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.
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.
