Achieving high strength and high ductility in magnesium alloys using severe plastic deformation combined with low-temperature aging

Kim, Woo Jin; Jeong, Hye Gwang; Jeong, Heon

doi:10.1016/j.scriptamat.2009.08.020

Cited by 156 publications

(50 citation statements)

References 22 publications

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“…Second, because of variations in atomic arrangement, grain boundaries can act as preferred sources for the nucleation and emission of dislocations (30,31). It has been reported that Mg alloys processed by equal channel angular processing or hot rolling for grain refinement exhibit not only higher yield stresses but larger elongations at room temperature (32)(33)(34). However, conflicting results have also been reported mainly due to the texture characteristics and complication of deformation twinning (35,36).…”

Section: Discussionmentioning

confidence: 99%

Reducing deformation anisotropy to achieve ultrahigh strength and ductility in Mg at the nanoscale

Qi²,

Mishra

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

115

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In mechanical deformation of crystalline materials, the critical resolved shear stress (CRSS; τ CRSS ) is the stress required to initiate movement of dislocations on a specific plane. In plastically anisotropic materials, such as Mg, τ CRSS for different slip systems differs greatly, leading to relatively poor ductility and formability. However, τ CRSS for all slip systems increases as the physical dimension of the sample decreases to approach eventually the ideal shear stresses of a material, which are much less anisotropic. Therefore, as the size of a sample gets smaller, the yield stress increases and τ CRSS anisotropy decreases. Here, we use in situ transmission electron microscopy mechanical testing and atomistic simulations to demonstrate that τ CRSS anisotropy can be significantly reduced in nanoscale Mg single crystals, where extremely high stresses (∼2 GPa) activate multiple deformation modes, resulting in a change from basal slip-dominated plasticity to a more homogeneous plasticity. Consequently, an abrupt and dramatic size-induced "brittleto-ductile" transition occurs around 100 nm. This nanoscale change in the CRSS anisotropy demonstrates the powerful effect of sizerelated deformation mechanisms and should be a general feature in plastically anisotropic materials.lightweight alloys | metallurgy | mechanical properties | in situ TEM | nanoparticle S trength and ductility are critical performance indicators of materials; in metals, both are associated with the activities of line defects in the crystalline lattice, called dislocation plasticity. Ductility is accomplished by the distributed and uniform multiplication and propagation of dislocations, whereas strengthening is often achieved by hindering their motion. Strength and ductility are fundamentally linked in materials, and a mechanism to improve one almost always leads to a decrease in the other (1, 2). How to achieve both high strength and high ductility is still a challenge of great importance in structural materials. Recent work has demonstrated that it is possible to improve both strength and ductility simultaneously, but this has mostly been limited to systems with ultrafine microstructures, such as nanotwinned Cu or twinning-induced plasticity steels (3, 4).As the lightest structural metal, Mg suffers from limited room temperature ductility and formability due to the highly anisotropic critical resolved shear stress (CRSS; τ CRSS ) of the different slip systems (5). In Mg, which has a hexagonal close-packed structure, the τ CRSS for nonbasal (prismatic and/or pyramidal) slip is ∼10 2 τ CRSS for basal slip (6, 7). Such an extreme plastic anisotropy of A ≡ τ nonbasal CRSS =τ basal CRSS ∼ 10 2 makes nonbasal slip very difficult to trigger. Without nonbasal slip or twinning (8), basal slip alone cannot accommodate arbitrary shape changes, and excessive basal slip in combination with unrelaxed tensile stresses in the plane-normal direction leads to spatially localized damage and rapid failure. Thus, a materials design strategy could be to enhanc...

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Section: Discussionmentioning

confidence: 99%

Reducing deformation anisotropy to achieve ultrahigh strength and ductility in Mg at the nanoscale

Qi²,

Mishra

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

115

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“…While the grain refinement by severe plastic deformation and slow-speed extrusion can induce high yield strengths over 300 MPa in various magnesium based alloys such as Mg-Al-Zn (AZ) [1][2][3][4][5], Mg-Zn-Zr (ZK) [6][7][8] and Mg-Sn (T) based alloys [9][10][11], the strengthening is achieved at the expense of ductility and formability due to their work hardened nature. On the other hand, precipitation hardenable alloys are formable after a solution treatment if recrystallized microstructure is optimized, and the final product can be strengthened by artificial aging [12].…”

Section: Introductionmentioning

confidence: 99%

Strong and ductile heat-treatable Mg–Sn–Zn–Al wrought alloys

et al. 2015

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a b s t r a c tA newly developed Mg-Sn-Zn-Al based alloy shows substantial strengthening by artificial aging. A Na-doped Mg-5.4Sn-4.2Zn-2.0Al-0.2Mn-0.1Na (TZAM5420À0.1Na) (wt.%) alloy exhibited a significant increase in yield strength from 243 to 347 MPa by a T6 treatment due to the uniform dispersion of nanoscale precipitates by aging. The trace addition of Na causes the formation of Sn-Na co-clusters in the early stage of aging, which provides heterogeneous nucleation sites for Mg 2 Sn precipitates. However, Na strongly segregates at grain boundaries and this degrades the ductility significantly. To overcome this problem, we developed a Na-free Mg-6.6Sn-5.9Zn-2.0Al-0.2Mn (TZAM6620) alloy, in which nano-scale MgZn 2 precipitates are uniformly dispersed by double aging. Pre-aging caused the formation of Zn-rich Guinier Preston zones, which acted as heterogeneous nucleation sites for the MgZn 2 precipitates. The double-aged TZAM6620 alloy exhibited a very high yield strength of 370 MPa with large elongation of 14%.

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“…Extensive effort has been made to improve their mechanical strength [3], formability [4], creep resistance [5] and corrosion resistance [6]. Reported strategies to enhance strength of Mg alloys include grain refinement [7], age hardening [8,9] and introduction of stacking faults [10][11][12]. Moreover, by combining these strengthening mechanisms, several high-strength magnesium alloys containing rare earth elements (Mg-RE) have been developed.…”

Section: Introductionmentioning

confidence: 99%