Comprehensive study of phase transformation in age-hardening of Mg–3Nd–0.2Zn by means of scanning transmission electron microscopy

Sanaty-Zadeh, A.; Luo, Alan A.; Stone, Donald S.

doi:10.1016/j.actamat.2015.05.001

Cited by 49 publications

(21 citation statements)

References 31 publications

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“…The aim of this study has been to resolve the :::: early studies [2,6,[9][10][11][12]. : We have shown that the early stages of the aging reaction in Mg-Nd alloys is dictated by the HCP convex hull.…”

Section: Discussionmentioning

confidence: 99%

“…Mg-RE (rare-earth) alloys are especially attractive due to their high strength at peak aging conditions [2]. As a result, a number of recent studies have investigated the precipitation sequence [4][5][6][7][8][9] :::::: [4][5][6][7][8][9][10][11][12] and hardening mechanisms [13][14][15][16][17][18][19][20][21] in magnesium alloys. Nd has proven to be an important alloying element in this respect.…”

Section: Introductionmentioning

confidence: 99%

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On the early stages of precipitation in dilute Mg–Nd alloys

et al. 2016

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We report on a first-principles and experimental study of precipitation in supersaturated solid solutions of Mg-rich Mg-Nd alloys. A cluster expansion Hamiltonian combined with Monte Carlo simulations was used to calculate a metastable HCP temperature-composition phase diagram. Aging studies were performed on dilute Mg-Nd alloys that were then characterized with HAADF-STEM. The early stage decomposition of these alloys is dominated by the thermodynamic stability of different Nd orderings on the HCP crystal structure. The observed progression of Guinier-Preston zones can be rationalized by a successive reduction of the Nd chemical potential. HAADF-STEM studies confirm the appearance of β ′ -like precipitates, however, the observed Nd ordering does not exclusively consist of zig-zag Nd rows but also contains linear strips of Nd-hexagons at non-periodic intervals. First-principles calculations predict that these orderings are stable compounds that belong to a family of hybrid phases formed by combining the β ′ ordering with the hexagons of β ′′ (D0 19 ) along the [010] β ′ direction. We label this hierarchy of orderings as β ′′′ . A high degree of degeneracy among the different β ′′′ phases is predicted. This along with a strong composition dependence of the misfit strain is likely responsible for the observed disorder of Nd-hexagon strips. Our experimental and first-principles evidence does not support the formation of Mg 3 Nd having D0 19 ordering. Rather, the features reported as D0 19 are isolated hexagonal GP rods that show no long-range periodicity. Our study suggests the following precipitation sequence for binary Mg-Nd alloys:Mg 41 Nd 5 ) (Emmanuelle A. Marquis), avdv@engineering.ucsb.edu (Anton Van der Ven) tion sequence during aging of Mg-Nd alloys, however, remains controversial, having been the topic of a number of experimental[22-28] and computational[29-33] studies. A recent review reports the following aging sequence[2]:During typical aging experiments, supersaturated solid solutions (SSSS) containing small amounts of Nd (of the order of 0.5 at. %) are solution treated around 800K and are then aged around 500K. Guinier-Preston zones (GP zones) containing Nd atoms arranged as rods along the HCP caxis form first in the early stages of precipitation [22,24]. These rods exhibit local orderings that resemble N's, V's, Λ's and hexagons when viewed along the ⟨0001⟩ direction (figs. 1a to 1c). At early times they appear as isolated columns (figs. 1a to 1c) but later self-organize into long chains along the ⟨1010⟩ direction (figs. 1d and 1e). All orderings proposed in past studies are illustrated as 2dimensional projections along the ⟨0001⟩ axis in fig. 1. The red circles in fig. 1 represent columns of Nd atoms along the c-axis of the HCP crystal. Two patterns have been proposed for the chains consisting of V's and Λ's,

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

On the early stages of precipitation in dilute Mg–Nd alloys

et al. 2016

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“…It has been reported that the strength of many age-hardened Mg-RE-Zr alloyscan be further enhanced by controlled addition of zinc (Zn) [5,15]. Smalladdition (< ~1 wt.%) of Zn promotes strength and creep resistance, while largeraddition (> ~1 wt.%) of Zn changes the prismatic ( 112 0 , or 101 0 ) β-series precipitates to basal ( 0001 ) γ-series, which are less effective in impeding basal dislocation motions [5].…”

Section: Introductionmentioning

confidence: 99%

Precipitation evolution and hardening in Mg Sm Zn Zr alloys

et al. 2016

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Precipitation evolution and its correspondence to mechanical properties in Mg-4Sm-xZn-0.4Zr (x=0, 0.3, 0.6, 1.3) (wt.%) alloys are systematically investigated in this work. Precipitation sequences at 200 o C are identified using transmission electron microscopy (TEM), highresolution TEM (HRTEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A new precipitate β' Z is found to present with the increase of Zn addition. With higher Zn content (> ~1 wt.%), basal γ-series precipitates dominate. The transitionin precipitation sequence agrees wellwith the change of mechanical properties. The microstructure and property evolution in the optimum Mg-4Sm-0.3Zn-0.4Zr alloy is modeled based on obtained experimental data through a combination of classical nucleation and growth model and the CALPHAD (CALculation of PhAse Diagram) simulation. The obtained

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“…Metals 2016, 6,196 3 of 10 Figure 2 shows the microstructure for Mg-2.7Nd-0.4Zn-0.5Zr alloy in as-cast condition. The microstructure of as-cast specimen consists of α-Mg matrix and eutectic compounds.…”

Section: Microstructuresmentioning

confidence: 99%

“…The precipitation sequence in this alloy during isothermal aging is commonly accepted as: supersaturated solid solution (SSSS) → ordered Guinier-Preston zones (G.P. zones) → β" → β → β 1 → β [6][7][8]. Despite many works regarding the precipitates evolution have been achieved in the past few years.…”

Section: Introductionmentioning

confidence: 99%

Enhanced Age Strengthening of Mg-Nd-Zn-Zr Alloy via Pre-Stretching

Guo

Zhao

Wang

et al. 2016

Metals

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Pre-stretching was carried out to modify the microstructure of Mg-Nd-Zn-Zr alloy to enhance its age strengthening. The results indicated that more heterogeneous nucleation sites can be provided by the high density of dislocations caused by the plastic pre-stretching deformation, as well as speeding up the growth rate of precipitates. Comparison of microstructure in non-pre-stretched specimens after artificial aging showed that pre-stretched specimens exhibited a higher number density of precipitates. The fine and coarse plate-shaped precipitates were found in the matrix. Due to an increase in the number density of precipitates, the dislocation slipping during the deformation process is effectively hindered, and the matrix is strengthened. The yield strength stabilizes at 4% pre-stretching condition, and then the evolution is stable within the error bars. The 8% pre-stretched specimens can achieve an ultimate tensile strength of 297 MPa. However, further pre-stretching strains after 8% cannot supply any increase in strength. Tensile fracture surfaces of specimens subjected to pre-stretching strain mainly exhibit a trans-granular cleavage fracture. This work indicated that a small amount of pre-stretching strain can further increase strength of alloy and also effectively enhance the formation of precipitates, which can expand the application fields of this alloy.

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Comprehensive study of phase transformation in age-hardening of Mg–3Nd–0.2Zn by means of scanning transmission electron microscopy

Cited by 49 publications

References 31 publications

On the early stages of precipitation in dilute Mg–Nd alloys

On the early stages of precipitation in dilute Mg–Nd alloys

Precipitation evolution and hardening in Mg Sm Zn Zr alloys

Enhanced Age Strengthening of Mg-Nd-Zn-Zr Alloy via Pre-Stretching

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