- Email: [email protected]
Dislocation arrays, precipitate bands and free zones in forged Mg-Gd-Y-Zr alloy
Journal Pre-proof Dislocation arrays, precipitate bands and free zones in forged Mg-Gd-Y-Zr alloy Bizheng Wang, Bei Tang, Chao You, Yingchun Wan, Yonghao Gao, Zhiyong Chen, Liwei Lu, Jian Wang, Chuming Liu PII:
S0921-5093(19)31574-6
DOI:
https://doi.org/10.1016/j.msea.2019.138789
Reference:
MSA 138789
To appear in:
Materials Science & Engineering A
Received Date: 22 July 2019 Revised Date:
5 December 2019
Accepted Date: 7 December 2019
Please cite this article as: B. Wang, B. Tang, C. You, Y. Wan, Y. Gao, Z. Chen, L. Lu, J. Wang, C. Liu, Dislocation arrays, precipitate bands and free zones in forged Mg-Gd-Y-Zr alloy, Materials Science & Engineering A (2020), doi: https://doi.org/10.1016/j.msea.2019.138789. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Dislocation arrays, precipitate bands and free zones in forged Mg-Gd-Y-Zr alloy
Bizheng Wanga, Bei Tanga, Chao Youa, Yingchun Wana,b,d∗, Yonghao Gaoa, Zhiyong Chena, Liwei Luc, Jian Wangd, Chuming Liua,c
a
School of Materials Science and Engineering, Central South University, Changsha 410083, China
b
School of Metallurgy and Environment, Central South University, Changsha 410083, China
c
College of Materials Science and Engineering, Hunan University of Science and Technology, Xiangtan,
411201, China d
Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588, USA
Abstract Dislocation arrays are extensively generated in the hammer forged and then annealed Mg-8.3 Gd-2.6 Y-0.4 Zr (wt. %) alloy. Microscopy characterizations reveal a high thermal stability of these dislocation arrays. During ageing treatment, dislocation arrays act as preferred sites for precipitation of
phase, promoting the formation of the bamboo-like precipitate bands, and
resulting in a depletion of solutes nearby and the formation of precipitate free zones (PFZs) along these bands. The forged samples after annealing and then ageing at an optimized temperature of 265 oC for different periods exhibit a similar strength and strain hardening but an increased elongation with ageing time, suggesting that the precipitate bands act as strong barriers for the motion of dislocations and twins while PFZs can effectively enhance deformability of Mg-Gd-Y-Zr alloy. Keywords: Mg-Gd-Y-Zr alloy, dislocation array, precipitate band, PFZ, mechanical property
∗
Corresponding Author: Yingchun Wan, Email: [email protected] 1
1 Introduction Magnesium alloys are promising lightweight structural materials owing to the merits of low density, high specific strength, ideal recyclability, etc. (1, 2) Mg-RE alloys (RE: rare earth) are typical ageing-hardening alloys because of the formation of precipitates during ageing process, for example, plate-shaped precipitates of basal plane
(7)
and
phases on prismatic planes (3-6) and γ
phase on
. It is well recognized that mechanical properties of precipitation-strengthening
alloys can be finely tuned through tailoring the morphology, characteristic dimension, distribution, and density of precipitates(8, 9). Precipitation behavior is a thermally activated process, and sometimes involves chemical reaction. Precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, and the temperature is high enough that diffusion can lead to segregation into precipitates. Ageing temperature is thus a dominant factor, modifying the dimension and distribution of precipitates because of the temperature dependence of phases and solute diffusion. For example, the increase of ageing temperature from 200 oC to 250 oC can result in the formation of coarser and sparser particles in peak-aged Mg-Gd-Y-Zr alloys because of the accelerated diffusion of Gd and Y elements (10). Nucleation, the first stage of the precipitation process, involves the creation of an interface between particle and matrix. Crystallographic defects are generally preferred sites for nucleating precipitates because of lower formation energy of solutes in these defects than that in perfect crystal, and higher diffusivity of solutes within/along these defects (11-16). For example, 1-nm Gd nano-fibers, with a 〈c〉-rod shape, are formed and hexagonally patterned in association with Gd segregations along dislocations that nucleated during hot extrusion
(12)
. Such self-assembled
patterns of Gd nano-fibers in Mg-Gd alloys act as the predictable inhibitor for basal slips because the glide of basal dislocations must cut them. Recently, ultra-dense and ultra-uniform precipitates with 1~2 nanometer sizes can be introduced in Al alloys during cyclic loading at room temperature, as dislocations repeatedly shear the solute segregation back and forth (16). Nie et al. (17) suggested to simultaneously increase the density and the aspect ratio of precipitates by introducing dislocations into the matrix prior to ageing. Dislocations can affect the precipitation behavior by offering an 2
additional driving force. As reported in WE54 alloy, dislocations introduced by cold work promote precipitation of
phase with larger diameter and number density, leading to a higher hardness.
In addition, the distribution of dislocations in many Mg-RE alloys promotes the heterogeneous nucleation of precipitates
(18-20)
, and changes the spatial arrangement of precipitates in local
regions of the matrix. It can also be of significance to precipitation strengthening, particularly for precipitates with special arrangements
(17)
. The formation of such well-arranged or patterned
precipitates relies on the initial structure of pre-introduced dislocations. However, it remains challenge for effectively regulating dislocations with desired component into proper structure via thermomechanical processes in magnesium alloys. In this work, we observed the formation of dislocation arrays in the hammer forged Mg-Gd-Y-Zr alloy, and characterized the formation of precipitates around dislocation arrays during ageing treatment. The characteristics and formation mechanism of such arrays, as well as their effect on ageing behavior and mechanical properties, are systematically investigated. The results may provide a guidance to produce a microstructure with patterned precipitates in other Mg-RE alloys. 2 Experimental procedure Mg-8.3Gd-2.6Y-0.4Zr (wt. %) ingots were prepared by melting intermetallic alloys (Mg-30%Gd, Mg-30%Y alloy), pure Zr, pure Mg in a mild steel crucible, and casting in an upright semi-continuous machine, and then being homogenized at 450 oC for 5 h and at 540 oC for 15 h. After preheated at 420 ℃ for 3 h, the ingot was subjected to unidirectional hammer forging at a strain rate of ∽ 10 s-1 and a total strain of ∽ 15 %, and then quenched in cold water. Slices that were cut from the central part of the ingot were annealed at 375 oC for 0 to 120 min in a heating furnace with a temperature fluctuation of ±1 oC. They were then ground with 400-1600 grit SiC papers and etched with citric acid for characterization using optical microscopy (OM) and scanning electron microscopy (SEM). OM and SEM characterizations were carried out on the Leica and FEI-PHENOM Pro equipment, respectively. The observing plane was parallel to the
3
forging direction. Discs for transmission electron microscopy (TEM) were also prepared through grinding, twin-jet electropolishing and ion thinning. TEM observation was performed on the FEI-TECNAI G2 F20 operated at 200 kV. Room temperature (RT) tensile tests were conducted on the Instron 3369 machine with a constant cross head speed of 1 mm/s. Cylindrical tensile specimens with a gauge dimension of Φ 4 mm * 20 mm were prepared from the central part of the ingot with tensile axis parallel to the forging direction. Elongation was obtained by calculating the ratio between the increment and initial length of the gauge. For each state, three samples were tested and their average value was displayed here.
Fig. 1 The microstructure of the sample hammer forged and then annealed by 375 ℃ / 15 min. (a) and (b) OM images, (c) SEM image, (d) and (e) TEM images.
3 Results 3.1 Dislocation arrays The microstructure of the sample annealed at 375 ℃ for 15 min is presented in Fig. 1. The OM image in Fig. 1 a shows a heterogeneous microstructure consisting of coarse grains (grain size = ∽ 100 µm) with gray contrast and few dynamic recrystallized grains (DRXed) (grain size = ∽ 10 µm) with white contrast. The magnified OM image in Fig. 1 b explores the densely distributed lines inside grains with the gray contrast. SEM image in Fig. 1 c reveals that these lines are parallel to
4
each other. TEM observation is further performed, and confirms that these lines found in OM and SEM images are river-like dislocation arrays (Fig. 1 d and 1 e). The Burgers vectors of these dislocations are identified in Fig. 2. First, two dislocation arrays were observed under double-beam operation with g
1010 in Fig. 2 a. The scattered
dislocations, serving as a fiduciary mark, are encircled by the red ellipse. When the double-beam operation with g
0002 is conducted, dislocations in the red ellipse are still visible, but both
of the two arrays become invisible. Hence, dislocations in the two arrays have the same Burgers vector based on the dislocation invisible criterion (21). The same results are obtained for dislocation arrays investigated in different grains.
Fig. 2 Identification of Burgers vectors for the dislocation arrays in Fig.1. (a) double-beam operation with
1010 , (b) double-beam operation with
0002 .
The formation of dislocation arrays is observed in the first 10 minutes of annealing, as shown in Fig. 3. The TEM image of the as-forged sample (Fig. 3 a) shows a high density of dislocations, but no apparent patterns. When the sample is annealed at 375 ℃ for 5 mins, a vast of dislocation cells appear in the TEM images (Fig. 3 c), and connect with each other (Fig. 3 c - f). The diameter of these cells ranges from 0.3 to 2.1 µm. High-amplified TEM image (Fig. 3 d) reveals that the cell walls consist of dislocations with a dense and ordered arrangement or pattern. Obviously, this is a typical static recovery process that deformation-induced dislocations rearrange themselves into ordered cells. Such process is generally associated with dislocation climbing during the annealing treatment (22). With longer annealing time of 10 min, dislocation arrays begin to appear. As shown 5
in Fig. 3 f, two segments of dislocation arrays are formed through annihilation of the shared walls of three cells. Normally, these cells will transform into new grains with low or high angle boundaries. However, a generally stable boundary requires three sets of dislocations with three non-coplanar Burgers vectors that could eliminate the long range stress field
(23-25)
. But the
as-forged microstructure, which is produced at a moderate temperature of 400-420 ℃ and a low strain, is unable to provide enough non-basal dislocations (26). As a result, the cells will turn into other forms, the dislocation arrays seem to be one of new forms. On one hand, dislocations arrays present a much stable configuration than the cells since they may greatly diminish the long range stress field, and each dislocation in the array can be firmly fixed by the strong attraction force from the others (27). On the other hand, through the climb of dislocations on the shared walls, the arrays can be easily and rapidly transformed from the interconnected cells. Hence, an irreversible transformation from the cells into dislocation arrays occurs with prolonged annealing time.
Table 1 Array width (D1) and array distance (D2) in the alloy annealed for 15 - 120 min at 375 ℃
375 ℃ / 15 min
375 ℃ / 30 min
375 ℃ / 60 min
375 ℃ / 120 min
Average value (µm)
0.17
0.20
0.20
0.16
Standard deviation
12 %
16 %
10 %
13 %
Average value (µm)
1.02
0.90
1.30
1.13
Standard deviation
45 %
71 %
55 %
50 %
Dimension D1
D2
The microstructure evolution in the 30 ~ 120 min annealing duration is shown in Fig. 4. The dimension and distribution of the arrays, obtained from 60 arrays in 15 grains based on the TEM observation, are summarized in Table 1. The array width D1 is defined to be the distance between the two boundaries of an array and the matrix as indicated by the red arrows in Fig. 4 a. The array spacing D2 is defined to be the distance between the adjacent arrays, as presented in Fig. 4 c. The average values of D1 and D2 do not vary obviously during 15 ~ 60 min. However, with the annealing time prolonged to 120 min, much less arrays were observed (as shown in Fig. 4 e and
6
4 f). This morphology variation is obtained through this process: the dislocations on parallel slipping planes re-orientated themselves or part of their lengths parallel with each other by climbing (diffusion) as the function of the elastic stress imposed on each other (28).This energetic procedure is immediately completed at the very initial of annealing.
Fig. 3 The microstructure evolution during the initial annealing period: (a) (b) the as-forged state (c) (d) 375 oC / 5 min (e)(f) 375 oC / 10 min.
7
Fig. 4 TEM (left) and SEM (right) images for the samples annealed at 375 ℃ with prolonged time (a) (b) 30 min, (c) (d) 60 min, (e) (f) 120 min.
3.2 The effect of dislocation arrays on ageing behavior and mechanical performance Fig. 5 a exhibits the engineering strain-stress curves for as-forged or as-annealed samples. The corresponding mechanical properties are summarized in Table 2 and Table 3. With regard to the as-forged samples, it has a strength of 223 MPa in yield stress (YS), 280 MPa in ultimate stress (UTS) and an elongation of 3.8 %. Ageing treatment largely enhances its strength but severely deteriorates the elongation, a highest strength of 310 MPa in YS, 369 MPa in UTS but an 8
elongation as low as 1.2 % for the sample peak-aged at 200 oC. When ageing temperature increases, the strength gradually decreases, the elongation, however, has a slight increase but lower than the initial state. As for the as-annealed sample, it has a slightly lower strength but higher elongation than the as-forged one. Alloy aged at 200 ~ 250 oC results in a very similar mechanical performance to that of the as-forged one regardless of the dislocation arrays. However, the sample annealed and peak-aged at 265 oC exhibits a simultaneous enhancement of strength and elongation, i.e., an increment of 53 MPa in YS, 52 MPa in UTS and 3.8 % in elongation compared to the as-annealed state.
Table 2. Corresponding mechanical properties for Fig.5 a State
UTS / MPa
YS / MPa
Elongation / %
As-forged
280
223
3.8
As-annealed (385 oC / 30 min)
273
215
4.1
200 oC / 72 h (forged)
369 (+ 89)
310 (+ 87)
1.2 (- 2.6)
200 oC / 66 h (annealed)
358 (+ 85)
304 (+ 89)
1.3 (- 2.8)
225 oC / 24 h (forged)
357 (+ 77)
295 (+ 72)
1.6 (- 2.2)
225 oC / 16 h (annealed)
352 (+ 79)
284 (+ 69)
1.6 (- 2.5)
336 (+ 56)
277 (+ 54)
3.5 (- 0.3)
330 (+ 57)
271 (+ 56)
3.7 (- 0.4)
327 (+ 47)
267 (+ 44)
3.6 (- 0.2)
325 (+ 52)
268 (+ 53)
7.9 (+ 3.8)
o
250 C / 14 h (forged) o
250 C / 8 h (annealed) o
265 C / 8 h (forged) o
265 C / 4 h (annealed)
Considering the remarkable mechanical properties of the sample annealed and then peak-aged at 265 oC, we conduct more mechanical tests and microstructure characterization of the 265 oC aged samples. Fig. 5 b shows the stress-strain curves of the 265 oC-aged samples at different aging times. It can be seen that a simultaneous enhancement of strength and elongation takes place at the initial ageing period of 1 h. With prolonged ageing time to 6 h, the strength and elongation continuously increase. The over-aged sample (265 oC / 6 h) has a strength of 273 MPa
9
in YS and 334 MPa in UTS, larger than those of the samples annealed and then peak-aged at either 265 oC or 250 oC. Meanwhile, its elongation reaches a value as high as 8.0 %, which is almost twice of that in the initial state. With further increased ageing time till 16 h, there is a slight decrease in strength but a slight increase in elongation.
Fig.5 Engineering stress-strain curves for (a) the initial and peak-ageing conditions, (b) ageing treatment at 265 ℃ for the annealed sample.
Table 3. Mechanical properties for the annealed sample aged at 265 oC
State
UTS / MPa
YS / MPa
Elongation / %
273
215
4.1
265 ℃/ 1 h
306 (+ 33)
246 (+ 29)
5.0 (+ 0.9)
265 ℃/ 2 h
311 (+ 38)
254 (+ 39)
5.8 (+ 1.7)
265 ℃/ 3 h
320 (+ 47)
256 (+ 41)
6.6 (+ 2.5)
265 ℃/ 4 h
325 (+ 52)
268 (+ 53)
7.9 (+ 3.8)
265 ℃/ 5 h
330 (+ 57)
266 (+ 51)
7.9 (+3.8)
265 ℃/ 6 h
334 (+ 61)
273 (+ 58)
8.0 (+ 3.9)
265 ℃/ 8 h
330 (+ 57)
270 (+ 55)
8.2 (+ 4.1)
265 ℃/ 16 h
326 (+ 53)
264 (+ 49)
8.6 (+ 4.5)
As-annealed (385 ℃ / 30 min)
10
Fig. 6 The microstructure of the annealed sample (375 oC / 30 min) aged by 265 oC / 15 min. (a) the low magnification bright field image (b) the HADDF-STEM mode of encircled area in Fig. 6 a, (c) the bright field image and (d) high resolution imaging and FFT of particles encircled in Fig. 6 c.
The microstructure of the annealed samples (375 oC / 30 min) and then aged at 265 oC is exhibited in Fig. 6 - 8. At the very beginning of 15 min, small particles largely appear within the matrix (Fig.6a). The image detected in the HADDF-STEM mode with higher magnification (Fig. 6 b) reveals that particles in the α-Mg matrix are mostly spheroidal with a diameter of ∽ 12 nm while those within dislocation arrays are basically needle-like with 14 - 30 nm in length and ∽ 5 nm in width. Meanwhile, it is noted that the needle-like particles are parallel with each other on the same dislocation array. Undoubtedly, these particles are all the precipitates formed during ageing treatment and the bright contrast implies their richness in heavy RE atoms of Gd and/or Y. In fact, these spheroidal particles can be inferred as β’’ phase based on their shape and size
(29)
.
But it is tough to estimate the type of the needle-like ones only depending on their morphology. In view of this, particles around a single dislocation (Fig. 6 c) are further characterized by the high resolution imaging and fast Fourier transformation (FFT) (Fig. 6 d). The same FFT patterns identify that particles with either spheroidal or needle-like shape are β’’ phases (D019 structure 11
(30)
). The parallel longitudinal surfaces of the needle-like particles are proved to be anastomotic
with the α / β’’ plane by the FFT result. The precipitation sequence of Mg-Gd-Y-Zr alloy is reported to be S. S. S. S → β’’ (D0 19)→ β’ (c b c o)→ β1 (f c c)→ β (f c c) (29). A possible continuous transformation from spheroidal
particle of β’’ into convex-lens like β’ is proposed by Honma et al
(31)
. In this work, the
needle-like particle is believed to act as a transition phase for the continuous transformation since it has a crystal structure like that of β’’ but a morphology and growth plane more similar to that of β’ phase (29). Being a kind of line lattice defect, the dislocations, especially their arrays, tend to
facilitate such transformation by providing more channels and extra stress field to accelerate the diffusion of solute atoms
(32)
. The needle-like particles are thus mostly located on them rather
than in the α-Mg matrix. In addition, the stress field of the dislocations will strongly interact with the precipitates and deeply influence their morphology and distribution. As reported in (18), due to the existence of stress filed, the mixed a-type dislocations often result in precipitate arrays comprising of particles with the same orientation variant. Similar result is also observed in the crept Mg-Gd-Zr alloy
(19)
. Hence, it is reasonably inferred that the parallel arrangement of the
needle-like particles is a result of the stress filed of the a-type dislocation arrays. The microstructure of the samples aged at 265 oC with longer time is presented in Fig. 7 and Fig. 8. The observation direction is parallel with < 0001 > for those aged from 1 h to 6 h and <2111> for that aged by 16 h. After ageing for 1h (Fig. 7), a lot of coarse and river-like precipitate bands appear within the grains. These bands have a quite similar running direction, length and adjacent distance to that of the dislocation arrays, implying that such bands are formed on the positions where dislocation arrays are located (Fig. 7 a). Certainly, there also exist some finer and well-ordered precipitates between these bands. Magnified TEM image (Fig. 7 c) reveals that the precipitate bands are composed of a lot of bamboo-like strips while the fine and well-ordered precipitates are bamboo-leaf-like and ladder-shape arranged. In addition, some much finer bamboo-leaf-like precipitates with 120o symmetrical distribution are also found within the matrix.
12
Fig. 7 The microstructure of the annealed sample (375 oC / 30 min) aged by 265 oC / 1 h, (a) Low magnification TEM image (b) the corresponding SAED pattern for Fig. 7 a, (c) (d) high magnification TEM images.
The petaloid deputy spots in the SAED pattern (Fig. 7 b) prove them to be the same phase regardless of their different morphology and distribution (30). With higher magnification (Fig. 8 d), it is known that each bamboo-like strip of the precipitate band (white arrow) is composed of two or more particles with the same orientation. These particles connect with each other through overlapping growth within one strip and through the bridge phase of β
(33)
between the adjacent
ones, forming a very dense precipitation structure. The ladder-shape arranged precipitates (blue arrow) are also interconnected through the linear bridge phase. It is well known that the homogeneous precipitation with 120o symmetrical distribution is the most common morphology of
phase in Mg-Gd-Y-Zr alloy(29, 30, 34, 35). However, dislocation arrays change the spatial
distribution of
phase. As mentioned above, the needle-like transition particles on the
dislocations have a parallel arrangement with each other and eventually transform into precipitates of
phase without changing their orientation, generating the precipitate bands on the arrays and
13
ladder-shape arranged particles on the single dislocation, respectively. In addition, due to the accelerating effect of the arrays on solute diffusion, solute atoms tend to strongly segregate to precipitate bands, resulting in a depletion of solutes nearby and formation of precipitate free zones (PFZs) along these bands (36). The precipitate bands are not homogeneously distributed within the grains. Under the same magnification, there may appear either only one or more than two precipitate bands in the view and this will actually result in different microstructure evolution with prolonged ageing time (Fig. 8). It should be recognized that most of the precipitate bands (yellow arrows), especially the PFZs along them, are narrow when aged for 1 h (Fig. 8 a and 8 e). As clarified above, the rest of the matrix is mostly occupied by the bamboo-leaf-like particles with ladder-shape arrangement (blue arrows). After ageing for 3 h (Fig. 8 b and 8 f), both the precipitate bands and PFZs become obviously wider. In the region where only one band exists, a width of ∽ 220 nm is obtained for the PFZ and the rest of the matrix has been largely taken up by the homogeneous precipitation with a dense and symmetrical distribution, as well as with a size of ∽ 90 nm in length and ∽ 18 nm in width for one precipitate particle. As for the region where more precipitate bands appear, it is interesting to find that particles between the bands have vanished a lot. The remaining ones, as encircled by the red ellipses, seem to stay away from these bands. Continued ageing till 6 h leads to a further size increase for the homogeneous precipitation (∽ 120 nm in length and ∽ 25 nm in width) and a slight width increase for the precipitate bands and PFZs (∽ 250 nm) (Fig.8c). In the region where more precipitate bands appear, it is noted that the remaining particles between the bands have almost disappeared (Fig. 8 g). After that, the ageing till 16 h only results in a size growth for the homogeneous precipitation. No remarkable change occurs for the size of precipitate bands. Based on the SAED patterns inserted, these precipitates are transformation from
to
phase till 16 h and since then the in situ
takes place (Fig. 8 h, red arrows)
(29)
. Such results imply that a
strong competition exists between the homogeneous and dislocation array induced precipitation. Considering the mutually reinforced stress filed for the array, the latter one has an obvious advantage, which contributes to the persistent width increase of the precipitate bands and PFZs at
14
the expense of redissolution of the homogeneous precipitation near them. When the PFZs reach a certain width or the homogeneous precipitates almost disappear, the precipitate bands will then keep unchanged for a long time without the supply of solute atoms. As for the homogeneous precipitation away from these bands, they will experience a continuous growth in size but a decrease in number density with prolonged ageing time just as reported elsewhere (37).
Fig. 8 The microstructure for the annealed sample (375 oC / 30 min) aged at 265 oC with prolonged time.
4 Discussion The effect of precipitates on dislocation gliding is shown in Fig. 9. It is observed that dislocations are strictly restricted in the regions separated by the precipitate bands (indicated by the red arrows) and largely piled up at the vicinity of these bands. This implies that the precipitate bands have a strong impediment on dislocation gliding. In addition, such impediment also happens in the region where homogeneous precipitation exists (Fig. 9 b). Fig. 10 shows that the deformation twins can propagate in the homogeneously precipitated regions but encounter tough obstacles of the precipitate bands. These twins are strictly hindered by the bands (Fig. 9 a and 9 b). These bands play a role quite similar to the grain boundary strengthening. In fact, the dislocation arrays themselves are exactly a certain kind of sub-grain boundaries
(38)
, separating the coarse
grains into much smaller parts. The precipitate bands formed on the arrays with similarly orientated and closely connected particles of
phases could extremely strengthen such
characteristic of being interfaces. 15
Fig. 9 TEM images: Interaction between the dislocations and precipitates in the sample (265 oC / 6 h) tensile to fracture.
Fig. 10 TEM images: interaction between the deformation twins and precipitates in the sample (265 oC / 6 h) tensile to fracture.
Due to the unchanged spacing between precipitate bands, the strength of the aged samples is almost retained. However, the widened PFZs enhance the ability of dislocation slips, increasing the elongation of the samples(7). It is believed that in the region where the PFZ is formed, the matrix is greatly softened due to the large absence of precipitates and solute atoms. In the regions where more precipitate bands appear, the homogeneous precipitates largely disappear due to the 16
existence of PFZs, which will undoubtedly further increase the ductility. Such strengthening and toughening mechanisms are apparently correlated to ageing temperature and time. For the sample over-aged by 265 ℃ / 6 h, the strength increment is mainly attributed to the precipitate bands and the elongation enhancement can be ascribed to the formation of PFZs along these bands. At the initial ageing period till 6 h, a continuous and simultaneous increment of strength and elongation is realized, which should be a result of the persistently widening of such bands as well as the PFZs. The proposed mechanisms are confirmed by the fact that the strength is unchanged but the elongation increases with ageing time at aging temperature of 265 ℃. Solute diffusion is more strongly dependent on aging temperature than aging time (39, 40). Fig. 11 exhibits the microstructure of the sample peak-aged at 200 ℃, fine and dense distribution of homogeneous precipitation are observed, but precipitate bands are very narrow. More significantly, apparent PFZs along precipitate bands don’t form. Hence, the material is still mainly strengthened by the homogeneous precipitation, resulting in their similar mechanical properties to the corresponding states of the as-forged sample.
Fig.11 TEM images for the annealed sample (375 oC / 30 min) peak-aged at 200 oC.
5 Conclusion We demonstrated the formation of dislocation arrays in Mg-8.3 Gd-2.6 Y-0.4 Zr alloy processed by hammer forging and subsequent annealing treatment. During aging, these dislocation arrays act as preferred sites for precipitation of
phase, promoting the formation of the bamboo
precipitate bands with only one variant orientation, and resulting in a depletion of solutes nearby 17
and the formation of precipitate free zones (PFZs) along these bands. The dislocation arrays are characterized with parallel arranged type dislocations, which are extensively generated in Mg-8.3 Gd-2.6 Y-0.4 Zr alloy through the recovery process during 375 oC annealing treatment. Precipitates with
phase preferentially grow on the dislocation arrays with obviously larger size
and only one variant orientation, forming the bamboo-like bands and promoting the formation of wide PFZs along them. An optimized aging temperature exists, corresponding to the formation of precipitate bands and PFZs. The forged samples after annealed and then aged at 265 oC for different periods exhibit a similar strength and strain hardening but an increased elongation with ageing time. The best combination of strength and ductility are 273 MPa in YS, 334 MPa in UTS and 8.0 % in Elongation. The results suggest that the precipitate bands strongly impede the motion of dislocation and twinning while PFZs can effectively enhance deformability of Mg-Gd-Y-Zr alloy. Acknowledgement This work is supported by National Natural Science Foundation of China (Grant number 51574291 and 51874367), China Postdoctoral Science Special Foundation [Grant number 2019T120712], China Postdoctoral Science Foundation [Grant number 2018M642999], State Key Laboratory for Powder Metallurgy of Central South University, China. Natural Science Foundation of Hunan Province [Grant number 2019JJ50787].
References 1.
E. Aghion, B. Bronfin, in Materials Science Forum. (Trans Tech Publ, 2000), vol. 350, pp. 19-30.
2.
B. Mordike, T. Ebert, Magnesium: properties—applications—potential. Materials Science and Engineering: A 302, 37-45 (2001).
3.
P. Apps, H. Karimzadeh, J. King, G. Lorimer, Precipitation reactions in magnesium-rare earth alloys
4.
S. M. He, X. Q. Zeng, L. Peng, X. Gao, J. F. Nie, W. J. Ding, Microstructure and strengthening mechanism of
containing yttrium, gadolinium or dysprosium. Scripta Materialia 48, 1023-1028 (2003). high strength Mg–10Gd–2Y–0.5 Zr alloy. Journal of Alloys and Compounds 427, 316-323 (2007). 5.
J. F. Nie, B. C. Muddle, Precipitation in magnesium alloy WE54 during isothermal ageing at 250 C. Scripta Materialia 40, 1089-1094 (1999).
18
6.
J. F. Nie, B. C. Muddle, Characterisation of strengthening precipitate phases in a Mg–Y–Nd alloy. Acta Materialia 48, 1691-1703 (2000).
7.
Y. Zhang, W. Rong, Y. Wu, L. Peng, J.-F. Nie, N. Birbilis, A comparative study of the role of Ag in microstructures and mechanical properties of Mg-Gd and Mg-Y alloys. Materials Science and Engineering: A 731, 609-622 (2018); published online Epub2018/07/25/ (https://doi.org/10.1016/j.msea.2018.06.084).
8.
J. F. Nie, Effects of precipitate shape and orientation on dispersion strengthening in magnesium alloys. Scripta Materialia 48, 1009-1015 (2003).
9.
Y.-H. Kang, H. Yan, R.-S. Chen, Effects of heat treatment on the precipitates and mechanical properties of sand-cast Mg–4Y–2.3Nd–1Gd–0.6Zr magnesium alloy. Materials Science and Engineering: A 645, 361-368 (2015); published online Epub2015/10/01/ (https://doi.org/10.1016/j.msea.2015.08.041).
10.
S. Liang, D. Guan, X. Tan, L. Chen, Y. Tang, Effect of isothermal aging on the microstructure and properties of as-cast Mg–Gd–Y–Zr alloy. Materials Science and Engineering: A 528, 1589-1595 (2011); published online Epub2011/01/25/ (https://doi.org/10.1016/j.msea.2010.10.082).
11.
R. G. Li, H. B. Shafqat, J. H. Zhang, R. Z. Wu, G. Y. Fu, L. Zong, Y. Su, Cold-working mediated converse age hardening responses in extruded Mg-14Gd-2Ag-0.5Zr alloy with different microstructure. Materials Science and Engineering: A 748, 95-99 (2019); published online Epub2019/03/04/ (https://doi.org/10.1016/j.msea.2019.01.082).
12.
Y. Li, J. Wang, K. Chen, M. Shao, Y. Shen, L. Jin, G.-z. Zhu, Self-patterning Gd nano-fibers in Mg-Gd alloys. Scientific reports 6, 38537 (2016).
13.
W. Sun, Y. Zhu, R. Marceau, L. Wang, Q. Zhang, X. Gao, C. Hutchinson, Precipitation strengthening of
14.
H. Xiao, B. Tang, C. Liu, Y. Gao, S. Yu, S. Jiang, Dynamic precipitation in a Mg–Gd–Y–Zr alloy during hot
aluminum alloys by room-temperature cyclic plasticity. Science 363, 972-975 (2019). compression. Materials Science and Engineering: A 645, 241-247 (2015); published online Epub2015/10/01/ (https://doi.org/10.1016/j.msea.2015.08.022). 15.
S. Yu, Y. Wan, C. Liu, J. Wang, Age-hardening and age-softening in nanocrystalline Mg-Gd-Y-Zr alloy. Materials Characterization 156, 109841 (2019); published online Epub2019/10/01/ (https://doi.org/10.1016/j.matchar.2019.109841).
16.
A. W. Zhu, E. A. Starke, Stress aging of Al–xCu alloys: experiments. Acta Materialia 49, 2285-2295 (2001); published online Epub2001/07/17/ (https://doi.org/10.1016/S1359-6454(01)00119-7).
17.
J.-F. Nie, Precipitation and Hardening in Magnesium Alloys. Metallurgical and Materials Transactions A 43,
18.
H. Liu, Y. Gao, Y. M. Zhu, Y. Wang, J. F. Nie, A simulation study of β1 precipitation on dislocations in an
3891-3939 (2012); published online Epub2012/11/01 (10.1007/s11661-012-1217-2). Mg–rare earth alloy. Acta Materialia 77, 133-150 (2014); published online Epub2014/09/15/ (https://doi.org/10.1016/j.actamat.2014.04.054). 19.
H. Liu, W. F. Xu, L. M. Peng, W. J. Ding, J. F. Nie, A simulation study of the distribution of β′ precipitates in a crept Mg-Gd-Zr alloy. Computational Materials Science 130, 152-164 (2017); published online Epub2017/04/01/ (https://doi.org/10.1016/j.commatsci.2016.12.021).
20.
G. Shi, K. Zhang, X. Li, Y. Li, M. Ma, J. Yuan, Precipitation behaviors, texture and tensile properties of an extruded Mg-7Y-1Nd-0.5Zr (wt%) alloy bar with large cross-section. Materials Science and Engineering: A 685, 300-309 (2017); published online Epub2017/02/08/ (https://doi.org/10.1016/j.msea.2016.12.076).
19
21.
S. Agnew, J. Horton, M. Yoo, Transmission electron microscopy investigation of< c+a> dislocations in Mg and α-solid solution Mg-Li alloys. Metallurgical and Materials Transactions A 33, 851-858 (2002).
22.
G. Falkinger, P. Simon, Static recovery of an AlMg4.5Mn aluminium alloy during multi-pass hot-rolling. Procedia Engineering 207, 31-36 (2017); published online Epub2017/01/01/ (https://doi.org/10.1016/j.proeng.2017.10.733).
23.
C. Hong, X. Huang, G. Winther, Dislocation content of geometrically necessary boundaries aligned with slip planes in rolled aluminium. Philosophical Magazine 93, 3118-3141 (2013).
24.
Y. L. Wei, A. Godfrey, W. Liu, Q. Liu, X. Huang, N. Hansen, G. Winther, Dislocations, boundaries and slip systems in cube grains of rolled aluminium. Scripta Materialia 65, 355-358 (2011); published online Epub2011/08/01/ (https://doi.org/10.1016/j.scriptamat.2011.05.005).
25.
G. Winther, J. P. Wright, S. Schmidt, J. Oddershede, Grain interaction mechanisms leading to intragranular orientation spread in tensile deformed bulk grains of interstitial-free steel. International Journal of Plasticity 88, 108-125 (2017); published online Epub2017/01/01/ (https://doi.org/10.1016/j.ijplas.2016.10.004).
26.
H. C. Xiao, S. N. Jiang, B. Tang, W. H. Hao, Y. H. Gao, Z. Y. Chen, C. M. Liu, Hot deformation and dynamic recrystallization behaviors of Mg–Gd–Y–Zr alloy. Materials Science and Engineering: A 628, 311-318 (2015); published online Epub2015/03/25/ (https://doi.org/10.1016/j.msea.2015.01.041).
27.
Y. Chou, Interaction of parallel dislocations in a hexagonal crystal. Journal of Applied Physics 33, 2747-2751 (1962).
28.
D. V. Bachurin, A. A. Nazarov, J. Weissmüller, Grain rotation by dislocation climb in a finite-size grain boundary. Acta Materialia 60, 7064-7077 (2012); published online Epub2012/12/01/ (https://doi.org/10.1016/j.actamat.2012.09.014).
29.
S. M. He, X. Q. Zeng, L. M. Peng, X. Gao, J. F. Nie, W. J. Ding, Precipitation in a Mg–10Gd–3Y–0.4Zr (wt.%) alloy during isothermal ageing at 250°C. Journal of Alloys and Compounds 421, 309-313 (2006); published online Epub2006/09/14/ (https://doi.org/10.1016/j.jallcom.2005.11.046).
30.
S. M. He, X. Q. Zeng, L. M. Peng, X. Gao, J. F. Nie, W. J. Ding, Microstructure and strengthening mechanism of high strength Mg–10Gd–2Y–0.5Zr alloy. Journal of Alloys and Compounds 427, 316-323 (2007); published online Epub2007/01/16/ (https://doi.org/10.1016/j.jallcom.2006.03.015).
31.
T. Honma, T. Ohkubo, K. Hono, S. Kamado, Chemistry of nanoscale precipitates in Mg–2.1 Gd–0.6 Y–0.2 Zr (at.%) alloy investigated by the atom probe technique. Materials Science and Engineering: A 395, 301-306 (2005).
32.
J. Čížek, I. Procházka, B. Smola, I. Stulíková, V. Očenášek, Influence of deformation on precipitation process in Mg–15wt.%Gd alloy. Journal of Alloys and Compounds 430, 92-96 (2007); published online Epub2007/03/14/ (https://doi.org/10.1016/j.jallcom.2006.03.097).
33.
H. Xie, H. Pan, Y. Ren, S. Sun, L. Wang, Y. He, G. Qin, Co-existences of the two types of β′ precipitations in
34.
Y. C. Wan, H. C. Xiao, S. N. Jiang, B. Tang, C. M. Liu, Z. Y. Chen, L. W. Lu, Microstructure and mechanical
peak-aged Mg-Gd binary alloy. Journal of Alloys and Compounds 738, 32-36 (2018). properties of semi-continuous cast Mg–Gd–Y–Zr alloy. Materials Science and Engineering: A 617, 243-248 (2014); published online Epub2014/11/03/ (https://doi.org/10.1016/j.msea.2014.07.042).
20
35.
S. Yu, C. Liu, Y. Gao, S. Jiang, Y. Yao, Microstructure, texture and mechanical properties of Mg-Gd-Y-Zr alloy annular forging processed by hot ring rolling. Materials Science and Engineering: A 689, 40-47 (2017); published online Epub2017/03/24/ (https://doi.org/10.1016/j.msea.2017.02.036).
36.
A. Zindal, J. Jain, R. Prasad, S. S. Singh, R. Sarvesha, P. Cizek, M. R. Barnett, Effect of heat treatment variables on the formation of precipitate free zones (PFZs) in Mg-8Al-0.5Zn alloy. Materials Characterization 136, 175-182 (2018); published online Epub2018/02/01/ (https://doi.org/10.1016/j.matchar.2017.12.018).
37.
W. Dongshu, L. Dejiang, X. Yancai, Z. Xiaoqin, HRTEM studies of aging precipitate phases in the Mg-10Gd-3Y-0.4 Zr alloy. Journal of Rare Earths 34, 441-446 (2016).
38.
D. Hull, D. J. Bacon, in Introduction to Dislocations (Fifth Edition), D. Hull, D. J. Bacon, Eds.
39.
S. K. Das, Y.-B. Kang, T. Ha, I.-H. Jung, Thermodynamic modeling and diffusion kinetic experiments of
(Butterworth-Heinemann, Oxford, 2011), pp. 171-204. binary Mg–Gd and Mg–Y systems. Acta Materialia 71, 164-175 (2014); published online Epub2014/06/01/ (https://doi.org/10.1016/j.actamat.2014.02.029). 40.
I. Stulikova, B. Smola, J. Cizek, T. Kekule, O. Melikhova, H. Kudrnova, Natural and artificial aging in Mg-Gd binary alloys. Journal of Alloys and Compounds 738, 173-181 (2018); published online Epub2018/03/25/ (https://doi.org/10.1016/j.jallcom.2017.12.026).
21
Bizheng Wang: Investigation; Roles/Writing - original draft; Bei Tang: Investigation; Methodology; Chao You: Investigation; Yingchun Wan: Investigation; Writing - review & editing; Yonghao Gao: Funding acquisition; Zhiyong Chen: Funding acquisition; Liwei Lu: Writing - review & editing; Jian Wang： ：Writing - review & editing; Chuming Liu： ：Funding acquisition;
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0921-5093(19)31574-6
DOI:
https://doi.org/10.1016/j.msea.2019.138789
Reference:
MSA 138789
To appear in:
Materials Science & Engineering A
Received Date: 22 July 2019 Revised Date:
5 December 2019
Accepted Date: 7 December 2019
Please cite this article as: B. Wang, B. Tang, C. You, Y. Wan, Y. Gao, Z. Chen, L. Lu, J. Wang, C. Liu, Dislocation arrays, precipitate bands and free zones in forged Mg-Gd-Y-Zr alloy, Materials Science & Engineering A (2020), doi: https://doi.org/10.1016/j.msea.2019.138789. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Dislocation arrays, precipitate bands and free zones in forged Mg-Gd-Y-Zr alloy
Bizheng Wanga, Bei Tanga, Chao Youa, Yingchun Wana,b,d∗, Yonghao Gaoa, Zhiyong Chena, Liwei Luc, Jian Wangd, Chuming Liua,c
a
School of Materials Science and Engineering, Central South University, Changsha 410083, China
b
School of Metallurgy and Environment, Central South University, Changsha 410083, China
c
College of Materials Science and Engineering, Hunan University of Science and Technology, Xiangtan,
411201, China d
Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588, USA
Abstract Dislocation arrays are extensively generated in the hammer forged and then annealed Mg-8.3 Gd-2.6 Y-0.4 Zr (wt. %) alloy. Microscopy characterizations reveal a high thermal stability of these dislocation arrays. During ageing treatment, dislocation arrays act as preferred sites for precipitation of
phase, promoting the formation of the bamboo-like precipitate bands, and
resulting in a depletion of solutes nearby and the formation of precipitate free zones (PFZs) along these bands. The forged samples after annealing and then ageing at an optimized temperature of 265 oC for different periods exhibit a similar strength and strain hardening but an increased elongation with ageing time, suggesting that the precipitate bands act as strong barriers for the motion of dislocations and twins while PFZs can effectively enhance deformability of Mg-Gd-Y-Zr alloy. Keywords: Mg-Gd-Y-Zr alloy, dislocation array, precipitate band, PFZ, mechanical property
∗
Corresponding Author: Yingchun Wan, Email: [email protected] 1
1 Introduction Magnesium alloys are promising lightweight structural materials owing to the merits of low density, high specific strength, ideal recyclability, etc. (1, 2) Mg-RE alloys (RE: rare earth) are typical ageing-hardening alloys because of the formation of precipitates during ageing process, for example, plate-shaped precipitates of basal plane
(7)
and
phases on prismatic planes (3-6) and γ
phase on
. It is well recognized that mechanical properties of precipitation-strengthening
alloys can be finely tuned through tailoring the morphology, characteristic dimension, distribution, and density of precipitates(8, 9). Precipitation behavior is a thermally activated process, and sometimes involves chemical reaction. Precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, and the temperature is high enough that diffusion can lead to segregation into precipitates. Ageing temperature is thus a dominant factor, modifying the dimension and distribution of precipitates because of the temperature dependence of phases and solute diffusion. For example, the increase of ageing temperature from 200 oC to 250 oC can result in the formation of coarser and sparser particles in peak-aged Mg-Gd-Y-Zr alloys because of the accelerated diffusion of Gd and Y elements (10). Nucleation, the first stage of the precipitation process, involves the creation of an interface between particle and matrix. Crystallographic defects are generally preferred sites for nucleating precipitates because of lower formation energy of solutes in these defects than that in perfect crystal, and higher diffusivity of solutes within/along these defects (11-16). For example, 1-nm Gd nano-fibers, with a 〈c〉-rod shape, are formed and hexagonally patterned in association with Gd segregations along dislocations that nucleated during hot extrusion
(12)
. Such self-assembled
patterns of Gd nano-fibers in Mg-Gd alloys act as the predictable inhibitor for basal slips because the glide of basal dislocations must cut them. Recently, ultra-dense and ultra-uniform precipitates with 1~2 nanometer sizes can be introduced in Al alloys during cyclic loading at room temperature, as dislocations repeatedly shear the solute segregation back and forth (16). Nie et al. (17) suggested to simultaneously increase the density and the aspect ratio of precipitates by introducing dislocations into the matrix prior to ageing. Dislocations can affect the precipitation behavior by offering an 2
additional driving force. As reported in WE54 alloy, dislocations introduced by cold work promote precipitation of
phase with larger diameter and number density, leading to a higher hardness.
In addition, the distribution of dislocations in many Mg-RE alloys promotes the heterogeneous nucleation of precipitates
(18-20)
, and changes the spatial arrangement of precipitates in local
regions of the matrix. It can also be of significance to precipitation strengthening, particularly for precipitates with special arrangements
(17)
. The formation of such well-arranged or patterned
precipitates relies on the initial structure of pre-introduced dislocations. However, it remains challenge for effectively regulating dislocations with desired component into proper structure via thermomechanical processes in magnesium alloys. In this work, we observed the formation of dislocation arrays in the hammer forged Mg-Gd-Y-Zr alloy, and characterized the formation of precipitates around dislocation arrays during ageing treatment. The characteristics and formation mechanism of such arrays, as well as their effect on ageing behavior and mechanical properties, are systematically investigated. The results may provide a guidance to produce a microstructure with patterned precipitates in other Mg-RE alloys. 2 Experimental procedure Mg-8.3Gd-2.6Y-0.4Zr (wt. %) ingots were prepared by melting intermetallic alloys (Mg-30%Gd, Mg-30%Y alloy), pure Zr, pure Mg in a mild steel crucible, and casting in an upright semi-continuous machine, and then being homogenized at 450 oC for 5 h and at 540 oC for 15 h. After preheated at 420 ℃ for 3 h, the ingot was subjected to unidirectional hammer forging at a strain rate of ∽ 10 s-1 and a total strain of ∽ 15 %, and then quenched in cold water. Slices that were cut from the central part of the ingot were annealed at 375 oC for 0 to 120 min in a heating furnace with a temperature fluctuation of ±1 oC. They were then ground with 400-1600 grit SiC papers and etched with citric acid for characterization using optical microscopy (OM) and scanning electron microscopy (SEM). OM and SEM characterizations were carried out on the Leica and FEI-PHENOM Pro equipment, respectively. The observing plane was parallel to the
3
forging direction. Discs for transmission electron microscopy (TEM) were also prepared through grinding, twin-jet electropolishing and ion thinning. TEM observation was performed on the FEI-TECNAI G2 F20 operated at 200 kV. Room temperature (RT) tensile tests were conducted on the Instron 3369 machine with a constant cross head speed of 1 mm/s. Cylindrical tensile specimens with a gauge dimension of Φ 4 mm * 20 mm were prepared from the central part of the ingot with tensile axis parallel to the forging direction. Elongation was obtained by calculating the ratio between the increment and initial length of the gauge. For each state, three samples were tested and their average value was displayed here.
Fig. 1 The microstructure of the sample hammer forged and then annealed by 375 ℃ / 15 min. (a) and (b) OM images, (c) SEM image, (d) and (e) TEM images.
3 Results 3.1 Dislocation arrays The microstructure of the sample annealed at 375 ℃ for 15 min is presented in Fig. 1. The OM image in Fig. 1 a shows a heterogeneous microstructure consisting of coarse grains (grain size = ∽ 100 µm) with gray contrast and few dynamic recrystallized grains (DRXed) (grain size = ∽ 10 µm) with white contrast. The magnified OM image in Fig. 1 b explores the densely distributed lines inside grains with the gray contrast. SEM image in Fig. 1 c reveals that these lines are parallel to
4
each other. TEM observation is further performed, and confirms that these lines found in OM and SEM images are river-like dislocation arrays (Fig. 1 d and 1 e). The Burgers vectors of these dislocations are identified in Fig. 2. First, two dislocation arrays were observed under double-beam operation with g
1010 in Fig. 2 a. The scattered
dislocations, serving as a fiduciary mark, are encircled by the red ellipse. When the double-beam operation with g
0002 is conducted, dislocations in the red ellipse are still visible, but both
of the two arrays become invisible. Hence, dislocations in the two arrays have the same Burgers vector based on the dislocation invisible criterion (21). The same results are obtained for dislocation arrays investigated in different grains.
Fig. 2 Identification of Burgers vectors for the dislocation arrays in Fig.1. (a) double-beam operation with
1010 , (b) double-beam operation with
0002 .
The formation of dislocation arrays is observed in the first 10 minutes of annealing, as shown in Fig. 3. The TEM image of the as-forged sample (Fig. 3 a) shows a high density of dislocations, but no apparent patterns. When the sample is annealed at 375 ℃ for 5 mins, a vast of dislocation cells appear in the TEM images (Fig. 3 c), and connect with each other (Fig. 3 c - f). The diameter of these cells ranges from 0.3 to 2.1 µm. High-amplified TEM image (Fig. 3 d) reveals that the cell walls consist of dislocations with a dense and ordered arrangement or pattern. Obviously, this is a typical static recovery process that deformation-induced dislocations rearrange themselves into ordered cells. Such process is generally associated with dislocation climbing during the annealing treatment (22). With longer annealing time of 10 min, dislocation arrays begin to appear. As shown 5
in Fig. 3 f, two segments of dislocation arrays are formed through annihilation of the shared walls of three cells. Normally, these cells will transform into new grains with low or high angle boundaries. However, a generally stable boundary requires three sets of dislocations with three non-coplanar Burgers vectors that could eliminate the long range stress field
(23-25)
. But the
as-forged microstructure, which is produced at a moderate temperature of 400-420 ℃ and a low strain, is unable to provide enough non-basal dislocations (26). As a result, the cells will turn into other forms, the dislocation arrays seem to be one of new forms. On one hand, dislocations arrays present a much stable configuration than the cells since they may greatly diminish the long range stress field, and each dislocation in the array can be firmly fixed by the strong attraction force from the others (27). On the other hand, through the climb of dislocations on the shared walls, the arrays can be easily and rapidly transformed from the interconnected cells. Hence, an irreversible transformation from the cells into dislocation arrays occurs with prolonged annealing time.
Table 1 Array width (D1) and array distance (D2) in the alloy annealed for 15 - 120 min at 375 ℃
375 ℃ / 15 min
375 ℃ / 30 min
375 ℃ / 60 min
375 ℃ / 120 min
Average value (µm)
0.17
0.20
0.20
0.16
Standard deviation
12 %
16 %
10 %
13 %
Average value (µm)
1.02
0.90
1.30
1.13
Standard deviation
45 %
71 %
55 %
50 %
Dimension D1
D2
The microstructure evolution in the 30 ~ 120 min annealing duration is shown in Fig. 4. The dimension and distribution of the arrays, obtained from 60 arrays in 15 grains based on the TEM observation, are summarized in Table 1. The array width D1 is defined to be the distance between the two boundaries of an array and the matrix as indicated by the red arrows in Fig. 4 a. The array spacing D2 is defined to be the distance between the adjacent arrays, as presented in Fig. 4 c. The average values of D1 and D2 do not vary obviously during 15 ~ 60 min. However, with the annealing time prolonged to 120 min, much less arrays were observed (as shown in Fig. 4 e and
6
4 f). This morphology variation is obtained through this process: the dislocations on parallel slipping planes re-orientated themselves or part of their lengths parallel with each other by climbing (diffusion) as the function of the elastic stress imposed on each other (28).This energetic procedure is immediately completed at the very initial of annealing.
Fig. 3 The microstructure evolution during the initial annealing period: (a) (b) the as-forged state (c) (d) 375 oC / 5 min (e)(f) 375 oC / 10 min.
7
Fig. 4 TEM (left) and SEM (right) images for the samples annealed at 375 ℃ with prolonged time (a) (b) 30 min, (c) (d) 60 min, (e) (f) 120 min.
3.2 The effect of dislocation arrays on ageing behavior and mechanical performance Fig. 5 a exhibits the engineering strain-stress curves for as-forged or as-annealed samples. The corresponding mechanical properties are summarized in Table 2 and Table 3. With regard to the as-forged samples, it has a strength of 223 MPa in yield stress (YS), 280 MPa in ultimate stress (UTS) and an elongation of 3.8 %. Ageing treatment largely enhances its strength but severely deteriorates the elongation, a highest strength of 310 MPa in YS, 369 MPa in UTS but an 8
elongation as low as 1.2 % for the sample peak-aged at 200 oC. When ageing temperature increases, the strength gradually decreases, the elongation, however, has a slight increase but lower than the initial state. As for the as-annealed sample, it has a slightly lower strength but higher elongation than the as-forged one. Alloy aged at 200 ~ 250 oC results in a very similar mechanical performance to that of the as-forged one regardless of the dislocation arrays. However, the sample annealed and peak-aged at 265 oC exhibits a simultaneous enhancement of strength and elongation, i.e., an increment of 53 MPa in YS, 52 MPa in UTS and 3.8 % in elongation compared to the as-annealed state.
Table 2. Corresponding mechanical properties for Fig.5 a State
UTS / MPa
YS / MPa
Elongation / %
As-forged
280
223
3.8
As-annealed (385 oC / 30 min)
273
215
4.1
200 oC / 72 h (forged)
369 (+ 89)
310 (+ 87)
1.2 (- 2.6)
200 oC / 66 h (annealed)
358 (+ 85)
304 (+ 89)
1.3 (- 2.8)
225 oC / 24 h (forged)
357 (+ 77)
295 (+ 72)
1.6 (- 2.2)
225 oC / 16 h (annealed)
352 (+ 79)
284 (+ 69)
1.6 (- 2.5)
336 (+ 56)
277 (+ 54)
3.5 (- 0.3)
330 (+ 57)
271 (+ 56)
3.7 (- 0.4)
327 (+ 47)
267 (+ 44)
3.6 (- 0.2)
325 (+ 52)
268 (+ 53)
7.9 (+ 3.8)
o
250 C / 14 h (forged) o
250 C / 8 h (annealed) o
265 C / 8 h (forged) o
265 C / 4 h (annealed)
Considering the remarkable mechanical properties of the sample annealed and then peak-aged at 265 oC, we conduct more mechanical tests and microstructure characterization of the 265 oC aged samples. Fig. 5 b shows the stress-strain curves of the 265 oC-aged samples at different aging times. It can be seen that a simultaneous enhancement of strength and elongation takes place at the initial ageing period of 1 h. With prolonged ageing time to 6 h, the strength and elongation continuously increase. The over-aged sample (265 oC / 6 h) has a strength of 273 MPa
9
in YS and 334 MPa in UTS, larger than those of the samples annealed and then peak-aged at either 265 oC or 250 oC. Meanwhile, its elongation reaches a value as high as 8.0 %, which is almost twice of that in the initial state. With further increased ageing time till 16 h, there is a slight decrease in strength but a slight increase in elongation.
Fig.5 Engineering stress-strain curves for (a) the initial and peak-ageing conditions, (b) ageing treatment at 265 ℃ for the annealed sample.
Table 3. Mechanical properties for the annealed sample aged at 265 oC
State
UTS / MPa
YS / MPa
Elongation / %
273
215
4.1
265 ℃/ 1 h
306 (+ 33)
246 (+ 29)
5.0 (+ 0.9)
265 ℃/ 2 h
311 (+ 38)
254 (+ 39)
5.8 (+ 1.7)
265 ℃/ 3 h
320 (+ 47)
256 (+ 41)
6.6 (+ 2.5)
265 ℃/ 4 h
325 (+ 52)
268 (+ 53)
7.9 (+ 3.8)
265 ℃/ 5 h
330 (+ 57)
266 (+ 51)
7.9 (+3.8)
265 ℃/ 6 h
334 (+ 61)
273 (+ 58)
8.0 (+ 3.9)
265 ℃/ 8 h
330 (+ 57)
270 (+ 55)
8.2 (+ 4.1)
265 ℃/ 16 h
326 (+ 53)
264 (+ 49)
8.6 (+ 4.5)
As-annealed (385 ℃ / 30 min)
10
Fig. 6 The microstructure of the annealed sample (375 oC / 30 min) aged by 265 oC / 15 min. (a) the low magnification bright field image (b) the HADDF-STEM mode of encircled area in Fig. 6 a, (c) the bright field image and (d) high resolution imaging and FFT of particles encircled in Fig. 6 c.
The microstructure of the annealed samples (375 oC / 30 min) and then aged at 265 oC is exhibited in Fig. 6 - 8. At the very beginning of 15 min, small particles largely appear within the matrix (Fig.6a). The image detected in the HADDF-STEM mode with higher magnification (Fig. 6 b) reveals that particles in the α-Mg matrix are mostly spheroidal with a diameter of ∽ 12 nm while those within dislocation arrays are basically needle-like with 14 - 30 nm in length and ∽ 5 nm in width. Meanwhile, it is noted that the needle-like particles are parallel with each other on the same dislocation array. Undoubtedly, these particles are all the precipitates formed during ageing treatment and the bright contrast implies their richness in heavy RE atoms of Gd and/or Y. In fact, these spheroidal particles can be inferred as β’’ phase based on their shape and size
(29)
.
But it is tough to estimate the type of the needle-like ones only depending on their morphology. In view of this, particles around a single dislocation (Fig. 6 c) are further characterized by the high resolution imaging and fast Fourier transformation (FFT) (Fig. 6 d). The same FFT patterns identify that particles with either spheroidal or needle-like shape are β’’ phases (D019 structure 11
(30)
). The parallel longitudinal surfaces of the needle-like particles are proved to be anastomotic
with the α / β’’ plane by the FFT result. The precipitation sequence of Mg-Gd-Y-Zr alloy is reported to be S. S. S. S → β’’ (D0 19)→ β’ (c b c o)→ β1 (f c c)→ β (f c c) (29). A possible continuous transformation from spheroidal
particle of β’’ into convex-lens like β’ is proposed by Honma et al
(31)
. In this work, the
needle-like particle is believed to act as a transition phase for the continuous transformation since it has a crystal structure like that of β’’ but a morphology and growth plane more similar to that of β’ phase (29). Being a kind of line lattice defect, the dislocations, especially their arrays, tend to
facilitate such transformation by providing more channels and extra stress field to accelerate the diffusion of solute atoms
(32)
. The needle-like particles are thus mostly located on them rather
than in the α-Mg matrix. In addition, the stress field of the dislocations will strongly interact with the precipitates and deeply influence their morphology and distribution. As reported in (18), due to the existence of stress filed, the mixed a-type dislocations often result in precipitate arrays comprising of particles with the same orientation variant. Similar result is also observed in the crept Mg-Gd-Zr alloy
(19)
. Hence, it is reasonably inferred that the parallel arrangement of the
needle-like particles is a result of the stress filed of the a-type dislocation arrays. The microstructure of the samples aged at 265 oC with longer time is presented in Fig. 7 and Fig. 8. The observation direction is parallel with < 0001 > for those aged from 1 h to 6 h and <2111> for that aged by 16 h. After ageing for 1h (Fig. 7), a lot of coarse and river-like precipitate bands appear within the grains. These bands have a quite similar running direction, length and adjacent distance to that of the dislocation arrays, implying that such bands are formed on the positions where dislocation arrays are located (Fig. 7 a). Certainly, there also exist some finer and well-ordered precipitates between these bands. Magnified TEM image (Fig. 7 c) reveals that the precipitate bands are composed of a lot of bamboo-like strips while the fine and well-ordered precipitates are bamboo-leaf-like and ladder-shape arranged. In addition, some much finer bamboo-leaf-like precipitates with 120o symmetrical distribution are also found within the matrix.
12
Fig. 7 The microstructure of the annealed sample (375 oC / 30 min) aged by 265 oC / 1 h, (a) Low magnification TEM image (b) the corresponding SAED pattern for Fig. 7 a, (c) (d) high magnification TEM images.
The petaloid deputy spots in the SAED pattern (Fig. 7 b) prove them to be the same phase regardless of their different morphology and distribution (30). With higher magnification (Fig. 8 d), it is known that each bamboo-like strip of the precipitate band (white arrow) is composed of two or more particles with the same orientation. These particles connect with each other through overlapping growth within one strip and through the bridge phase of β
(33)
between the adjacent
ones, forming a very dense precipitation structure. The ladder-shape arranged precipitates (blue arrow) are also interconnected through the linear bridge phase. It is well known that the homogeneous precipitation with 120o symmetrical distribution is the most common morphology of
phase in Mg-Gd-Y-Zr alloy(29, 30, 34, 35). However, dislocation arrays change the spatial
distribution of
phase. As mentioned above, the needle-like transition particles on the
dislocations have a parallel arrangement with each other and eventually transform into precipitates of
phase without changing their orientation, generating the precipitate bands on the arrays and
13
ladder-shape arranged particles on the single dislocation, respectively. In addition, due to the accelerating effect of the arrays on solute diffusion, solute atoms tend to strongly segregate to precipitate bands, resulting in a depletion of solutes nearby and formation of precipitate free zones (PFZs) along these bands (36). The precipitate bands are not homogeneously distributed within the grains. Under the same magnification, there may appear either only one or more than two precipitate bands in the view and this will actually result in different microstructure evolution with prolonged ageing time (Fig. 8). It should be recognized that most of the precipitate bands (yellow arrows), especially the PFZs along them, are narrow when aged for 1 h (Fig. 8 a and 8 e). As clarified above, the rest of the matrix is mostly occupied by the bamboo-leaf-like particles with ladder-shape arrangement (blue arrows). After ageing for 3 h (Fig. 8 b and 8 f), both the precipitate bands and PFZs become obviously wider. In the region where only one band exists, a width of ∽ 220 nm is obtained for the PFZ and the rest of the matrix has been largely taken up by the homogeneous precipitation with a dense and symmetrical distribution, as well as with a size of ∽ 90 nm in length and ∽ 18 nm in width for one precipitate particle. As for the region where more precipitate bands appear, it is interesting to find that particles between the bands have vanished a lot. The remaining ones, as encircled by the red ellipses, seem to stay away from these bands. Continued ageing till 6 h leads to a further size increase for the homogeneous precipitation (∽ 120 nm in length and ∽ 25 nm in width) and a slight width increase for the precipitate bands and PFZs (∽ 250 nm) (Fig.8c). In the region where more precipitate bands appear, it is noted that the remaining particles between the bands have almost disappeared (Fig. 8 g). After that, the ageing till 16 h only results in a size growth for the homogeneous precipitation. No remarkable change occurs for the size of precipitate bands. Based on the SAED patterns inserted, these precipitates are transformation from
to
phase till 16 h and since then the in situ
takes place (Fig. 8 h, red arrows)
(29)
. Such results imply that a
strong competition exists between the homogeneous and dislocation array induced precipitation. Considering the mutually reinforced stress filed for the array, the latter one has an obvious advantage, which contributes to the persistent width increase of the precipitate bands and PFZs at
14
the expense of redissolution of the homogeneous precipitation near them. When the PFZs reach a certain width or the homogeneous precipitates almost disappear, the precipitate bands will then keep unchanged for a long time without the supply of solute atoms. As for the homogeneous precipitation away from these bands, they will experience a continuous growth in size but a decrease in number density with prolonged ageing time just as reported elsewhere (37).
Fig. 8 The microstructure for the annealed sample (375 oC / 30 min) aged at 265 oC with prolonged time.
4 Discussion The effect of precipitates on dislocation gliding is shown in Fig. 9. It is observed that dislocations are strictly restricted in the regions separated by the precipitate bands (indicated by the red arrows) and largely piled up at the vicinity of these bands. This implies that the precipitate bands have a strong impediment on dislocation gliding. In addition, such impediment also happens in the region where homogeneous precipitation exists (Fig. 9 b). Fig. 10 shows that the deformation twins can propagate in the homogeneously precipitated regions but encounter tough obstacles of the precipitate bands. These twins are strictly hindered by the bands (Fig. 9 a and 9 b). These bands play a role quite similar to the grain boundary strengthening. In fact, the dislocation arrays themselves are exactly a certain kind of sub-grain boundaries
(38)
, separating the coarse
grains into much smaller parts. The precipitate bands formed on the arrays with similarly orientated and closely connected particles of
phases could extremely strengthen such
characteristic of being interfaces. 15
Fig. 9 TEM images: Interaction between the dislocations and precipitates in the sample (265 oC / 6 h) tensile to fracture.
Fig. 10 TEM images: interaction between the deformation twins and precipitates in the sample (265 oC / 6 h) tensile to fracture.
Due to the unchanged spacing between precipitate bands, the strength of the aged samples is almost retained. However, the widened PFZs enhance the ability of dislocation slips, increasing the elongation of the samples(7). It is believed that in the region where the PFZ is formed, the matrix is greatly softened due to the large absence of precipitates and solute atoms. In the regions where more precipitate bands appear, the homogeneous precipitates largely disappear due to the 16
existence of PFZs, which will undoubtedly further increase the ductility. Such strengthening and toughening mechanisms are apparently correlated to ageing temperature and time. For the sample over-aged by 265 ℃ / 6 h, the strength increment is mainly attributed to the precipitate bands and the elongation enhancement can be ascribed to the formation of PFZs along these bands. At the initial ageing period till 6 h, a continuous and simultaneous increment of strength and elongation is realized, which should be a result of the persistently widening of such bands as well as the PFZs. The proposed mechanisms are confirmed by the fact that the strength is unchanged but the elongation increases with ageing time at aging temperature of 265 ℃. Solute diffusion is more strongly dependent on aging temperature than aging time (39, 40). Fig. 11 exhibits the microstructure of the sample peak-aged at 200 ℃, fine and dense distribution of homogeneous precipitation are observed, but precipitate bands are very narrow. More significantly, apparent PFZs along precipitate bands don’t form. Hence, the material is still mainly strengthened by the homogeneous precipitation, resulting in their similar mechanical properties to the corresponding states of the as-forged sample.
Fig.11 TEM images for the annealed sample (375 oC / 30 min) peak-aged at 200 oC.
5 Conclusion We demonstrated the formation of dislocation arrays in Mg-8.3 Gd-2.6 Y-0.4 Zr alloy processed by hammer forging and subsequent annealing treatment. During aging, these dislocation arrays act as preferred sites for precipitation of
phase, promoting the formation of the bamboo
precipitate bands with only one variant orientation, and resulting in a depletion of solutes nearby 17
and the formation of precipitate free zones (PFZs) along these bands. The dislocation arrays are characterized with parallel arranged type dislocations, which are extensively generated in Mg-8.3 Gd-2.6 Y-0.4 Zr alloy through the recovery process during 375 oC annealing treatment. Precipitates with
phase preferentially grow on the dislocation arrays with obviously larger size
and only one variant orientation, forming the bamboo-like bands and promoting the formation of wide PFZs along them. An optimized aging temperature exists, corresponding to the formation of precipitate bands and PFZs. The forged samples after annealed and then aged at 265 oC for different periods exhibit a similar strength and strain hardening but an increased elongation with ageing time. The best combination of strength and ductility are 273 MPa in YS, 334 MPa in UTS and 8.0 % in Elongation. The results suggest that the precipitate bands strongly impede the motion of dislocation and twinning while PFZs can effectively enhance deformability of Mg-Gd-Y-Zr alloy. Acknowledgement This work is supported by National Natural Science Foundation of China (Grant number 51574291 and 51874367), China Postdoctoral Science Special Foundation [Grant number 2019T120712], China Postdoctoral Science Foundation [Grant number 2018M642999], State Key Laboratory for Powder Metallurgy of Central South University, China. Natural Science Foundation of Hunan Province [Grant number 2019JJ50787].
References 1.
E. Aghion, B. Bronfin, in Materials Science Forum. (Trans Tech Publ, 2000), vol. 350, pp. 19-30.
2.
B. Mordike, T. Ebert, Magnesium: properties—applications—potential. Materials Science and Engineering: A 302, 37-45 (2001).
3.
P. Apps, H. Karimzadeh, J. King, G. Lorimer, Precipitation reactions in magnesium-rare earth alloys
4.
S. M. He, X. Q. Zeng, L. Peng, X. Gao, J. F. Nie, W. J. Ding, Microstructure and strengthening mechanism of
containing yttrium, gadolinium or dysprosium. Scripta Materialia 48, 1023-1028 (2003). high strength Mg–10Gd–2Y–0.5 Zr alloy. Journal of Alloys and Compounds 427, 316-323 (2007). 5.
J. F. Nie, B. C. Muddle, Precipitation in magnesium alloy WE54 during isothermal ageing at 250 C. Scripta Materialia 40, 1089-1094 (1999).
18
6.
J. F. Nie, B. C. Muddle, Characterisation of strengthening precipitate phases in a Mg–Y–Nd alloy. Acta Materialia 48, 1691-1703 (2000).
7.
Y. Zhang, W. Rong, Y. Wu, L. Peng, J.-F. Nie, N. Birbilis, A comparative study of the role of Ag in microstructures and mechanical properties of Mg-Gd and Mg-Y alloys. Materials Science and Engineering: A 731, 609-622 (2018); published online Epub2018/07/25/ (https://doi.org/10.1016/j.msea.2018.06.084).
8.
J. F. Nie, Effects of precipitate shape and orientation on dispersion strengthening in magnesium alloys. Scripta Materialia 48, 1009-1015 (2003).
9.
Y.-H. Kang, H. Yan, R.-S. Chen, Effects of heat treatment on the precipitates and mechanical properties of sand-cast Mg–4Y–2.3Nd–1Gd–0.6Zr magnesium alloy. Materials Science and Engineering: A 645, 361-368 (2015); published online Epub2015/10/01/ (https://doi.org/10.1016/j.msea.2015.08.041).
10.
S. Liang, D. Guan, X. Tan, L. Chen, Y. Tang, Effect of isothermal aging on the microstructure and properties of as-cast Mg–Gd–Y–Zr alloy. Materials Science and Engineering: A 528, 1589-1595 (2011); published online Epub2011/01/25/ (https://doi.org/10.1016/j.msea.2010.10.082).
11.
R. G. Li, H. B. Shafqat, J. H. Zhang, R. Z. Wu, G. Y. Fu, L. Zong, Y. Su, Cold-working mediated converse age hardening responses in extruded Mg-14Gd-2Ag-0.5Zr alloy with different microstructure. Materials Science and Engineering: A 748, 95-99 (2019); published online Epub2019/03/04/ (https://doi.org/10.1016/j.msea.2019.01.082).
12.
Y. Li, J. Wang, K. Chen, M. Shao, Y. Shen, L. Jin, G.-z. Zhu, Self-patterning Gd nano-fibers in Mg-Gd alloys. Scientific reports 6, 38537 (2016).
13.
W. Sun, Y. Zhu, R. Marceau, L. Wang, Q. Zhang, X. Gao, C. Hutchinson, Precipitation strengthening of
14.
H. Xiao, B. Tang, C. Liu, Y. Gao, S. Yu, S. Jiang, Dynamic precipitation in a Mg–Gd–Y–Zr alloy during hot
aluminum alloys by room-temperature cyclic plasticity. Science 363, 972-975 (2019). compression. Materials Science and Engineering: A 645, 241-247 (2015); published online Epub2015/10/01/ (https://doi.org/10.1016/j.msea.2015.08.022). 15.
S. Yu, Y. Wan, C. Liu, J. Wang, Age-hardening and age-softening in nanocrystalline Mg-Gd-Y-Zr alloy. Materials Characterization 156, 109841 (2019); published online Epub2019/10/01/ (https://doi.org/10.1016/j.matchar.2019.109841).
16.
A. W. Zhu, E. A. Starke, Stress aging of Al–xCu alloys: experiments. Acta Materialia 49, 2285-2295 (2001); published online Epub2001/07/17/ (https://doi.org/10.1016/S1359-6454(01)00119-7).
17.
J.-F. Nie, Precipitation and Hardening in Magnesium Alloys. Metallurgical and Materials Transactions A 43,
18.
H. Liu, Y. Gao, Y. M. Zhu, Y. Wang, J. F. Nie, A simulation study of β1 precipitation on dislocations in an
3891-3939 (2012); published online Epub2012/11/01 (10.1007/s11661-012-1217-2). Mg–rare earth alloy. Acta Materialia 77, 133-150 (2014); published online Epub2014/09/15/ (https://doi.org/10.1016/j.actamat.2014.04.054). 19.
H. Liu, W. F. Xu, L. M. Peng, W. J. Ding, J. F. Nie, A simulation study of the distribution of β′ precipitates in a crept Mg-Gd-Zr alloy. Computational Materials Science 130, 152-164 (2017); published online Epub2017/04/01/ (https://doi.org/10.1016/j.commatsci.2016.12.021).
20.
G. Shi, K. Zhang, X. Li, Y. Li, M. Ma, J. Yuan, Precipitation behaviors, texture and tensile properties of an extruded Mg-7Y-1Nd-0.5Zr (wt%) alloy bar with large cross-section. Materials Science and Engineering: A 685, 300-309 (2017); published online Epub2017/02/08/ (https://doi.org/10.1016/j.msea.2016.12.076).
19
21.
S. Agnew, J. Horton, M. Yoo, Transmission electron microscopy investigation of< c+a> dislocations in Mg and α-solid solution Mg-Li alloys. Metallurgical and Materials Transactions A 33, 851-858 (2002).
22.
G. Falkinger, P. Simon, Static recovery of an AlMg4.5Mn aluminium alloy during multi-pass hot-rolling. Procedia Engineering 207, 31-36 (2017); published online Epub2017/01/01/ (https://doi.org/10.1016/j.proeng.2017.10.733).
23.
C. Hong, X. Huang, G. Winther, Dislocation content of geometrically necessary boundaries aligned with slip planes in rolled aluminium. Philosophical Magazine 93, 3118-3141 (2013).
24.
Y. L. Wei, A. Godfrey, W. Liu, Q. Liu, X. Huang, N. Hansen, G. Winther, Dislocations, boundaries and slip systems in cube grains of rolled aluminium. Scripta Materialia 65, 355-358 (2011); published online Epub2011/08/01/ (https://doi.org/10.1016/j.scriptamat.2011.05.005).
25.
G. Winther, J. P. Wright, S. Schmidt, J. Oddershede, Grain interaction mechanisms leading to intragranular orientation spread in tensile deformed bulk grains of interstitial-free steel. International Journal of Plasticity 88, 108-125 (2017); published online Epub2017/01/01/ (https://doi.org/10.1016/j.ijplas.2016.10.004).
26.
H. C. Xiao, S. N. Jiang, B. Tang, W. H. Hao, Y. H. Gao, Z. Y. Chen, C. M. Liu, Hot deformation and dynamic recrystallization behaviors of Mg–Gd–Y–Zr alloy. Materials Science and Engineering: A 628, 311-318 (2015); published online Epub2015/03/25/ (https://doi.org/10.1016/j.msea.2015.01.041).
27.
Y. Chou, Interaction of parallel dislocations in a hexagonal crystal. Journal of Applied Physics 33, 2747-2751 (1962).
28.
D. V. Bachurin, A. A. Nazarov, J. Weissmüller, Grain rotation by dislocation climb in a finite-size grain boundary. Acta Materialia 60, 7064-7077 (2012); published online Epub2012/12/01/ (https://doi.org/10.1016/j.actamat.2012.09.014).
29.
S. M. He, X. Q. Zeng, L. M. Peng, X. Gao, J. F. Nie, W. J. Ding, Precipitation in a Mg–10Gd–3Y–0.4Zr (wt.%) alloy during isothermal ageing at 250°C. Journal of Alloys and Compounds 421, 309-313 (2006); published online Epub2006/09/14/ (https://doi.org/10.1016/j.jallcom.2005.11.046).
30.
S. M. He, X. Q. Zeng, L. M. Peng, X. Gao, J. F. Nie, W. J. Ding, Microstructure and strengthening mechanism of high strength Mg–10Gd–2Y–0.5Zr alloy. Journal of Alloys and Compounds 427, 316-323 (2007); published online Epub2007/01/16/ (https://doi.org/10.1016/j.jallcom.2006.03.015).
31.
T. Honma, T. Ohkubo, K. Hono, S. Kamado, Chemistry of nanoscale precipitates in Mg–2.1 Gd–0.6 Y–0.2 Zr (at.%) alloy investigated by the atom probe technique. Materials Science and Engineering: A 395, 301-306 (2005).
32.
J. Čížek, I. Procházka, B. Smola, I. Stulíková, V. Očenášek, Influence of deformation on precipitation process in Mg–15wt.%Gd alloy. Journal of Alloys and Compounds 430, 92-96 (2007); published online Epub2007/03/14/ (https://doi.org/10.1016/j.jallcom.2006.03.097).
33.
H. Xie, H. Pan, Y. Ren, S. Sun, L. Wang, Y. He, G. Qin, Co-existences of the two types of β′ precipitations in
34.
Y. C. Wan, H. C. Xiao, S. N. Jiang, B. Tang, C. M. Liu, Z. Y. Chen, L. W. Lu, Microstructure and mechanical
peak-aged Mg-Gd binary alloy. Journal of Alloys and Compounds 738, 32-36 (2018). properties of semi-continuous cast Mg–Gd–Y–Zr alloy. Materials Science and Engineering: A 617, 243-248 (2014); published online Epub2014/11/03/ (https://doi.org/10.1016/j.msea.2014.07.042).
20
35.
S. Yu, C. Liu, Y. Gao, S. Jiang, Y. Yao, Microstructure, texture and mechanical properties of Mg-Gd-Y-Zr alloy annular forging processed by hot ring rolling. Materials Science and Engineering: A 689, 40-47 (2017); published online Epub2017/03/24/ (https://doi.org/10.1016/j.msea.2017.02.036).
36.
A. Zindal, J. Jain, R. Prasad, S. S. Singh, R. Sarvesha, P. Cizek, M. R. Barnett, Effect of heat treatment variables on the formation of precipitate free zones (PFZs) in Mg-8Al-0.5Zn alloy. Materials Characterization 136, 175-182 (2018); published online Epub2018/02/01/ (https://doi.org/10.1016/j.matchar.2017.12.018).
37.
W. Dongshu, L. Dejiang, X. Yancai, Z. Xiaoqin, HRTEM studies of aging precipitate phases in the Mg-10Gd-3Y-0.4 Zr alloy. Journal of Rare Earths 34, 441-446 (2016).
38.
D. Hull, D. J. Bacon, in Introduction to Dislocations (Fifth Edition), D. Hull, D. J. Bacon, Eds.
39.
S. K. Das, Y.-B. Kang, T. Ha, I.-H. Jung, Thermodynamic modeling and diffusion kinetic experiments of
(Butterworth-Heinemann, Oxford, 2011), pp. 171-204. binary Mg–Gd and Mg–Y systems. Acta Materialia 71, 164-175 (2014); published online Epub2014/06/01/ (https://doi.org/10.1016/j.actamat.2014.02.029). 40.
I. Stulikova, B. Smola, J. Cizek, T. Kekule, O. Melikhova, H. Kudrnova, Natural and artificial aging in Mg-Gd binary alloys. Journal of Alloys and Compounds 738, 173-181 (2018); published online Epub2018/03/25/ (https://doi.org/10.1016/j.jallcom.2017.12.026).
21
Bizheng Wang: Investigation; Roles/Writing - original draft; Bei Tang: Investigation; Methodology; Chao You: Investigation; Yingchun Wan: Investigation; Writing - review & editing; Yonghao Gao: Funding acquisition; Zhiyong Chen: Funding acquisition; Liwei Lu: Writing - review & editing; Jian Wang： ：Writing - review & editing; Chuming Liu： ：Funding acquisition;
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: