PrecipitationStrengthened AlScTi Alloys

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 (lt200ºC ) because of the
  dissolution and/or coarsening of their
  precipitates.1
• Al-Sc alloys, however, form nanosize, coherent
  Al3Sc (L12 structure) precipitates which exhibit
  low coarsening 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.
3
Al-Sc Phase Diagram

• 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.
4
Ternary alloying elements

• 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.
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 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.
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 640ºC.
• It is in the three phase region during aging at
  300ºC and 350ºC.
• No Al3Ti precipitates were observed.

J.L. Murray, ALCOA
7
Vickers Microhardness
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
8
Vickers Microhardness
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
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 Al3Sc Precipitates

• 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.
11
Coarsening Models

• LSW Coarsening Theory predicts for binary alloys
  for steady-state1,2
• Average precipitate radius, ltRgt ? 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.
12
Precipitate Size vs. Time at 300ºC

• 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
13
3-Dimensional Atom Probe (3DAP)
14
3DAP Microscopy Results
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.
15
3DAP Microscopy ResultsTi Concentration vs. Time
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.

precipitate
matrix
16
3DAP Microscopy ResultsConcentration 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 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.
19
Trends in Segregation ofTi to Interface

• 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.

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
1Marquis EA, Seidman DN, Dunand DC, Acta Mat. 50
(2002) 4021-4035. 2Fuller, CB, DN Seidman, DC
Dunand, Acta Materialia 51 (2003) 4803-4814.
21
Creep of Al-0.06 Sc-0.06 Ti at 300ºC

• 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.

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-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.
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 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.