Materials Science Metals and alloys

Types of imperfections • Vacancy atoms • Interstitial atoms • Substitutional atoms Point defects (0D) • Dislocations Line defects (1D) • Grain Boundaries Area defects (2D) • Precipitates / cracks / porosity Volume defects (3D)

Line defects in solids (1D) 1-D defect in which atoms are mispositioned.

Dislocation motion • Incrementally breaking bonds • If dislocations don't move, deformation doesn't happen! (But it will fracture like a ceramic)

Dislocations in materials • Metals: Disl. motion easier. -non-directional bonding -close-packed directions for slip. electron cloud ion cores • Covalent Ceramics (Si, diamond): Motion hard. -directional (angular) bonding • Ionic Ceramics (NaCl): Motion hard. -need to avoid ++ and -- neighbors.

Stress-Strain curves σUTS σYIELD σFAILURE or σFRACTURE εYIELD εUTS Necking starts STRESS σUTS REGION I REGION III σYIELD REGION II HARDENING OCCURS DISLOCATION MOTION AND GENERATION ! l0 + le σFAILURE or σFRACTURE Region I : Elastic Deformation Hooke’s Law Region II: Uniform Plastic Deformation Strain is uniform across material Region III: Non-uniform Plastic Deformation Deformation is limited to “neck” region E l0 + le + lp STRAIN εYIELD εUTS l0

Plastic deformation (Metals) 1. Initial 2. Small load 3. Unload p lanes still sheared F d elastic + plastic bonds stretch & planes shear plastic F d linear elastic plastic

Atomic origin of elastic properties Slope of stress strain plot (which is proportional to the elastic modulus) depends on bond strength of material s e High E Low E

Modulus vs Yield Strength

Mechanical properties – objectives Strength (tensile and yield strength) Stiffness (elastic or Young’s modulus) Ductility (plastic deformation) vs Brittleness Resilience (capacity to absorb and recover elastic energy) Hardness Toughness (energy absorption / impact)

Dislocations – local strain fields Edge dislocation: compression (above dislocation line) & tension (below dislocation line) Screw dislocation: shear Stress & strain fields decrease with radial distance from dislocation line Symbol Symbol

Dislocation interaction Strain field from one dislocation can affect a neighboring dislocation Two like dislocations can repel each other Unlike dislocations attract and annihilate each other

Strengthening mechanisms (metals) Macroscopic plastic deformation corresponds to the motion of large numbers of dislocations The ability of a metal to plastically deform depends on the ability of dislocations to move Virtually all strengthening techniques rely on restricting or hindering dislocation motion We will look at 4 such mechanisms Reduce grain size Solid-solution strengthening Precipitation strengthening Strain hardening (or cold working)

Strategy 1: Grain size reduction • Grain boundaries are barriers to slip - dislocation has to change directions - grain boundary region disordered, so discontinuity in slip planes • Barrier "strength” increases with misorientation • Smaller grain size: more barriers to slip

Strategy 1: Grain size reduction Small grains increase strength, but decrease ductility (many barriers to dislocations) Controlled by casting conditions (slow/ fast cooling) Can also be altered by processing – Can be induced by rolling a polycrystalline metal -before rolling -anisotropic since rolling affects grain orientation and shape. -after rolling -isotropic since grains are approx. spherical & randomly oriented. 235 mm rolling direction

Strategy 2: Solid-solution strengthening Deliberately alloy metal with impurity atoms Substitutional or interstitial Impurity atoms produce lattic strains Strain fields interact with dislocations restricting movement ‘Dislocation atmosphere’ pins the dislocations OR Substitutional solid soln. (e.g., Cu in Ni) Interstitial solid soln. (e.g., C in Fe)

Strategy 2: Solid-solution strengthening • Impurity atoms distort the lattice & generate stress. • Stress can produce a barrier to dislocation motion. • Smaller substitutional impurity • Larger substitutional impurity

Strategy 2: Solid-solution strengthening Degree of strengthening depends on relative atomic size Size difference between Cu and Sn is large Large lattice strain Greater strengthening by concentration

Strategy 3: Precipitation strengthening Hard precipitates are difficult to shear E.g. Ceramics in metals (SiC in Iron or Aluminum) • Result:

Strategy 3: Precipitation strengthening E.g. Aluminum is strengthened with precipitates formed by alloying. 1.5mm Internal wing structure on Boeing 767

Strategy 4: Cold work strengthening Dislocations entangle with one another during cold work. Dislocation motion becomes more difficult. Ti alloy after cold working:

SUMMARY • Dislocations are observed primarily in metals and alloys. • Here, strength is increased by making dislocation motion difficult. • Particular ways to increase strength are to: --decrease grain size --solid solution strengthening --precipitate strengthening --cold work • Heating (annealing) can reduce dislocation density and increase grain size. 29