Learning Objectives. Chapter Outline. Solidification of Metals. Solidification of Metals

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1 Learning Objectives Study the principles of solidification as they apply to pure metals. Examine the mechanisms by which solidification occurs. - Chapter Outline Importance of Solidification Nucleation and Growth Cooling Curves Cast Structure Solidification of Metals Solidification of metals and their alloys is an important industrial process: Metals and alloys start with the casting of ingots for processing into semi-finished or finished products (bars, rods, and other shapes) During welding, a small region of the metal near the weld melts and resolidify During soldering, the whole solder alloy melts and resolidify Other processes which involve solidification: casting, glass forming - - Solidification of Metals G G L Solid phase will form when a liquid metal is cooled below T m. Solidification of metals is a phase transformation from liquid to solid It occurs because the original state of the metal is unstable relative to the final state G S G S < G L G Solid is stable G L < G S Liquid is stable But how is phase stability measured? The answer is provided by thermodynamics. The driving force promoting this transformation is the difference between the free energy of the solid and liquid phases G G L G S T Solidification T m T -5-6

2 Solidification of Metals Solidification proceeds in steps: Solidification of Metals Nucleation of a new phase (formation of small clusters (embryos or nuclei) in the melt and reach a critical size) Growth of the new phase: growth of the stable nuclei Thermal gradients define the shape of each grain Stages of solidification of a pure metal -7-8 Nucleation - The physical process by which a new phase is produced in a material. Critical radius (r*) - The minimum size that must be formed by atoms clustering together in the liquid before the solid particle is stable and begins to grow. Undercooling - The temperature to which the liquid metal must cool below the equilibrium freezing temperature before nucleation occurs. Homogeneous nucleation - Formation of a critically sized solid from the liquid by the clustering together of a large number of atoms at a high undercooling (without an external interface). Heterogeneous nucleation - Formation of a critically sized solid from the liquid on an impurity surface. Nucleation There are types of nucleation: Homogeneous: the solid nuclei appears spontaneously within the undercooled liquid Heterogeneous: the new solid phase nucleates at the mould wall, impurities etc -9-0 Homogeneous Nucleation Homogeneous Nucleation First and simplest case Metal itself will provide atoms to form nuclei. Metal, when significantly undercooled, has several slow moving atoms which bond each other to form nuclei. Cluster of atoms below critical size (r*) is called embryo. If the cluster of atoms reach critical size, they grow into crystals. Else get dissolved. Cluster of atoms that are greater than critical size are called nucleus 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. - -

3 Homogeneous Nucleation Heterogeneous Nucleation Mould wall Solid Liquid Heterogeneous nucleation occurs when there are foreign or special objects inside a phase which can cause nucleation Such objects include: mould wall, impurities, other metal additions (grain refiner) 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. - - Solidification of Metals Stages in the formation of a grain structure during solidification Liquid Liquid Liquid Solid Solid Nucleation Growth Undercooled Liquid Homogeneous nucleation Heterogeneous Nucleation During solidification, nucleation involves the formation of small solid particles surrounded by liquid Growth Grain boundaries -5-6 Cooling Curves 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. a) Cooling curve of a pure metal with undercooling (metal is well inoculated) b) without undercooling (no inoculation) Cooling Curves Recoalescence - The increase in temperature of an undercooled liquid metal as a result of the liberation of heat during nucleation. Thermal arrest - A plateau on the cooling curve during the solidification of a material caused by the evolution of the latent heat of fusion during solidification. Total solidification time - The time required for the casting to solidify completely after the casting has been poured. Local solidification time - The time required for a particular location in a casting to solidify once nucleation has begun. -7-8

4 Cast Structure Cast Structure 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Development of the ingot structure of a casting during solidification: (a) Nucleation begins, (b) Chill zone forms, (c) Preferred growth produces the columnar zone, (d) Additional nucleation creates the equiaxed zone 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Cast ingot -9-0 Cast Structure: Dendrites Cast Structure: Dendrites 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. - - Cast Structure: Dendrites Cast Structure: Dendrites - -

5 Learning Objectives Introduce the three basic types of imperfections: point defects, line defects (or dislocations), and surface defects. Explore the nature and effects of different types of defects. - Crystal Defects An ideal crystalline material is described in terms of a - D periodic arrangement of lattice points and an atom or group of atoms associated with each lattice point called basis Types of Crystal Defects & their Scale Point, Line, Surface (Planar), and volumetric defects Vacancies, impurities dislocations Grain and twin boundaries Voids Inclusions precipitates However, when there is deviation from this ideality, the material is said to have crystal defects From Chawla and Meyers, Mechanical Behavior of Materials - - Crystal Defects Point defects - Imperfections, such as vacancies, that are located typically at one (in some cases a few) sites in the crystal. Extended (Line) defects - Defects that involve several atoms/ions and thus occur over a finite volume of the crystalline material (e.g., dislocations, stacking faults, etc.). Vacancy - An atom or an ion missing from its regular crystallographic site. Interstitial defect - A point defect produced when an atom is placed into the crystal at a site that is normally not a lattice point. Substitutional defect - A point defect produced when an atom is removed from a regular lattice point and replaced with a different atom, usually of a different size. Importance of Defects Effect on Mechanical Properties via Control of the Slip Process Strain Hardening Solid-Solution Strengthening Grain-Size Strengthening Effects on Electrical, Optical, and Magnetic Properties -5-6

6 Types of Crystal Defects Vacancy atoms Interstitial atoms Substitutional atoms Dislocations Edges, Screws, Mixed Point defects Line defects (c) 00 Brooks/Cole Publishing / Thomson Learning Grain Boundaries Stacking Faults Twin Boundaries Area/Planar defects We need to describe them and understand their effects. -7 Point defects: (a) vacancy, (b) interstitial atom, (c) small substitutional atom, (d) large substitutional atom, (e) Frenkel defect, (f) Schottky defect. All of these defects disrupt the perfect arrangement of the surrounding atoms. -8.Point Defects Defects in ionic solids/crystals vacancy Interstitial impurity Frenkel defect Cation vacancy + cation interstitial Substitutional impurity The fact is there MUST be vacancies in a crystal!!!! -9 Schottky defect Cation vacancy + anion vacancy -0. Line Defects: Dislocations Missing half plane A Defect Dislocation - A line imperfection in a crystalline material. Screw dislocation - A dislocation produced by skewing a crystal so that one atomic plane produces a spiral ramp about the dislocation. Edge dislocation - A dislocation introduced into the crystal by adding an extra half plane of atoms. Mixed dislocation - A dislocation that contains partly edge components and partly screw components. Slip - Deformation of a metallic material by the movement of dislocations through the crystal. - -

7 An extra half plane Edge dislocation or a missing half plane - - F b S 6 5 Map the same Burgers circuit on a real crystal S F A closed Burgers Circuit in an ideal crystal Burgers Vector t Burger s vector Johannes Martinus BURGERS Burgers vector -7 b b t -8

8 Line Defects: Dislocations Line Defects: Dislocations Comparison between EDGE and SCREW Dislocations: Edge Dislocation: dislocation line (t) is normal to the burgers vector (b) Screw dislocation: dislocation line (t) is parallel to the Burgers vector (b) -9-0 Line Defects: Dislocations Movement of Edge Dislocation Dislocations are formed solidification plastic deformation thermal stresses from cooling Movement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals in much lower than that predicted by theory Planar Defects Burger s Vector = b All defects increase energy (energy is higher than perfect crystal) Surfaces, grains, interphase and twin boundaries, stacking faults Planar Defect Energy is Energy per Unit Area (J/m ) Surfaces: missing or fewer number of optimal or preferred bonds. - -

9 Planar Defects: Grain Boundaries GB: missing or fewer number of optimal or preferred bonds. (c) 00 Brooks/Cole Publishing / Thomson Learning Planar Defects: Twinning Application of a stress to the perfect crystal (a) may cause a displacement of the atoms, (b) causing the formation of a twin. Note that the crystal has deformed as a result of twinning Planar Defects: Twinning Occurs in metals with BCC or HCP crystal structure Occurs at low temperatures and high rates of shear loading (shock loading) Occurs in conditions in which there are few present slip systems (restricting the possibility of slip). Volume Defects Volume defects include undesirable defects such as: voids or porosity found in casting Inclusions or foreign particles But, they also include precipitates or second phase particles which contribute to strengthening of the material Small amount of deformation when compared with slip

10 Learning Objectives Explain what is diffusion and why is it an important part of metal processing? How does diffusion in metals occur? How can the diffusion rate can be predicted Factors affecting diffusion (rate of diffusion) - Chapter Outline Diffusion: definition, importance of diffusion in metal processing Types of Diffusion Mechanisms of diffusion Steady-state diffusion: Fick s First Law Example of metal processing using diffusion Diffusion What is diffusion? Diffusion is the atom movement (migration) in materials (solids, liquids, or gases) Why is diffusion important? Diffusion plays an important role in many areas of materials science It is important in processes such as: Heat treatment (phase transformation from solid to solid) Solidification (liquid to solid) Surface hardening of steel (carburising, nitriding, carbo-nitriding etc ) Corrosion and oxidation Coatings (galvanising, electroplating, anodising) - - Types of Diffusion There are types of diffusion in crystalline solids:. Impurity diffusion also called inter-diffusion Occurs in a solid material with more than one type of element (such as an alloy). Self-diffusion Different Types of flows in materials Electrical Current : flow of electric charge Heat transfer : flow of temperature Diffusion : flow of matter Occurs in chemically pure materials (only one type of atoms) For diffusion to occur point defects (vacancy) must be present in a crystal (atomic diffusion is in a way the migration of these defects) -5-6

11 Mechanisms of Diffusion Self-diffusion - The random movement of atoms within an essentially pure material. Mechanisms of Diffusion Vacancy Interstitial impurity Vacancy diffusion (also called Substitutional diffusion)- Diffusion of atoms when an atom leaves a regular lattice position to fill a vacancy in the crystal. Interstitial diffusion - Diffusion of small atoms from one interstitial position to another in the crystal structure. Substitutional impurity -7-8 Mechanisms of Diffusion Activation Energy for Diffusion Diffusion couple - A combination of elements involved in diffusion studies 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Diffusion mechanisms in a material: (a) vacancy or substitutional atom diffusion and (b) interstitial diffusion 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. A high energy is required to squeeze atoms past one another during diffusion. This energy is the activation energy Q. Generally more energy is required for a substitutional atom than for an interstitial atom -9-0 Vacancy Diffusion Occurs in substitutional solid solutions Requires vacancies and activation energy A substitutional atom will only be able to move if there is a vacancy Vacancy moves in opposite direction of atomic movement Rate of vacancy diffusion (fast or slow) depends on: Concentration of vacancies (number of vacancies increases as the temperature increases) Activation energy for atom migration Example of vacancy diffusion: Ni in Cu alloy Interstitial Diffusion Migration of an interstitial atom to an adjacent vacant interstitial site Occurs in interstitial solid solutions Rate of diffusion depends on: Concentration of interstitial atoms Interstitial diffusion is faster than vacancy diffusion because: more empty interstitials are present no vacancy is required Example of interstitial diffusion: C in Fe (steel) - -

12 Driving Force for Diffusion Cu Ni T o C The driving force (why diffusion occurs) for diffusion is a concentration gradient Atoms will diffuse only if there is a concentration gradient. Atoms can diffuse from regions of high concentration towards regions of low concentration and vice versa. 00 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Concentration of Cu 00 0 Cu Distance (x) Ni Time Concentration of Cu 00 0 Distance (x) - - Rate of Diffusion: Fick s First Law Fick s first law - The equation relating the flux of atoms by diffusion to the diffusion coefficient and the concentration gradient. Diffusion coefficient (D) - A temperature-dependent coefficient related to the rate at which atoms, ions, or other species diffuse. Concentration gradient - The rate of change of composition with distance in a nonuniform material, typically expressed as atoms/cm. cm or at%/cm. Rate of Diffusion: Fick s First Law The diffusion rate follows the Arrhenius equation (valid for any thermally activated process) Q D = D0 exp RT Where: D: is the diffusivity, which is proportional to the diffusion rate D o : is the temperature independent constant (m.s - ) Q: is the activation energy (J. mole - ) R: is the gas constant (8. J. mole - K - ) T: is the absolute temperature (K) -5-6 Fick s First Law: Steady-State Diffusion Steady-state diffusion means that the diffusion flux does not vary with time Factors Affecting Diffusion. Temperature dc J = D dx The negative sign indicates that the direction of diffusion flux is down the concentration gradient from high to low concentration J: is the diffusion flux (atoms/ m / s or kg / m / s) D: is the diffusivity (or diffusion coefficient) (m s - ) C: is the concentration X: is the distance dc/ dx: concentration gradient The diffusion rate increases with increase in temperature. Diffusing species (substitutional or interstitial atom) and host material (size, bonding) Smaller atoms diffuse faster than bigger atoms. Microstructure Diffusion at defects such as grain boundaries and dislocations is much faster than in perfect crystals Vacancy concentrations are also higher near these defects -7-8

13 Example of Processing using Diffusion Case Hardening of gears: Diffuse carbon atoms into the host iron atoms at the surface. Result is the case (surface) is harder and so is difficult to deform and crack -9