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.
Cobalt, chromium, molybdenum, tungsten, tantalum and rhenium are the principal solid-solution strengthening elements. The heavier, larger-atomic-radius refractory additions (Mo, W, Ta, Re) partition mainly to the $\gamma$ matrix, where their size and modulus mismatch with nickel produce a strong lattice friction (solute-drag) resistance to dislocation glide; cobalt and chromium also partition largely to the matrix and add a smaller solid-solution increment while chromium's main role is protective-oxide formation.
Cobalt-base superalloys lack a stable, high-volume-fraction coherent ordered precipitate equivalent to nickel's $\gamma'$, so carbide precipitation is their primary strengthening mechanism rather than a secondary one. MC-type carbides (e.g. TaC, TiC) precipitate finely within the matrix during solidification and subsequent heat treatment, obstructing dislocation glide directly; on aging/service exposure these can partially transform to a network of $M_{23}C_6$ (chromium-rich) carbides that decorate the grain boundaries. That grain-boundary carbide network is the more important contribution at high temperature: it pins the boundaries against sliding and migration, which is the dominant softening/creep mechanism in a polycrystalline alloy at elevated homologous temperature, so carbides act largely as a grain-boundary strengthener/creep inhibitor rather than as a matrix-dislocation obstacle in the way $\gamma'$ acts in Ni-base alloys.
Nickel-base superalloys derive the majority of their high-temperature strength from a coherent, ordered $\gamma'$ (Ni⊂3⊂(Al,Ti), L1⊂2⊂ structure) precipitate that can be developed to a very high volume fraction (60–70 percent in the most advanced alloys); because the ordered structure resists dislocation shearing (an antiphase boundary must be created), $\gamma'$ gives a strengthening increment that actually increases with temperature up to roughly 800 °C, an unusual and highly valuable property. Cobalt-base superalloys, by contrast, do not form a comparably stable, high-volume-fraction coherent precipitate in the classical Co-Cr-W/Co-Cr-Mo compositions (the ordered Co⊂3⊂(Al,W) $\gamma'$ analogue is only stable in a narrower, more recently developed composition space and has historically been far less exploited); their strength at temperature instead comes predominantly from solid-solution strengthening of the matrix plus the carbide network described in part (b). The practical consequence is that classical Co-base alloys generally have lower yield strength than Ni-base alloys at a given temperature, but they retain useful ductility and, notably, better resistance to hot corrosion and thermal-fatigue cracking, which is why they remain the material of choice for some combustor and vane hardware even where blades use Ni-base alloys.
Hot corrosion is an accelerated, catastrophic form of high-temperature attack that occurs when a molten salt deposit — classically sodium sulphate, Na⊂2⊂SO⊂4⊂, formed from ingested sea-salt sodium and fuel-borne sulphur — condenses on a hot component surface (turbine blades/vanes) and chemically fluxes the normally protective Cr⊂2⊂O⊂3⊂ or Al⊂2⊂O⊂3⊂ scale, dissolving it and exposing bare metal to rapid, often catastrophic sulphidation/oxidation attack; it is distinguished from ordinary high-temperature oxidation by the presence of the molten salt film and by attack rates far faster than dry oxidation alone would predict. Resistance can be increased by raising the alloy's chromium content (or applying a Cr-rich diffusion coating, or an MCrAlY overlay coating), because a higher Cr level promotes a faster-reforming, more stable Cr⊂2⊂O⊂3⊂ scale that is more resistant to fluxing by the molten sulphate and re-heals more readily if locally disrupted.
