Early Stage Decomposition of AA6111: II. Small Angle X-ray Scattering

Transcription

1 Early Stage Decomposition of AA6111: II. Small Angle X-ray Scattering B.J Diak 1, M.A. Singh 2, L.A. Westfall 1, S. Saimoto 1 1. Dept. of Mechanical and Materials Engineering, Queen s University, Kingston, Ontario, K7L 3N6, Canada 2. Dept. of Physics, Engineering Physics and Astronomy, Queen s University, Kingston, Ontario, K7L 3N6, Canada 1. Introduction In the continuing drive for automobile weight reduction, Al-Mg-Si, or AA6XXX series aluminum alloys have been developed as the most promising candidates for heat treatable body sheet materials [1]. In the automobile manufacturing process, these solutionized alloys are subject to a room temperature ageing during storage (T4) and an artificial aging during an elevated temperature paint-bake cycle. The latter treatment is ascribed to components often painted and dried for a period of time in an oven at a temperature in the range 150 to 180ºC. This paint-bake temperature is used to achieve some of the heat treatment strengthening required [2]. The ageing time and temperature are often chosen to optimize the strength and ductility of the alloy. Recent attention has focused on the latter stages of ageing which identify an extended precipitation sequence between β and β [3, 4] as SSSS Mg/Si Clusters GPZ β (β +U2+U1+B ) β (1) Still, the important strengthening contributions to this alloy system come from the pre-β / β stages, starting at the super-saturated solid solution (SSSS), which remain difficult to characterize, and are worthy of further study. Recently there has been HRTEM and first principal calculations of the possible phases forming after quenching and artificial ageing in Cu free Al-Mg-Si alloys with the main hardening phase β templated from Si-based pillars [5]. In the quaternary alloy AA6111, the presence of Cu results in a final Q-phase from its precursors. Recent resistivity study of the early stage decomposition of this solid solution at 60 C indicates a continually increasing resistivity with ageing time, and 3D atom probe confirms the presence of Cu in the Mg/Si clusters [6]. In the basic textbook description, the strengthening mechanism for precipitation hardening is mainly due to the impeding of dislocations by lattice obstacles. Dislocations can overcome these obstacles either by shearing or bypassing them. At the earliest stages of solid solution decomposition during natural ageing in AA6111, strengthening involves dislocation interactions with forest dislocations and a complicated solid solution. In the accompanying paper [7] solute clusters are identified as obstacles compared to dislocation, but stronger than the general solid solution of AA6111. Examination of the early stages of solid solution decomposition into GP zones has been done in the past by small angle x-ray scattering (SAXS) studies. SAXS studies of Al-Cu [8] and Al-Zn-Mg-Cu [9] alloys have revealed important size and number density information about GP zone formation. There have been very few SAXS studies of Al-Mg-Si alloys, due to the challenge of small scattering contrast among the very similar atoms involved in this alloy. In one of the few reported studies, Tsao et al. [10] characterized the evolution of β during artificial ageing using synchrotron SAXS,

2 but the early stages of natural ageing were not reported. Recently, we have started a program to produce nano-voids by quenching and deformation in nominally pure aluminum alloys and characterize them using SAXS [11]. Our initial success in identifying nano-voids using SAXS, and our characterization of the early stages of solid solution decomposition in AA6111 using DDA motivates the present work which presents a preliminary SAXS study on the early stage decomposition of AA6111 during natural ageing. 2. Experimental Procedure Alloy AA6111 specimens containing (wt.%) 0.79Mg, 0.6Si, 0.70Cu, 0.25Fe, 0.20Mn, 0.05Cr, and 0.06Ti were prepared for tensile testing [7] and SAXS, and then solutionized at 560 C for two minutes and ice-water quenched before storing in liquid nitrogen. Specimens for transmission mode SAXS were pre-thinned to 120µm thickness by grinding and final electropolishing before the solutionizing and quenching. SAXS experiments were carried out on beam-line G1 at the Cornell High Energy Synchrotron Source (CHESS). Figure 1 illustrates the experimental set-up in the hutch. A beam of kev with spot-size of 0.5 x 0.5 mm was directed onto the specimen and data collected with a 72x72mm (69.8um/pixel) Flicam CCD detector positioned 1200 mm from the specimen allowing for a maximum possible q range of 3.21 nm -1 over one quadrant of the scattering. The reported flux was 5x10 13 /s/mm 2. The specimen was removed from the liquid nitrogen storage tank, brought to room temperature, and placed continuously in the x-ray beam for the first 10 hours, and then periodically over a period approaching two days. The continuous scans guaranteed that the same volume was observed during ageing. Each data point is an average of multiple scans for scan times ranging from 2 to 8 seconds to avoid detector saturation. The scattering was normalized for changes in incident intensity and specimen thickness by dividing by the transmitted intensity registered on a photo-diode directly in front of the beam-stop. This instrumentation allowed the background to be removed from the specimen scattering. Figure 1. Photo of the CHESS-G1 beam-line showing (from right to left) upstream evacuated beam-path (EBP), shaping slits, proportional ionization counter, guard slits, specimen stage and stage motor, downstream EBP, and CCD detector. Kapton windows enclose the upstream and downstream EBP (not visible).

3 3. Results and Discussion The variation in tensile yield strength at 78 K with natural ageing of as-quenched AA6111 [7] is re-plotted in Fig. 2. The first data point is at 30 minutes, but the initial yield strength increase over the first three hours appears fastest. Beyond three hours there is a continuous decrease in the slope of the curve as the yield strength increases and approaches the final measurement at 168 hours. At 78 K the strengthening signal from the solid solution is significant, and so the yield strength is a better indicator of the strengthening effect than hardness as the latter includes a work hardening contribution. Figure 2. Time dependence of the tensile 0.02% offset yield strength measured at 78K during natural ageing of SSSS AA6111. The inset plot shows the results out to the longer time of 168 hours. Figure 3 shows the SAXS scattering profiles from the natural ageing AA6111 over two days. The inset plot shows the overall trend in the data, and the larger plot is a closer examination of the intensity increase with ageing over a finite q range. The curves are characterized by an increasing low-q integrated intensity with increasing ageing time that stabilizes before increasing slightly at longer times. For this preliminary report we will simply quantify the evolution of the volume fraction of the scattering inhomogeneities, loosely defined as atom clusters, which contribute to the change in the observed scattering. The volume fraction of the scattering particles can be determined precisely from SAXS experiments using the invariant if the electron density contrast between particles is known and the scattering intensity I(q) can be measured in absolute units [12] over all of q-space. In lieu of this information, an alternative approach is to calculate the effective invariant, Q eff, such that

4 qmax 2 Q eff = I( q) q dq (1) qmin where the integration limits represents the range of characteristic length scales between 2π/q max to 2π/q min [13]. Figure 3. Close-up of SAXS profile trends of SSSS AA6111 undergoing natural ageing at room temperature. Inset plot shows complete range of data up to detector limit q=3.21 nm -1. Figure 4 plots the time dependence of Q eff with natural ageing. The integration of equation (1) was performed between q limits of and 1.0 nm -1, corresponding to length scales of 50 and 6.3 nm, respectively. This means that length scale information outside this range does not contribute to the reported particle volume fraction change; changes associated with length scales outside of this range are not visible. The absence of q-data outside of the range quoted is a result of instrument limitations. Extrapolation of the observed scattering to q = 0 and q is prone to introduce large uncertainties in the calculation of the true invariant and is not done here. Figure 4 indicates a rapid increase in Q eff up to four hours, followed by a slower increase without saturation to the end of the observed ageing time, which correlates with the observed trend in the measured yield strength. The data indicates that there is an increase in concentration, or volume fraction of scattering clusters, which are resolved within the length scales spanned by distances of 6.3 to 50 nm. The variability in the measurement can be seen from data beyond 20 hours. An increase in the volume fraction of strengthening centers able to sustain strain was identified in the naturally ageing AA6111 by DDA in the accompanying paper [7], which implies that Q eff contributes in a similar nature. The kinetics of the change in volume fraction can be estimated by fitting simple power law relationships between Q eff and ageing time (Fig. 4). Up to four hours Q eff is proportional to t, and after four hours the growth in Q eff is proportional to t suggesting fast and slow kinetic regimes, respectively. Recent resistivity measurements on AA6111 containing less Cu than in

5 the present alloy showed a rapid increase in resistivity within the first few hours of quenching and holding at 60 C [6]. Working with Al-0.85wt.%Mg 2 Si alloy Kovács et al. [14] detected fast (within one minute), medium (under 60 minutes) and slow (to longer times) kinetic regimes after quenching and ageing at 30 C. In the present experiments the ex situ solutionizing and quenching before SAXS makes the fast region unobservable, but the existence of a transition in Q eff between latter regimes matches past observations in resistivity. It remains difficult to ascertain what the identity and geometry of the x-ray scatters is with the present data, but the SAXS signal is expected to be particularly sensitive to vacancy clusters and Cu atoms, which have larger difference in electron density than the Mg, Al, Si species. This implies that all the nuclei already existed when the SAXS measurements were made and that short-range movement of solute atoms (and vacancies) from the supersaturated solid solution into the nuclei and their growth into larger clusters could be responsible for the observed increase in Q eff. Whether Q eff correlates to the volume fraction of hardening species and its strengthening contribution can be estimated by plotting the yield strength (at time) to the fitted Q eff versus time relationships (Fig. 4) as shown in Fig.5. The result can be interpreted as two distinct linear dependent regimes of yield strength to Q eff, which implies: (1) strengthening by natural ageing is related to the increase in the detected concentration of scattering inhomogeneities; (2) the concentration of scatterers in the first regime have less effect on strengthening than those in the second stage. The rapid motion of vacancies and their trapping by solute atoms at room temperature make vacancy clusters a likely contributor to Q eff in the first stage, and with the depletion of vacancies by substitutional elements could result in greater strengthening of the solid solution by atomic misfit into the second regime. Fleischer [15] was the first to indicate the existence of a linear dependence of critical stress with solute atom concentration in substitutional solid solutions. Recent molecular dynamics analysis by Proville et al. [16] also suggests linear hardening is operative in more concentrated solid solutions such as examined here. Figure 4. Q eff versus ageing time of AA6111 at room temperature. The data shows two distinct kinetic regimes above and below 4 hours. Figure 5. Offset yield strength measured at 78 K versus corresponding Q eff with natural ageing of AA6111. The fit lines are determined by first order linear regression.

6 4. Conclusions SAXS of AA6111 undergoing early stage decomposition of its solid solution at room temperature after solutionizing and quenching reveals an increase in the effective invariant, Q eff, which is proportional to the concentration of scattering inhomogeneities. The increase in Q eff shows medium and slow kinetic regimes below and above four hours, respectively. Strengthening during natural ageing appears to correlate linearly to the Q eff regimes, and is therefore likely due to the increases in the concentration of the inhomogeneities responsible for the scattering intensity change. 5. Acknowledgements This project was funded under the proposal Formability enhancement in light weight structural materials to advance flexible manufacturing of transport vehicles with the generous support of the Natural Science and Engineering Research Council of Canada and GM Canada Ltd. We would also like to especially thank CHESS for access to their facility, and Dr. Arthur Woll for his technical support at CHESS-G1. 5. References 1. G.B. Burger, A.K. Gupta, P.W. Jeffrey, D.J. Lloyd, Matls. Char. 1995, 35, M. Murayama, K. Hono, Acta Mater. 1999, 47, K. Matsuda, Y. Sakaguchi, Y. Miyata, Y. Uetani, T. Sato, A. Kamio, S. Ikeno, J. Mater. Sci. 2000, 35, S.J. Anderson, C.D. Marioara, A. Froseth, R. Vissers, H. W. Zandbergen, Matl. Sci. Eng. A. 2005, 390, M.A. van Huis, J.H. Chen, M.H.F. Sluiter, H.W. Zanderbergen, Acta Mater. 2007, 55, S. Esmaeli, D. Vaumousse, M.W. Zanderbergen, W.J. Poole, A. Cerezo, D.J. Lloyd, 2007, 87, S. Saimoto, C. Gabryel, J. Cooley, R.K. Mishra, this volume. 8. K. Osamura, N. Otsuka, Y. Murakami, Phil. Mag. 1982, 45B, A. Deschamps, F. Bley, F. Livet, D. Fabregue, Phil. Mag. 2003, 83, C.-S. Tsao, U.-S. Jeng., C.-Y. Chen, Y.-T. Kuo, Scripta Mat., 2005, 53, L. Westfall, B.J. Diak, M.A. Singh, S. Saimoto, ASME J. Matls. Tech. 2008, 130, G. Porod, in Small Angle X-ray Scattering (Ed. O. Glatter and O. Kratky) Adademic Press, Toronto, 1982, M.A. Singh, C. Barberato, La Physique au Canada, 1997, septembre à octobre, I. Kovács, J. Lendvai, E. Nagy, Acta Metall. 1972, 20, R.L. Fleischer, in The Strengthening of Metals (Ed. D. Peckner) Reinhold Press, New York, 1964, L. Proville, D. Rodney, Y. Brechet, G. Martin, Phil. Mag. 2006, 25-26,