Engineering Materials
Which of the following methods is the most effective in reducing galvanic corrosion between faying objects? (A) manufacturing both parts from the same material (B) eliminating moisture in the atmosphere (C) plating or painting one or both parts with epoxy primer (D) lubricating the parts
(A) manufacturing both parts from the same material Faying parts rub against each other. Any paint, plating, coating, or lubricant will be abraded off. Since the parts are already in contact, an electrolyte (moisture) is not needed to complete the circuit. Galvanic action will be reduced if the parts are manufactured from the same material.
A composite material consists of 20 kg of material A, 10 kg of material B, and 5 kg of material C. The densities of materials A, B, and C are 2 g/cm^3, 3 g/cm^3, and 4 g/cm^3, respectively. Most nearly, what is the density of the composite material? (A) 2.1 g/cm^3 (B) 2.4 g/cm^3 (C) 2.7 g/cm^3 (D) 3.3 g/cm^3
(B) 2.4 g/cm^3 V_A= m_A / p_A V_A= (((20 kg) (1000 g/kg)) / (2 g/cm^3)) = 10,000 cm^3 V_B= m_B / p_B = 3,333 cm^3 V_C= m_C / p_C = 1,250 cm^3 The total volume is: V_tot = V_A + V_B + V_C = 10,000 cm^3 + 3,333 cm^3 + 1,250 cm^3 = 14,583 cm^3 The density of the composite material is: P_c = Σf_i * p_i = f_A * P_A + f_B * P_B + f_C * P_C P_c = ((V_A * p_A + V_B * p_B + V_C * p_C) / (V_tot)) P_c = ((10,000 cm^3 * 2 g/cm^3) + (3,333 cm^3 * 3 g/cm^3) + (1,250 cm^3 * 4 g/cm^3)) / (14,583 cm^3)) P_c = 2.4 g/cm^3
Corrosion of iron can be inhibited with a more electropositive coating, while a less electropositive coating tends to accelerate corrosion. Which of these coatings will contribute to corrosion of iron products? (A) zinc (B) gold (C) aluminum (D) magnesium
(B) gold Zinc, aluminum, and magnesium are all more electropositive (anodic) than iron and will corrode sacrificially to protect it. Gold is more cathodic than iron and the iron will corrode first. The iron is lower than gold on the galvanic series.
What metal can be used as a sacrificial anode on a small ocean fishing boat with an aluminum hull? (A) tin (B) magnesium (C) iron (D) copper
(B) magnesium Aluminum is near the bottom of the galvanic scale. Of the answer choices, only magnesium is lower on the scale, so that magnesium is more anodic and has a greater anode half-cell potential. Therefore, magnesium will become the anode, while the aluminum will become the cathode. The less noble magnesium gives up its electrons to the more noble aluminum. During this process, the magnesium breaks down (corrodes), resulting in magnesium oxide plus free electrons flowing through the electrolyte (sea water) to the aluminum.
A mixture of ice and water is warmed from 0°C to 10°C. How many degrees of freedom does the mixture have? (A) -1 (B) 0 (C) 1 (D) 2
(C) 1 Solid and liquid phases are present simultaneously, so the number of phases, P, is 2. Only water is involved, so the number of compounds, C, is 1. Gibbs phase rule is applicable when both temperature and pressure can be varied. When the temperature is varied, Gibbs phase rule becomes: P + F = C + 2 F = C + 2 - P = 1 + 2 - 2 = 1
Galvanic corrosion between a steel pipe and an attached copper fitting is to be counteracted electrically with a monitored direct current power supply. Most nearly, what should be the applied voltage to eliminate the corrosion? (A) 0.10 V (B) 0.34 V (C) 0.44 V (D) 0.78 V
(D) 0.78 V The oxidation potential of the iron in the steel pipe is 0.440 V; the oxidation potential of the copper is -0.337 V. The difference in potentials is 0.440 V - (-0.337 V) = 0.777 V (0.78 V).
Which of the following characteristics describes martensite? I. high ductility II . formed by quenching austenite HI. high hardness (A) l only (B) I and II (C) I and III (D) II and III
(D) II and III Martensite is a hard, strong, and brittle material formed by rapid cooling of austenite. Ductility is the property of a material to be able to change its shape easily. Because quenching makes ferrous metals hard, and martensite is a very hard metal, it has very low ductility.
Which of the following processes can increase the deformation resistance of steel? I. tempering II. hot working III adding alloying elements IV. hardening (A) I and II (B) I and IV (C) II and III (D) III and IV
(D) III and IV Steel's hardness, or resistance to deformation, can be increased by surface hardening processes and by some alloying metals. However, tempering and hot working increase the ductility (deformation capability of steel, not its hardness.
Pearlite is a mixture of ferrite and cementite present in cast iron and steel, formed by the cooling of austenite. Ferrite is a form of iron that contains only a very small amount of carbon. Bainite is an aggregate of iron carbide and ferrite, formed from austenite below the temperature at which pearlite forms and above the temperature at which martensite forms. Rapid quenching of austenite can result in martensite. What is the hardest form of steel? (A) pearlite (B) ferrite (C) bainite (D) martensite
(D) martensite Hardness in steel is obtained by rapid quenching. Martensite is quenched rapidly, so it has a high hardness (and low ductility).
What does the cooling curve of a pure metal look like?
First graph on pg.2: https://uomustansiriyah.edu.iq/media/lectures/5/5_2021_12_05!04_21_16_PM.pdf ChatGPT: The cooling curve of a pure metal typically shows a gradual decrease in temperature as heat is dissipated into the environment. As the metal cools, its internal energy decreases, and the atoms in the lattice begin to lose their thermal energy and move closer together. As a result, the metal undergoes a phase transition from the liquid phase to the solid phase, and its temperature drops more rapidly until it reaches its solidification temperature. At this point, the temperature of the metal remains constant until the entire sample has solidified. This plateau on the cooling curve is called the solidification or freezing point. After solidification is complete, the temperature of the metal continues to decrease until it reaches the ambient temperature of its surroundings. The resulting curve on a graph of temperature vs. time is typically a "U" shape.
What are some common features of a binary phase diagram? This question is being asked because the book asks three separate questions about binary phase diagrams.
Pg. 105 of the handbook University of Illinois: https://fabweb.ece.illinois.edu/gt/equations/phases.aspx ChatGPT: 1. Liquidus line: The line that separates the liquid phase from the solid phase(s) at equilibrium. 2. Solidus line: The line that separates the solid phase(s) from the liquid phase at equilibrium. 3. Phase boundaries: The lines that separate two different solid phases at equilibrium. 4. Eutectic point: The lowest temperature at which a liquid phase can exist for a given composition. At this point, the liquid phase transforms into two different solid phases. 5. Eutectoid point: The temperature at which a single solid phase transforms into two different solid phases at a given composition. 6. Peritectic point: The temperature at which a solid and a liquid phase combine to form a new solid phase at a given composition. 7. Solubility limit: The maximum or minimum concentration of one component that can dissolve in the other component to form a homogeneous solid solution.
Do problem 11 of the chapter, which tests knowledge of the lever rule and binary phase diagrams. Problem: https://drive.google.com/file/d/1VV9uIkge_LUnFmnV9LAOZ3xCo6_cbBFv/view?usp=share_link
Solution: https://drive.google.com/file/d/1VV9uIkge_LUnFmnV9LAOZ3xCo6_cbBFv/view?usp=share_link
Which of the following statements is NOT true about creep failure?
The correct answer is most nearly, (b) Creep failure occurs at a point slightly above yield strength. In creep failure, the constant stress applied to a material is below the yield strength. However, due to the elevated temperatures and the constant stress applied over time, creep failure occurs BELOW the yield strength. This is important because you may design your system to be below the yield strength of all your materials, but it could still fail, if you do not take into account creep.