14ME406/ME 226. Material science &Metallurgy. Hall Ticket Number: Fourth Semester. II/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION

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1 Hall Ticket Number: 14ME406/ME 226 April, 2017 Fourth Semester Time: Three Hours Answer Question No.1 compulsorily. Answer ONE question from each unit. II/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION Mechanical Engineering Material science &Metallurgy Maximum : 60 Marks (1X12 = 12 Marks) (4X12=48 Marks) 1 Answer all questions (1X12=12 Marks) a) What are types of crystal imperfections? b) What is atomic packing factor & coordination number for Iron? c) Define Gibbs phase rule. d) What is the purpose of Normalising? e) What is hardening? f) What is purpose of Martempering? g) What is the purpose of strain hardening? h) Write any two applications of Particle reinforcement composite? i) What is the difference between Particle reinforcement composites and Metal matrix composites? j) What is the composition of aluminium? k) Give any two applications of powder metallurgy. l) Write any two applications of nano materials? UNIT I 2 a) Define the term coordination number. What is the significance of coordination number? Calculate 6M the coordination number of cubic lattice. b) Name the different crystal systems and explain their distinguishing characteristics. 6M (OR) 3 a) Sketch and label a binary phase diagram whose components are completely soluble in each other in 6M liquid state and completely insoluble in solid state. b) Explain the factors governing the formation of substitutional solid solutions. 6M UNIT II 4 a) Construct Iron-Iron Carbide equilibrium diagram and explain various reactions. 6M b) Sketch isothermal transformation diagram for hypo-eutectoid steel and identify different 6M transformed products. (OR) 5 a) Define the process of heat treatment.explain about the Austempering and Martempering processes 6M with neat sketches. b) Briefly explain various chemical hardening techniques. 6M UNIT III 6 Write short notes on the following. a) Strain hardening 4M b) Grain refinement 4M c) Dispersion strengthening 4M (OR)

2 7 a) Give the classification of composites along with their applications. 6M b) Explain in detail about laminar composites. 6M UNIT IV 8 a) Discuss the various advantages and applications of power metallurgy. 6M b) Define sintering. State and describe various stages of sintering process. 6M (OR) 9 a) Explain the following with neat sketch. (i ) White Cast-iron (ii) Gray Cast-iron 6M b) Explain the properties and applications of copper alloy? 6M

3 II/IV B. Tech (Regular/Supplementary) DEGREE EXAMINATION Mechanical Engineering April, 2017 Material Science & Metallurgy (14ME406/226) (key) 1 a).crystal imperfections are : Point defect, Line defect, Surface defect and volume defect b). Atomic Packing factor=(volume of atom in unit cell) / (Volume of unit cell) Coordination Number for iron is 8 c). P+F=C+2 d). Normalising is used for increasing ductility & toughness with moderate increases of hardness and avoid excessive softness in the material. e). Hardening is performed to impart strength and hardness to alloys by heating up to a certain temperature, depending on the material, and cooling it rapidly. f). In Martempering, Martensitic transformation takes place under lower cooling rate and therefore the internal stress are reduced to greater extent and used for heavy sections and pieces of irregular shape g). Work hardening, or strain hardening, results in an increase in the strength of a material due to plastic deformation. h).applications: cutting tools for hardened steels and high temperature applications i). Metal Matrix composite Soft and Ductile Inexpensive Particle reinforcement composite Strong and hard Costly j).composition of Aluminium: Al-97.42, Mg-0.9, Si-0.83, Fe-0.3 and Zn-0.65 k). Automobiles, Aerospace, high temperature applications and defences l).medical treatments, manufacturing areas and construction parts

4 2 a). Coordination number: Coordination number is the number of nearest neighbor to a particular atom in the crystal Significance: Higher coordination number gives higher strength and density to that material. Calculation of Coordination of cubic lattice: FCC: In the FCC lattice each atom is in contact with 12 neighbor atoms. FCC coordination number Z = 12 The face centered atom in the front face is in contact with four corner atoms and four other facecentered atoms behind it (two sides, top and bottom) and is also touching four face-centered atoms of the unit cell in front of it. BCC: The coordination number of BCC crystal is 8. The body centered atom is in contact with all the eight corner atoms. Each corner atom is shared by eight unit cells and hence, each of these atoms is in touch with eight body centered atoms. Simple Cube: Coordination number of simple cube is 6

5 2b). Different crystal systems:

6 3 a). In Eutectic phase diagram whose components are completely soluble in each other in liquid state and completely insoluble in a solid state In the eutectic system between two metals A and B, two solid solutions, one rich in A (α) and another rich in B (β) form. In addition to liquidus and solidus lines there are two more lines on A and B rich ends which define the solubility limits B in A and A in B respectively. These are called solvus lines. 3 b). Factors governing the formation of substitutionall solid solutions Extent of solid solubility in a two element system can be predicted based on Hume-Ruthery conditions. If the system obeys these conditions, then complete solid solubility can be expected. Hume-Ruthery conditions: - Crystal structure factor: Crystal structure of each element of solid solution must be the same. - Atomic Size factor: Size of atoms of each two elements must not differ by more than 15%. - Electro-negativity factor : Elements should not form compounds with each other i.e. there should be no appreciable difference in the electro- negativities of the two elements. - Relative Valence factor: Elements should have the same valence.

7 4 a). Iron-Iron Carbide equilibrium Diagram Reactions: peritectic reaction at 1495 C and 0.16%C, δ-ferrite + L γ-iron (austenite) monotectic reaction 1495 C and 0.51%C, L L + γ-iron (austenite) eutectic reaction at 1147 C and 4.3 %C, L γ-iron + Fe 3 C (cementite) [ledeburite] eutectoid reaction at 723 C and 0.8%C, γ-iron α ferrite + Fe 3 C (cementite) [pearlite]

8 4 b). Isothermal Transformation Diagram: A-Austenite P-Pearlite B-Bainite M-Martensite

9 5a). Process of heat treatment: Heat treatment is a process of heating, holding and cooling a component to get desired properties. Martempering: Applicable to steel with %C=0 to 0.4% The process consist of heating the stee to the hardening temperature and then cooled suddenly (at above the critical rate of cooling) down to a temperature just above Ms point (temperature at which martensite formation begins and is nearly equal to 240oC). It is held there sufficient time to equalize the temperature throughout the section i.e., whole volume convert to martensite. The martensitic transformation takes place under lower cooling rate and therefore the internal stress are reduced to greater extent. This method can be used for heavy sections and the pieces of irregular shape. ]

10 Austempering : Applicable to steel with %C=0.8 Austempering is also a kind of interrupted quenching in a tempering process.but in austempering instead of martensite structure formation during quenching.bainite structure is produced. Austempering of steel is carried out by heating the steal above the upper critical temperature to make it all austenite form. It is then quenched at a critical cooling rate in a oil bath maintaining a temperature range 723 o C The whole austenite steel is converted to bainite. Then the second stage of gradual cooling to room temperature is done. The steel is produced by this process have greater ductility and toughness. Residual stress will be minimised No crack in the austempering component.

11 5 b). Various chemical hardening techniques:

12 6 a). Strain hardening: Phenomenon where ductile metals become stronger and harder when they are deformed plastically is called strain hardening or work hardening. Increasing temperature lowers the rate of strain hardening. Hence materials are strain hardened at low temperatures,thus also called cold working. 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 follows: During strain hardening, in addition to mechanical properties physical properties also changes:- a small decrease in density - an appreciable decrease in electrical conductivity - small increase in thermal coefficient of expansion -increased chemical reactivity (decrease in corrosion resistance). Deleterious effects of cold work can be removed by heating the material to suitable temperatures Annealing. It restores the original properties into material. It consists of three stages recovery, recrystallization and grain growth. In industry, alternate cycles of strain hardening and annealing are used to deform most metals to a very great extent. b). Grain refinement : Grain boundary barrier to dislocation motion: slip plane discontinues or change orientation. Small angle grain boundaries are not very effective in blocking dislocations. High-angle grain boundaries block slip and increase strength of the material. A stress concentration at end of a slip plane may trigger new dislocations in an adjacent grain. The finer the grains, the larger the area of grain boundaries that impedes dislocation motion. Grain-size reduction usually improves toughness as well. Usually, the yield strength varies with grain size d according to Hall-Petch equation: σ y = σ 0 + k y d where σ o and k y are constants for a particular material, d is the average grain diameter.

13 c). Dispersion strengthening: In dispersion hardening, fine second particles are mixed with matrix powder, consolidated, and pressed in powder metallurgy techniques. For dispersion hardening, second phase need to have very low solubility at all temperatures. E.g.: oxides, carbides, nitrides, borides, etc. Dislocation moving through matrix embedded with foreign particles can either cut through the particles or bend around and bypass them. Cutting of particles is easier for small particles which can be considered as segregated solute atoms. Effective strengthening is achieved in the bending process, when the particles are submicroscopic in size. Stress (τ) required to bend a dislocation is inversely proportional to the average interspacing (λ) of particles: Optimum strengthening occurs during aging once the right interspacing of particles is achieved. - Smaller the particles, dislocations can cut through them at lower stresses - larger the particles they will be distributed at wider distances. 7 a). Classification of composites Applications : Automobile applications Aerospace applications Defence applications Manufacturing applications

14 7b). Laminar composites A laminar composite is composed of two-dimensional sheets or panels that have a preferred highstrength direction such as is found in wood and continuous and aligned fiber-reinforced plastics. The layers are stacked and subsequently cemented together such that the orientation of the highstrength direction varies with each successive layer. For example, adjacent wood sheets in plywood are aligned with the grain direction at right angles to each other. Laminations may also be constructed using fabric material such as cotton, paper, or woven glass fibers embedded in a plastic matrix. Thus a laminar composite has relatively high strength in a number of directions in the twodimensional plane; however, the strength in any given direction is, of course, lower than it would be if all the fibers were oriented in that direction. 8 a). Advantages of powder metallurgy : Metal and non metal can be produced by this process Metal which never undergo molten state can be produced by this method Production of refractory metals like W,Mo, Ti, Th etc., is possible Close control over the dimensions of the finished components can easily obtained. Density and thickness of the component can be controlled by varying the compaction pressure(p) so that the strength can be changed. Application of Powder metallurgy Automotive applications Defence applications High temperature applications Aerospace applications Atomic energy applications 8 b). Sintering : Sintering is carried out to increase strength and hardness of a green compact and consists of heating the compact to some temperature under controlled conditions with or without pressure for a definite time Sintering process is concerned with: (a). Diffusion (b). Densification (c). Recrystallization and grain growth 9 a). (i) Grey Cast Iron : Grey cast iron contains graphite in the form of flakes Named after its grey fractured surface. C: wt%, Si: % Microstructure: graphite flakes in a ferrite or pearlite matrix Weak & brittle in tension (the graphite flake tips act as stress concentration sites).stronger in compression, Excellent damping capacity, wear resistance. Microstructure modification by varying silicon content and cooling rate Casting shrinkage is low

15 (ii).white Cast-iron: White cast iron C: wt.%, Si: %. Most of the carbon is in the form of cementite. Named after its white fracture surface. Results from faster cooling. Contains pearlite + cementite, not graphite. Thickness variation may result in nonuniform microstructure from variable cooling Very hard and brittle Used as intermediate to produce malleable cast iron.

16 9 b). Copper Alloys