7249-Based Custom Aluminum Alloy (T73 Temper)

We designed a custom aluminum alloy for the MSE 3331L Aluminum Alloy Design Competition at The Ohio State University.

What we made:

Modified 7249-T73 aluminum alloy optimized for high yield strength, elongation, & electrical conductivity.

Why 7249-T73:

This aerospace-grade alloy is used for aircraft main landing gear forgings where both extreme strength & superior corrosion resistance are critical. We selected 7249 as our base because it offers the best combination of high strength (from Zn-Cu alloying) & excellent stress corrosion resistance in the 7xxx family. We modified the composition to meet the competition's ≥90% aluminum requirement. The T73 temper (two-stage artificial aging per AMS 2770) maximizes corrosion resistance while achieving target properties: 380-430 MPa yield strength, 9-12% elongation, & 33-35% IACS conductivity.

Processing route:

Casting → hot rolling → solution treatment → water quench → two-stage artificial aging

Supplies

Step 1: Alloy Casting

Starting Microstructure:

1. Dendritic cast structure with segregation of alloying elements
2. Primary α-aluminum matrix with inter-dendritic eutectic phases
3. Large, coarse intermetallic particles at grain boundaries
4. Non-uniform distribution of Zn, Mg, and Cu
5. Grain size: Large & irregular (typical of as-cast material)

The as-cast structure has poor mechanical properties and non-uniform composition. The casting process creates regions of varying composition (micro-segregation), with Zn, Mg, & Cu concentrated in the last areas to solidify. This must be homogenized before achieving optimal heat treatment response.

Step 2: Homogenization

Process Parameters:

1. Temperature: 750-800°F (399-427°C)
2. Time: 2 hours (casting thickness dependent)
3. Cooling: Furnace cool at 50°F/hr (28°C/hr) to 450°F (232°C), then air cool
4. Reference: AMS2770R Table 8 (Full Anneal for 7000 Series)

Metallurgical Changes:

1. Diffusion of alloying elements from dendrite cores and inter-dendritic regions.
2. Dissolution of non-equilibrium eutectic phases formed during casting.
3. Spheroidization of intermetallic particles (they become more rounded & evenly distributed).
4. Reduction of micro-segregation, composition becomes more uniform throughout the material.
5. Some grain growth, but this is acceptable at this stage.

Why This Step Matters:

Homogenization creates a uniform starting composition throughout the material. Without this step, different regions of the casting would respond differently to solution heat treatment, leading to inconsistent mechanical properties.

Visual indicator: The microstructure transitions from dendritic to more uniform with dispersed particles.

Step 3: Hot Rolling

Process Overview:

1. Hot rolling: ~890°F (475°C)
2. Total reduction: Varies based on final thickness requirement (2-3mm for competition)

Metallurgical Changes:

1. Grain elongation in the rolling direction
2. Work hardening from cold rolling increases dislocation density
3. Breakup of coarse intermetallic particles into smaller, more dispersed particles
4. Stored energy from deformation provides driving force for recrystallization during solution heat treatment

Why This Step Matters:

Rolling refines the microstructure, breaks up coarse particles, & creates the final part geometry. Fragmentation and redistribution of intermetallic particles improve homogeneity, reduce stress concentrators, and give more uniform mechanical properties. The stored deformation energy will influence grain structure during solution heat treatment.

Step 4: Solution Heat Treatment

Process Parameters:

1. Temperature: 875°F ± 10°F (468°C ± 6°C)
2. Soaking Time: Based on thickness (per AMS2770R Table 3)
3. Up to 0.020" (0.51mm): 20 min (air furnace)
4. 0.063-0.090" (1.6-2.29mm): 35 min (air furnace)
5. 0.125-0.250" (3.18-6.35mm): 50 min (air furnace)
6. 0.250-0.500" (6.35-12.7mm): 60 min (air furnace)
7. Temperature Control: Class 2 instrumentation (±10°F uniformity)
8. Atmosphere: Air
9. Reference: AMS2770R Table 2 & Table 3, Section 3.3

Critical Requirements:

1. Furnace must be stabilized at temperature before loading parts (Section 3.3.2)
2. Soaking time starts when all temperature sensors reach minimum of uniformity range (Section 3.2.5.1)
3. No interruptions allowed during soaking (Section 3.2.5.3)
4. Parts must be spaced minimum 1 inch (25mm) apart, plus part thickness (Section 3.2.3.1.1)

Metallurgical Changes:

Dissolves into solution:

1. η (MgZn₂) precipitates → Zn & Mg atoms enter α-Al matrix
2. S (Al₂CuMg) precipitates → Cu & Mg atoms enter α-Al matrix
3. Small GP zones and clusters → Complete dissolution
4. T (Al₂Mg₃Zn₃) phase (if present) → Dissolves

Remaining:

1. Large Fe-rich intermetallics (insoluble at this temperature)
2. Cr-containing dispersoids (Al₃Cr, etc.) - these are deliberately stable to prevent grain growth
3. Primary grain boundaries (though some grain growth may occur)

Resulting microstructure:

1. Homogeneous α-Al solid solution supersaturated with Zn, Mg, and Cu
2. Uniform composition throughout the matrix
3. Dissolved elements are randomly distributed on substitutional sites in the FCC aluminum lattice
4. Some recrystallization may occur in cold-worked areas, creating new strain-free grains

Why This Temperature?

875°F (468°C) is above the solvus line for η (MgZn₂) and S (Al₂CuMg) phases for our composition. This ensures maximum dissolution of strengthening elements. Going higher risks:

1. Incipient melting (eutectic melting at grain boundaries)
2. Excessive grain growth
3. Surface oxidation

Going lower would leave some phases undissolved, reducing our maximum achievable strength.

Visual Indicator: Under microscope: Transition from visible precipitates to relatively clean matrix with only insoluble intermetallics visible.

Step 5: Water Quenching

Process Parameters:

1. Quenchant: Water
2. Starting Temperature: ≤90°F (32°C) for general parts; 130-160°F (54-71°C) for forgings
3. Maximum Delay: 15 seconds from furnace door opening to complete immersion (AMS2770R Table 5)
4. Agitation: Mechanical or hydraulic agitation of quenchant and/or parts required (Section 3.4.5)
5. Immersion Time: Minimum 1 minute per inch of maximum thickness, OR until boiling ceases, whichever is longer (Section 3.4.7)
6. Temperature Rise: Quenchant shall not exceed starting temp by more than 10°F during quench (Section 3.4.3)
7. Reference: AMS2770R Section 3.4, Tables 4 and 5

Critical Requirements:

Must be ~15 seconds; Al-Zn-Mg-Cu alloys form precipitates rapidly in the range 750-550°F (400-290°C). Any delay allows:

1. Heterogeneous precipitation at grain boundaries
2. Coarse precipitate formation that won't contribute to strengthening
3. Loss of supersaturation - reducing maximum achievable strength

Cooling Rate Requirements:

1. Target cooling rate: >100°F/second (>56°C/second) through critical range (750-400°F)
2. This suppresses diffusion and 'freezes' the supersaturated solution

Metallurgical Changes:

During Quench:

1. 750-550°F (400-290°C) - CRITICAL RANGE: If cooling is too slow: η and S phase precipitation at grain boundaries. With proper quench, supersaturation is retained.
2. 550-200°F (290-93°C): Vacancy formation "frozen in" - these will aid low-temperature diffusion during aging. Thermal stresses develop from differential cooling rates. Some very fine clustering may begin (pre-GP zone formation).
3. Below 200°F (93°C): Essentially no diffusion; supersaturated solid solution is stable.

Resulting Microstructure:

1. Supersaturated α-Al solid solution with Zn, Mg, Cu in solution
2. High vacancy concentration (10⁴-10⁶ times equilibrium concentration)
3. Residual quench stresses from thermal gradients
4. Metastable condition - thermodynamically unstable, wants to precipitate

This is the "W" temper - as-quenched condition

Quench Stresses:

Rapid cooling creates:

1. Thermal gradients: Surface cools faster than center.
2. Differential contraction: Creates internal stresses.
3. Residual stress pattern: Surface in tension, core in compression (for thick sections).

These stresses can cause distortion and must be relieved (done during aging or by stretching in T76511 temper).

Why Agitation?

During quenching, a vapor blanket (Leidenfrost effect) can form on the surface, dramatically reducing heat transfer. Agitation:

1. Disrupts vapor formation,
2. Ensures uniform cooling across all surfaces,
3. Prevents soft spots from slow-cooled regions.

Step 6: Natural Aging Hold

Process Parameters:

1. Temperature: Room temperature (60-80°F / 15-27°C)
2. Time: minimum 48 hours before aging can begin
3. Maximum Time: Should not exceed 72 hours (risk of stress corrosion cracking in thick sections)
4. Reference: AMS2770R Table 7, Note 6 - "The aging treatment for 7049, 7149, and 7249 parts shall not be initiated until at least 48 hours after quenching"

Why This Wait is Mandatory:

(Unique for 7049, 7149, and 7249 alloys not shared by other 7xxx alloys like 7075.)

Metallurgical Reasons:

1. Stress Equilibration: Quench stresses relax through micro-plastic deformation; vacancy concentration begins to equilibrate - reduces risk of quench cracking.
2. Natural Pre-Aging: Zn & Mg atoms begin clustering; formation of extremely fine pre-GP zones. This creates more uniform nucleation sites for subsequent aging.
3. Stabilization: The metastable supersaturated solution reaches a more stable intermediate state to reduce variability in aging response, improving reproducibility of final properties.

What's Happening Microscopically:

0-24 Hours:

1. Vacancy migration: Excess vacancies migrate to sinks (dislocations, grain boundaries)
2. Solute clustering: Zn and Mg atoms begin to cluster due to favorable interactions
3. Stress relaxation: Micro-yielding in highly stressed regions

24-48 Hours:

1. Pre-GP zone formation: Small (1-2nm) clusters of Zn and Mg atoms form (these are not yet true GP zones (too small), but they create preferred nucleation sites.
2. Further vacancy redistribution

After 48 Hours:

1. The structure is relatively suitable for controlled aging. Supersaturation still high enough for effective precipitation hardening; more uniform distribution of nucleation sites.

Temperature Control During Wait:

Room temperature variations (65-75°F) have minimal effect on the alloy. (Refrigeration would suppress the beneficial pre-aging, while elevated temperatures (>85°F) cause excessive natural aging).

This is Still "W" Temper:

Even after 48 hours at room temperature, the alloy is considered to be in the as-quenched (W) temper. It has some natural aging but has not yet achieved stable T4 condition (still requires artificial aging for T73 properties).

Step 7: First Stage Artificial Aging

Process Parameters

Temperature: 250°F ± 5°F (121°C ± 3°C)

Time: 10-12 hours continuous

Temperature Control: Class 2 instrumentation acceptable (±10°F uniformity) per AMS2770R Section 3.1.1.2.3

Heating Rate: Should reach temperature within 30 minutes of loading parts

Starting Condition: Parts must be at room temperature for at least 48 hours after quenching (as required by AMS2770R Table 7, Note 6)

Reference: AMS2770R Table 7, Page 20, 7249 alloy row

Furnace Requirements:

1. Furnace must be stabilized at 250°F before loading parts
2. Soaking time starts when all temperature sensors reach minimum temperature (245°F)
3. Maximum of 4 interruptions allowed (door openings), each not exceeding 2 minutes

First 2 Hours at 250°F

Initial Precipitate Formation: Guinier-Preston (GP) Zones

On initial insertion of the alloy into the 250°F furnace, the supersaturated aluminum matrix is unstable. Zinc and magnesium atoms that were "frozen" in solution during quenching now have enough thermal energy to move around and cluster together.

GP Zone Characteristics:

1. Size: Extremely small, only 1-4 nanometers in diameter
2. Thickness: Just 1-2 atomic layers thick (about 0.3-0.6 nanometers)
3. Shape: Thin disc-shaped clusters
4. Composition: Mostly zinc atoms with some magnesium mixed in
5. Location: They form on crystallographic {111} planes in the aluminum crystal structure
6. Atomic Structure: GP zones are coherent with the aluminum matrix

(Coherence: Say the aluminum atoms arranged in a perfect 3D grid; the GP zones fit perfectly into this grid - the zinc and magnesium atoms sit exactly where aluminum atoms would normally be. However, zinc and magnesium atoms are slightly different sizes than aluminum, so they create stress/strain in the surrounding aluminum lattice.)

How GP Zones Form: The excess vacancies (empty atomic sites) created during quenching make it easier for zinc and magnesium atoms to move around. These atoms are attracted to each other and begin clustering together in specific crystallographic orientations. At this stage, GP zones are too small to see even with most electron microscopes. The material would still look relatively uniform.

Effect on Properties:

1. Hardness increases
2. Electrical conductivity decreases (Zn and Mg in clusters reduce electron flow)
3. Material is getting stronger but is still far from final strength

2-6 Hours at 250°F

Transition to Metastable η' (Eta-Prime) Precipitates

As time progresses, the GP zones continue to grow and transform into a more organized structure called eta-prime (η').

η' Precipitate Characteristics:

1. Size: Larger than GP zones, now 4-10 nanometers in diameter
2. Thickness: 2-5 nanometers thick (about 5-15 atomic layers)
3. Shape: Still disc-shaped, but thicker and more defined
4. Composition: Much closer to the chemical formula MgZn₂ (one magnesium atom for every two zinc atoms)
5. Crystal Structure: These now have their own distinct hexagonal crystal structure, different from aluminum's face-centered cubic structure
6. Interface: Semi-coherent with the aluminum matrix

Semi-Coherence: Unlike GP zones that fit perfectly into the aluminum lattice, η' precipitates have their own crystal structure. However, they still maintain some atomic matching with the aluminum matrix (mostly fits, but there are some gaps and mismatches at edges - misfit dislocations.)

The Transformation Process:

The Zn and Mg atoms in the GP zones rearrange themselves into η'

1. More zinc and magnesium atoms from the surrounding aluminum migrate to the growing clusters.
2. The clusters reorganize from simple atomic clusters into a more ordered MgZn₂ structure.
3. The precipitates grow larger and adopt the hexagonal crystal structure characteristic of η'.

Many tiny η' precipitates are distributed throughout each grain of aluminum. These are oriented on the {111} crystallographic planes and create a pattern throughout the material.

How These Precipitates Strengthen the Aluminum:

When the aluminum tries to deform (like when you bend it or pull on it), the deformation happens by movement of dislocations (defects in the crystal structure that move through the material). The η' precipitates block these dislocations in several ways:

1. Coherency Strain: The stress fields around each precipitate make it harder for dislocations to move past.
2. Modulus Mismatch: The η' precipitates are stiffer than aluminum, so dislocations have trouble moving through them.
3. Ordered Structure: The η' has an ordered atomic arrangement that dislocations have difficulty cutting through.

Effect on Properties:

1. Material is noticeably harder than after 2 hours.
2. If you could see at the nanometer scale, you'd see a uniform distribution of small disc-shaped precipitates throughout the aluminum grains.
3. The aluminum matrix between precipitates is now depleted of zinc and magnesium (they've moved into the precipitates).

6-12 Hours at 250°F

Precipitate Refinement and Continued Strengthening

During this period, the microstructure continues to evolve, though fewer changes

Ongoing Processes:

1. Continued Nucleation (Slowing Down): New η' precipitates continue forming, but at a much slower rate than earlier. Most of the available nucleation sites (favorable locations for precipitates to form) are already occupied.

2. Growth of Existing Precipitates: Zinc and magnesium atoms continue diffusing from the aluminum matrix to existing precipitates, making them larger. The average precipitate size increases from about 5 nanometers to 10-15 nm.

3. Ostwald Ripening Begins: This is a process where larger precipitates grow at the expense of smaller ones:

1. Smaller precipitates have more surface area relative to their volume. This high surface area means they have higher energy (atoms at surfaces are less stable). To reduce total energy, the system favors larger precipitates.
2. Small precipitates begin dissolving, releasing zinc and magnesium atoms. These atoms diffuse through the aluminum and join larger precipitates.
3. Result: Fewer total precipitates, but the remaining ones are larger.

4. Partial Transformation to Stable η (MgZn₂): A small fraction of the η' precipitates begin transforming to the fully stable η phase. However, at 250°F, this transformation is very slow. Most precipitates remain as η' at the end of this first aging step.

Microstructure at End of First Aging Step (12 Hours):

What's Present:

1. Very high density of η' precipitates: approximately 100 million to 1 billion precipitates per mm^3.
2. Average precipitate size: 5-15 nanometers in diameter.
3. Relatively uniform distribution throughout the interior of aluminum grains.
4. Some precipitation beginning at grain boundaries, but this is minimal.

The aluminum between precipitates is now significantly depleted of zinc and magnesium. Most of the alloying elements have moved into the precipitates. However, there's still enough supersaturation remaining for the second aging step. Very early-stage precipitation of a different phase called S' (Al₂CuMg) may be starting at grain boundaries. This phase involves the copper in the alloy and will become more important during the second aging step.

Why Stop at 12 Hours? Why Not Continue to Peak Strength?

At this temperature (250°F), if we continued aging for much longer (say 20-30 hours), the peak strength (T6 condition) would be reached. The precipitates would reach their optimal size for maximum strengthening, fine enough to be numerous, but large enough to effectively block dislocations.

However, we deliberately stop at 12 hours because:

1. We want to create a specific precipitate size distribution that will respond correctly to the second aging step
2. We're not trying to reach peak strength but setting up the microstructure for over-aging.
3. The second step at higher temperature will complete the transformation to the desired coarser and more corrosion-resistant microstructure.

Step 8: Second Stage Artificial Aging

Process Parameters

Temperature: 325°F ± 5°F (162°C ± 3°C)

Time: 7 hours continuous

Temperature Control: Class 1 instrumentation acceptable (±5°F uniformity) per AMS2770R Section 3.1.1.2.2

Heating Rate: Should reach temperature within 40 minutes of loading parts

Starting Condition: Parts must come directly from first stage aging (250°F for 12h) with no intermediate delay longer than continuous operation (see note 14)

Reference: AMS2770R Table 7, Page 20, 7249 alloy row

Microstructural Evolution

Transformation of η′ precipitates:

Many of the fine η′ discs formed in the first stage begin coarsening.

Partial transformation into stable η (MgZn₂) phase occurs, especially at grain boundaries.

Ostwald Ripening:

Larger precipitates grow instead of smaller ones, reducing total precipitate density.

Grain Boundary Precipitation:

More pronounced precipitation along boundaries, including η and S′ (Al₂CuMg).

This reduces susceptibility to stress corrosion cracking (SCC).

Matrix Changes:

Further depletion of Zn, Mg, and Cu from the aluminum matrix.

Electrical conductivity increases as matrix approaches purer aluminum.

Why This Step Matters

1. This second stage deliberately overages the alloy to trade some peak strength for corrosion resistance.
2. Coarser precipitates and grain boundary phases reduce SCC susceptibility, a critical requirement for aerospace applications.
3. The T73 temper balances mechanical performance with durability, ensuring the alloy maintains strength while resisting environmental degradation.

Step 9: Preliminary Mechanical Testing

Step 10: Preliminary Mechanical Test Results

The animated visualization reveals the temporal development of each stress-strain curve during testing. Several important observations about test setup and sample behavior are apparent.

Some samples show negative force readings at test initiation (ranging from -200 to -500 N). This occurs when grips are first clamped onto specimens.

Step 11: Competition Results

The competition sample exhibited premature failure with significantly reduced elongation compared to preliminary test samples. Microscopic examination revealed dark bands in the microstructure, indicating the presence of porosity or inclusions that likely served as stress concentrators during tensile testing.

Premature core fracture occurred at 6.4% elongation—before normal necking could complete. Electrical conductivity measurements confirmed the bulk microstructure remained unchanged at 32.5% IACS, proving the T73 temper state was preserved. Preliminary samples achieved ~8.9% elongation.

Step 12: Improvements

Understanding Non-Linear Behavior in Early Testing

What You're Seeing

The plot above shows the first few seconds of each tensile test, zoomed in to reveal non-linear behavior at the very beginning. The dotted lines show where the stress-strain relationship is non-linear, while the solid lines show where true elastic (linear) behavior begins. The large circles mark the exact transition point for each sample.

Why This Happens

This non-linearity is not a material property - it's a testing artifact called the "toe region." When a tensile test begins, several things need to happen before the sample is truly under uniform tension:

1. Grips settle and align as hydraulic pressure builds
2. Sample straightens - heat-treated aluminum often warps slightly during quenching, and this warping gets pulled straight at the start of testing
3. Slack is taken up in the testing system

Dramatic Differences Between Samples

The extent of this toe region varied significantly:

1. Sample 1B (red): Transitioned to linear behavior in only 0.56 seconds at ~5 MPa
2. Sample 2A (green): Quick transition at 1.66 seconds and ~17 MPa
3. Sample 2B (orange): Moderate toe region, transitioning at 2.36 seconds and ~30 MPa
4. Sample 1A (blue): Extended non-linearity lasting 5.60 seconds up to ~64 MPa

Sample 1A's prolonged toe region occurred because the test was interrupted and restarted when we initially forgot to attach the extensometer. This required reseating the sample in the grips, which created additional alignment challenges.

Why This Matters

This variability makes it impossible to calculate reliable elastic modulus values from the early data. The different transition points (ranging from 0.56 to 5.60 seconds) aren't telling us about the aluminum's properties - they're telling us about how each sample was seated in the testing machine.

For accurate material property measurements, we must exclude this toe region and use only the linear portion of the curve where true elastic behavior is observed.

Step 13: Probable Causes

1. Non-Uniform Quenching Effects

The location of sample extraction from the parent material is critical. Samples taken from regions that experienced:

- Slower cooling rates during quenching may develop:

- Coarse precipitates along grain boundaries

- Hydrogen porosity from trapped moisture

- Incomplete solution treatment effects

- Edge effects where heat extraction varies can create:

- Residual stress concentrations

- Variable microstructure across the thickness

2. Hydrogen-Induced Porosity

Per AMS2770R Section 3.3.3, hydrogen-induced porosity is a known issue in aluminum heat treatment:

- Trapped moisture during solution treatment can cause hydrogen gas evolution

- Creates microscopic voids that appear as dark regions in micrographs

- These voids act as crack initiation sites during tensile testing

- More prevalent in thick sections or areas with poor quench uniformity

3. Quench Delay Effects

If the sample location experienced delayed quenching:

- Precipitation begins during transfer from furnace to quench

- Creates coarse, incoherent precipitates

- These appear as dark bands and reduce ductility

- AMS2770R specifies maximum 15-second quench delay for thick sections

4. Segregation and Banding

The dark bands may represent:

- Microsegregation from the original casting/forging process

- Banded structure from non-uniform deformation during prior working

- These bands concentrate alloying elements differently

- Creates planes of weakness perpendicular to loading direction

The horizontal dark bands (voids) observed in the fracture surface image are consistent with:

1. Linear porosity - Aligned voids from hydrogen evolution or shrinkage

2. Inclusion stringers - Oxide or intermetallic particles aligned during working

3. Precipitate-free zones - Areas depleted of strengthening phases

These features create a "perforated" microstructure where cracks can easily:

- Initiate at void/matrix interfaces

- Propagate along weakened planes

- Link up rapidly, causing premature failure

Step 14: True Stress-Strain

Step 15: Acknowledgements

We thank our instructor, Professor Elvin Beach, lab supervisors Peter Fallon and Wayne Papageorge, as well as our TAs Nicole Hudak, Liz Kuebel, Ziyao Su, and Kerry Ulm for facilitating this insightful laboratory experience.

Step 16: References

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