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Metals and alloys

Check carbon steel designations, the types of stainless steel, the characteristics of aluminum, titanium and copper alloys, solid-solution strengthening and work hardening, recovery and recrystallization, and solidification structures of castings with multiple-choice questions, and practice calculations of specific strength and solidification time.

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Metals and alloys: key points

Steel is the mainstay of engineering metals. In carbon steels, the carbon content sets the balance between hardness and ductility, and designations such as S45C (JIS) indicate the carbon content. Stainless steels, containing about 11% Cr or more, are divided by microstructure into austenitic, ferritic and martensitic grades, which differ in magnetism and in whether they harden on quenching. Among nonferrous metals, organize light aluminum alloys, titanium alloys with excellent specific strength and corrosion resistance, and readily worked copper alloys by alloy system and designation.

The basic way to strengthen a metal is to obstruct dislocation motion. Typical methods are solid-solution strengthening by interstitial or substitutional solute atoms and work hardening, which multiplies dislocations by cold working; when a work-hardened metal is heated, it changes through recovery, recrystallization and grain growth in that order. Hot and cold working are distinguished by the recrystallization temperature. In castings, chill grains, columnar grains and equiaxed grains form in that order from the mold wall, and the solidification time can be estimated with Chvorinov's rule.

All Metals and alloys questions (12)
  1. What does the JIS steel designation “S45C” stand for?
  2. Which correctly describes the properties of the 18%Cr-8%Ni austenitic stainless steel SUS304 (JIS; equivalent to AISI 304)?
  3. Which correctly pairs a copper alloy with its main alloying element?
  4. In the aluminum alloy designation “A6061-T6” (JIS; 6061-T6 in Aluminum Association notation), what treatment does “T6” indicate?
  5. Which statement about the crystal structure of pure titanium is correct?
  6. Which pair of solute elements forms interstitial solid solutions in iron?
  7. Which correctly describes recovery, which occurs when a cold-worked metal is heated to a temperature too low for recrystallization?
  8. Which sequence of solidification structures is generally seen in the cross section of an ingot made by pouring molten metal into a mold?
  9. A titanium alloy has a tensile strength of 950 MPa and a density of 4.43 g/cm³, and a high-strength steel has a tensile strength of 1200 MPa and a density of 7.85 g/cm³. About how many times the specific strength (tensile strength/density) of the steel is that of the titanium alloy?
  10. For annealed copper, the relation between true stress σ and true strain ε in the plastic region is σ = Kεⁿ with K = 530 MPa and n = 0.44. Which value is closest to the true stress at a true strain of 0.10?
  11. Taking the recrystallization temperature as roughly 0.4 times the melting point (in absolute temperature), which of these counts as hot working? Take the melting points as 327 °C for lead, 3422 °C for tungsten and 1538 °C for iron.
  12. Chvorinov's rule gives the solidification time t of a casting in terms of the ratio of its volume V to surface area A as t = B(V/A)² (B is a constant set by the mold and metal). Under the same mold conditions, a cube casting 50 mm on a side solidified in 4.0 minutes. Which value is closest to the solidification time of a cube casting 100 mm on a side?

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