Material Properties and Processing practice questions

Answer: B) Creep

Creep is slow, time-dependent strain under constant stress at high homologous temperature. Stress relaxation is the opposite case: strain held constant while stress decays.

• AIncorrect. Fatigue is crack initiation and growth driven by fluctuating (cyclic) stresses. Here the stress is constant, and the response is gradual strain build-up, not cracking from load cycles.
• BCorrect. Creep is irreversible, time-dependent deformation under sustained stress at elevated temperature, usually above about 40% of the melting temperature. It goes through primary, secondary (steady-state) and tertiary stages.
• CIncorrect. Stress relaxation is a drop in stress over time while the strain is held constant, as in a bolted joint. In this case the stress is constant and the strain increases.
• DIncorrect. Strain hardening is the rise in strength caused by plastic deformation, usually cold working. It describes strengthening, not slow elongation over time under a steady load.

Answer: B) Pitting corrosion

• AIncorrect. The passive film usually resists general corrosion.
• BCorrect. Chlorides break down the passive film locally.
• CIncorrect. That isn't the main chloride concern.
• DIncorrect. That isn't a corrosion mode.

Answer: D) Precipitation (age) hardening

• AIncorrect. That's a zinc coating.
• BIncorrect. Aluminum doesn't form martensite.
• CIncorrect. That's for steel surfaces.
• DCorrect. Fine precipitates block dislocations.

Answer: C) With little plastic deformation

• AIncorrect. Brittle materials are weak in tension.
• BIncorrect. That's ductile failure.
• CCorrect. Cast iron and glass are examples.
• DIncorrect. Brittleness doesn't need high temperature.

Answer: D) Ferrite, with a body-centered cubic structure

• AIncorrect. Steel is crystalline.
• BIncorrect. That's stable at high temperatures.
• CIncorrect. Martensite forms from rapid quenching.
• DCorrect. Austenite is face-centered cubic and stable at high temperatures.

Answer: A) Depth of indenter penetration under a major load, measured relative to a minor preload

Rockwell hardness comes from the depth of penetration under load. Brinell and Vickers instead measure the size of the indentation to find hardness as force per area. Charpy measures impact energy.

• ACorrect. Rockwell is a one-step test. The hardness value comes from how deep the indenter goes during the test, so no optical measurement of the indent is needed. That is why it is fast and popular in production.
• BIncorrect. This describes the Brinell test. It uses a ball indenter with a load-on, load-off procedure, and the hardness is calculated from the size of the impression.
• CIncorrect. This describes the Vickers test, where hardness is the test force divided by the surface area of the indentation.
• DIncorrect. This describes the Charpy impact test. It measures impact energy (toughness), not indentation hardness.

Answer: A) Lowest melting temperature

• ACorrect. Liquid transforms directly to two solids.
• BIncorrect. It's the lowest.
• CIncorrect. It's the lowest melting point.
• DIncorrect. It's an alloy composition.

Answer: A) High strength-to-weight ratio and good corrosion resistance

Titanium alloys are chosen for aerospace because of their high specific strength (strength/density) and excellent corrosion resistance, even though they are expensive and less stiff than steel.

• ACorrect. Titanium alloys combine moderate density with high strength, giving a high specific strength, and they resist corrosion well. Both matter for weight-critical aerospace parts.
• BIncorrect. Titanium costs more to produce than aluminum and is harder to machine. It is chosen despite its cost, not because of it.
• CIncorrect. Titanium (about 4.5 g/cm³) is denser than aluminum (about 2.7 g/cm³). Its advantage is strength per unit weight, not lower density.
• DIncorrect. Titanium's elastic modulus (about 110 GPa) is roughly half that of steel (about 200 GPa), so it is not chosen for stiffness.

Answer: B) High strength-to-weight ratio

• AIncorrect. They're relatively brittle and costly.
• BCorrect. And high stiffness-to-weight.
• CIncorrect. Carbon fiber conducts.
• DIncorrect. They're stiff.

Answer: B) Base metal that stayed solid but whose microstructure was altered

A fusion weld has three regions: the fusion zone (melted weld metal), the HAZ (base metal that did not melt but whose microstructure and properties changed), and the unaffected base metal.

• AIncorrect. Base metal that melted and mixed with filler forms the fusion zone (weld metal), not the HAZ.
• BCorrect. The HAZ is base metal next to the fusion zone. It never melted, but the welding thermal cycle changed its grain structure and properties, such as hardness, toughness, and strength.
• CIncorrect. This is the unaffected base metal. Its peak temperature stayed too low to change its microstructure.
• DIncorrect. Solidified filler metal is part of the weld metal (fusion zone), wherever it froze first. The HAZ is made only of base metal that did not melt.