# Presentation: 398095

## 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.

## Introduction: Al-Sc alloys

- 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 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.

## Al-Sc Phase Diagram

- 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.

- -Al + Al3Sc

- 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

## 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.

## 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.

## 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

## Vickers Microhardness

- 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

## Vickers Microhardness

- 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.

- 1 hr

- 1 day

- 1 week

- Triple Aged Sample: 300oC/24 h - 400oC/10 days - 450oC/48 h
- Double Aged Sample: 300oC/24 h - 425oC/48 h

## 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.

## 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.

## Coarsening Models

- 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.

## 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.

## 3-Dimensional Atom Probe (3DAP)

## 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.

## 3DAP Microscopy Results:Ti Concentration vs. Time

- 3DAP Microscopy Results:Ti Concentration vs. Time

- 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.

- Proximity Histogram of Ti for various aging times

- precipitate

- matrix

## 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.

## 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.

## 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.

## Trends in Segregation ofTi 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.

## 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.

## 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.

## 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.

## 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.