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.
Aluminum (solid-solution and precipitation strengthening, the base of the AZ/AM series), zinc (added with aluminum for extra strength and to counter the corrosion effect of iron impurities, AZ series; also alloyed with zirconium in the ZK series), manganese (controls iron content and improves corrosion resistance rather than providing strength), zirconium (a potent grain refiner used in aluminum-free alloys such as ZK and ZE, since Zr and Al cannot coexist — they form a stable intermetallic), and rare earths/thorium/silver (added for creep resistance and strength retention at elevated temperature, e.g. the WE and EZ series) are the major alloying additions used in engineering magnesium alloys.
Grain refinement in sand- or permanent-mold-cast magnesium (by superheating, carbon inoculation, or Zr additions) exists to counteract the naturally slow, coarse-grained solidification of those slower-cooling processes. High-pressure die casting injects molten metal into a water-cooled steel die under high pressure, giving an extremely high solidification rate and correspondingly severe undercooling; this alone nucleates a very fine as-cast grain structure without any chemical or process grain-refining step, so the additional refining practice needed for sand/permanent-mold castings would be redundant (and, for aluminum-bearing die-cast alloys, is precluded anyway because Zr cannot be used in the presence of Al).
On quenching from the single-phase solid-solution field, an Mg–Al alloy such as AZ91 is supersaturated in aluminum. Two competing precipitation reactions then occur during aging: a continuous reaction, in which fine, plate-shaped, semicoherent $\beta'$ precipitates nucleate homogeneously on the basal planes throughout the grain interior; and a discontinuous (cellular) reaction, in which lamellar colonies of coarse, incoherent equilibrium $\beta$ (Mg⊂17⊂Al⊂12⊂) nucleate at the grain boundaries and sweep inward, consuming solute as the cell front advances. The overall precipitation hardening response in Mg–Al alloys is comparatively small: the discontinuous cellular reaction dominates the transformed volume, and its widely spaced, incoherent lamellae produce little coherency strain and offer only a modest Orowan-type obstacle to dislocations, while the continuous reaction — the one that could in principle give a fine, closely spaced, more effective coherent dispersion — is starved of solute by the faster-advancing cellular front before it can develop fully. This is why Mg–Al alloys age-harden only modestly compared with a system like Al–Cu, where the equivalent continuous, fully coherent precipitation sequence is not competing with a discontinuous reaction for solute.
Magnesium is the most anodic (electrochemically active) common structural metal, with a standard electrode potential of about −2.37 V for Mg²♠/Mg — far more negative than iron (−0.44 V) or copper (+0.34 V). Direct metal-to-metal contact between magnesium and either steel or copper, in the presence of any electrolyte (moisture, condensation, road salt), forms a galvanic couple in which the enormous potential difference drives magnesium to corrode rapidly as the sacrificial anode while the steel or copper cathode is protected; because the anodic Mg area is often much smaller than the cathodic area in a typical fastener/bracket joint, the attack is also intensely localized. Engineering practice therefore isolates the joint electrically — insulating gaskets, coatings or plating on the mating part, sealed fasteners, and paint/primer barriers — rather than allowing bare Mg-to-steel or Mg-to-Cu contact in service.
