21-Mat-A6 Materials Selection and Design for Materials Processing · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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. Solution-treat the as-cast alloy (e.g. Cu–2%Be, Cu–Cr) above its solvus, in the single-phase $\alpha$ field, to dissolve the alloying element; quench to room temperature to retain a supersaturated solid solution; then age at an intermediate temperature. Microstructural change. Fine, initially coherent second-phase particles (e.g. $\gamma$-CuBe or Cr-rich precipitates) nucleate and grow throughout the matrix during ageing, obstructing dislocation glide (particle cutting at fine sizes, Orowan looping once coarser) and substantially raising strength and hardness relative to the as-solutionized (or as-cast, coarse-second-phase) condition. Overageing coarsens the particles and softens the alloy again.
Procedure. For an alloy system with a miscibility gap and a genuine spinodal (e.g. Cu–Ni–Sn or Cu–Ti spinodal alloys), solution-treat in the single-phase field and quench directly into the spinodal region of the miscibility gap, then hold (age) at a low-to-intermediate temperature. Microstructural change. Because the homogeneous solution is intrinsically unstable inside the spinodal, no nucleation barrier need be crossed: the alloy decomposes continuously by uphill diffusion into a fine, coherent, periodically composition-modulated two-phase structure (a characteristic wavelength rather than discrete particles), which coarsens with further ageing. The resulting strengthening (coherency-strain hardening from the modulation) can rival conventional precipitation hardening while avoiding some of the interfacial/particle-cutting mechanics of discrete precipitates.
Procedure. As-cast copper alloys solidify dendritically under non-equilibrium (Scheil-type) conditions, leaving a cored, spatially varying composition between dendrite cores and interdendritic regions. Homogenizing holds the casting at a high temperature, just below the solidus (to maximize solid-state diffusivity without incipient melting), for an extended time. Microstructural change. Solid-state diffusion progressively erases the composition gradients, converting the cored dendritic structure into a compositionally uniform single-phase (or equilibrium multiphase) grain structure, which removes the risk of interdendritic incipient melting during subsequent hot working and substantially improves hot ductility/workability.
Procedure. A relatively low-temperature anneal, held well below any recrystallization, solvus or transformation temperature, applied after casting, cold work or machining. Microstructural change. The bulk grain structure, phase amounts and mechanical properties are essentially unchanged; only limited dislocation rearrangement and partial annihilation (recovery-level processes, not recrystallization) occur, sufficient to relax the internal (residual) stresses locked in by non-uniform cooling, cold deformation or machining. This is particularly important for copper–zinc brasses, which are susceptible to stress-corrosion ("season") cracking in service if residual tensile stresses are left unrelieved in a corrosive (e.g. ammoniacal) environment; stress-relieving removes that driving force without sacrificing the strength gained from cold work.
