SOLIDIFICATION suicide-wine- Fundamentals of solidification . Introduction . Homogeneous nucleation . Heterogeneous nucleation . Growth and microstructure . Summary Introduction There are two types of solicitation Glass formation: physical properties such as viscosity change smoothly across the solidifying region Phase transition:some physical properties change abruptly such as viscosity, heat capacity Temperature vs time in glass solidification and phase transition Solidification by phase transition is modelled as two stage Nucleation Homogeneous nucleation: is responsible for the formation of particles from the Vapor phase Heterogeneous nucleation: involves the introduction of a foreign phase or surface,, typically the nucleation agent, for the crystal to grow on The only difference between homogenous nucleation and heterogenous nucleation is that, homogenous nucleation occurs away from the surface of the system whereas heterogenous nucleation occurs at the surface of the system Growth No preferred nucleation sites Random, spontaneous Those of preferred sites Boundary, surface, inclusion Local free energy change G- energy - Spherical nucleus ✓ volume Area A- (two forms = = E- / energy Critical radius Single nucleus Heterogenous nucleation Nucleation site More walls, inclusion, interface, surface, impurity Growth and microstructure Outer chilled zones Microstructure of ingot Chilled zone Fine equiaxed grains, pure substance: continuous shell, solution: particles and particles flushed away from wall into the central Re-melted Survived-nucleus Intermediate columnar zone Equiaxed grain Nucleation: supercooling, falling particles, and dendrite fragments Elevated pouring Temperature: larger equiaxed grains Structure and properties Growth and overtaken More columnar zone Anisotropic properties: magnetic materials, and turbo blade s More equiaxed zone Isotropic properties Less segregation Anisotropic and isotropic properties The directionality of properties is termed as anisotropy The properties that are independent of the direction of measurements are isotropic Heat treatment Heating and cooling operation applied to metals and alloys in solid state to obtain the desired properties Significance of heat treatment Relieve internal stress, improve ductility, refine grain size, increase hardness and tensile strength, and modify surface composition and properties A) Heating the metal part to ensure uniform heating Thermal conductivity of treated part Size and shape of the part Pre-stressed condition of the part B) Soaking the metal parts of uniform temperature for a given time Depends on chemical composition and mass of the parts Intricate shaped parts need soaking and preheating Preheating: for intricate shaped parts or for large section of part, heating to a high temperature is not gonna happen in one operation, the part s heated up to a lower temperature just below where the change takes place, and held at his temperature for some time so that heat is the same throughout that part, and this holding lower temperature is called preheating C: cooling a heat metal part in contact with a cooling medium Controls of rate of cooling depends on the type of metals and alloys and desired properties Cooling medium: solid(cast iron chips, line, sand,ashes), liquid(water, oil, soda solution), and gas(air) Heat treatment processes Crystallography crystallography is the science that examines the arrangement of atoms in solid Crystalline/noncrystalline solids The solid materials may be classified according to the regularity with which atoms or ions are arranged with respect to one another A crystalline materials is one in which the atoms are situated in a repeating or periodic away over large atomic distances Unit cell The atomic order in crystalline solids indicates that small groups of atoms form a repetitive pattern A unit cell is chosen to represent the symmetry of crystal structure Unit cell is the basic structural unit BCC FCC When dealing with crystalline materials, it is often becomes necessary to specify some particular crystallographic plane of atoms or a crystallographic direction Coordinate system: A crystallographic direction is defined as a line between two points, or a vector IMPERFECTION IN SOLIDS Types of imperfection Point defects Vacancy atoms Interstital atoms Substitutional atoms Line defects Dislocation Area defects Grain boundaries Vacancies Self-interstital Linear defects(dislocation) Edge dislocation Screw dislocation When dislocation move(slip between crystal planes) permanent(plastic) deformation occurs Edge dislocation Burgers vector A measure of lattice distortion Solidification mechanism Solidification mechanism Polycrystalline materials Grain boundaries Dislocations are visible in electron microgaphs Other defects External surface Bulk defects Examining defects Grains and grain boundaries very considerably in size Can be large Large single crystal of quartz Can be quite small(mm)- generally need to be observed with a microscope Material failure Mechanical Chemical Combination of both Fatigue Failure by fatigue is the most cause of engineering failure Fatigue in engineering, microscopic cracking of materials, especially metals, after repeated application of stress Fatigue occurs most often in moving machinery parts Characterising fatigue properties:S-N curve Material exhibiting a fatigue limit: i.e. a stress below which a component will last an infinite number of cycles A material which does not exhibit a fatigue limit typically non ferrous materials Creep Creep is the continuous, gradual, permanent, extension of material of material which occurs under a steady force at high temperature As the temperature is raised, materials under loads which caused no permanent deformation at low temperature start to creep Many structures such as those associated with energy conversion operate at high temperature High and low temperature that is talked about is dependent on the melting point of the materials Low temperature creep materials Lead has a melting point which is low to a point where at room temperature it is about half its absolute melting temperature Deformation at a constant stress vs time: Secondary creep rate An empirical relationships has been developed which relates this strain to stress being applied: Where k and n are material constant, and n can only vary between 3 and 8 Fracture mechanisms:failure of a pipe Ductile failure Brittle failure Moderately ductile failure Nature of crack growth Trans crystalline/trans granular: Growth through the grain, general mechanism are shear or cleavage Inter crystalline: this type of fracture occurs generally off the grain boundaries are embrittlement by precipitates or impurity phases Concentration of stress at crack tip Ideal vs real materials Fast fracture Failure occurs when the stress around an existing crack, flaw or design feature exceeds the critical stress Crack then grows at the speed of sound through the material Overall stress level lower than yield stress Fracture toughness K= fracture toughness A property we can measure although Crack propagation Basic consideration involves the energy balance in the system The net energy Surface energy:U(s) Elastic strain energy:U(e) Corrosion Corrosion is defined as the destruction or deterioration of material because of reaction with its environment
