Most metals and alloys have either bcc or close-packed (fcc or hcp) crystal structures Close packed (hexagonal or cubic) hcp ccp Metals and Alloys: Mechanical Properties (Ch. 7)

Mechanical Properties of Metals and Alloys Hypothetical situation: Upon graduation, you go to work for Boeing. Your job – select a high-strength Al alloy for jet airplanes. 50 tons cargo Airplane: 500 tons } 150 tons plane structure 300 tons fuel If you can triple the alloy strength, you can triple cargo load (to 150 tons). MaterialTensile Yield Stress (psi) pure (99.45%) annealed Al 4 x 10 3 pure (99.45%) cold drawn Al24 x 10 3 Al alloy - precipitated, hardened50 x 10 3 big improvement But, “perfect” single crystal Al as a yield stress of ca psi!

Defects in Metallic Crystals Defects are responsible for important mechanical properties of metals: malleability, yield stress, etc. Non-directional bonding, large number of nearest neighbor atoms  metallic structures readily tolerate “mistakes” vacancy dislocation (missing atom) (extra plane of atoms) point defect line defect Not important Very important

Dislocations Move Under Stress Key point: Moving a dislocation breaks/makes a line of metal-metal bonds (easy) Shearing a perfect crystal means we have to break a plane of bonds (requires much more force) shear force

Hardening of Alloys Structural alloys - e.g., girders, knife blades, airplane wings Need to minimize movement of dislocations. How? 1.Use annealed single crystals (expensive) Some specialty applications – e.g. jet turbine blade Impossible for large items (airplane wings, bridges…) 2.Work hardening - moves dislocations to grain boundaries planar defect (stronger under stress) “Cold working” or “drawing” of a metal increases strength and brittleness (e.g., iron beams, knives, horseshoes)

Hardening of Alloys (contd.) Work Hardening and Annealing have opposite effects Annealing : crystal grains grow, dislocations move (metal becomes more malleable) 3.Alloying – homogeneous or heterogeneous Impurity atoms or phases “pin” dislocations.

Malleability of Metals and Alloys Some metals are soft and ductile (Au, Ag, Cu, Al, etc.) Others are hard (Fe, W, Cr, etc.) Why? Crystal structure is important. Two types: body centered cubic (bcc) - 8-coordinate - hard close packed (fcc and hcp) - 12-coordinate - soft Close-packed planes slip easily Non-close packed - “speed bumps” Cu (fcc) CuZn alloy (brass) Zn (hcp)

Iron and Steels Below 900 o C, iron has bcc structure - “hard as nails” Above 900 o C, iron is close packed (fcc) - soft Can be worked into various shapes when hot Steelmaking: Carbon steel contains ~ 1% C by weight (dissolves well in fcc iron but not in bcc) Slow cooling (tempering): fcc Fe/1%C  mixture of bcc Fe and Fe 3 C (pearlite) Fe 3 C (cementite) grains stop movement of dislocation in high carbon steel - very hard material

Amorphous (Glassy) Alloys Metals are typically polycrystalline Amorphous alloys have superior mechanical properties because dislocations cannot move.