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Solid solution strengthening Precipitation hardening (age hardening)

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  1. Strengtheningmechanismsin aluminium alloys • Solid solution strengthening • Precipitation hardening (age hardening) • 3. Precipitation strengthening • 4. Grain size effect • Transition metals addition (dispersoids, Zener Drag) • Work hardening Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  2. 1. Solid solution strengthening MScEng Honorata Kazimierczak Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  3. Solid solution strengthening • type of alloying that can be used to improve the strength of a pure metal. The technique works by adding atoms of one element (the alloying element) to the crystalline lattice of another element (the base metal). Strategy for strengthening: • Make dislocations hard to move Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  4. Solid solution strengthening • Impurity atoms generate stress by distorting the lattice • This stress can produce a barrier to dislocation motion smaller substitutional impurity larger substitutional impurity Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  5. Solid solution strengthening mechanism in aluminium alloys • Alloy elements, such as Mg, Mn and Cu can 'pin' dislocations, thereby strengthening the material. • Solute atoms are barrier for dislocation Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim


  6. Solid solution strengthening mechanism in aluminium alloys • Alloy elements, such as Mg, Mn and Cu can 'pin' dislocations, thereby strengthening the material. • Solute atoms are barrier for dislocation Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  7. Solid solution strengthening mechanism in aluminium alloys • Alloy elements, such as Mg, Mn and Cu can 'pin' dislocations, thereby strengthening the material. • Solute atoms are barrier for dislocation Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  8. Solid solution strengthening mechanism in aluminium alloys • Alloy elements, such as Mg, Mn and Cu can 'pin' dislocations, thereby strengthening the material. • Solute atoms are barrier for dislocation For successful strengthening, alloy additions must satisfy 2 criteria: • high solid solubility; • atomic misfit to create local compressive or tensile strains. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  9. Magnesium is most likely to be used as a solid solution strengthener because: • the atomic misfit of Mg is quite high at 13% • it also has a high solid solubility in Al. • The principal alloys that are strengthened by alloying elements in solid solution are those in the aluminium-magnesium (5xxx) series, ranging from 0,5 to 6 wt% Mg. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  10. Potency of alloying Elements Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  11. Potency of alloying Elements Three final points are worthy of note: • In addition to atomic misfit and solubility, other factors, such as electronegativity can also affect the overall effectiveness of alloying elements on solution hardening. • As a strengthening mechanism, solidsolution strengthening is effective at all temperatures - indeed the solubility increases with temperature. This means that alloys such as 5xxx (Al-Mg) are more difficult to process at higher temperatures. • The degree of solid solution strengthening is NOT dependent on process history, simply on composition. This is not true of other hardening mechanisms. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  12. 2. Precipitation hardening (age hardening) MScEngKatarzyna Stan Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  13. Aluminium soft metal, insufficient strength for most engineeringapplications has to be strengthened The strongest aluminium alloys (2xxx, 6xxx and 7xxx) are produced by precipitation (age) hardening. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  14. Precipitation hardening– strengthening by precipitation of fine particles of a second phase from a supersaturated solid solution. Another name for this process is „Age Hardening” – because some metals form precipitates at room temperature over time. Saturatedα CuAl2 Room temperature microstructures in Al-4%Cu alloy. Slow cooling Agehardening Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  15. Basic requirement: Alloy contains soluble alloying elements, but their solid solubility decrease with decreasing temperature. Precipitation Hardening Steps Solution treatment at a relatively high temperature within the single phase region to dissolve the alloying elements. 600 α +L Rapid cooling or quenching (usually at room temperature), to obtain a super-saturated solid solution (solid solution in non-equilibrium state) of these elements in aluminium α 400 T α + β 200 A B % Ageing - controlled decomposition of the saturated solid solution to form finely dispersed precipitate, usually by ageing for convenient times at one and sometimes two intermediate temperatures. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  16. single phase solid solution Solutionheat treatment Ageing Quench supersaturated solid solution TM www.substech.com Afterageing TF Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  17. Precipitation Sequences Guinier-Preston (GP) zones Coherent with matrix, ordered, solute-rich clusters of atoms which often form on one or more atomic planes – formation influenced by the presence of excess vacancies retained in the matrix by quenching, Depending on the system they are disc, rod or spherical shaped Intermediate precipitate Larger than a GP zone, only partly coherent with the matrix. Usually composition and crystal structure differ only slightly from those of the equilibrium precipitate. May be nucleated from, or at, the sites of stable GP zones. This phase nucleates also heterogeneously at lattice defects such as dislocations. Equilibrium precipitate Formation of the equilibrium precipitate involves complete loss of coherency with the parent lattice. It forms only at relatively high ageing temperatures and because it is coarsely dispersed, little hardening results. Maximum hardening normally occurs when there is present a critical dispersion of GP zones, or an intermediate precipitate, or a combination of both. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  18. Precipitation Sequences http://www.eaa.net/eaa/education/TALAT TEM micrographs of an Al-4wt%Cu alloy after increasing heat treatment times,showing (a) GP zones, (b) GP zones and Θ’ precipitates and (c) Θ’ and Θ precipitates Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  19. Natural ageing - the aging is conducted at the room temperature and may continue almost indefinitely, the rate of change becomes extremely slow after months or years Artificial ageing- because the diffusion coefficient is strongly dependent on the temperature, precipitation from supersaturated solution is much faster at elevated temperatures. Such process is called artificial aging It takes usually a time from several hours to one day Hardnessorstrength http://www.eaa.net/eaa/education/TALAT Ageing time inhours ‘Overageing’ – at a sufficiently elevated temperature hardness usually increases to a maximum and then decreases Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  20. 3. Precipitation strengthening MScEng Piotr Bobrowski Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  21. Mechanisms of precipitation strengthening Precipitation strengthening results from inhibition of dislocation mobility by interaction with internal strain zones and precipitates. • Mechanisms: • coherent strain field • chemical hardening • dispersion hardening Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  22. Coherency strain hardening Interaction of internal strain fields caused by precipitates and dislocation line. Idealspacing Coarsespacing Finespacing τ t Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  23. Chemical hardening Increaseinstressrequired to forcea dislocationmovementthrougha coherentprecipitate • increase in precipitate-matrix interfacial area • creation of antiphase boundaries • change in separation distance between dislocations due to difference of stacking fault energy Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  24. Dispersion hardening Dislocationscan not cutthrough an incoherentprecipitates and ithas to by-pass it Dislocationclimbing Cross slip Dislocationbending Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  25. Orowan effect Dislocationbending v – Poissons ratio G – shear modulus b – Burgers vector λ – interparticle spacing D – particle planar diameter r0 – dislocation radius Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  26. The ageing curve Solutehardening Coherencystrain Chemical hardening Dispersionhardening Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  27. 4. Grain size effect MScEngMarta Gajewska Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  28. Grain boundary effect • The grain boundary acts as a barrier to dislocation motion for two reasons: • dislocation passing into grain B will have to change its direction of motion (more difficult as the crystallographic misorientation increases) • atomic disorder within a grain boundary region will result in a discontinuity of slip planes from one grain into the other For high-angle grain boundaries dislocations tend to “pile up” (or back up) at grain boundaries rather than traverse grain boundaries during deformation. These pile-ups introduce stress concentrations ahead of their slip planes, which generate new dislocations in adjacent grains. Fine-grained material has a greater total grain boundary area to impede dislocation motion Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  29. Hall-Petch equation σy = σ0 + kyd-1/2 d – is the average grain diameter σ0 – intrinsicyieldstress ky– constant for a particular material Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  30. Grain refinement advantages • The grain refinement offers: • improved mechanical properties, • consistent properties after heat treatment, • uniform properties after extrusion, • improved machinability, • reduced chemical segregation and porosity, • increased density. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  31. Grain refinement methods • During solidification of the molten metal: • - addition of trace elements • addition of borides, carbides, intermetallic compounds etc. • use of ultrasonic treatment or electromagnetic field • introduction of fine gas bubbles … • During thermomechanical treatments involving recovery and recrystallization of the deformed material • During severe plastic deformation using processes such as equichannel angular processing (ECAP), hydrostatic extrusion and roll bonding inoculation Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  32. Grain refinement by inoculation Inoculation is the addition of solid particles (or trace elements) – grain refiners - to a metallic melt to act as nucleationcatalysts for the formation of fine equiaxed, rather than columnar, Al grains. • The requirements of an inoculant, so it could act as an effective nucleating site are: • it should have a melting point higher than the alloy being solidified • it should be able to initiate freezing at very small undercooling • a sufficient number of nucleating particles should be uniformly distributed • nucleating particles should be larger than a critical size, which depends on the undercooling of the melt Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  33. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  34. Transition metals addition (dispersoids, Zener Drag) MScEng Jagoda Poplewska Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  35. Periodic tableof elements with Al, main alloying elements in Al-alloys Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  36. Periodic tableof elements with Al, main alloying elements in Al-alloys Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  37. Grain growth Grain size as a function of annealing temperature (alloys after ECAP, route Bc) S. Lee, A. Utsunomiya, H. Akamatsu, M. Furukawa, Z. Horita, K. Neishi, T.G. Langdon; Influence of scandium and zirconium on grain stability and superplasticductilities in ultrafine-grained Al–Mg alloys, Acta Materialia 50 (2002) 553–564 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  38. Grain growth Grain size as a function of annealing temperature (alloys after ECAP, route Bc) Vickers hardness measurements of alloys after cold rolling (z=60%) and annealing for 1h in different temperatures S. Lee, A. Utsunomiya, H. Akamatsu, M. Furukawa, Z. Horita, K. Neishi, T.G. Langdon; Influence of scandium and zirconium on grain stability and superplastic ductilities in ultrafine-grained Al–Mg alloys, Acta Materialia 50 (2002) 553–564 L.Lityńska-Dobrzyńska; Rola cyrkonu i skandu w procesach tworzenia struktur metastabilnych w stopach Al.-Mg-Si-Cu, IMIM PAN, Kraków 2009 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  39. Grain growth Precipitate growth in Al-0.25wt.%Sc during a 1 hanneal M.J. Jones, F.J. Humphreys; Interaction of recrystallization and precipitation: The effect ofAl3Sc on the recrystallizationbehaviour of deformedaluminium, Acta Materialia 51 (2003) 2149–2159 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  40. Grain growth Recrystallization temperature (50% recrystallization) ofcold-rolled sheets ofbinary alloys of Al–Mn, Al–Cr, Al–Zr, andAl – Sc systems as a function of the content of transition metal (AE) M.J. Jones, F.J. Humphreys; Interaction of recrystallization and precipitation: The effect ofAl3Sc on the recrystallizationbehaviour of deformedaluminium, Acta Materialia 51 (2003) 2149–2159 V. V. Zakharov; Effect of scandium on thestrcture and properties of aluminium alloys, Metal Science and Heat Treatment, Vol. 45, Nos. 7 – 8, 2003 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  41. Zener drag A dispersion of particles will exert a retarding force or pressure on a low angle or high angle grain boundary and this may have a profound effect on the processes of recovery, recrystallization and grain growth. The effect is known as Zener drag after the original analysis by Zener which was published by Smith (1948). The magnitude of this interaction depends the nature of the particle and interface, and the shape, size, spacing and volume fraction of the particles. The interaction between a grain boundary and a spherical particle (incoherent) If the boundary meets the particle at an angle β the restraining force on the boundary is: The maximum restraining effect (FS) is obtained when β =45°, so: γ - energy It should be noted that when a boundary intersects a particle, the particle effectively removes a region of boundary equal to the intersection area and thus the energy of the system is lowered, and boundaries are therefore attracted to particles. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  42. Zener drag • The interaction between a coherent particle and a high angle grain boundary • The boundary by-passes the particle; • The boundary halts at theparticles; If a high angle grain boundary moves past a coherent particle then the particle will generally lose coherence during the passage of the boundary. As the energy of the incoherent interface is greater than that of the original coherent interface, energy is required to cause this transformation, and this energy must be supplied by the moving boundary. Therefore coherent particles will be more effective in pinning boundaries than will incoherent particles. The drag force from single particle is given by: FC is a maximum when Θ=α/2 and α=0, giving: Thus coherent particles are twice as effective in pinning a grain boundary as incoherent particles of the same size Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  43. Zener drag • Interaction of particles with • a rigid planar boundary (b) a flexible boundary For a volume fraction FV of randomly distributed spherical particles of radius r, thenumber of particles per unit volume (NV) is givenby: The Zenerpinning pressure exerted by the particles on unit area of the boundary: Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  44. Interaction of particles with grain boundary Backscattered electron SEM micrograph showing heterogeneous precipitation in 0.12wt.%Sc annealed 5 m at 380°C M.J. Jones, F.J. Humphreys; Interaction of recrystallization and precipitation: The effect ofAl3Sc on the recrystallizationbehaviour of deformedaluminium, Acta Materialia 51 (2003) 2149–2159 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  45. Interaction of particles with grain boundary Backscattered electron SEM micrograph showing heterogeneous precipitation in 0.12wt.%Sc annealed 5 m at 380°C Transmission electron micrograph showing migrationof a high angle boundary through a dispersion of semi-coherentparticles in Al-0.25wt.%Sc annealed 5 m at 575 °C. M.J. Jones, F.J. Humphreys; Interaction of recrystallization and precipitation: The effect ofAl3Sc on the recrystallizationbehaviour of deformedaluminium, Acta Materialia 51 (2003) 2149–2159 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  46. 6. Work hardening MScEng Grażyna Kulesza Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  47. Work hardening • Wrought alloys that do not respond to age hardening (e.g. 1xxx, 3xxx, 5xxx) are usually strengthened by strain hardening. This generally involves cold-working at ambient temperatures, at which the multiplication of dislocations occurs at a faster rate than they are annihilated by dynamic recovery. • The strain field surrounding a dislocation in the aluminium lattice can be sufficiently large to inhibit the passage of other dislocations gliding on intersecting slip planes. • A stress must therefore be applied in addition to the lattice friction stress in order for dislocations to intersect. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  48. Initially the material deform elastically – i.e. strain is proportional to stress. The gradient of the slope is the Youngs Modulus, E Above the yield stress, the material starts to deform plastically. If the load is removed, the material will show some permanent deformation. Dislocations multiply rapidly and their strain fields interact. This stage corresponds to the ultimate tensile stress (UTS). Strain localises so that a „neck” starts to form on the testpiece. Final failure occurs when the reduced cross section can no longer support the load. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  49. Commercial purity (CP) aluminium, such as EN AW-1050 has a low solute content and therefore relatively low strength when annealed. • This can be increased to some extent by strain hardening. Note however that the rate of work hardening is quite low. At 20% strain the dislocation density has increased further. However the dislocations have now undergone dynamic recovery – they have arranged into cell walls (or sub boundaries) in order to minimise the total strain energy. The crystal between these sub-boundaries is relatively free of dislocations At 1% strain the dislocation density is quite low and individual dislocations are easily resolved At 5% strain the dislocation density is increasing Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

  50. By introducing 5% Mg into solid solution, the proof stress, work hardening rate and thus Rm(UTS) can be significantly increased. • From the stress-strain curve, it is clear that the effects of solid solution hardening and strain hardening are more than additive, as can be seen by comparing with the curve for99.5% Al. Elastic limit Solute pinning of dislocation sources raises yield stress compared to Al Strain without load increase in number of dislocations unpinned from their solute atmospheres. Onset of serrated yielding – Lüders line propagation. Serrated flow continues to maximum engineering stress. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim