopy with an Imago LEAP microscope, was used to determine the concentrations of Zr and Al ions at grain boundaries; the results confirmed those from the STEM. INTRODUCTION The effect of solutes on the migration of boundaries has long been a topic of investigation due to its strong impact on industrial materials processing. The lack of measurement of the solute drag effect can be attributed in part to the limitations of current imaging capabilities of electron microscopes. Thus, there exists a limited amount of experimental investigation with respect to solute drag. There has also been an extended discussion of the relative importance of solute drag versus particle pinning in recrystallization. [10] Recent comparisons of experimental data to computer current molecular dynamics simulation show that both microscopy and simulation are limited in terms of size and duration. New theoretical developments [1-3] suggest that boundary motion should be jerky in certain regimes of solute diffusivity and driving force because of repeated pinning and unpinning of boundaries. These results prompted a detailed investigation using in-situ heating in a TEM, which suggested that the 'jerky motion' revealed in velocity profiles taken from SEM experiments are a result of both Zr in solid solution and Al 3 Zr precipitation at subgrain boundaries. The improved resolution at the atomic level allows for the identification of solute segregation at individual boundaries. To obtain a more detailed analysis of solute segregation, Atom Probe Microscopy in an Imago LEAP microscope was used to prove for concentrations of Zr at grain boundaries. Previous experiments [4, 5] have demonstrated that Atom Probe Microscopy

2 EE6.5.2/BB6.5.2 is a useful tool for the subnanometer scale investigation of solute segregation. Unfortunately, the limited volumes of material that can be measured at such a scale means that relating segregation to specific boundary types is difficult. In this study, LEAP is combined with EBSD/OIM [6, 7] and STEM [8] to study the trends in both segregation and precipitation at several different length scales. The progression from mesoscopic (EBSD) to atomistic (LEAP) is intended to probe the mechanisms by which a solute actually influences boundary motion. EXPERIMENTAL Bulk samples were first prepared by heating a previously deformed and scratched single crystal at 350 C in a vacuum furnace for minutes, following the nucleation and growth techniques described in previous experiments [6-8]. Following procedures of Krakauer et al. [4], samples were then chemically polished into Atom Probe tips. Specifically, bulk plates were ground to 250 µm 2 bars and subsequently electropolished in 10% perchloric acid solution to form 8 mm needle-shaped atom probe specimens with an approximate tip radius of 50 nm (figure 1). Figure 1: Needle-like sample to which a large electric field is applied (electrode on right) during a typical LEAP experiment. Two compositions were studied: High purity Aluminum, and high purity Al containing 0.03 wt % Zr. Because results from STEM showed varying composition at different length scales in the experiment, samples were taken from various areas in a bulk sample, as shown in Figure 2, where both images are from the same sample. The principle of operation of the LEAP instrument is to use a high electric field gradient to evaporate atoms from the sharpened tip and count them individually by mass spectroscopic methods. The position in the sample from which each ion originated is also known within +/- 0.2nm. We take advantage, for example, of the different ionization state for Zr atoms typically found for dissolved Zr (Zr ++ ) versus those from the Al 3 Zr compound (Zr +++ ).

3 EE6.5.3/BB6.5.3 (a) (b) Figure 2: (a) EBSD map with marked locations for needle preparation, where 1 = 30% deformed (before recrystallization process), 2 = partially recrystallized grain located away from nucleation site, 3 = partially recystallized located at the nucleation site. An needle was also prepared from unalloyed Al as a baseline comparison for composition. (b) STEM Image of sample in 2 showing high solute contrast at subgrain boundaries to the right of a high angle boundary, suggesting strong Zr segregation to these low angle boundaries. RESULTS & DISCUSSION Because LEAP microscopy observes specimens with atomic resolution, thus possessing a limited field of view of about 100 x 100 nm, the probability of locating a microstructural feature such as a grain boundary with the above method of preparation is small. Direct detection/measurement of a grain boundary is not available in this instrument, therefore we present results for regions believed to be close to boundaries, based on secondary evidence and experience in other investigations. There are clues to locating such features of interest, which include orientation changes in the field evaporation spectrum (Field Ion Image), and sharp increases in evaporation rate. In this experiment, rate changes caused discontinuous experiments, and at these intervals, shifts in ion evaporation spectrum (Field Ion Microscope image) were noted. It was inferred that these discontinuities, or fractures that did not harm the needle were possibly grain boundaries normal to the needle direction, as studied previously by Krakauer and Seidman [8]. It was noted that these discontinuities occurred at localized increases in solute in the Al-Zr alloy. Figure 3 displays reconstructed elemental maps showing the segregation of Zr in solid solution in the fully recrystallized sample of Al-Zr, and a fully recrystallized high purity Al sample. In figure 3a, the left image shows the segregation of Al and Zr 2+ by atomic layers; we assume this is a grain boundary that is normal to the needle long axis direction, and thus the evaporation. The right image in figure 3a shows the top of the needle, and contains all three ion types (Al +, Zr 2+, and Zr 3+ ). There is some noise in the mass to charge data collected, from which the elemental maps are reconstructed. To decrease uncertainty, maps of the Al-Zr alloy (high purity %Zr) were compared to a high

4 EE6.5.4/BB6.5.4 purity (unalloyed) Al. The concentration of Zr 2+ observed is significant evidence of segregation despite the low signal-to-noise ratio, as verified by the absence of this species in a sample of unalloyed Al (0.1% Zr 2+ was detected in the high purity when using the same mass to charge ratios). (a) (b) Figure 3: Reconstructed elemental maps (in nm) from APFIM data of (a) Al-Zr and (b) HPAl; colored circles correspond to specific ions detected during field evaporation. Both maps show the surface of the needle. Zr 3+ is an indication of Al 3 Zr precipitation, and was present in all Al-Zr samples except the fully recrystallized matrix. Figure 5 below shows that the concentration of Zr 3+ varied from one sample to another, but was present in similar ratios to Zr 2+ at what were believed to be grain boundaries. Figure 4 below shows the variation of Zr 3+ in a partially recrystallized matrix. Figure 4: Reconstructed elemental map (in nm) showing Zr 3+ ions (presumed to originate in Al 3 Zr precipitates) observed in partially recrystallized matrix, exhibiting layering. Image represents majority of needle specimen measured, where the X axis is the evaporation direction, from 0 to 650nm.

5 EE6.5.5/BB6.5.5 There is some Zr 2+ present, but only in very small amounts, whereas the Zr 3+ is present in a layer, which suggests that that the sample contained a precipitate or some segregation. Figure 5 shows composition results for the series of needles studied, with amounts of Zr 2+ and Zr 3+ segregated to boundaries; unalloyed (high purity) Al ( HP )was used as a baseline for ion detection, and is plotted with Al-Zr alloy needles for a comparison. Zr 2+, representative of Zr in solid solution, was highly segregated in a fully recrystallized needle (number 1) and the partially recrystallized needle (number 2) at what were presumed to be grain boundaries, with some variation in concentration, but was below detectable limits in the areas of no evaporation discontinuities, or the matrix. Zr 2+ is also below detectable limits in the scratch region (number 3) that contains the highest density of nuclei, subgrains and small grains. Figure 5: Ion concentration, by needle type according to Figure 1, in the matrices (a) and the grain boundaries (b). Red denotes Al, grey and green are Zr 3+ and Zr 2+, respectively. From the results above, it is clear that Zr 2+ in solid solution segregates to boundaries at longer annealing times (in partially to fully recrystallized regions), whereas clusters of Zr 3+ are present throughout the sample. Zr has not been detected in fully recrystallized regions using STEM. In constrast, clusters of segregated Zr in Al 3 Zr precipitates were detected in small grains and subgrains in a partially recrystallized sample using STEM with Z-contrast imaging. If Zr 3+ represents Al 3 Zr precipitates and is present throughout samples in the LEAP experiments, does this negate Al 3 Zr precipitation and indicate spherical clustering of Zr atoms at short annealing times? Is this the reason for Zr 2+ presence at solely long annealing times? CONCLUSIONS Zr 3+ was associated with dislocation networks/subgrain boundaries based on TEM observations, and cluster size varies in amount and location within partially recrystallized samples in LEAP results, whereas Zr 2+ was found only at certain locations after annealing. Zr 2+ varies in recrystallized samples, suggesting variable segregation to different boundary types. This may help to explain mesoscopic results where low activation energies for migration of 38 <111> boundaries suggest different behavior of these with respect to general boundary types [8].

6 EE6.5.6/BB6.5.6 The fact that both Zr 2+ and Zr 3+ were found at boundaries in recrystallized samples means that it is not clear whether drag is controlled by precipitates or by (segregated) Zr in solid solution. ACKNOWLEDGEMENTS This work was supported in part by the Alcoa Technical Center, in part by the commonwealth of Pennsylvania, and in part by the MRSEC program of the National Science Foundation under Award Number DMR The author would like to thank Moneesh Upmanyu for useful discussions of grain boundary segregation. REFERENCES 1. Srolovitz, D.J., Informal Communication on Irregular Grain Boundary Motion at the Computational Materials Science Network Workshop, Golden, CO, October Mendelev, M.I. and Srolovitz, D.J., Modelling Simul. Mater. Sci. Eng. 10 (2002) R79-R M.I. Mendelev and D.J. Srolovitz: Acta materiala Vol. 49 (2001), p Krakauer,B.W. et al.: Rev.Sci.Instrum.61(11), November 1990, pp Krakauer, B.W. and Seidman, D.N.: Acta mater. Vol.46, No. 17, pp , M.L. Taheri, A.D. Rollett, and H. Weiland, In-Situ Quantification of Solute Effects on Grain Boundary Mobility and Character in Aluminum Alloys During Recrystallization, Materials Science Forum (2004) M.L. Taheri, A.D. Rollett and H. Weiland, In-Situ Investigation of Grain Boundary Mobility and Character in Aluminum Alloys in the Presence of a Stored Energy Driving Force, Mat. Res. Soc. Symp. Proc., 819 (2004) N M.L. Taheri, Eric Stach, V.R. Radmilovic, H. Weiland and A.D. Rollett, In-Situ Electron Microscopy Studies of the Effect of Zr on Grain Boundary Anisotropy and Mobility in an Aluminum Alloy, Mat. Res. Soc. Symp. Proc., (2004) P Boutin, F.R., Journal de Physique, colloque C4, supplement No.10, vol.36, October 1975, pp. C4355-C Doherty et al., Materials Science and Engineering (A) 238 p.219, 1997