What Microstructures Occur in Aluminum and Its Alloys?

Cast Aluminum Alloys Microstructures

Al-Si Alloys Microstructures

The silicon mass fraction w_Si is generally 5%-13%. These hypoeutectic and eutectic alloys cannot be strengthened by heat treatment. A simple binary Al-Si eutectic alloy such as ZL102 has an α + Si eutectic structure in the as-cast condition. Silicon appears as coarse needles and polygons. Modification with sodium salts, phosphorus, or other elements refines the coarse eutectic silicon and improves mechanical properties and machinability.

Al-Si alloys contain relatively few strengthening elements, and their quenching temperature is far from the eutectic point, so overheating is uncommon during solution treatment. If an excessive solution temperature causes incipient melting, the structure develops remelted globules, grain-boundary melting, and agglomerated, coarsened silicon.

Al-Cu Alloys Microstructures

The copper mass fraction w_Cu is 4%-11%. These alloys can be strengthened by heat treatment, principally through the θ phase, Al₂Cu. Small magnesium and zinc additions can form the S phase, Al₂CuMg, or T phase, AlCuZn, for supplementary strengthening. High mechanical properties at room and elevated temperatures make this system a basis for high-strength, heat-resistant cast aluminum alloys.

Al-Mg Alloys Microstructures

Al-Mg alloys have the lowest density and highest corrosion resistance among aluminum alloys, particularly against electrochemical corrosion. In the quenched condition, the structure is single-phase α. If Mg₂Si is present, its lower electrode potential makes it anodic to α, so Mg₂Si is continually consumed during electrochemical corrosion until a single-phase α surface remains and corrosion stops. These alloys have poor castability, a complex melting process, and low hot strength. High-magnesium grades may become brittle during long service, limiting their use.

Al-Zn Alloys Microstructures

In the as-cast condition, phases include α, silicon, Mg₂Si, and a small amount of harmful black needle-like β phase, Al₉Si₂Fe₂. Mg₂Si is often difficult to observe because of its low content.

Wrought Aluminum and Aluminum Alloys Microstructures

Commercially Pure Aluminum Microstructures

The aluminum mass fraction w_Al in commercially pure aluminum is generally 98.80%-99.99%; iron and silicon form the principal impurity phases. At low iron and silicon contents, silicon dissolves in the matrix and iron forms needle-like or fine-strip Al₃Fe. At higher contents, ternary compounds form. If iron exceeds silicon, an irregular plate-like or skeletal α phase, Al₁₂Fe₃Si, forms. If silicon exceeds iron, needle-like or fine-strip β phase, Al₉Si₂Fe₂, forms.

Corrosion-Resistant Aluminum Alloys

Corrosion-resistant aluminum comprises Al-Mn and Al-Mg alloy systems.

Al-Mn Alloys Microstructures

A common grade is 3A21, with manganese as the principal addition. When w_Mn > 1.6%, abundant brittle Al₆Mn compounds make the alloy prone to cracking during deformation. In addition to α solid solution and Al₆Mn, phases may include Al₆(FeMn), T(Al₂Mn₃Si₂), α(Al₁₂Fe₃Si), and β(Al₉Si₂Fe₂).

Al-Mg Alloys

After annealing, an Al-Mg alloy with w_Mg < 2% is a single-phase α solid solution. As magnesium increases, β(Al₈Mg₅) appears. Above 5% Mg, the structure consists of α + β(Al₈Mg₅), and ductility decreases. Principal phases may include α, β(Al₈Mg₅), Al₆Mn, Al₇Cr, and Al₃Ti, with impurity phases such as Al₃Fe, (FeMn)Al₆, and Mg₂Si. The β phase in the eutectic is skeletal and appears light gray before etching.

Duralumin Alloys

Duralumin alloys are heat-treatable and include the Al-Cu-Mg and Al-Cu-Mn systems.

Al-Cu-Mg Alloys Microstructures

At the aluminum-rich corner of the equilibrium Al-Cu-Mg ternary diagram, solidification can produce S(Al₂CuMg), θ(Al₂Cu), T(Al₆CuMg₄), β(Al₈Mg₅), and α. Actual magnesium contents are relatively low, placing alloys in the α + S, α + θ + S, or α + θ phase fields. The two principal strengthening phases are therefore θ and S.

Al-Cu-Mn Alloys

The copper mass fraction can reach 6%-7%. Copper and aluminum form the strengthening phase Al₂Cu, which strengthens the alloy after quenching and artificial aging. At w_Mn = 0.4%-0.5%, finely dispersed T(Al₂CuMn₂) improves heat resistance. Excess manganese increases and coarsens the T phase, enlarging phase boundaries and accelerating diffusion. Heat resistance then falls and hot-cracking tendency during welding increases.

Super-Duralumin Alloys

Common super-duralumin alloys contain α, MgZn₂, T(Al₂Mg₃Zn₃), an α + T(Al₂Mg₃Zn₃) eutectic, and impurity phases such as (FeMn)Al₆ and Mg₂Si.

Forging Aluminum Alloys

Forging aluminum alloys are generally divided into two systems.

Al-Mg-Si-Cu Alloys

When copper is very low and the magnesium-to-silicon mass ratio is 1.73, Mg₂Si forms and all copper dissolves in the matrix. At higher copper contents and a magnesium-to-silicon ratio below 1.08, ω(AlCu₄Mg₅Si₄) can form, with the remaining copper forming θ(Al₂Cu). Adding copper to Al-Mg-Si alloys can therefore introduce ω, S, and θ in addition to Mg₂Si.

Al-Cu-Mg-Fe-Ni Alloys

These alloys contain less copper and magnesium than duralumin and therefore contain more S phase, Al₂CuMg. Iron and nickel form Al₉FeNi, which has good thermal stability. Excess iron forms Al₇Cu₂Fe, while excess nickel can form AlCuNi.