Materials HW key

The crystallographic direction denoted by [010] will be perpendicular to _____ plane.

xz plane

One would like to select a material for a mechanical gear device that engages with other gears and rotates with more than 1000 rpm. What properties should the gear teeth material possess? What type of material you would recommend for the manufacturing of gear? Would Al2O3 be a good choice? Explain

for specifying correct material property - Strength, hardness, impact resistance, durability 1 point for correct choice of a material based on above properties 1 point each for Al2O3 (Alumina) answer 1 point for right justification - Al2O3 would not be a good choice, even though it is hard it will also easily break under the applied loading conditions

In ionic solids, the smaller cations occupy ____________ locations formed between the larger anions.

interstitial

Calculate the atomic radius for: a) BCC iron with a0=2.866 Å. Name examples of two engineering materials that form BCC crystal structure. b) FCC aluminum with a0=4.049 Å. Name examples of two engineering materials that form FCC crystal structure.

) For BCC: 4𝑟 = √3𝑎0 ∴ 𝑟 =√3𝑎0/4 = 1.241 Å Other examples of BCC materials include chromium, tungsten, molybdenum a) For FCC: 4𝑟 = √2𝑎0 ∴ 𝑟 =√2𝑎0/4 = 1.431 Å Other examples of FCC materials include copper, silver, nickel

Which of the following statement(s) are true about dislocation movement. Please select all those are true. Wrong answers carry partial negative credit. 1- Dislocations preferentially move along the directions that are not close packed 2- Dislocations do not move easily in ceramic materials 3- Dislocations move on a certain plane and along a certain direction together known as slip system 4- Dislocation movement under application of shear force is called 'slip'. 5- Grain boundaries resist the movement of dislocations The particular combination of direction and plane along which the dislocation movement is active is not dependent on the direction of the applied force

2,3,4,5 are correct

Compare and contrast metallic and covalent primary bonds in terms of (a) the fundamental mechanism of bond formation due to valence electrons, (b) the number of valence of the atoms involved, and (c) the ductility of the materials bonded in these ways. Give two specific material examples for each bond type.

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C in F B doped in Si Porosity in cast Al alloy part grain boundaries in brass micro structure A. interstitial B. volume defect C. substitutional D. planar/surface defect

A C B D

Iron forms a BCC structure with a0=2.866 Å. a. Find the repeat distance and linear packing fraction along [11̅1] and [101] directions for BCC iron b. Find the planar density for (110) and (010) planes.

Atomic arrangement along [11̅1] will be as follows in a BCC structure: Hence, repeat distance will be √3𝑎0 2 = 2.482 Å And linear packing fraction will be 1 as whole linear segment is occupied by atoms. (Closed-packed direction) MEEG 2303 FALL 2019 HW 3 SECTION III KEY Atomic arrangement along [101] will be as follows in a BCC structure: Hence, repeat distance will be √2𝑎0= 4.053 Å And linear packing fraction will be: 2𝑟 √2𝑎0 = 0.612 b. [11̅1] is close packed c. For (110) plane: Planar density =2 𝑎𝑡𝑜𝑚𝑠/(√2𝑎0∗𝑎0) =2/(√2×(2.866×10−8)^2 = 1.721 × 1015 𝑎𝑡𝑜𝑚𝑠/𝑐𝑚2 For (010) plane: Planar density = 1 𝑎𝑡𝑜𝑚/(𝑎0∗𝑎0) =1/(2.866×10−8)^2 = 1.217 × 1015 𝑎𝑡𝑜𝑚𝑠/𝑐𝑚2

Steel blade of a turbine is coated with a thin layer of ceramic to help protect against corrosion. What would you expect to happen to the coating when the temperature of the steel is increased significantly to 500 °C or above? Explain based upon a specific material property.

Ceramics are expected to have a low coefficient of thermal expansion due to strong ionic/covalent bonds; steel has a high thermal expansion coefficient. When the structure heats, steel expands more than the coating, which may crack and expose the underlying steel to corrosion.

_______________________ unit cell has highest packing factor among the three cubic unit cell types.

FCC

A body centered cubic unit cell has a total of 4 atoms/unit cell

False

Strength of material increases with increase in average grain size. T,F

False

A face centered cubic unit cell is an example of a closed packed structure. This unit cell is formed by placing one atom at each corner lattice point and one atom at the Correct center of each face. The total number of atoms per unit cell can be calculated as: Correct 8*1/8 (from shared corner atoms) + Correct 6*1/2 (shared between faces) = Correct 4 total atoms/unit cell. In this arrangement, the atoms are touching along the Correct face diagonal. As a result, the relationship between the atomic radius r and the lattice parameter a0 can be written as 4r=Correct √2a0. From this relationship, the packing factor for the unit cell packed with same type of atoms can be computed to be equal to Correct 0.74. The Miller indices of the directions along with atoms are touching can be represented by {Correct 110} family of directions. The linear packing fraction for this direction in an FCC structure is Correct 1 while the repeat distance is Correct √2 /2 a0 .

Know these terms

) Would you expect iron or silicon nitride (SiN) to have a higher modulus of elasticity? Why?

SiN has covalent bonding as opposed to metallic bonding in iron. As a result, SiN will likely have higher modulus of elasticity due to stronger bonding. Stronger bonds are harder to break and thus will resist the elongation increasing the elastic modulus.

.One would like to be able to physically separate different materials in a scrap recycling plant. Describe one possible method that could be designed to separate materials such as polymers, aluminum alloys and steels from one another based on their inherent properties. Explain the scientific principle/material property(ies) that enables the selected method.

Steels can be magnetically separated from the other materials; steel (or carbon containing iron alloys) are ferromagnetic and will be attracted by magnets. Density differences could be used— polymers have a density near that of water; the specific gravity of aluminum alloys is around 2.7; that of steels is between 7.5 and 8. Electrical conductivity measurements could be used—polymers are insulators while aluminum has a particularly high electrical conductivity.

Strengthening of a material by increasing the number of dislocations by deformation, or cold working is known as ______________________.

Strain Hardening

A point defect produced when an atom or ion is replaced by a different type of atom or ion at a regular lattice position is called ________________ defect.

Substitutional

A closed packed direction will have a linear packing fraction of 1.

True

Defects such as grain boundaries or impurity atoms resist the movement of dislocations within the lattice which leads to increase in material strength. T-F

True