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Precipitation-Strengthened Al-Sc-Ti Alloys Marsha van Dalen David Dunand, David Seidman Northwestern University Dept. of Materials Science and Engineering.

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Presentation on theme: "Precipitation-Strengthened Al-Sc-Ti Alloys Marsha van Dalen David Dunand, David Seidman Northwestern University Dept. of Materials Science and Engineering."— Presentation transcript:

1 Precipitation-Strengthened Al-Sc-Ti Alloys Marsha van Dalen David Dunand, David Seidman Northwestern University Dept. of Materials Science and Engineering Evanston, IL This study is supported by the US Department of Energy through grant DE-FG02-98ER45721.

2 Introduction: Al-Sc alloys Most current Al alloys are limited to low temperature usage (<200 º C ) because of the dissolution and/or coarsening of their precipitates. 1 Al-Sc alloys, however, form nanosize, coherent Al 3 Sc (L1 2 structure) precipitates which exhibit low coarsening rates at 300 º C-350 º C. Significant improvement in creep resistance over pure Al. 2 1 Polmear IJ, Light Alloys: Metallurgy of the Light Metals, Edward Arnold Marquis EA, Seidman DN, Dunand DC, Acta Mat. 50 (2002) L1 2 Structure Al atoms Sc atoms

3 Al-Sc Phase Diagram 1 Hyland, Met. Trans. A, 23A (1992) Drits M Ye., Ber LB, Bykov YG, Toropova LS, Anastas'eva GK, Phys. Met. Metall., 57 (6) (1984)  -Al + Al 3 Sc Sc has limited solid solubility in  -Al. Sc is most potent strengthener on a per atom basis. More potent than Zn, Cu, Mg, Li and Si. 2

4 Ternary alloying elements Ternary additions can alter the properties of Al-Sc alloys. Mg for solid solution strengthening Zr partitions to Al 3 Sc phase Diffusivity of Zr is over 4 orders of magnitude smaller than Sc 1 at 300 º C which leads to a lower coarsening rate compared to the binary. Reduces the lattice parameter mismatch 2 between Al and Al 3 Sc which also leads to a lower coarsening rate. Segregates to the  -Al/Al 3 Sc heterophase interface. 3 1 Fujikawa SI, Defect and Diff. For (1997) Harada & Dunand, Mater. Sci. & Eng. A, (2002) C.B. Fuller, J.L. Murray, D.N. Seidman, to be submitted for publication, 2005.

5 Al-Sc-Ti alloys Ti as a ternary alloying element: Low diffusion rate in Al Smaller than Zr by factor of ca. 20 at 300 º C 1 High solubility in Al 3 Sc 2 Replacing up to 50% of Sc atoms. Ti reduces the lattice parameter mismatch between  -Al and Al 3 (Sc,Ti) precipitates. Has the potential of reducing the coarsening rate since the diffusion and elastic strain energy are reduced. 1 Bergner D, Van Chi N, Wissens. Zeit. der Padag. Hochschule “N.K. Krupskaja” Halle XV (1977), Heft 3. 2 Harada & Dunand, Mater. Sci. & Eng. A, (2002)

6 Al-Sc-Ti Ternary Phase Diagram Composition analyzed: Al-0.06at.%Sc-0.06at.%Ti The composition is in the single phase  -Al region during homogenization at 640 º C. It is in the three phase region during aging at 300 º C and 350 º C. No Al 3 Ti precipitates were observed. J.L. Murray, ALCOA 350ºC300ºC

7 Vickers Microhardness Sc is more effective strengthener at room temperature than Ti. Even the addition of at.% Zr increases the hardness to several hundred MPa over the alloy with Ti additions. E.A. Marquis, D.N. Seidman, D.C. Dunand, Acta Mater. 51 (2003) E.A. Marquis, D.N. Seidman, Acta Mater. 49 (2001) C.B. Fuller, PhD Thesis, Northwestern University, hr1 day1 week

8 Vickers Microhardness Significant hardening at 300 º C Overaging occurs after 16 days. Decrease in hardness with increasing temperature due to coarsening of ppts. No significant hardening above 320 º C Due to heterogeneous nucleation at higher temperatures Still significant hardening for samples aged at 300 º C first before aging at higher temperatures likely due to diffusion of Ti into the precipitates. 1 hr1 day1 week Triple Aged Sample: 300 º C/24 h º C/10 days º C/48 h Double Aged Sample: 300 º C/24 h º C/48 h

9 Precipitate Morphology Dark Field TEM images showing changes in precipitate size, shape and distribution with aging treatment: (a) 300  C / 64 days [110] zone axis; (b) 320  C / 1 day. [100] zone axis; (c) 330  C / 1 day. [211] zone axis; (d) 300  C / 1 day, 400  C / 10 days, 450  C / 2 days, [110] zone axis.

10 Coherency of Al 3 Sc Precipitates The Al 3 Sc precipitates remain coherent up to temperatures of 320ºC The precipitates display Ashby-Brown strain contrast typical of coherent precipitates. Consistent with binary alloys in which precipitates remained coherent up to 40 nm in diameter. 1 BF TEM image of Al-0.06Sc-0.06Ti aged at 320ºC for 24 h. 1 E.A. Marquis, D.N. Seidman, Acta Mater. 49 (2001)

11 Coarsening Models LSW Coarsening Theory predicts for binary alloys for steady-state: 1,2 Average precipitate radius,  t 1/3 Precipitate Number Density  t -1 Supersaturation  t -1/3 For ternary alloys the time exponents are the same. 3 Assumptions: Negligible volume fraction. No elastic interaction among ppts. Ppts. have spherical shape and are randomly distributed. Only takes into account diffusion - not coagulation or coalescence of precipitates. Composition of precipitates and matrix is in quasi-steady-state, i.e. dC/dt  0 Off-diagonal terms of diffusion tensor neglected. 1 Lifshitz IM, Slyozov VV, J Phys. Chem. Solids, 19 (1961) Wagner C, Z. Elektrochem, 65, (1961) Kuehmann CJ, Voorhees PW, Met. Mat. Trans. A, 27A (1996)

12 1 C.B. Fuller, PhD Thesis, Northwestern University, 2003 Precipitate Size vs. Time at 300 º C Average precipitate radius only increases slightly with time for aging at 300 º C. Much smaller time exponent than predicted. Similar trends observed for Al-Sc-Zr alloys. 1 Indicates coarsening is occurring more slowly than predicted by coarsening models.

13 3-Dimensional Atom Probe (3DAP)

14 3DAP Microscopy Results 3D reconstruction showing Al 3 Sc precipitate in sample aged for 96 h. at 300 º C ~125,000 atoms Sc atoms Ti atoms Al atoms omitted for clarity.

15 3DAP Microscopy Results: Ti Concentration vs. Time Ti concentration in Al 3 Sc precipitates increases with time at 300 º C. Only small amount incorporated into the ppts. since the diffusion of Ti in Al is slow. Apparent interfacial segregation at longer aging times. Similar to results obtained for Al-Sc-Zr alloys. Based on 9 at.% Sc isosurface. Proximity Histogram of Ti for various aging times precipitatematrix

16 3DAP Microscopy Results: Concentration vs. Time Sc concentration in precipitate phase decreases over time. Sc atoms replaced by Ti atoms. System thus not in equilibrium.

17 Ti concentration in matrix Decreases slowly with aging time. Far from equilibrium value of 0.01 at.% At 0.04 at.% after 64 days. Concentration changing significantly thus not in equilibrium.

18 High Temperature Coarsening Increased Ti in precipitate after double aging 24 hrs. at 300 º C 120 hrs. at 400 º C Diffusion distance for 64 days at 300 º C: 3 nm Diffusion distance for double aging treatment: 48 nm Data for Double Aging Taken with Imago Scienentific LEAP microscope.

19 Trends in Segregation of Ti to Interface Segregation increases with aging time at 300 º C Due to slower diffusion in ppt. Interfacial energy is reduced. Less segregation than Zr Possibly because Ti is more effective at reducing the lattice parameter. Less segregated after aging at 400 º C Lower mismatch at higher temperatures.

20 Room Temperature Strengthening Mechanisms Orowan looping seems to be the dominant mechanism. All other mechanisms lead to stresses that would be much too high at the radii measured. order strengthening modulus mismatch coherency strains Fairly good agreement with previous studies. 1,2 Calculated Orowan Stress 1 Marquis EA, Seidman DN, Dunand DC, Acta Mat. 50 (2002) Fuller, CB, DN Seidman, DC Dunand, Acta Materialia 51 (2003)

21 Creep of Al-0.06 Sc-0.06 Ti at 300 º C High apparent stress exponents indicative of threshold stress. For radii in the range nm, creep resistance and threshold stress increases with increasing precipitate size. At largest average precipitate radius (16.9 nm), however, the interprecipitate distance is so large that the creep resistance has decreased.

22 Normalized Threshold Stress Most climb related models predict normalized threshold stress to be constant with radius. Increase of  norm with increasing radius due to lattice and elastic misfits. 1 Consistent with Al-Sc, Al- Sc-Mg 2 and Al-Sc-Zr 3 Slight decrease in creep properties for the Al-Sc-Ti alloy due to lower lattice misfit. 1 Marquis EA, Dunand DC, Scripta Mat. 47 (2002) Marquis EA, Seidman DN, Dunand DC, Acta Mater. 51 (2003) Fuller CB, Seidman DN, Dunand DC, Acta Mater. 51 (2003)  norm =  th /  or

23 Conclusions Ti does not provide as much of a strengthening effect at room temperature as an equal addition of Sc or Zr to pure aluminum. Ti partitions to the precipitates, although this is a very slow kinetic process and at the aging times analyzed, most of the Ti remains in solid solution in the matrix. The coarsening of the precipitates does not agree exactly with coarsening model - slower than predicted. A creep threshold stress is found at 300 º C, which when normalized by the Orowan stress, increases with increasing precipitate radius. Qualitative agreement is found with a model considering climb with elastic interactions with the precipitate.


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