Strengthening mechanisms in aluminium alloys

Strengthening mechanisms in 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

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

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

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

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

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

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

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

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, solid solution 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

2. Precipitation hardening
(age hardening) MSc Eng Katarzyna Stan Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

soft metal, insufficient strength for most engineering applications
Aluminium soft metal, insufficient strength for most engineering applications 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

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. Saturated α CuAl2 Room temperature microstructures in Al-4%Cu alloy. Slow cooling Age hardening Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

Precipitation Hardening Steps
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

Solution heat treatment Ageing Quench single phase solid solution
supersaturated solid solution TM After ageing TF Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

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

Precipitation Sequences
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

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 Hardness or strength Ageing time in hours ‘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

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

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

Coherency strain hardening
Interaction of internal strain fields caused by precipitates and dislocation line. Fine spacing Ideal spacing Coarse spacing τ t Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

Chemical hardening Increase in stress required to force a dislocation movement through a coherent precipitate 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

Dispersion hardening Dislocations can not cut through an incoherent precipitates and it has to by-pass it Cross slip Dislocation climbing Dislocation bending Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

Orowan effect Dislocation bending 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

The ageing curve Solute hardening Coherency strain Chemical hardening
Dispersion hardening Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

4. Grain size effect MSc Eng Marta Gajewska

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

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

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

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

Grain refinement by inoculation
Inoculation is the addition of solid particles (or trace elements) – grain refiners - to a metallic melt to act as nucleation catalysts 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

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

Periodic table of elements with Al, main alloying elements in Al-alloys

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 superplastic ductilities in ultrafine-grained Al–Mg alloys, Acta Materialia 50 (2002) 553–564 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

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

Grain growth Precipitate growth in Al-0.25wt.%Sc during a 1 h anneal
M.J. Jones, F.J. Humphreys; Interaction of recrystallization and precipitation: The effect of Al3Sc on the recrystallization behaviour of deformed aluminium, Acta Materialia 51 (2003) 2149–2159 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

Grain growth Recrystallization temperature (50% recrystallization) of cold-rolled sheets of binary alloys of Al–Mn, Al–Cr, Al–Zr, and Al – 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 of Al3Sc on the recrystallization behaviour of deformed aluminium, Acta Materialia 51 (2003) 2149–2159 V. V. Zakharov; Effect of scandium on the strcture 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

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

Zener drag The interaction between a coherent particle and a high angle grain boundary The boundary by-passes the particle; The boundary halts at the particles; 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

Interaction of particles with
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, the number of particles per unit volume (NV) is given by: The Zener pinning pressure exerted by the particles on unit area of the boundary: Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

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 of Al3Sc on the recrystallization behaviour of deformed aluminium, Acta Materialia 51 (2003) 2149–2159 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

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 migration of a high angle boundary through a dispersion of semi-coherent particles in Al-0.25wt.%Sc annealed 5 m at 575 °C. M.J. Jones, F.J. Humphreys; Interaction of recrystallization and precipitation: The effect of Al3Sc on the recrystallization behaviour of deformed aluminium, Acta Materialia 51 (2003) 2149–2159 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

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

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

Initially the material deform elastically – i. e
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 test piece. 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

At 20% strain the dislocation density has increased further.
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 1% strain the dislocation density is quite low and individual dislocations are easily resolved At 5% strain the dislocation density is increasing 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 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

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 for 99.5% 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

At 20% strain, dense ‘forests’ of dislocations have formed.
TEM microstructure at different strains. At 20% strain, dense ‘forests’ of dislocations have formed. The high alloy content reduces the tendency for dislocations to form cell walls (c.f. the purer EN AW-1050) At 5% strain, the dislocation density is still quite low and individual dislocations are easily resolved. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

In comparison with 99.5% Al, the Mg solute allows very little dynamic recovery at room temperature, thus preventing dislocation rearrangement into a cell structure. Of course, since recovery is a thermally activated process, a cell structure can be produced in Al- 5%Mg by deforming at a higher temperature, e.g. at 400 °C. Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

References Joseph R. Davis „Aluminum and aluminum alloys”, J. R. Davis & Associates, ASM International. Handbook Committee Ian Polmear „Light Alloys-From Traditional Alloys to Nanocrystals” Elsevier 2006 „High performance materials in aerospace”, ed. By Harvey M. Flower, Published by Chapman and Hall, London 1995 „Aluminum properties and physical metallurgy”, edited by John E. Hatch, American Society for Metals, Metals Park, Ohio,1984 A. L. Greer „Grain refinement of alloys by inoculation of melts” Phil. Trans. R. Soc. Lond., A , P. Moldovan and G. Popescu „The Grain Reﬁnement of 6063 Aluminum Using Al-5Ti-1B and Al-3Ti-0.15C Grain Reﬁners”, JOM, 2004, 59-61 T.R. Ramachandran, P.K. Sharma and K. Balasubramanian, „Grain Refinement of Light Alloys”, 68th WFC - World Foundry Congress, 7th - 10th February, 2008, B. S. Murty, S. A. Kori, and M. Chakraborty „Grain refinement of aluminium and its alloys by heterogeneous nucleation and alloying”, International Materials Reviews 2002 Vol. 47 No. 1, 3-29 William D. Callister Jr., David G. Rethwisch „ Materials Science and Engineering”, John Wiley & Sons, 2010, 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 V. V. Zakharov „Effect of scandium on the strcture and properties of aluminium alloys”, Metal Science and Heat Treatment, Vol. 45, Nos. 7 – 8, 2003; M.J. Jones, F.J. Humphreys „Interaction of recrystallization and precipitation: The effect of Al3Sc on the recrystallization behaviour of deformed aluminium” Acta Materialia 51 (2003) 2149–2159 F.J. Humphreys, M. Hatherly „Recrystallization and Related Annealing Phenomena” PERGAMON 2002 Kun Yu, Songrui Li, Wenxian Li „Recrystallization Behavior; Recrystallization Behaviour in an Al.-Cu-Mg-Fe-Ni Alloy with Trace Scandium and Zirconium”, Materials Transactions, JIM, Vol. 41, No. 2, (2000) G. Gottstein, L.S. Shvindlerman „Grain Boundary Migration in Metals” CRC Press, 2010 Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim

Strengthening mechanisms in aluminium alloys

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