MET 2110 STUDY GUIDE EXAM 1 General characteristics and bonding Metals metallic bonding ductile and conductive Ceramics covalent/ionic bonding brittle, insulators, stiff, high melting point Polymers covalent, van der Waals bonding weak, low temperatures, not stiff, insulators Metallic bonds nondirectional, free electrons that give conductivity Electron Sea Covalent bonds directional, strong, no free electrons sharing of electrons Ionic bonds involves two elements, strong, no free electrons, donating and accepting electrons Crystal structures simple cubicFCCBCCHCP atom locationscornerscorners and facescorners and centerforget! atoms/cell1422 relationshipA=2rA=4r / !2A=4r / !3forget! coordination number612812 packing factor0.520.740.680.74 stacking sequencenoneABCABCABCnoneABABAB close packed directionsforget! close packed planesnone(111)none(0001) = basal critical resolved shearforget!100 psi10,000 psi100 psi major propertyforgetductilestrongbrittle typical metalsnoneAl, Cu, NiFe, WMg, TI, Be Miller indices Directions subtract tail from head, clear fractions, reduce to lowest integers Planes find intercepts, take reciprocals, clear fractions find a corner of the cube that makes iteasiest to find the intercepts 0,1,0 x=1, y=1/2, z=1 x=-1, y=1, z=-1 - (1,0,1) 1/x=1, 1/y=2, 1/z=1 1/x=-1, 1/y=1, 1/z=-1 -1,1,-1 [! 1 1 ! 1](! 11! 1)(121) Lattice Defects Line defect:dislocations:movement (slip) permits metals to have ductility! Blocking slip or dislocation movement causes strengthening! Increasing number of dislocations = strain hardening (cold working) Surface defects:grain boundaries, stacking faults (ABCBCBCABC) Small grains give large amount of grain boundaries and higher strength N = 2n-1where n = ASTM grain size number; big n means small size Point defects:substitutional, interstitial (small atoms), vacancies More defects block dislocations and increase strength Substitutional and interstitial = solid solution strengthening Number of substitutional and interstitial independent of temperature Number of vacancies increases with increasing temperature The secret to metallurgy is: INCREASING NUMBER OF LATTICE DEFECTS MAKES IT HARDER FOR DISLOCATIONS TOSLIP AND INCREASES STRENGTH!!!!!!! Diffusion Movement of atoms through crystal structure D = Doexp(-Q/RT) where Q = activation energy and D = diffusion coefficient Increasing T or decreasing Q gives more rapid diffusion! Low Q (fast diffusion) for:interstitial, low packing factor, along surface or grain boundary, weak bonding, low melting point metals, metals vs. ceramics Flux J is how many atoms pass a given area per unit time: J = -D(!c/!x) where !c/!x is concentration gradient High flux means a heat treatment will be done quickly Diffusion important for: grain growth, recrystallization, age hardening, tempering, solidification, creep, manufacturing processes such as sintering, diffusion bonding, etc.NOT important in room temperature strength, fatigue, hardness, martensite reaction, etc. Diffusion becomes important when T > 0.4 Tm(in Kelvin) Mechanical properties Know stress-strain curve and what various points mean Tensile test gives: yield strength (dislocations move) tensile strength (maximum load, necking occurs) modulus of elasticity (stiffness) slope of elastic region %elongation = (lf -lo)/ lo X 100 = ductility %reduction in area = (Ao Af )/AoX 100 = another measure of ductility YS, TS, Modulus decrease with high temperature; %E and %RA increase Hardness test: measure of penetration of surface by hard object; good indication of wear resistance; correlates with other properties (TSsteel = 500*BHN); Brinell and Rockwell tests. Impact test gives: energy absorbed in sudden blow (high for FCC, increases with temperature). transition temperature (in BCC metals) ductile above this T; we want a materialto have a transition temperature below the service temperature.Charpy is typical test. Fatigue test gives: ability to withstand a cyclical load or vibration.Stress < yield strength endurance limit = stress below which failure does not occur endurance ratio = endurance limit / tensile strength ~ 0.5 get an S-N curve (stress vs. number of cycles) Creep test gives: ability to resist deformation at high temperature.Stress < yield strengthcreep rate = strain or elongation per unit time slope of creep curve rupture time = time required to break during creep stress-rupture curve = time to break as function of stress and temperature rupture time decreases and creep rate increases as stress and T increase Fracture toughness: ability of material to resist crack growth when stress is applied.Anything we do to permit the material to absorb more energy before a flaw causes failure improves toughness. Allotropic = material exists in more than one crystal structure Isotropic = same properties in all directions Anisotropic = different properties in different directions