A materials scientist measures the ultimate tensile strength (UTS) of four…
CHEMISTRY · Single correct · CBSE · Chemistry · Metals and Non-metals · Metals and Non-metals
Problem
A materials scientist measures the ultimate tensile strength (UTS) of four nickel-based specimens and obtains the following data:
Specimen A: pure Ni, UTS = 420MPa
Specimen B: Ni + 5at.% Co (atomic radius Co ≈ Ni), UTS = 450MPa
Specimen C: Ni + 1at.% B (atomic radius B ≪ Ni), UTS = 610MPa
Specimen D: Ni + 1at.% B, annealed at high temperature followed by slow cooling, UTS = 500MPa
Which of the following most plausibly accounts for the variation in UTS among the specimens?
(a) Substitutional cobalt atoms are larger than Ni, producing the greatest lattice strain; this strain impedes dislocation motion, so Specimen B should be far stronger than the others, while the much smaller boron atoms have little effect.
(b) Slow cooling after annealing in Specimen D generates a finer grain structure than the rapid-quenched Specimen C; the extra grain boundaries should strengthen D beyond C, so its lower UTS must arise from experimental error.
(c) Interstitial boron in Specimen C creates significant lattice strain that strengthens the metal, while slow cooling in Specimen D allows some strain to relax, reducing its strength compared with C.
(d) Annealing introduces a high vacancy concentration that clusters with boron to form hard Ni3B precipitates. These precipitates should pin dislocations more effectively, so Specimen D ought to have a higher UTS than Specimen C.