* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download TYPES OF IMPERFECTIONS
Condensed matter physics wikipedia , lookup
High-temperature superconductivity wikipedia , lookup
Shape-memory alloy wikipedia , lookup
Geometrical frustration wikipedia , lookup
Spinodal decomposition wikipedia , lookup
Low-energy electron diffraction wikipedia , lookup
Heat transfer physics wikipedia , lookup
X-ray crystallography wikipedia , lookup
Paleostress inversion wikipedia , lookup
Bose–Einstein condensate wikipedia , lookup
Radiation damage wikipedia , lookup
Electromigration wikipedia , lookup
State of matter wikipedia , lookup
Electronic band structure wikipedia , lookup
Crystallographic defects in diamond wikipedia , lookup
Colloidal crystal wikipedia , lookup
Work hardening wikipedia , lookup
Crystal structure wikipedia , lookup
TYPES OF IMPERFECTIONS A perfect crystalline material usually does not exist rather a various number of defects develop in the crystalline structure mainly upon solidification. A molten metal is amorphous and upon solidification it crystallizes but during the process some defects happens. • Vacancy atoms • Interstitial atoms • Substitutional atoms Point defects • Dislocations Line defects • Grain Boundaries Area defects 1. Point Defects: A. Vacancy defect: An atom missing from a normally occupied lattice site. -vacant atomic sites in a structure. distortion of planes Vacancy • Equilibrium number of vacancies Nv concentration depends on and increase with temperature for a given quantity of material Where: Nv = N exp ( -Qv / KT) Nv: Equilibrium number of vacancies for a given quantity of material (units usually is vacancies / m3) N: total number of atomic sites NA: Avagdro’s Number: 6.023 x 10 23 atoms/mole ρ: density A:atomic weight (g/mole) K: Boltzmann’s constant =1.38x10-23 J/atom. K or 8.62x10-5 ev/atom K T: Absolute temperature in kelvin (T k = T oC +273) Qv : Energy required for the formation of a vacancy. ESTIMATING VACANCY CONC. • Find the equil. # of vacancies in 1m 3 of Cu at 1000C. • Given: ρ = 8.4 g/cm3 ACu = 63.5g/mol QV = 0.9eV/atom NA = 6.02 x 1023 atoms/mole Mass of Cu is: (ρ x v) = 8.4 g/cm3 x 106 (cm3/m3)x 1= 8.4x 106 g 63.5 g/mole ---------Æ 6.023 x1023 atoms 8.4x106 g ------Æ N? N : total number of atomic sites: 8.4x106 x 6.023x1023 / 63.5 = 8 x 1028 atoms/m3 Nv = 8 x1028 exp (-0.9 / 8.62x10-5 x 1273)= 2.2x1025 vacancies /m3 Remarks: •As the temperature increase the number of vacancies also increase. •For most metals , just below melting temperature Nv/N = 10-4 , which means that one lattice site out of 10000 site will be empty. B. Self-Interstitials: An atom from the crystal that is crowded in a small void space that under normal conditions is not occupied. This type of defect introduces large distortions in the surrounding lattice, consequently the formation of this defect is not highly probable. distortion of planes selfinterstitial C. Impurities: Either impurity atom or intentionally added atoms (alloying) Find their way into the host crystal through two ways: 1. Producing solid solutions, 2. Producing new phases Solid Solution: It take place when different atoms take part in building a crystal lattice, solute atoms are added and no new phases are formed. Phase: A homogeneous portion of a system that have uniform physical and chemical characteristics Types of solid solutions: A. Substitutional Solid solution : Solute atoms replace a host atom. B. Interstitial Solid solution: Solute atoms fill the voids among the host atoms. Two outcomes if impurity (B) added to host (A): • Solid solution of B in A (i.e., random dist. of point defects) rCu=.128nm rC=0.071 nm OR rFe=0.124 nm rNi=0.125nm Substitutional alloy (e.g., Cu in Ni) Interstitial alloy (e.g., C in Fe) • Solid solution of B in A plus particles of a new phase (usually for a larger amount of B) Second phase particle --different composition --often different structure. Factors that affect the degree to which the solute atoms dissolves in the host metal to form a substitutional solid solution: 1. Atomic size factor. Appreciable quantities of a solute may be accommodated in this type of solid solution (substitutional ) only when the difference in atomic radii between the two atom types is less than about : 15% .Otherwise the solute atoms will create substantial lattice distortions and a new phase will form. ∆r% =[ (r solute – r solvent) / r solvent ] x 100 ≤ ±15% 2. Crystal structure. For appreciable solid solubility the crystal structures for metals of both atom types must be the same. 3. Electronegativity. The more electropositive one element and the more elec- tronegative the other, the greater is the likelihood that they will form an intrmetallic compound instead of a substitutional solid solution. Similar electronegativity increases solubility 4. Valences. Other factors being equal, a metal will have more of a tendency to dissolve another metal of higher valency than one of a lower valency. Similar valency increases solubility. Remarks regarding interstitial solid solutions: •The atomic radius of an interstitial atom must be substantially smaller than that of the host atom. •Metallic materials that have relatively high atomic packing factors crystal structures, the interstitial positions are relatively small. •Normally the maximum allowable concentrations of interstitial atoms is low (less than 10%) due to the accompanied lattice distortions. 4 In this problem we are asked to cite which of the elements listed form with Cu the three possible solid solution types. For complete substitutional solubility the following criteria must be met: 1) the difference in atomic radii between Cu and the other element (∆R%) must be less than ±15%, 2) the crystal structures must be the same, 3) the electronegativities must be similar, and 4) the valences should be the same, or nearly the same. Below are tabulated, for the various elements, these criteria. El Composition how do you specify the composition of an alloy? •Weight percent: (wt %) , C2= m2 x100 [4.3] m1+m2 Weight ( or mass) of a particular element relative to the total alloy weight. C1+C2 =100 •Atom percent: (at%) Number of moles of an element to the total number of the moles of the elements in the alloy nm1= m1’/A1 (4.4) , where: nm1: No. of moles of element 1 m1’ : mass in g of element 1 A1: Atomic weight (g/mole) C1’= nm1 x100 (4.5) nm1+nm2 C2’= nm2 nm1+nm2 C1’+C2’=100 x100 Composition Conversions: 4.6a 4.6b 4.7a 4.7b Divide the numerator and denomenator by M’/100. Multiply the numerator and denomenator by A1A2 To derive equation 4.7a Since: nm1= m1’ (4.4) A1 Since: C1’= nm1 x100 (4.5) nm1+nm2 C1= [C1’ (nm1+nm2)/100] A1 [C1’ (nm1+nm2)/100]A1 + [C2’ (nm1+nm2)/100] A2 x 100 multiply the numerator and denominator by [100/ (nm1+nm2)] LINE DEFECTS Dislocations: They are introduced in the crystalline structure during solidification (specially with rapid cooling) and during plastic deformation. • are line defects, • cause slip between crystal plane when they move, • produce permanent (plastic) deformation. Edge dislocation: Extra ½ plane of atoms Burgers vector: distance between adjacent atoms that defines the magnitude and the direction the dislocation slip each unit step motion LINE DEFECTS Edge dislocation: Extra ½ plane of atoms Dislocation line perpendicular to burgers vector Dislocation line motion parallel to burgers vector Lattice Above dislocation line distortions Compressive strain field Dislocation line perpendicular to the plane of the page Lattice distortions Below dislocation line Tensile strain field Motion of dislocation line Screw dislocation: A portion of the crystal is skewed one atom spacing with respect to the other portion of the crystal. Dislocation line motion is perpendicular to burgers vector Dislocation line motion Dislocation line b Dislocation line parallel to burgers vector on ne m oti Li b Line motion Edge dislocation Screw dislocation b AREA DEFECTS: GRAIN BOUNDARIES Grain boundaries: • • • • are boundaries between crystals. are produced by the solidification process, for example. have a change in crystal orientation across them. impede dislocation motion. Solidification of metals: liquid nucleation growth Grain with different lattice orientations nucleation Growth Grains are formed Angle of misalignment grain boundaries Different grains have different orientations of atoms Grain boundaries separate grains that have different lattice orientations and impede dislocation motion. When the orientation mismatch is on the order of few degrees then a small angle grain boundary is generated . Small angle boundary is formed when edge dislocation are aligned in the manner shown here.