Precipitation-Strengthened Al-Sc-Ti Alloys - PowerPoint PPT Presentation

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.

L12 Structure

Al atoms

Sc atoms

• Most current Al alloys are limited to low temperature usage (<200ºC ) because of the dissolution and/or coarsening of their precipitates.1
• Al-Sc alloys, however, form nanosize, coherent Al3Sc (L12 structure) precipitates which exhibit lowcoarsening rates at 300ºC-350ºC.
• Significant improvement in creep resistance over pure Al.2

1Polmear IJ, Light Alloys: Metallurgy of the Light Metals, Edward Arnold 1981.

2Marquis EA, Seidman DN, Dunand DC, Acta Mat. 50 (2002) 4021-4035.

• 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

-Al + Al3Sc

1Hyland, Met. Trans. A, 23A (1992) 1947-1955.

2Drits M Ye., Ber LB, Bykov YG, Toropova LS, Anastas'eva GK, Phys. Met. Metall., 57 (6) (1984) 118-126.

Ternary additions can alter the properties of Al-Sc alloys.

• Mg for solid solution strengthening
• Zr partitions to Al3Sc phase
• Diffusivity of Zr is over 4 orders of magnitude smaller than Sc1 at 300ºC which leads to a lower coarsening rate compared to the binary.
• Reduces the lattice parameter mismatch2 between Al and Al3Sc which also leads to a lower coarsening rate.
• Segregates to the -Al/Al3Sc heterophase interface.3

1Fujikawa SI, Defect and Diff. For. 143-147 (1997) 115-120.

2Harada & Dunand, Mater. Sci. & Eng. A, 329-331 (2002) 686-695.

3C.B. Fuller, J.L. Murray, D.N. Seidman, to be submitted for publication, 2005.

Ti as a ternary alloying element:

• Low diffusion rate in Al
• Smaller than Zr by factor of ca. 20 at 300ºC1
• High solubility in Al3Sc2
• Replacing up to 50% of Sc atoms.
• Ti reduces the lattice parameter mismatch between -Al and Al3(Sc,Ti) precipitates.
• Has the potential of reducing the coarsening rate since the diffusion and elastic strain energy are reduced.

1Bergner D, Van Chi N, Wissens. Zeit. der Padag. Hochschule “N.K. Krupskaja” Halle XV (1977), Heft 3.

2Harada & Dunand, Mater. Sci. & Eng. A, 329-331 (2002) 686-695.

350ºC

300ºC

• Composition analyzed:
• Al-0.06at.%Sc-0.06at.%Ti
• The composition is in the single phase -Al region during homogenization at 640ºC.
• It is in the three phase region during aging at 300ºC and 350ºC.
• No Al3Ti precipitates were observed.

J.L. Murray, ALCOA

1 hr

1 day

1 week

• Sc is more effective strengthener at room temperature than Ti.
• Even the addition of 0.005 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) 4751-4760.

E.A. Marquis, D.N. Seidman, Acta Mater. 49 (2001) 1909-1919.

C.B. Fuller, PhD Thesis, Northwestern University, 2003

1 hr

1 day

1 week

• Significant hardening at 300ºC
• Overaging occurs after 16 days.
• Decrease in hardness with increasing temperature due to coarsening of ppts.
• No significant hardening above 320ºC
• Due to heterogeneous nucleation at higher temperatures
• Still significant hardening for samples aged at 300ºC first before aging at higher temperatures likely due to diffusion of Ti into the precipitates.

Triple Aged Sample: 300ºC/24 h - 400ºC/10 days - 450ºC/48 h

Double Aged Sample: 300ºC/24 h - 425ºC/48 h

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.

• The Al3Sc 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.

1E.A. Marquis, D.N. Seidman, Acta Mater. 49 (2001) 1909-1919.

• LSW Coarsening Theory predicts for binary alloys for steady-state:1,2
• Average precipitate radius, <R>  t1/3
• 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.

1Lifshitz IM, Slyozov VV, J Phys. Chem. Solids, 19 (1961) 35-50.

2Wagner C, Z. Elektrochem, 65, (1961) 581-591.

3Kuehmann CJ, Voorhees PW, Met. Mat. Trans. A, 27A (1996) 937-943.

• Average precipitate radius only increases slightly with time for aging at 300ºC.
• 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.

1C.B. Fuller, PhD Thesis, Northwestern University, 2003

3D reconstruction showing Al3Sc precipitate in sample aged for 96 h. at 300ºC

~125,000 atoms

• Sc atoms
• Ti atoms

Al atoms omitted for clarity.

Proximity Histogram of Ti for various aging times

• Ti concentration in Al3Sc precipitates increases with time at 300ºC.
• 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.

matrix

precipitate

• Sc concentration in precipitate phase decreases over time.
• Sc atoms replaced by Ti atoms.
• System thus not in equilibrium.

• 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.

• Increased Ti in precipitate after double aging
• 24 hrs. at 300ºC
• 120 hrs. at 400ºC
• Diffusion distance for 64 days at 300ºC: 3 nm
• Diffusion distance for double aging treatment: 48 nm

Data for Double Aging Taken with Imago Scienentific LEAP microscope.

• Segregation increases with aging time at 300ºC
• 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 400ºC
• Lower mismatch at higher temperatures.

• 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

1Marquis EA, Seidman DN, Dunand DC, Acta Mat. 50 (2002) 4021-4035.

2Fuller, CB, DN Seidman, DC Dunand, Acta Materialia 51 (2003) 4803-4814.

• High apparent stress exponents indicative of threshold stress.
• For radii in the range 5.8-10.8 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.

norm = th/or

• 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-Mg2 and Al-Sc-Zr3
• Slight decrease in creep properties for the Al-Sc-Ti alloy due to lower lattice misfit.

1Marquis EA, Dunand DC, Scripta Mat. 47 (2002) 503-508

2Marquis EA, Seidman DN, Dunand DC, Acta Mater. 51 (2003) 4751-4760.

3Fuller CB, Seidman DN, Dunand DC, Acta Mater. 51 (2003) 4803-4814.

• 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 300ºC, 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.