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Nonferrous metals and alloys

Among copper alloys for springs and connectors, which is the correct pairing of the strengthening mechanisms of phosphor bronze and beryllium copper?

  1. APhosphor bronze is strengthened by precipitation hardening with phosphides, and beryllium copper by solid-solution strengthening with Be and work hardening by cold work
  2. BBoth are strengthened by martensitic transformation on quenching and then tempered to adjust toughness and spring properties
  3. CPhosphor bronze is strengthened by solid-solution strengthening with Sn and work hardening by cold work, and beryllium copper by precipitation hardening through solution treatment and aging
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Answer: C. Phosphor bronze is strengthened by solid-solution strengthening with Sn and work hardening by cold work, and beryllium copper by precipitation hardening through solution treatment and aging

Phosphor bronze is copper with a few percent of Sn, deoxidized with a little P; its spring strength comes from solid-solution strengthening by Sn and work hardening by cold rolling, and it is not hardened by heat treatment. Beryllium copper contains about 2% Be; when solution treated at about 800 °C, quenched and aged at about 300 °C, fine precipitates form and its tensile strength exceeds 1 GPa, the highest among copper alloys. Its conductivity is also fairly good, and it is used for spark-resistant tools as well, but Be dust and fumes are harmful, so precautions against inhaling them are needed in processing. The option citing phosphide precipitation and Be in solid solution mixes up the mechanisms of the two alloys. Neither alloy is strengthened by martensitic transformation on quenching as steel is.

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  1. The corrosion resistance of magnesium alloys depends strongly on the amounts of the impurities Fe, Ni and Cu, and in high-purity alloys such as AZ91D the upper limits for Fe and Ni are set very low, at about 0.005% or less. Which is the correct reason?
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