21-Mat-B7 Structure and Properties of Polymers · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 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.
Nothing on this paper is a polymer question; the syllabus actually examined is the physical metallurgy, strengthening and heat treatment of non-ferrous engineering alloys — aluminum, magnesium, copper-base alloys (brasses and bronzes), nickel- and cobalt-base superalloys, titanium, and the refractory/noble metals and intermetallic compounds.
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.
Procedure. An as-cast copper alloy containing a solute whose solubility falls sharply with temperature (e.g. Be in Cu–Be, Cr in Cu–Cr, or Ni–Si in Cu–Ni–Si) is solution-treated above its solvus to dissolve the second phase, quenched to trap a supersaturated solid solution, then aged at an intermediate temperature. Microstructural change. The coarse, equilibrium second-phase particles present in the as-cast structure are replaced, after solution treatment and quench, by a single-phase supersaturated matrix, and then by a fine dispersion of coherent/semicoherent precipitate particles that nucleate homogeneously through the grains during aging; this fine dispersion is what raises strength and hardness well above the as-cast or annealed condition, while the underlying grain structure is otherwise little changed (unless combined with prior working).
Procedure. An alloy such as Cu–Ni–Sn is solution-treated into a single-phase field that lies within a metastable miscibility gap on cooling, quenched to retain the single phase, then aged at a relatively low temperature. Microstructural change. Because the alloy is inside the spinodal region, decomposition proceeds by uphill diffusion with no nucleation barrier, spontaneously developing a fine, periodic (roughly sinusoidal), compositionally modulated structure on a nanometre length scale — two coherent regions of the SAME crystal structure differing only in composition, rather than a discrete precipitate in a distinct matrix. The associated coherency strain field impedes dislocation motion and raises strength, without the incoherent-particle/matrix interfaces that can promote embrittlement or intergranular attack in a conventional discrete-precipitate alloy; spinodally hardened Cu–Ni–Sn is used as a high-strength, non-toxic substitute for beryllium copper in some applications for exactly this reason.
Procedure. The as-cast part is held at a high temperature, just below the solidus, for an extended time (hours) to allow solid-state diffusion to proceed. Microstructural change. As-cast copper alloys solidify with cored dendrites — a composition gradient from dendrite core to periphery — and, in alloys with a wide freezing range, low-melting interdendritic eutectic constituents at the grain/dendrite boundaries. The extended high-temperature soak lets diffusion even out these composition gradients, eliminating or greatly reducing coring and redistributing the interdendritic constituent into solid solution, producing a much more compositionally uniform structure. This matters practically because it removes the localized low-melting films that would otherwise cause hot-shortness (intergranular cracking) during subsequent hot working, and it makes the alloy's response to any later solution/aging treatment more uniform.
Procedure. The part (as-cast, or after cold working) is given a low-temperature anneal, well below the recrystallization temperature, for a moderate hold time. Microstructural change. No recrystallization and essentially no change in grain size or bulk strength occurs; instead, limited dislocation rearrangement and annihilation (recovery/polygonization) relieves the internal residual stresses left by non-uniform solidification shrinkage/cooling or by prior cold work, without sacrificing the strength that a full anneal would remove. The practical purpose is to reduce distortion during subsequent machining and, importantly in high-zinc brasses, to reduce susceptibility to stress-corrosion cracking (historically "season cracking") that residual tensile stress combined with an ammoniacal or humid atmosphere can otherwise cause.
