MATERIAL SCIENCE QUESTION AN AND D ANSWER Q-1: A-1:
(CONVENTIONAL TYPE)
What are coordinati coordination on number of of BCC, FCC and and HCP crystal crystal structure? structure? CN, the coordination number, which is the number of closest neighbors to which an atom is bonded. CN of BCC structure is 8 CN of FCC structure is 12 CN of HCP structure is 12
BCC
FCC HCP
Q-2: A-2:
Q-3: A-3:
What are packing packing factors factors of BCC, FCC and HCP HCP crystal structure? structure? APF, the atomic packing factor, which is the fraction of the volume of the cell actually occupied by the hard spheres. APF = Sum of atomic volumes/Volume of cell. APF of BCC BCC structure is 0.68 APF of FCC structure is is 0.74 APF of HCP structure is is 0.74 How many slip planes are there in BCC, FCC and HCP crystal structure? Crystal BCC FCC HCP
Q-4: A-4:
[1 2 0]
Slip Planes {110}, {112}, {123} {111} Basal plane, Prismatic & Pyramidal planes
Show crystalographic directions [1 2 0], [1 3 3], [1 1 0 0], [1 2 0] The length of the vector projection on the axis x, y and z respectably a/2, b, 0c
Crystalographic Directions
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[1 3 3]
a/3, b, c
[1 1 0 0]
0.866a, -0.866a, 0a, 0c
[1 2 0]
a/2, -b, 0c
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Q-5: A-5:
Show crystalographic planes [1 0 2], [2 2 1], [6 3 2], [10 1 0] Crystalographic Planes [1 0 2]
[ 2 2 1]
[632]
[10 1 0] Plane ABCD
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Q-6: A-6:
Show Burger’s vector in edge and screw dislocations. Burger’s vector in edge dislocations Burger’s vector in screw dislocations
Q-7: A-7:
Why fine grained grained structure structure is harder harder than coarse coarse grain structure? structure? The smaller the grain size, the more frequent is the pile up of dislocations. With decrease in grain size, the mean distance of a dislocation can travel decreases, and soon starts pile up of dislocations at grain boundaries. This leads to increase in yield strength of the material.
Q-8: A-8:
What is the type of solid solution (a) copper and nickel (b) Iron and carbon (a) copper and nickel nickel Cu-Ni forms a sunstitutional solid solution. If a melt of Cu and Ni with any composition is cooled, a solid solution begins to freeze out. This solid solution is richer in Ni than the liquid solution. As the two phase system of solid plus melt is cooled further, the mole fractin of Ni decreases in both the solid solution and the liquid melt. (b) Iron and carbon Fe-C forms an interstitial solid solution; the C atoms occupy interstices in the crystal structure of substance Fe. The Fe-Fe 3C is characterized by five individual phases. Five phases that exist in the Fe-C diagram are: α –ferrite (BCC) Fe-C solid solution, γ-austenite (FCC) Fe-C solid solution, δ -ferrite -ferrite (BCC) Fe-C solid solution, Fe 3C (iron carbide) or cementite - an inter-metallic compound and liquid Fe-C solution.
Q-9: A-9:
Differentiate between the following; a) age hardening b) strain hardening c) precipitation hardening. a) Age hardening hardening or c) precipitation precipitation hardening hardening.. Age hardening hardening is produced produced by solution solution treating treating and quenching quenching an alloy. alloy. Term ‘Age hardening’ hardening’ is is used to describe the process because strength develops with time. Requisite for precipitation hardening to take place is that second phase must be soluble at an elevated temperature but precipitates upon quenching and aging at a lower temperature. This limits the alloy systems which can be strengthened by precipitation hardening. For example: Al-alloys, Cu-Be alloys, Mg-Al alloys, Cu-Sn alloys. If the precipitation occurs at normal ambient temperatures, it is called natural aging. Some alloy systems needed to be aged at higher temperatures and the process is known as artificial aging. Most precipitation hardened alloys are limited in their maximum service temperatures, which may lose their strength at elevated temperatures due to over-aging. b) Strain hardening • Phenomenon where ductile metals become stronger and harder when they are deformed plastically is called strain hardening or work hardening. • During plastic deformation, dislocation density increases. And thus their interaction with each other resulting in increase in yield stress. •
Dislocation density ( ρ) and shear stress ( τ) are related as,
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τ
=
τo
+
A
ρ
Q-10: A-10:
Explain the effect of mean stress on fatigue life. Failure that occurs under fluctuating/cyclic loads – Fatigue. Fatigue occurs at stresses that considerable smaller than yield/tensile yield/tensile strength of the material. material. S-N testing is done under alternating (completely (completely reversed) loading and stress. Here mean stress ( σm) is zero. If mean stress is present then fatigue life will change according to the following diagram.
Following Following empirical curves are used to estimate mean stress effects on fatigue life a. Soderberg (USA, 1930) b. Goodman (England, 1899) c. Gerber (Germany, 1874) d. Morrow (USA, 1960s) Q-11: A-11:
Explain the difference between Soderberg line and Goodman line.
Alternating stress (σ a ) = Meanstress (σ m )
σ max
=
σ max
− σ
min
2
+ σ
min
2
Yield strength = σ y UltimateTensileStrength = σ u
1.
2. • •
Q-12: A-12:
σa
Goodman Line
+
σe σa
Soderberg Line
σe
σ m
=
1
=
1
σ ut +
σ m σ y
Most actual test data tend to fall above the Goodman line. The Soderberg line is very conservative and seldom used.
What are creep resistant alloy? Give composition of Nimonic 90 and Vitallium HS 21. Creep resistant resistant alloy To make creep resistance alloy we have to strengthen the solid solution by mechanisms which cause dislocation locking and those which contribute to lattice friction hardening. The alloy can also be hardened by precipitation. Some solute alloying elements is added in reducing the rate of climb and cross-slip processes. Page 5 of 14
Example: The nickel alloy (Inconol, Nimonic), ferritic steel, austenitic steel 16-25-6, etc. Composition of Nimonic 90 Cr-20%, Co-16%, Ti-2.3% Al-1.40 %, Fe-0.5%, C-0.08%, Mn-0.06%, Si-0.017% and Ni -58% Composition of Vitallium HS 21 C – 0.25%, Cr – 27%, Ni –3 %, Mo – 5%, Fe – 1%, Mn – 1%, Si -1%, Co - bal Q-13: A-13:
Differentiate between temper embrittlement and hydrogen embrittlement. Temper embrittleme embrittlement nt Tempering of some steels may result in a reduction of toughness what is known as temper embrittlement. This may be avoided by (1) compositional control, and/or (2) tempering above 575 oC or below 375oC , followed by quenching to room temperature. The effect is greatest in Martensite structures, less severe in bainitic structures and least severe in pearlite structures. It appears to be associated with the segregation of solute atoms to the grain boundaries lowering the boundary strength. Impurities responsible for temper brittleness are: P, Sn, Sb and As. Si reduces the risk of embrittlement by carbide formation. Mo has a stabilizing effect on carbides and is also used to minimize the risk of temper brittleness in low alloy steels. Hydrogen embrittlement Hydrogen embrittlement is more failure than a form of corrosion, but it is often results from the hydrogen, produced from corrosion. Atomic hydrogen produced during corrosion diffuses interstitially through crystal lattice, and interferes with dislocation motion, leading to failure. It is similar to stress corrosion in the sense that ductile materials experience brittle failures as a result. Counter measures to hydrogen embrittlement include: heat treatment to reduce strength of the alloy; removal of source of hydrogen; baking the component to drive out any dissolved hydrogen.
Q-14: What is diffusion couple? Give two examples. A-14: Diffusion Diffusion couple is made by two metals A and B. Two containers of two metals are joined together by removing the barrier between them. This couple is heated for an extended period at a higher temperature, but certainly lower than the melting points of A and B, and then cooled to room temperature. It is observed that atoms A have migrated into atoms B and atoms B have migrated into atoms A. There is a net flow of atoms from higher concentration to lower concentration regions. This type of diffusion is known as inter-diffusion or impurity diffusion. Example (i) Copper and Nickel couple (ii) Gold and Silver couple
Fig. Diffusion couple of two metals
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Q-15: A-15:
Explain how annealing and normalizing are diffusion controlled processes? Annealing can be defined as a heat treatment process in which the material is taken to a high temperature, kept there for some time and then cooled. Carbon atoms diffuse in BCC and FCC by interstitial interstitial diffusion process. High temperatures allow diffusion processes to occur fast. The time at the high temperature (soaking time) must be long enough to allow the desired transformation to occur. Normalizing is used to refine the grains and produce a more uniform and desirable size distribution. It involves heating the component to attain single phase (e.g.: austenite in steels), then cooling in open air atmosphere. In normalizing also high temperature allows diffusion process to occur fast.
Q-16: A-16:
What is a Eutectic system? Explain copper/silver and lead/tin eutectics. Many binary systems have components which have limited solid solubility, e.g.: Cu-Ag, Pb-Sn. The regions of limited solid solubility at each end of a phase diagram are called terminal solid solutions as they appear at ends of the diagram. Many of the binary systems with limited solubility are of eutectic type, which consists of specific alloy composition known as eutectic composition that solidifies solidifies at a lower temperature than all other compositions. compositions. This low temperature which corresponds to the lowest temperature at which the liquid can exist when cooled under equilibrium conditions is known as eutectic temperature. The corresponding point on the phase diagram is called eutectic point. When the liquid of eutectic composition is cooled, at or below eutectic temperature this liquid transforms simultaneously into two solid phases (two terminal solid solutions, represented by α and β). This transformation is known as eutectic reaction and is written symbolically symbolically as: Liquid (L (L) ↔ solid solution-1 ( α) + solid solution-2 ( β) This eutectic reaction is called invariant reaction as it occurs under equilibrium conditions at a specific temperature and specific composition which can not be varied. Thus, this reaction is represented by a thermal horizontal arrest in the cooling curve of an alloy of eutectic composition. A typical eutectic type phase diagram is shown in figure-4 along with a cooling curve.
Eutectic system of Copper and Silver In the Copper-silver binary eutectic system, system, the invariant point is located at 71.9 wt% Ag + 28.1 wt% Cu at 779oC
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Eutectic reactions for copper-silver copper-silver cooling
L (71.9 wt% Ag + 28.1 wt% Cu)
α(8.0 wt% Ag + 92 wt% Cu) +
heating
β(91.2 wt% Ag + 8.8 wt% Cu)
Eutectic system of Lead and Tin In the lead-tin binary eutectic system, the invariant invariant point is located at 61.9 wt% Sn + 38.1 wt% Pb at o 183 C
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Eutectic reactions for Lead and tin cooling
L (61.9 wt% Sn + 38.1 wt% Pb)
α(18.3 wt% Sn + 81.7 wt% Pb) +
heating
Q-17: A-17:
β(97.8 wt% Sn + 2.2 wt% Pb)
What are hypoeutectoid and hypereutectoid steels, explain. Hypoeutectoid Hypoeutectoid Steel Steel Plain carbon steels in which carbon percentage is less than 0.8% are called hypoeutectoid steel. Hypereutectoid Steel Plain carbon steels in which which carbon percentage is more than 0.8% are called hypoeutectoid hypoeutectoid steel.
Q-18: A-18:
What is 0.8% C, steel, what are its special properties? Steel which contains contains 0.8% C is known as eutectoid composition. composition. In the solid state when o cooled below 723 C a eutectic reaction takes place one solid phase ( γ-iron) having eutectoid (0.8% C) composition transforms into two different solid phases α – ferrite and Fe3C (cementite). This particular composition composition of ferrite and cementite is known as pearlite.
Q-19: A-19:
What is tempered martensite? Cooling the austenized steel to temperature just above Ms temperature, holding holding it there until temperature is uniform, followed by cooling at a moderate rate to room temperature before austenite-to-bainite transformation begins. The final structure is tempered Martensite
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Q-20: A-20:
What is the driving force in the formation of Spheroidite? The driving force for the formation of spheroidite is the net reduction in ferrite-cementite phase boundary area.
Q-21: A-21:
What is the difference between martempering and austempering? Martempering Martempering is a modified quenching procedure used to minimize distortion and cracking that may develop during uneven cooling of the heat-treated material. It involves cooling the austenized steel to temperature just above Ms temperature, holding it there until temperature is uniform, followed by cooling at a moderate rate to room temperature before austenite-to-bainite transformation begins. The final structure of martempered steel is tempered Martensite.
Austempering Austempering is different from martempering in the sense that it involves austenite-to bainite transformation. transformation. Thus, the structure of austempered steel is bainite. Advantages of austempering are improved ductility; decreased distortion and disadvantages are need for special molten bath; process can be applied to limited number of steels.
Q-22: A-22:
What is the difference between nitriding and carbonitriding processes? Nitriding Nitriding Nitriding is carried out in the ferritic region. No phase change occurs after nitriding. The part to be nitrided should posses the required core properties prior to nitriding. During nitriding, pure ammonia decomposes to yield nitrogen which enters the steel. The solubility of nitrogen in ferrite is small. Most of the nitrogen, that enters the steel, forms hard nitrides (e.g., Fe 3N). The temperature of nitriding is 500-590 oC. The time for a case depth of 0.02 mm is about 2 hr. In addition to providing outstanding outstanding wear resistance, the nitride layer increases the resistance of carbon steel to corrosion in moist atmospheres. Page 10 of 14
Carbonitriding Carbonitriding is a lower cost surface hardening process that provides a thin, high hardness case on lower hardenability steels. Carbonitriding involves the diffusion of both carbon and nitrogen into the base steel. The carbon provides the base metal with a high carbon surface, and the nitrogen provides the case with an added boost of hardenability to insure full case hardness. The addition of nitrogen makes the carbonitriding process especially suited to plain, low carbon steel that would not otherwise respond to standard carburizing. Carbonitriding is usually carried out in a temperature range of 820-900°C in a gaseous atmosphere adding between 0.5 to 0.8% carbon and 0.2-0.4% (< 5%) nitrogen to the surface of plain carbon steel or low alloy steel. Q-23: A-23: •
•
• •
• •
Q-24:
What are the high-strength low-alloy steel? High-strength low-alloy steel ( steel (HSLA HSLA ) is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather to specific mechanical properties. They have low carbon content between 0.05–0.25% to retain formability and weldability. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for strengthening strengthening purposes. These steels are not strengthened by heat treatment due to low carbon content. Give composition and use of (a) Hadfield steel (b) Maraging steel (c) Spring Steel (d) Rail Steel (e) Invar Steel
A-24: (a) Hadfield steel (b) Maraging steel
(c) Spring Steel
Composition C 1.1 to 1.4%, Mn 11-14%, rest Fe
Use Jaw crusher plate, Nuts and bolts, Chains C <0.03%, Ni-25%, Co 7-10%, Mo 3 -5%, Ti – Aircraft under carriage parts, 1.75 %, Al- 0.2%, other trace, rest Fe portable bridges and booster motor in missile C 0.55 – 0.65%, Si 0.1 – 0.35%, Mn 0.7 – 1.0 Spring %, Cr 0.4 – 0.6%, Ni 0.4 – 0.7%, Mo 0.15 – 0.25%, rest Fe
(d) Rail Steel
C 0.4 – 0.6 %, Mn -1.5%, rest Fe
(e) Invar Steel
Ni 32%, Fe-68%
Rail Precision measuring instrument, survey measuring tapes
Q-25: A-25:
What is nodular cast Iron? How it is made? Nodular (or ductile) cast iron: iron: Alloying additions are of prime importance in producing these materials. Small additions of Mg / Ce to the gray cast iron melt before casting can result in graphite to form nodules or sphere-like particles. Matrix surrounding these particles can be either ferrite or pearlite depending depending on the heat treatment. These are stronger and ductile than gray cast irons.
Q-26: A-26:
What is Superalloy? Give composition and use of Waspalloy, and Inconel? Superalloys as a class constitute constitute the currently currently reigning reigning aristocrats aristocrats of the metallurgical metallurgical world. world. They are the alloys which have made jet flight possible, and they show what can be achieved by drawing together and exploiting all the resources of modern physical and process metallurgy in the pursuit of a very challenging challenging objective. Page 11 of 14
Applications Applications of Superalloy? Superalloy? • Gas Turbine Engines − Blades, vanes, disks, combustors • Space Vehicles − Rocket motors • Nuclear Reactors • Submarines • Petroleum Equipment Waspalloy Inconel
Q-27: A-27:
Use For high temperature application upto 900oC For high temperature application upto 820oC
What are PTFE, Nylon 6, Nylon 610, Perspex, where they are used?
PTFE
Q-28: A-28:
Composition Cr-19%, Co-13%, Ti-3%, Al 1.4%, Zr 0.06 %, C 0.08%, rest Ni Cr 15%, Ti 2.4%, Al 1%, Nb 1%, Ta 1%, Fe 7%, C 0.04%, rest Ni
What is? Use Fluorocarbons (PTFE or TFE) or Teflon. It Anticorrosive Anticorrosive seals, chemical chemical is chemically inert in almost all pipes and valves, bearings, anti environments, excellent electrical adhesive coatings, high properties; low coefficient of friction; may be temperature electronic parts. used to 260oC; relatively weak and poor cold-flow properties.
Nylon 6
Nylon 6 or polycaprolactam or cast nylon is polymer developed to reproduce the properties of nylon 6,6. Unlike most other nylons, nylon 6 is not a condensation polymer, but instead is formed by ringopening polymerization. polymerization.
Synthetic fibers
Nylon 610
Polyhexamethylene Polyhexamethylene sebacamide
Flexible tubes
Perspex
PMMA-polymethyle PMMA-polymethyle methacrylate
Domestic article
What are conducting polymers and conducting ceramics? Give 2 examples of each. Conducting Polymers Due to the kind of bonding, polymers are typically electrical and thermal insulators. However, conducting polymers can be obtained by doping, and conducting polymer-matrix composites can be obtained by the use of conducting fillers. They decompose at moderate temperatures (100 – 400 oC), and are lightweight. Other properties vary greatly. . The most recent research in this has been the development of highly conducting polymers with good stability and acceptable processing attributes. Polyphenylene, Polypyrroles Example: Polyacetylene, Polyphenylene, Conducting Ceramics Conductive ceramics, advanced industrial materials that, owing to modifications in their structure, serve as electrical conductors. Like metals, conducting ceramics have overlapping electron energy bands and are therefore excellent electronic conductors. They constitute complex systems based on oxide and non-oxide phases. ruthenium dioxide dioxide (RuO2), bismuth ruthenate (Bi 2Ru2O7) Examples: lead oxide (PbO), ruthenium
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Q-29: A-29:
What are silicon carbide and silicon nitride, what are their strength and hardness? Silicon carbide carbide (SiC) (SiC) It is known as one of best ceramic material for very high temperature applications. It is used as coatings on other material for protection from extreme temperatures. It is also used as abrasive material. It is used as reinforcement in many metallic and ceramic based composites. It is a semiconductor semiconductor and often used in high temperature electronics. Ultimate tensile strength of SiC is 300 MPa Hardness of SiC is 2500 VPN (Vickers Pyramid Number) Silicon nitride (Si 3N4) It has properties similar to those of SiC but is somewhat lower, and found applications in such as automotive and gas turbine engines. Ultimate tensile strength of Si 3N4 is 580 MPa Hardness of Si 3N4 is 2300 VPN (Vickers Pyramid Number)
Q-30: A-30:
What are dispersion strengthened and particulate composites? Give two examples of each. Dispersion-strength Dispersion-strengthened ened composites composites • In this composite, particles are of 0.01-0.1 μm in size. • Strengthening occurs as a result of dislocation motion hindrance. It is similar to that of precipitation hardening in metals. Matrix bears the major portion of the applied load, while dispersoids obstruct the motion of • dislocations. Example: Example: thoria (ThO 2) dispersed Ni-alloys (TD Ni-alloys) with high-temperature strength; SAP (sintered aluminium powder) – where aluminium matrix is dispersed with extremely small flakes of alumina (Al 2O3). Particulate composites • These composites contain large number of coarse particles. These composites are designed to produce combination of properties rather than increase the • strength. Mechanical properties are characterized by rule-of-mixtures. rule-of-mixtures. • • Particulate composites are usually made of all three conventional engineering materials, namely – metals, polymers polymers and ceramics. ceramics. Example: tungsten carbide (WC) (WC) or titanium carbide (TiC) embedded embedded cobalt or nickel based cutting tools. Aluminium alloy castings containing dispersed SiC particles are widely used for automotive applications including pistons and brake applications.
Q-31:
A-31:
Describe the following (a) Ceramic matrix composite (b) Metal matrix composite (c) Carbon Carbon Composite (a) Ceramic Ceramic matrix matrix composites composites (CMCs) They are a subgroup of composite materials as well as a subgroup of technical ceramics. They consist of ceramic fibers embedded in a ceramic matrix, thus forming a ceramic fiber reinforced ceramic (CFRC) material. The matrix and fibers can consist of any ceramic material, whereby carbon and carbon fibers can also be considered a ceramic material. (b) Metal Matrix Composites (MMC) Metal Matrix Composites are composed of a metallic matrix (aluminium, magnesium, iron, cobalt, copper) and a dispersed ceramic (oxides, carbides) or metallic (lead, tungsten, molybdenum) molybdenum) phase.
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(c) Carbon Carbon Composite It is a composite material consisting of carbon fibre reinforcement in a matrix of graphite. It was developed for the nose cones of intercontinental ballistic missiles. It has been used in the brake systems of Formula Formula One racing cars. Carbon–carbon is well-suited to structural applications at high temperatures, or where thermal shock resistance and/or a low coefficient of thermal expansion is needed. Q-32:
A-32:
Explain the following in corrosion (a) Redox (b) Electrolyte (a) Redox corrosion corrosion Corrosion of metals is the most common type of corrosion and is a process involving involving an exchange of electrons between two substances, one of them being the metal. In this process, the metal usually loses electrons, becoming oxidized, while the other substance gains electrons, becoming reduced. For this reason, corrosion is classified as an oxidation-reduction or redox reaction. While many redox reactions are extremely important and beneficial to society (for example, those that are used to make batteries), the redox reactions involved in corrosion are destructive. (b) Electrolyte corrosion Electrochemical corrosion takes place in the presence of an electrolyte, which is simply a fluid conducting electricity, electricity, by migration of ions. Water generally contains mineral ions, hydrogen ions and hydroxyl ions. In this case of atmospheric corrosion, humidity in the air does the job of an electrolyte.
Q-33: A-33:
Explain (a) Degradation of polymer (b) Corrosion of ceramics (a) Degradation Degradation of polymer polymer • As other engineering materials, polymers polymers also deteriorated during their service. However, in contrast to electrochemical nature of metal corrosion, polymer degradation is of physiochemical in nature. • As polymer structures are are complex, so are the mechanisms mechanisms involved involved in their their deterioration. deterioration. • Many factors involved in degradation of polymers, like – temperature, radiation, environment, environment, moisture, bacteria or external loads/stress. • Polymers degrade mainly in three forms – swelling and dissolution, bond rupture, and weathering. (b) Corrosion of ceramics • As ceramics are made of metals and non-metals, non-metals, they can be considered as already corroded. corroded. • Ceramics do get deteriorated during their service under extreme temperatures and external loads. • Factors effecting life of ceramic components include: temperature, external loads, vibrations, environment, environment, etc. • Life span of ceramics can be increased by controlling the environment they are exposed to; operational loads and temperatures; altering the component design.
Q-34: A-34:
Give composition and uses of Permalloy and Cammalloy
Permalloy
Composition 45 Permalloy (55%Fe-45%Ni), (55%Fe-45%Ni), 79 Permalloy (79% Ni-4% Mo-17 %Fe),
Cammalloy
66.5% Ni, 30% Cu, 3.5% Fe
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Use Application of soft magnets (Permalloy) include: cores for electro-magnets, electric motors, transformers, generators, and other electrical equipment. Soft magnetic material, Curie point is 100oC
