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.
Distinctive features. The refractory metals — tungsten, molybdenum, tantalum, niobium and rhenium — are the BCC transition metals with the highest melting points in the periodic table (W 3422 °C, Ta 3017 °C, Mo 2623 °C, Nb 2477 °C), giving them exceptionally high elastic modulus, hot strength and creep resistance retained well above the temperature at which conventional steels and superalloys have already melted or lost most of their strength.
Limitations. Two limitations dominate. First, most refractory metals (W, Mo especially) have a ductile-to-brittle transition temperature (DBTT) that is at or above room temperature in the recrystallized/coarse-grained condition, so they can be brittle and difficult to fabricate or machine cold unless worked and used in a fine-grained, stress-relieved condition. Second, they oxidize catastrophically at high temperature — W and Mo form volatile oxides (WO3, and especially MoO3 which sublimes above about 700 °C, "catastrophic oxidation") rather than a protective scale — so they must be used in vacuum, an inert/reducing atmosphere, or with a protective coating whenever service temperature is high. They are also dense (W, Ta) and, being difficult to melt and cast, are typically consolidated by powder metallurgy, which adds cost.
Applications. Incandescent and vacuum-tube filaments and rocket-nozzle throat inserts (W); furnace heating elements and glass-melting electrodes operated in protective/inert atmospheres (Mo, W); high-temperature structural and fastener applications plus chemical-process equipment for its excellent corrosion resistance to acids (Ta, Nb); and, alloyed into nickel-base superalloys, as the solid-solution strengtheners already described in Question 6(a) (Mo, W, Ta, Re).
Distinctive features. The noble metals — gold, silver, platinum, palladium and the other platinum-group metals (Rh, Ir, Ru, Os) — are defined by very positive standard electrode potentials (e.g. Au3+/Au +1.50 V, Ag+/Ag +0.80 V, Pt2+/Pt +1.19 V), meaning they are thermodynamically resistant to oxidation and corrosion in most environments without needing a protective passive film the way, say, aluminum or stainless steel does. They also have excellent electrical and thermal conductivity (silver is the best electrical conductor of all metals) and, for the platinum-group metals, strong catalytic activity.
Limitations. In the pure state they are comparatively soft and low-strength, so structural or wear applications require alloying (which somewhat compromises the very corrosion resistance that makes them attractive) or use in thin films/coatings rather than bulk structural form. Their high cost, driven by scarcity, is the dominant practical limitation and restricts their use to applications where the property they provide cannot be obtained more cheaply any other way.
Applications. Electrical contacts and connectors, where freedom from oxide/corrosion films is essential for reliable low-resistance contact (Au, Ag, Pd); catalytic converters and industrial catalysis, exploiting the platinum-group metals' catalytic surface activity (Pt, Pd, Rh); jewelry and historically coinage, for tarnish resistance and appearance (Au, Ag, Pt); and corrosion-resistant laboratory ware, dental and medical implant components, and high-temperature thermocouples (Pt, Pt–Rh).
An intermetallic compound forms at (or very near) a specific atomic ratio of its two constituent elements, with its own distinct crystal structure different from either parent metal. The distinction the question asks for is how much composition range that compound can tolerate while remaining a single phase — equivalently, how it appears as a phase field on a binary phase diagram.
A stoichiometric intermetallic exists only at (or within a fraction of an atomic percent of) its ideal ratio $A_mB_n$; on the phase diagram it appears as a near-vertical "line compound," often with a single sharp congruent melting point at the top of the line, because its ordered crystal structure has essentially no tolerance for excess A or B atoms without destabilizing the structure. A non-stoichiometric intermetallic instead exists over a finite range of composition around its ideal ratio — a wide, dome-shaped single-phase field bounded by solvus/solidus curves on both sides — because its ordered structure CAN accommodate a controlled excess of one constituent through point defects: antisite atoms (an A atom sitting on a B sublattice site, or vice versa) or constitutional (structural) vacancies on one sublattice. The width of that dome is a direct measure of how much point-defect disorder the ordered structure can tolerate before a competing phase becomes more stable.
The classic structural intermetallic is Ni3Al ($\gamma'$), the ordered L12 (FCC-derived, Cu3Au-type) precipitate phase that is the primary strengthener of nickel-base superalloys (introduced in Question 6). It is close to, but not perfectly, stoichiometric — it tolerates a modest homogeneity range on the Ni-rich side, accommodated by Ni antisite defects on the Al sublattice, which is why it is normally classed as a (narrow) non-stoichiometric intermetallic rather than a perfect line compound.
Its primary strengthening mechanism, and the reason it gives good high-temperature creep resistance specifically, is that the ordered L12 lattice cannot be sheared by an ordinary single dislocation the way a disordered solid solution can: passing one $\tfrac{1}{2}\langle110\rangle$ dislocation through the lattice would leave a high-energy antiphase boundary (APB) — a plane across which the A/B sublattice registry is disrupted — in its wake. Dislocations are therefore forced to move in coupled pairs ("superdislocations") connected by a ribbon of APB, so that the second partial restores the correct ordering immediately behind the first; creating and dragging that APB ribbon requires substantially more stress than moving a single dislocation through a disordered matrix. A further, unusual consequence of this same APB-coupled mechanism is that screw-dislocation segments in $\gamma'$ can cross-slip from the closest-packed {111} glide plane onto the lower-energy-APB {100} plane and become locked there (Kear–Wilsdorf locking), which is thermally assisted and therefore becomes MORE effective as temperature rises — giving $\gamma'$, and the Ni-base superalloys strengthened by it, a yield strength that actually increases with temperature up to roughly 800 °C rather than falling as it would in a normal metal. It is precisely this rising, temperature-resistant flow strength — combined with a very high coherent volume fraction and the requirement for boundary-crossing dislocations to keep re-forming APB ribbons at every particle — that gives Ni3Al-strengthened superalloys their outstanding resistance to dislocation-creep deformation at elevated homologous temperature.
