--- title: "Precipitation hardening of Al-Cu" subject: "Material Science" chapter: 9 key: "semester_3/material_science:age-hardening-of-aluminium-copper-alloy" source: https://www.bctengineer.com/q/semester_3/material_science/age-hardening-of-aluminium-copper-alloy site: BCT Engineering --- # Precipitation hardening of Al-Cu Material Science · Chapter 9: Non-ferrous Alloys - Asked in: year not given, 8 marks - Video: [Age hardening I: Introduction](https://www.youtube.com/watch?v=X_qpURDNikc) — Introduction to Materials Science and Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/material_science/age-hardening-of-aluminium-copper-alloy - Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-9 ## Question Explain precipitation (age) hardening of an Al-4% Cu alloy. Describe the three steps of the heat treatment, the sequence of precipitates formed, and why the alloy becomes stronger. Why does over-ageing reduce the strength? ## Short answer Age hardening: solution-treat at ~520°C, quench to form supersaturated solid solution, then age at 130–190°C. Sequence: GP zones → theta'' → theta' → theta (CuAl2). Maximum strength from fine coherent theta''/theta' particles impeding dislocations. Over-ageing coarsens to incoherent theta, enabling easy dislocation bypass and strength loss. ## Answer **Age hardening** raises the strength of alloys whose solid solubility decreases with falling temperature, by forming very fine precipitates that block dislocations. Al-4% Cu is the classic example (duralumin). #### Conditions for age hardening The phase diagram must show a solvus line, i.e., the solute (Cu) is soluble at high temperature (up to 5.65% at 548 °C) but only about 0.2% at room temperature. #### Heat-treatment steps ```text T | solution ______ | treat. | | (500-550 C) | | | quench | | `--. ageing | | `----_____ (130-190 C, hours) +------------------------------------> time ``` 1. **Solution treatment**: heat to about 520-540 °C, a single-phase $\alpha$ region, and hold to dissolve all Cu. 2. **Quenching** in water to room temperature: Cu is trapped in a **supersaturated** solid solution. 3. **Ageing**: hold at room temperature (natural ageing) or 130-190 °C (artificial ageing). Cu atoms diffuse and cluster to form precipitates. #### Precipitation sequence Supersaturated $\alpha$ $\rightarrow$ **GP zones** (Cu-rich discs, coherent) $\rightarrow$ $\theta''$ $\rightarrow$ $\theta'$ (semi-coherent) $\rightarrow$ $\theta$ (CuAl$_2$, incoherent, equilibrium). #### Why strength increases Coherent GP zones and $\theta''$ distort the surrounding lattice. Dislocations must cut through the particles or bow around them, which needs a higher stress. Maximum hardness occurs with a fine, dense dispersion of $\theta''$/$\theta'$. #### Over-ageing Long time or high temperature lets precipitates coarsen into large, widely spaced incoherent $\theta$ particles. Dislocations can easily bypass them (Orowan looping), so hardness and strength fall. | Stage | Hardness | |---|---| | Quenched | Low | | Under-aged (GP zones) | Rising | | Peak aged ($\theta''$, $\theta'$) | Maximum | | Over-aged ($\theta$) | Falling | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What phase-diagram feature makes an alloy age-hardenable? A. A eutectoid point B. A solvus line: solubility decreasing strongly with falling temperature C. A peritectic reaction D. Complete solid solubility at all temperatures Answer: B. Cu is soluble at high temperature but only about 0.2% at room temperature, so a supersaturated solution can be created. (Key point: Condition for age hardening) 2. What is the correct order of age hardening? A. Ageing, quenching, solution treatment B. Quenching, solution treatment, ageing C. Solution treatment, slow cooling, ageing D. Solution treatment, quenching, ageing Answer: D. Dissolve Cu in the single-phase alpha region, quench to trap a supersaturated solution, then age to precipitate. (Key point: Three steps) 3. Why does peak ageing give maximum strength? A. Fine, dense coherent/semi-coherent precipitates (GP zones, theta'', theta') distort the lattice and obstruct dislocations B. Large CuAl2 particles are formed C. All Cu leaves the matrix instantly D. Grain size increases Answer: A. Dislocations must cut through or bend around closely spaced fine particles, needing higher stress. (Key point: Why strength rises) 4. Why does over-ageing reduce strength? A. Precipitates dissolve completely B. Cu is lost by evaporation C. Precipitates coarsen into widely spaced incoherent theta particles that dislocations bypass easily D. Grain boundaries disappear Answer: C. Coarse, sparse particles are bypassed by Orowan looping, so hardness falls. 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