21-Mat-B7 Structure and Properties of Polymers · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2016 — 10-Met-B7, Physical Metallurgy of Non-Ferrous Metals and Alloys. Three hours, closed book, approved Casio/Sharp calculator only. Eight questions of 20 marks each; the rubric states that any five questions constitute a complete paper (100 marks total) and that only the first five appearing in the answer book are marked. All eight are answered here, because this set is a study resource rather than an exam script. The rubric explicitly notes that most questions require an essay-format answer and that clarity and organization are marked, so the answers below are written as structured prose rather than as note form.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Step 1 — solution heat treatment. The alloy is held at a temperature above the solvus but below the eutectic (for Al–4%Cu, roughly 515–540 °C) long enough for the equilibrium $\theta$ (Al&sub2;Cu) phase present after casting to dissolve completely, producing a single-phase, copper-saturated $\alpha$ solid solution.
Step 2 — quenching. The alloy is rapidly cooled (typically a water quench) to room temperature fast enough that copper has no time to diffuse and precipitate on the way down. This traps the alloy as a supersaturated solid solution (SSSS) together with a high concentration of quenched-in vacancies, which will later assist solute diffusion during aging.
Step 3 — aging (precipitation heat treatment). The supersaturated solution is held at room temperature (natural aging) or at an elevated temperature (artificial aging) to let the excess copper precipitate out through a sequence of metastable, progressively less coherent products — GP zones → $\theta''$ → $\theta'$ → equilibrium $\theta$ — with peak hardness reached while the particles are still small, coherent or semi-coherent and closely spaced, so that dislocations are impeded by an Orowan bowing/particle-shearing mechanism rather than passing freely.
Natural aging takes place at room temperature over days to weeks after quenching. Diffusion is slow at this temperature, so the process stalls at the earliest, most highly coherent stage — GP zones — giving the T4 temper: a moderate, stable strength increment with good ductility, reached without any furnace step. Artificial aging uses a deliberate elevated-temperature soak (roughly 150–190 °C for a few to several hours) that speeds diffusion enough to carry the precipitation sequence through the semi-coherent $\theta'$ stage, at which the strengthening contribution peaks, giving the T6 temper: a higher peak strength than T4, but one that must be timed carefully because continued heating (over-aging) coarsens the precipitates into the incoherent equilibrium $\theta$ phase and strength falls again. In short, natural aging is a slow, low-temperature process that self-limits at an under-aged condition, while artificial aging is a fast, controlled process capable of reaching (or, if mistimed, overshooting) the true strength peak.
A representative susceptible grade is 7075-T6 (Al–Zn–Mg–Cu), the highest-strength common structural aluminum alloy and one of the classic SCC failure cases in aircraft structure. In the T6 (peak-aged) condition, the grain-boundary precipitate (MgZn&sub2;, $\eta$ phase) is more electrochemically anodic than the adjacent, solute-depleted precipitate-free zone (PFZ) and the bulk matrix, so a continuous, preferentially corrodible path exists along the grain boundaries. When this microstructure is combined with a sustained tensile stress in the short-transverse direction — the direction most exposed in a rolled or forged product because the grains are flattened (pancaked) parallel to the rolling plane, so a boundary path runs nearly straight across the section — anodic dissolution at the boundary (assisted by hydrogen generated at the crack tip) advances an intergranular crack with very little visible ductility, well below the material's tensile strength and often without warning. The standard mitigation is to overage the alloy to the T73 (or T76) temper: holding longer/hotter than the T6 peak coarsens the grain-boundary precipitate and widens/discontinues the PFZ path, trading a modest amount of strength (roughly 10–15 percent below T6) for a large improvement in SCC resistance, which is why aerospace substructure exposed to sustained tensile stress and humidity is specified in T73/T76 rather than T6.
