AUTOMOTIVEMATERIALSANDMANUFACTURINGS3.ppt

1、85-500-01Structure and Deformation in Materials,Dr. W. Altenhof, P. Eng. Department of Mechanical, Automotive and Materials Engineering Friday, August 14, 2020,2,Engineering Materials,Large selection of materials for supporting loads. Metals and alloys Polymers Ceramics and glasses Composites,3,Bond

2、ing in Solid Materials,Primary bonding types in solids are: Ionic Covalent Metallic,Ionic Bonding Involves the transfer of electrons between atoms Non-directional as transfer of electrons generate “stable configurations” NaCl (Na+ + Cl- NaCl),Covalent Bonding Sharing of electrons (in outer shell) be

3、tween atoms Due to sharing of electrons atoms orientate themselves accordingly for electrostatic charge strongly directional behaviour results (molecule chains) H2O, polyethylene C2H4n, carbon,Metallic Bonding Atoms donate outer shell electrons to form a cloud or sea of electrons that are shared by

4、all atoms Intermetallic compounds (in alloys) involving metals and alloying agents typically illustrate a combination of metallic and ionic or covalent types Steel, aluminum alloys,4,Crystalline Structure of Materials,Metals, ceramics, polymers crystal structure Metals and ceramics are polycrystals,

5、 in which each grain is a crystal with its own particular orientation. Polymers may have molecule chains arranged in a regular pattern, in which case their structure is considered crystalline (amorphous structure can also exist for polymers),Primitive cubic (rare),BCC (chromium, iron, Mo, Na, W,),FC

6、C (Al, Pb, Cu, Ni,),HCP (Mg, Be, Zn,.),5,Defects in Crystals,Polycrystalline solids are not perfect structures but exhibit different types of defects: Point defects Line defects (dislocations) Surface defects Grain boundaries Lattice planes change orientation by a large angle Twin boundary Lattice p

7、lanes are mirror images of each other 3D defects Precipitates (second phase particles) Inclusions (oxides, sulfides etc.),Point defects,Line defects,6,Elastic Deformation of Materials,Associated with the stretching of bonds between the atoms of solids. Does not involve the breaking of these bonds St

8、rong atomic bonds are resistant to stretching and result in a high value of elastic modulus (E). Diamond has strong covalent bonds and E = 1000 GPa. Metal alloys have weaker bonds, and E = 210 GPa for steel and 70 GPa for Al Polymers have an elastic modulus around E = 3 GPa (at low temperatures) As

9、temperature increase, the elastic modulus of polymers decreases rapidly (glass transition temperature).,7,Elastic Deformation,xe equilibrium atomic spacing,Elastic deformations typically occur in the realm of 1% strain,8,Inelastic Deformation,Inelastic deformations are irreversible and result from a

10、 rearranging of atoms in the crystal lattice. “Changing of neighbors” for atoms Requiring energy which cannot be recovered Plastic deformation is a result of dislocation motion. Applied shear stresses cause edge and screw dislocation motion along slip plane resulting in a slip step In actuality, pla

11、stic deformation occurs as a result of the combined motion of edge and screw dislocations,9,Inelastic Behaviour Edge and Screw Dislocation Motion,Edge Dislocation Motion,Screw Dislocation Motion,Concentration of Slip Planes forms Slip Bands (regions of intense shear deformation),10,Inelastic Behavio

12、ur,Slip typically occurs on preferred planes and directions Atoms are relatively close together (close- packed planes) Distances between atoms is smallest (close-packed directions) Dislocations are easier to move if the distance to the next atom is small,11,Plastic Deformation Strength Consideration

13、s,Elastic deformation stretch of atomic bonds Inelastic deformation position change of atoms (through dislocation motion) (Generally) cannot influence atomic bond strength thus cannot influence elastic behaviour One can influence dislocation motion and hence the strength (in plastic deformation) of

14、materials This is why Eall steels = 208 GPa but yield and post yield behaviour can be very different.,12,Plastic Deformation Strength Considerations - continued,Inhibiting dislocation motion results in an increase in material strength Can be accomplished through Fine grain size (increased grain boun

15、daries dislocation motion difficult) Precipitates Addition of more dislocations to “tangle” dislocations and inhibit motion (cold working),13,Plastic Deformation Strength Considerations Nonmetals and Compounds,Where bonding is covalent (partially) dislocation motion is inhibited due to directional n

16、ature of bonding Dislocation motion in ionic bonding is difficult as electrical charges must balance In both cases: Brittle fracture typically results Little or no signs of yielding/post yielding behaviour Deformation is dominated by stretching of atomic bonds Strength is significantly influenced by

17、 presence of cracks and/or pores (stress risers),14,Mechanism of Creep Deformation,Creep occurs as a result of the diffusion of vacancies (point defects). Vacancies diffuse to grain boundaries which are perpendicular to applied stress direction Diffusion of vacancy (in one direction) also implies mo

18、tion of atom in opposite direction Resulting in microscopic geometry change of grain Resulting in measurable deformation of component,15,Mechanism of Creep Deformation,Thermal energy (temperature) is a catalyst for diffusion As temperature increases the engineering concern for creep is more prevalent