Crystalline Defects (7.4) to Phase Diagrams (7.8)

74 Crystalline Defects Le real materials are not pure, in most cases materials have been alloyed in other atoms or we have impurity atoms that are incorporated into our materials Crystalline defects ها a deviation from a perfect crystal diameter or a lattice irregularity on the order of an atomic Lo crystalline defects can be divided into -Point defects associated with one or two atamic positions - Linear defects one-dimensional defects Interfacial defects two-dimensional defects. Pant Defects Vacancy-atom is missing in the atom lattice site Lo -all crystalline solids contain vacancies impossible to create a material that is free of vacancy defech Self interstitial atom from the crystal is positioned mho an intershhial site - self.interstitial atom is substantially larger than the interstitial position, which introduces major distortions in the surrounding lattice interstitial site is the space between aroms -formanon of this defect is not highly probade & only exists in very small concentrations (significantly lower than the vacancy concentration) Lo Impurities in solids can be present inherently or added intentionally intentionally added into metals to form solid sol alloys to enhance mechanical properties of a material pure metal consisting of only one atom type is not possible approximately 10 impurity atoms will be present in puring of 99.9999% one cubic meter of material even at Linear, Defects Edge dislocations Screw dislocations La disiacation most dislocations is an extended one dimensional defect that is incorporated into the crystal are termed mixed dislocation which exhibil both edge & screw dislocations La dislocation density (no of dislocations) in a material is the total dislocation length per unit volume (am") Lo dislocation density Lo 105 mm carefully solidified metal crystals - 10-10 mm-heavily deformed metal Edge Dislocation extra half-plane of atoms that is incorporated into the crystal Interfacial Defects they separate regions of the materials that have different crystal structures and/or crystallographic orientations these imperfections include external surfaces, grain boundaries and phase boundaries. -surface atoms aren't bonded to the mar. number of atome & have a higher energy than the interior atoms grain boundaries separate 2 grains in polycrystalline materials (most common imerfacial defect) 7.5 Diffusion Lo diffusion is required when annealing or heat treating a component, or when diffusion Le diffusion is the process of material transport by atomic motion L requirements for diffusion to occur - an emply adjacent site e defects sufficient energy for atom to breale bonds provided through vibrational energy Bulk Diffusion Pathways Lo diffusion mechanisms in metallic systems. substitutional & interstinal Subshtuhanal intershhal Le interstitial aloms need to be small eg H.CAN to interstitial diffusion is faster than substitutional diffusion Since the atoms are smaller & more mobile Lo interstitial atoms migrate from an interstitial position to an empty neighbouring posinon A Diffusion Coefficient diffusion coefficient (D) rate at which atoms diffuse unit: m²/s D = Do * exp(-Qd/RT) Do = temp. independent pre-exponential Qd = activation energy for diffusion R = gas constant T = absolute temperature diffusion activation energy energy required to produce the diffusive motion of one mole of atoms given by gradient of graph when you plot D against T D depends on the temperature, host material and diffusing species Fast Diffusion Pathways diffusion occurs through defects in materials other defects in materials: dislocation lines grain boundaries 4 dominant diffusion pathways: interstitial, substitutional, grain boundary & dislocations all of these are thermally activated 7.6 Grain Structure Development material properties non structure-sensitive properties: density, modulus structure sensitive properties: yield & tensile strength, ductility, fracture toughness, creep & fatigue strength materials selection narrow the list of candidates down investigate detailed material's specifications detailed specifications structure-sensitive properties cannot be guessed from the general data sheets investigate supplier's data sheets listing the properties of the alloy you intend to use, with the same mechanical forming & heat treatment history Le when considering structure-sensitive properties, need to consider both the composition & history Le material history refers to any solidification that the material has gone through & any subsequent forming processes and/or heat treatment Solidification: Metal Casting to during solidification, stable nuclei form through nucleation 78 Phase Diagrams: Introduction Possible Solid Structures solid structure for 2 mixed metas takes one of these forms. Two separated and pure Single solid Two separated solid components solution salutions A chemical compound A chemical compound together with a solid solution Solid Solution (ss) formed when solute atoms are Lo crystal structure is maintained & added to the parent material no new structures are formed oss is compositionally homogeneous solute atoms are randomly & uniformly dispersed within the solid Lo substitutional & nierstitial SS