Chapter 9 - Microstructure of a lead‐tin alloy 1 • –...

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Unformatted text preview: Microstructure of a lead‐tin alloy 1 • – – – – – – – – • – Example: In a copper‐zinc brass, the components are Cu and Zn • • – Consists of atoms of at least two different types – Solute atoms occupy either substitutional or interstitial positions – Crystal structure of solvent is maintained • • • • Phase: a homogeneous portion of a system that has uniform physical and chemical characteristics The following are considered phases: – Solid solution – Liquid solution – Gaseous solution • • • • • • If more than one phase is present in a system, each will have its own distinct properties Phases exist over a range of compositions, temperatures and pressures A separating boundary exists discontinuous and abrupt change in physical/chemical characteristics Example: Water and Ice two phases physically dissimilar, but chemically the same Homogeneous: a single‐phased system Mixtures/Heterogeneous: two or more phases are present in a system 6 • Phase depends on physical state Phase diagram for water • Phase depends on crystal structure Phase diagram for iron 7 8 • – The elements or compounds that are mixed to make the alloy • Al and Cu • MgO and Al2O3 • styrene and butadiene – The composition is the concentration of each component present in an alloy • (lighter phase) (darker phase) Al-Cu alloy Phase α – Physically and/or chemically distinct regions in the material Phase β Aluminum atom Copper atom 9 • Initially, sugar is added to water and there is a sugar‐ water solution or syrup • Solution becomes more concentrated with sugar until the solubility limit is reached saturated • System cannot dissolve any more sugar at the specified temperature and further additions settle to the bottom of the container • System now contains two phases; syrup and solid undissolved sugar crystals • ALS: What is the solubility of sugar in water at 20oC? Answer: 65 wt% sugar Previous Answer: 65 wt% sugar ALS: If you have 1 L 1000 g of water, how much sugar can you dissolve in it? a 650 g of sugar b 2000 g of sugar c 1350 g of sugar http://www.flickr.com/photos/mathewpeet/217576805/ • The mechanical behaviour of a material is dependent on it’s microstructure • Metal alloys – Microstructure characterized by: • Number of phases present • Proportions of phases • Manner in which the phases are distributed or arranged – Microstructure dependent on: • Alloying elements present • Concentrations of elements • Heat treatment of alloy Martensite and Bainite Microstructure 14 • • • 15 • • • • 3 externally controllable parameters that affect phase structure: – Temperature – Pressure usually scaled logarithmically – Composition Unary phase diagrams only deal with pressure‐temperature graphs composition remains constant Along the phase boundaries, the phases on either side are in equilibrium Triple point: all three states are in equilibrium at 273.16 K at a pressure of 6.04x10‐3 atm in the example H20 Each of the phases will exist in equilibrium conditions over the corresponding temperature 16 17 • • • • • • – • • – – – • Three phase regions or fields: – Alpha α • Substitutional solid solution consisting of Cu and Ni and both have an FCC crystal structure – Liquid L • Homogeneous liquid solution composed of both copper and nickel – Two phase α L • The copper‐nickel system is termed isomorphous due to the complete liquid and solid solubility of the two components Phase Boundary • • – • – • composition of phase is the same as the overall composition of the alloy – • A Tie line is constructed across two‐phase region at a specified temperature of the alloy • Intersections of tie lines and phase boundaries on both sides are noted • Perpendiculars vertical lines are dropped from these intersections to the horizontal composition axis • Composition of each phase is read • Determining Phase Amounts: – One phase present: • Alloy is composed entirely of that phase; phase fraction is 1.0 or percentage is 100% – Two phases present: • Use the lever rule; tie line is constructed across the two‐phase region at a specified temperature of the alloy • Overall alloy composition is located on the tie line • Fraction of one phase is determined by taking the length of the tie line from the overall alloy composition to the phase boundary for the other phase, and dividing by the total lie line length • The fraction of the other phase is determined in the same manner • Segment lengths can be determined either by direct measurement from the phase diagram using a linear scale or by subtracting compositions from the composition axis • C C0 S WL R S C C L • C0 C L R W R S C C L • – • – – • TS for pure Ni %EL for pure Ni %EL for pure Cu TS for pure Cu • • • • copper‐silver phase diagram 27 • Solvus line: solid solubility limit line separating the and regions Eutectic isotherm • Solidus line: line separating the and L regions BEG represents the lowest temperature at which a liquid phase may exist for any copper‐silver alloy at equilibrium • Eutectic Reaction: L(CE ) (CE ) (CE ) cooling copper‐silver phase diagram heating 28 One common type of alloy system is called the eutectic system. Binary eutectic systems occur when 2 elements completely dissolve into each other in the liquid state, but have only limited solubility in the solid state. More than one solid phase exists in these systems. There are multiphase regions between single‐phase regions. Which phases are present depends, as before, on the temperature and the composition of the material. 29 • Consider Cu‐Ag alloys: The copper‐silver alloy system is binary eutectic. How many phases are there in the system? 3 (L, , ) What are the two solid phases? : mostly Cu : mostly Ag Where is the eutectic point? eutectic temperature: TE eutectic composition: CE 30 The lead‐tin system is also binary eutectic • o • T(°C) 3 00 • Answer: There are 2 phases: a and b. Ca 11 wt% Sn Cb 99 wt% Sn L (liquid) L + 2 00 18.3 150 97.8 1 00 0 31 L+ 183 °C 61.9 20 40 Co 60 80 10 0 C o , wt% Sn • T(°C) • Answer: 3 00 L (liquid) From the lever rule: L + 2 00 59 W 67% 88 18.3 150 32 97.8 1 00 0 W 1 W 33% L+ 183 °C 61.9 20 40 Co 60 80 10 0 C o , wt% Sn • 400 o • – Temperature oC L 300 200 100 0% Co 33 Adapted from Fig. 9.9, Callister 6e. L 10% %Sn 20% 30% • L: Cowt%Sn T(°C) 400 L o 300 • L + 200 TE – 34 + 0 10 2 (sol. limit at Troom) Adapted from Fig. 9.10, Callister 6e. : Cowt%Sn 100 • • L 20 Pb-Sn system 30 Co Co, wt% 18.3 (sol. limit at TE) Sn • When the initial concentration Co equals the eutectic composition CE the result is... – T(°C) L: C o wt%Sn 3 00 L Pb-Sn system 2 00 TE L+ 0 0 + 20 18.3 40 Adapted from Fig. 9.11, 35 L+ 183°C 1 00 Callister 6e. Micrograph of Pb-Sn eutectic microstructure : 97.8wt%Sn : 18.3wt%Sn 60 CE 61.9 80 100 97.8 C o , wt% Sn 160 m Adapted from Fig. 9.12, Callister 6e. Fig. 9.12 from Metals Handbook, Vol. 9, 9th ed., Metallography and Microstructures, American Society for Metals, Materials Park, OH, 1985. • Co between 18.3 and 97.8 wt% Sn results in... – … a combination of crystals and eutectic regions L+ 200 TE R R 100 0 36 0 C = 18.3 wt% Sn L+ S C L = 61.9 wt% Sn S W = + 20 18.3 E L L 300 Pb-Sn system L L: C owt%Sn T(°C) 40 Co 60 61.9 Adapted from Fig. 9.14, Callister 6e. 80 primary eutectic eutectic 100 97.8 wt% Sn S = 50 wt% R+S W L = (1-W) = 50 wt% • Co between 18.3 and 97.8 wt% Sn results in... – … a combination of crystals and eutectic regions T(°C) Pb-Sn system C = 18.3 wt%Sn L+ 2 00 TE 0 0 L+ S R R 1 00 C = 97.8 wt%Sn S + 20 18.3 E: L L 3 00 37 L L: C o wt%Sn 40 Co 60 61.9 80 C o , wt% Sn Adapted from Fig. 9.14, Callister 6e. primary eutectic eutectic 100 97.8 W = S = 73 wt% R+S W = 27 wt% • Eutectoid reaction: a reaction wherein upon cooling, one solid phase transforms isothermally and reversibly into two new solid phases that are intimately mixed cooling heating • Peritectic reaction: a reaction wherein upon cooling, a solid and a liquid phases transform isothermally and reversibly to a solid phase having a different composition L Copper‐zinc phase diagram cooling heating 38 39 • – – – • Cementite, Iron Carbide ‐ Fe3C . It is 25at% C or 6.7 wt% C. Eutectic reaction for the iron‐iron carbide system: L cooling heating Euctectic Point Fe3C Eutectoid Point Eutectoid reaction for the iron‐iron carbide system: (0.76 wt %C ) cooling heating (0.022 wt %C ) Fe3C (6.7 wt %C ) • The microstructure depends on: – Concentration of carbon – Heat treatment • Pearlite: a two‐phase microstucture results from the transformation of austenite and consists of alternating layers or lamellae of ‐ferrite and cementite Pearlite 43 • Hypoeutectoid alloy : less than eutectoid between 0.022 and 0.76wt% C • Proeutectoid ferrite: that formed above the eutectic temperature 44 • Hypereutectoid alloys: containing between 0.76 and 2.14 wt% C • Proeutectoid cementite: that which forms before the eutectoid reaction cementite composition remains constant as temperature changes 45 • • In most situations, the cooling rates are impractically slow and unnecessary Nonequilibrium effects of practical importance: – Phase changes or transformations at temperatures other than those predicted in phase diagrams – The existence of non‐equilibrium phases that do not appear on the phase diagram 46 • • • • • • 47 1. Salt is often spread on roads during the winter in order to depress the freezing point of water. A binary phase diagram for water and salt NaCl is shown below. Liquid Brine Field “Z” a What are the two phases that exist in equilibrium in the field “Z” region. b At ‐10°C determine the overall composition for which equal amounts of these phases exist in equilibrium. c What are the compositions ofeach phase under these conditions? d What is the lowest temperature at which adding salt can prevent water from freezing completely? 3 α p α p α p α p 3. Use the lead‐Sn phase diagram, to answer this question. a For a 30 wt%Sn alloy, determine the phases present and their compositions at 200°C, 150°C and 75°C. b For the same alloy, 30 wt%Sn, determine the amount/weight fraction of each phase at 200°C, 150°C and 75°C. c What is the amount/weight fraction of α formed during the eutectic reaction? ...
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This note was uploaded on 10/14/2011 for the course ENGINEER CHEM ENG 3 taught by Professor Ghosh during the Spring '11 term at McMaster University.

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