Measuring and modeling the long-term sensitization behavior of al 5xxx alloys

Gaosong Yi
2018
T h e U n i v e r s i t y o f U t a h G r a d u a t e S c h o o l STATEMENT OF DISSERTATION APPROVAL The dissertation of Gaosong Yi has been approved by the following supervisory committee members: Michael L. Free , Chair ABSTRACT Al 5xxx alloys are widely used in marine and offshore structures for their excellent balance of weight, strength, ductility, weldability, and corrosion resistance. However, they can become sensitized when exposed to elevated temperature for a long time, which is
more » ... ime, which is caused by the precipitation of intergranular β phase. β phase is anodic to Al matrix and can be selectively dissolved by corrosive solutions, such as sea water, and cause intergranular corrosion and stress corrosion cracking. In the present study, Al 5xxx alloys (Al 5083, Al 5456, Al 5050, Al 5052, and Al 5154) were aged at constant temperatures (40, 50, 60, 70℃) and cyclic temperatures (40-45, 30-70, 50-70℃) for as long as 57.5 months. The microstructure was investigated using electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), atom probe tomography (APT), and small angle neutron scattering (SANS). Experimental results reveal that a phase transformation process from GP zones to β'/β phases occurs for precipitates formed in both Al 5083 H131 and H116 aged at 70℃. The size of intergranular and intragranular β phase increase with aging time. In addition, a model based on local equilibrium of chemical potential and multiclass precipitates number evolution was adopted to predict the multiphase precipitation process in the Al-Mg binary system. The overall trend of precipitate radius and number density predicted by the model match well with experimental results. Moreover, the particle size TABLE OF CONTENTS ABSTRACT . Chapters ix 3.9 Schematic diagram for the imaging mechanism of atomic force microscopy.........55 4.1 EBSD inverse pole figure (IPF) maps of (a) Al 5083 H131 and (b) Al 5083 H116, (c) grain size distribution of Al 5083 H131 and Al 5083 H116, and (d) grain boundary misorientation of Al 5083 H131 and Al 5083 H116............................................77 4.2 Geometrically necessary dislocation density maps of as-received (a) Al 5083 H131 and (b) Al 5083 H116 alloy .............................................................................78 4.3 EBSD inverse pole figure (IPF) maps of (a) Al 5456 H116, (b) grain size distribution of Al 5456 H116 obtained from the intercept per line method, and (c) grain boundary misorientation angle distribution results of Al 5456 H116.....................................78 4.4 AFM results of Al 5083 H131 aged at 70℃ for 30 months (a) and (b) vertical trace image, (c) height trace image of the area within the dash frame in (b) and the height distribution along the line scan, (d) 3D image of (c)............................................79 4.5 AFM results of Al 5456 H116 aged at 50℃ for 18 months (a) vertical and (b) horizontal trace image, (c) and (d) vertical and 3D image of a triple junction of three grain boundaries............................................................................................80 4.6 AFM results of Al 5083 H131 aged at 70℃ for 30 months (a) height trace image (tapping mode) of a grain boundary and height distribution along line scan 1 and 2, (b) 3D image of (a)...........................................................................................81 4.7 BF-STEM images of the as received (a) Al 5083 H131 and (c) Al 5083 H116 sample. High resolution TEM images of (b) Al 5083 H131 and (d) Al 5083 H116, and the corresponding fast Fourier transform (FFT) patterns (inserted)...............................82 4.8 BF-STEM images of Al 5083 H131 aged at 70℃ for (a)1.5 months, and DF-STEM image of Al 5083 H131 aged at 70℃ for (c) 9 months, (e) 18 months, and (g) 30 months, and the corresponding EDS maps for the (b) 1.5 months, (d) 9 months, (f) 18 months, and (h) 30 months samples..............................................................................83 4.9 BF-STEM images of Al 5083 H116 aged at 70℃ for (a) 3 months and (c) 9 months, and the DF-STEM images of Al 5083 H116 aged at 70℃ for (e) 18 months and (g) 30 months, and the corresponding EDS maps for the (b) 3 months, (d) 9 months (the area within the dash frame of (c)), (f) 18 months, and (h) 30 months samples....................84 4.10 Size distribution and the cumulative log-normal fitting of Mg-rich precipitates formed in Al 5083 H116 alloy aged at 70℃ for 18 months.............................................85 4.11 BF-STEM images of Al 5083 H116 aged at 50℃ for (a) 9 months, (c) 24 months, and (e) 41 months, and the corresponding EDS maps for the (b) 9 months, (d) 24 months, and (f) 41 months sample.................................................................................86 4.12 BF-STEM images of Al 5456 H116 aged at 70℃ for (a) 1.5 months, (b) 9 months, (c) 18 months, and (d) 30 months. Fig. (a) is obtained using Hitachi HF-3300 S/TEM, and Fig. x (b)-(d) are obtained using JEM 2800. The inserted image in (a) is a high-angle annular dark-field (HAADF) image of the area within the square frame, and the corresponding STEM-EDS line scan results (indicated by the arrow in the frame) are shown in Appendix A.1. EDS mapping results of Mg for (b), (c), and (d) have been inserted, and the position of Mg rich precipitates and the grain boundary has been highlighted by arrows.............87 4.13 STEM results of precipitates formed in Al 5456 H116 aged at 70℃ for 30 months (a) HAADF-STEM image of the grain matrix obtained from JEM 2800, and (b) size distribution of intragranular Mg rich precipitates in Al 5456 H116 aged at 70℃ for 30 months................................................................................................87 4.14 TEM results of Al 5456 H116 aged at 70℃ for 30 months (a) TEM image of 4 intragranular Mg-rich precipitates formed in the matrix obtained from JEM 2800, the inserted image is the FFT of Al matrix, (b) High resolution TEM image of precipitate 2 as highlighted in (a), (c) Inverse FFT results of the area within the frame of (b), the inserted images are the EDS line scan results across the precipitate as highlighted by the arrow in (b) and the FFT of the Mg-rich precipitate, (d) TEM image of some other Mg-rich precipitates formed in the matrix of Al 5456 H116 aged at 70℃ for 30 months, (e) TEM image of the precipitate within the frame of (d), and (f) the FFT results of Al matrix the area within the frame in (e)........................................... ..............................88 4.15 STEM and EDS results of Al 5456 H116 aged at 70℃ for 30 months (a) HAADF-STEM image of a grain boundary obtained from JEM 2800, (b) EDS mapping results of (a) obtained from the ultrafast EDS system of JEM 2800, (c) EDS line scan results across the grain boundary as indicated by arrows in (a). (d) HAADF-STEM image of the same area in (a) obtained from FEI Talos F200X, (e) EDS mapping results of (d) obtained from Talos, and (f) EDS line scan results across the pre-existing particle as indicated by arrows in (a). There are 5 intragranular precipitates identified by numbers in (a) and (d), and the corresponding EDS line scan results obtained from JEM ultrafast EDS system and FEI Talos F200X are shown in Appendix. A.3........................................................89 4.16 HAADF-STEM images of Mg rich precipitates formed (a) at pre-existing particle, (b) triple junction of grain boundaries, (c) grain boundary of the navy ship sample, (d) HAADF-STEM image of two precipitates formed at the grain boundary of the navy ship sample, inserted images shows the FFT of the precipitate within the frame of (d), (e) EDS line scan results of the precipitate in (d) (as highlighted by an arrow), and (f) high resolution STEM image and the corresponding FFT pattern of the Navy ship sample. All the images are obtained using the JEM-2200FS S/TEM.....................................................90 4.17 DF-STEM image of β phase formed at the grain boundary of Al 5083 aged for different times (a) 1 month, (b) 1.5months, (c) 3 months, (d) 12 months, (e) 20 months and (f) 30 months.........................................................................................91 4.18 STEM results of Al 5083 H116 (a) DF-STEM image of Al 5083 H116 aged at (d) 50℃ for 41 months, (b) 70℃ for 18 months and (c) 30 months......................................92 xi 4.19 DF-STEM image of Al 5083 H131 aged at 30-70℃ for (a) 6 months, (b) 8 months, and (c) 10 months....................................................................................92 First of all, I would like to express my sincere gratitude to my advisor, Dr. Michael L. Free, for providing me the opportunity to work on this wonderful project. I am thankful to him for his great guidance, fruitful discussions, and valuable suggestions which helped a lot to solve the academic problems encountered during the research. Moreover, thanks to him for teaching me how to be a nice person in the past four years, and I could not have imagined having a better advisor and mentor for my Ph.D study.
doi:10.26053/0h-mdyc-pmg0 fatcat:odifzbz5qvdg7l5bbdo7bgzfie