High strength magnesium alloy with α-Mg and
W-pha procesd by hot extrusion
YANG Wen-peng1, GUO Xue-feng2
1. School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China;
2. School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China
Received 20 December 2010; accepted 26 April 2011
Abstract: Fine-grained Mg−6Zn−4Y alloy was prepared by an ingot metallurgy process with hot extrusi
on at 300 °C. The microstructure was studied by XRD, OM, SEM and TEM, and the tensile properties were tested at room temperature. The results show that the alloy is compod of α-Mg and W-pha. The microstructure of the as-extruded alloy has a bimodal grain size distribution. The fine grains with the mean size of 1.2 μm are formed by dynamic recrystallization. The coar grains (about 23% in area fraction) are unrecrystallized regions which are elongated along extrusion direction. The engineering stress—strain curve shows a pronounced yield point. The ultimate tensile strength, yield strength, and elongation are (371±10) MPa, (350±5) MPa and (7±2)%, respectively. The high strengths are attributed to the fine-grained matrix structure enhanced by W-pha particles, nano-scaled precipitates, and strong basal plane texture.
Key words:Mg−6Zn−4Y alloy; extrusion; W-pha; high strength; yield phenomenon
1 Introduction
Mg-bad alloys are attractive for many engineering structural applications owing to their low density and high specific strength [1]. However, Mg and most Mg alloys have hexagonal clo-packed crystal structure that does not have sufficient slip systems; hence it is hard to increa their strengths and ductilities simultaneously. Recently, Mg alloys containing Zn and Y have attracted a gr
eat deal of attention as they have high strengths and reasonable ductilities [2−3]. In general, depending on the Zn/Y ratio ternary equilibrium phas in as-cast and rapidly solidified Mg−Zn−Y system include I-pha (Mg3YZn6), W-pha (Mg3Y2Zn3), and long-period stacking order (LPSO) structures (including 6H, 14H and 18R etc) [4−5]. The alloys with the novel LPSO structures have excellent properties becau the hard LPSO structures and the coherent LPSO/α-Mg interface can provide with high strength and fractural resistance to Mg alloys. As an example, Mg−2Y−1Zn (mole fraction, %) alloy strengthened by 6H LPSO structure consolidated at various temperatures from gas-atomized powder exhibits yield strengths of 480−610 MPa in company with elongations of 5%−16% at room temperature [6]. I-pha is a stable icosahedral quasicrystalline pha with high lattice symmetry. By far, five orientation relationships between I-pha and Mg matrix have been identified [7−8]. Each has strong interfacial properties due to the low interfacial energy. The Mg−Zn−Y alloys containing I-pha have yield strengths of 190−320 MPa and elongations of 15%−25% at room temperature after being procesd by hot rolling and extrusion [2−3]. W-pha with face-centered cubic structure has weak adhesive force with Mg matrix due to their limited structural symmetry [9]. Nevertheless, the strengthening mechanisms for high strength Mg alloys containing W-pha have more possibilities except W-pha strengthening. The matrix grain size [10], texture [11] and volume fraction of strengthening particles [12] are perhaps also the key factors to strength a
nd ductility. However, few reports investigated the effects of above factors on the mechanical properties. In this study, we report a high strength two-pha alloy containing α-Mg and W-pha, which was prepared by hot extrusion from as-cast Mg−6Zn−4Y ingot, and the
Foundation item: Project (50271054) supported by the National Natural Science Foundation of China; Project (20070700003) supported by the Doctorate Programs Foundation of Ministry of Education of China; Project (102102210031) supported by the Science and Technologies Foundation
of Henan Province, China; Project (2010A430008) supported by the Natural Science Foundation of Henan Educational Committee of
China
Corresponding author: YANG Wen-peng; Tel: +86-29-82314009; E-mail:
DOI: 10.1016/S1003-6326(11)61020-0
YANG Wen-peng, et al/Trans. Nonferrous Met. Soc. China 21(2011) 2358−2364 2359
strengthening mechanism is discusd.
2 Experimental
The as-cast alloy with the nominal composition of Mg−6Zn−4Y was prepared by melting Mg (99.9%), Zn (99.9%) and Mg−47%Y master alloy in an electric resistance furnace at 720 °C under Ar+SF6 atmosphere. The ingots with a diameter of 52 mm and a length of 120 mm were prepared by pouring melt into graphite moulds.
The as-cast ingots were machined into rods with a diameter of 50 mm and a length of 50 mm for extrusion. Before extrusion, the billets and extrusion die were heated to 300 °C for 60 min in order to homogenize the temperature of the billets. The billets were extruded into bars with a diameter of 12 mm at an extrusion ratio of 17:1 and a ram speed of 1 mm/min.
The pha constitutions were analyzed by a Rigaku D/max-C X-ray diffraction (XRD) instrument with Cu Kα radiation. Metallographic specimen was polished to mirror and then etched using a solution of 0.5 mL nitric acid + 5 g picric acid + 10 mL acetic acid + 10 mL water + 100 mL alcohol. The specimen was examined by a Niko Epiphot optical microscope (OM). The grain size (d) was estimated using an image analysis software bad on OM micrograph. During evaluation, over 2 000 grains were measured. The distribution of broken W-pha was obrved using a JSM−6700F scan
ning electron microscope (SEM). Transmission electron microscopy (TEM) was employed to obtain detailed information about microstructure including features of strengthening particles and dislocations. TEM sample was cut from as-extruded bar along the extrusion direction (ED). The sample was twin-jet electron- polished to perforation, then ion-milled to remove oxide film with an ion accelerating voltage of 4 keV. TEM obrvation was carried out using a JEM−3010 operating at 300 kV.
Tensile specimen with a gauge length of 30 mm and a diameter of 6 mm was machined from the as-extruded bar along ED. Tensile tests were carried out using a HT2008 machine at a constant strain rate of 5 × 10−4 s−1 at room temperature.
3 Results
Figure 1 shows the XRD pattern of Mg−6Zn−4Y alloy. The alloy is mainly compod of α-Mg and W-pha. This result agrees well with the previous studies [4−5], in which it was found that the predominant intermetallic compound is W-pha when Zn/Y ratio is between 1 and 2.
Figure 2 shows the microstructures of as-extruded alloy on longitudinal ction. The as-extruded alloy consists of fine equiaxed grains and some coar grains (Fig. 2(a)). This kind of microstructure
has typical bimodal grain size distribution and is commonly obrved in the alloys extruded at relatively low temperatures [13]. According to the previous reports, the fine grains with an average grain size of 1.2 μm are attributed to the dynamic recrystallization (DRX). The coar grains are the ones that did not undergo recrystallization, and kept the elongated form along ED with an aspect ratio of more than two order of magnitude and rrated boundaries. The area fraction of the unrecrystallized regions is measured to be about 23%.
深海勇士
Fig. 1 XRD pattern of Mg−6Zn−4Y alloy
Fig. 2 Microstructures of as-extruded Mg−6Zn−4Y alloy: (a) OM micrograph showing fine recrystallized grains and elongated unrecrystallized grains; (b) SEM micrograph showing broken intermetallics
YANG Wen-peng, et al/Trans. Nonferrous Met. Soc. China 21(2011) 2358−2364 2360
The dark regions in Fig. 2(a) are the broken W-pha particles formed during extrusion from grain boundary networks of the as-cast alloy. The SEM micrograph shown in Fig. 2(b) provides good contrast between the intermetallics and matrix. It is clear from Fig. 2(b) that the broken W-pha (white regions) particles are distributed as streamlines in parallel to ED. The area fraction of W-pha is measured to be about 6.7%.孔敏
Figure 3(a) shows a bright-field TEM image taken from the streamline region. The average grain size in this region is smaller than 1 µm. Most of grains are dislocation free; some grains containing high density of tangled dislocations are indicated by white arrows. W-pha particles with large size of 1.3 μm and small size of 50 nm are distributed in the matrix. The int in Fig. 3(a) shows the lected area electron diffraction pattern of the W-pha recorded from [111] zone axis.
Fig. 3 TEM bright-field images of as-extruded Mg−6Zn−4Y alloy: (a) Taken from streamline region showing broken intermetallics and matrix microstructure (the int is [111] lected area electron diffraction pattern taken from W-pha);
(b) Taken from a region without large broken particles showing coar grains and nano-scaled precipitates The large particles are broken eutectic W-pha. The broken process can be deduced from the crack and detached particles, as indicated by the black arrow and dotted ellip. The small particles are precipitates which mainly are disperd along grain boundaries.
A bright-field TEM image taken from large-particle free region is shown in Fig. 3(b). The grains in this region are coar in comparison with tho in streamline regions. A large number of small particles of 50 nm are homogeneously disperd within the matrix. Tho precipitates distributed along grain boundaries (indicated by the white arrows) can pin grain boundaries movements and retard grain growth at elevated temperatures. Some dislocations locked by the small particles can be obrved within the grains, as indicated by the black arrows.
The engineering stress—strain curves of as- extruded Mg−6Zn−4Y alloy consistently show pronounced yield points. A reprentative one is shown in Fig. 4. According to the engineering stress—
strain curves, the yield strength (YS), ultimate tensile strength (UTS) and elongation (δ) are (350±5) MPa, (371±10) MPa and (7±2)%, respectively.
多动症的表现
Fig. 4 Engineering stress—strain curve of as-extruded Mg−6Zn−4Y alloy
Figure 5 shows the tensile fracture of the as-extruded Mg−6Zn−4Y alloy. The fracture consists most of dimples and a few cleavage surfaces (Fig. 5(a)). The high magnification fractograph (Fig. 5(b)) reveals that the distribution of intermetallic particles on the fracture surface is not uniform. In some regions many particles crowd together, as indicated by the dotted circles. The regions are the streamline areas on the cross-ctions, as shown in Fig. 2(b). It is reasonable to conclude from Fig. 5 that during tensile test the dislocations pile up around the strengthening particles and result in microvoids forming at the interfaces between the intermetallic particles and the matrix. Therefore, the microvoids initially derive from the
YANG Wen-peng, et al/Trans. Nonferrous Met. Soc. China 21(2011) 2358−2364 2361
regions containing high density of particles, for example, the dotted circle regions as marked in Fig. 5(b), then the microvoids grow and coalesce into large cavities and finally result in fracture occurring.
Fig. 5 SEM images of fracture surfaces of as-extruded Mg−6Zn−4Y alloy: (a) Surface morphology; (b) Distribution of strengthening particles
4 Discussion
清明诗句古诗
4.1 Microstructure
Thermomechanical treatments are effective to: 1) refine matrix by DRX; 2) break intermetallic particles within grains and networks at grain boundaries, and make the strengthening particles well-distributed; 3) obtain uniform precipitates during hot working and following cooling process; and 4) form strong basal plane texture. It is suggested that microstructural refinement is much connected with processing parameters, such as wrought temperature, deformation ratio and deformation speed. Working with small deformation ratio and at low temperature, inadequate recrystallization is obrved sometimes, and uniform microstructure is hard to be achieved. As shown in Fig. 2(a), the microstructure of as-extruded Mg−6Zn−4Y alloy consists of 23% unrecrystallized regions; the regions have rrated boundaries. This kind of microstructure is considered to be the result of discontinuous DRX. The nucleation of recrystallized grains originates from the regions with high density of dislocation and large lattice misorientation, becau tho regions store much energy nee
ded for recrystallization. TEM obrvation found that at the initiation of hot deformation, the tangled dislocations focus on the original grain boundaries and the grain boundaries become wavy and corrugated [14]. Therefore, the recrystallized grains are firstly formed on the original grain boundaries by boundary bulging. As the strain is incread, more DRX grains are formed, and the original boundaries become more rrated. Since the coarr grains have larger grain boundary lengths, they have a great tendency to develop into rrated grain boundaries. Therefore, it can be concluded that the unrecrystallized grains are the residuals of original coarr grains. It was reported that the unrecrystallized grains have strong basal texture along ED [15]. However, they do not have enough stored energy to trigger recrystallization. As a conquence, the grains are deformed in shape, have large aspect ratio and conrve the coar form as a whole.
会计工作内容有哪些In addition, the strengthening particles can pin and prevent the movement of dislocations which are tangled and stored around the strengthening particles. Hence, the W-pha particles distributed in the alloy could act as the nucleation sources of DRX. At the same time, the strengthening particles have strong pinning effect to prevent the growth of recrystallized grains (Fig. 3). Therefore, the strengthening particles disperd in the matrix play an important role in refining process during hot deformation.
4.2 Tensile properties
In general, alloys containing W-pha are suppod to have low strengths due to weak interface bonding between the matrix and W-pha. However, in the prent experiment, Mg−6Zn−4Y alloy containing W-pha exhibits high tensile strengths. In order to compare with the previous results, the data of Mg−Zn−Y(−Zr) and Mg−Zn−Gd alloys are summarized in Table 1. It is apparent from Table 1 that the strengths of Mg−6Zn−4Y alloy are higher than tho of Mg−Zn−Y alloys, and are as high as tho of Mg−2.3Zn−14Gd alloy which has ultrafine-grained matrix enhanced by 14H LPSO structure. It is emphasized that grain refinement is the main strengthening mechanism for Mg alloys. For Mg−6Zn−4Y alloy, in addition to the contribution of grain refinement, high volume fraction of W-pha and strong fiber texture have large contributions as well.
The interface between the W-pha and α-Mg is incoherent. Hence, the W-pha particles are hard to shear by dislocation during tension and have strong pinning effect on the moving of dislocation in comparison with the pha having coherent interface
YANG Wen-peng, et al/Trans. Nonferrous Met. Soc. China 21(2011) 2358−2364
2362
Table 1 Tensile properties of Mg−6Zn−4Y alloy and previous data of Mg−Zn−Y and Mg−Zn−Gd alloys
Material Pha
t EX*/°C d/μm YS/MPa UTS/MPa δ/% Mg−6.4Zn−1Y [16] I-pha 250 10 213 321 18 Mg−5.53Zn−1.08Y−0.83Zr [11] I-pha 390 8 200 345 10.8
Mg−5.47Zn−3.69Y−0.86Zr [17] W-pha 390 4 216 318 12 Mg−6.4Zn−1Y [18] I-pha 400 3 252 335 12
电脑相机
Mg−6.7Zn−1.4Y [2] I-pha 210 1.0 315 337 16.8
Mg−2.3Zn−14Gd [19] 14H 350 0.5 345 380 6.9 Mg−6Zn−4Y W-pha
300
1.2
350
371
7
*t
EX
is the extrusion temperature
with the matrix. Thus, it is reasonable to conclude that materials containing W-pha also have high strengths.
At room temperature, the values of critical shearing stress for dislocation slip on {1010} 〈1120〉 prismatic and {1012} 〈1120〉 or {1012} 〈1120〉 pyramidal systems are about 100 times larger than tho on the {0001} 〈1120〉 basal slip system in pure magnesium [3]. Hence, the basal slip system is the easiest system to be activated and play the dominant role in plastic deformation at room temperature or moderately elevated temperatures. The prismatic and pyramidal slip systems only can be activated at high temperatures [20] or with fine grain size smaller than 10 μm at room temperature [21]. According to the Schmid’s law, when the slip plane and the slip direction are at 45°
relative to the stress axis, a maximum Schmid factor occurs and the shearing stress on the slip system reaches the maximum. However, AZEEM et al [22] found that the elongated grains are aligned along 〈1120〉 crystal direction with their c-axis perpendicular (within ±5°) to ED, thus the Schmid factor values on {0001} 〈1120〉 for the unrecrystallized grains are very low [23]. Furthermore, since the basal slip modes are restricted due to the strong fiber texture, the unrecrystallized regions may deform by twinning. The {1012} 〈1011〉 twinning system is the easiest system to be activated. Unfortunately, it is not active when the tensile direction is parallel to the basal plane [24]. Therefore, the grains have high resistance against yielding during tensile test. The as-extruded Mg−6Zn−4Y alloy contains 23% unrecrystallized regions, which have a significant improvement on the yield strength.撒的拼音和组词
4.3 Yield phenomenon
dnf多键连发
The engineering stress—strain curve for the as-extruded Mg−6Zn−4Y alloy shows a pronounced yield point, which is not often obrved in Mg alloys. To the best of our knowledge, the earliest explanation about yield phenomenon was propod by CLARK [25] in an Mg−Zn alloy. The yield point is attributed to the pinning effect on dislocations by β′1 precipitates at initial state, which are nucleated previously along the dislocation lines. Recently, the yield phenomena have been obrved
in Mg−Zn−Y [10] and Mg−Zn−Y−Ce [26] alloys procesd by powder metallurgy at consolidation temperatures below 300 °C. The alloys have fine grain sizes between 500 nm and 1.5 µm. Interestingly, when the wrought temperatures are above 300 °C, the yield phenomenon disappears. MORA et al [10] suggested that the yield phenomenon may be due to the weak texture of the alloy extruded at low temperatures becau the stress required for twinning increas with decreasing grain size more rapidly than the stress required to activate slip. But this yield phenomenon associated with the inhibition of twinning only maybe occurs in compression. Further, the yield phenomenon is rarely obrved in as-extruded alloys with typical basal texture prepared from as-cast ingot.
The prent authors suggest that the yield phenomenon could be related to the interaction of the residual dislocations with the strengthening particles. At high extrusion temperatures, the motion of dislocations is active and dislocations can easily move to grain boundaries and be absorbed, thus the DRX grains are dislocation free. However, at low extrusion temperatures, the motion of dislocations is not so active; some dislocations remain in grains and are pinned by the strengthening particles, as shown in Fig. 3(b). As a result, at the initial stage of tensile test, excess breakaway stress is required to overcome the pinning effects of the strengthening particles. When the dislocatio
ns break through the influence of strengthening particles, slip could occur at low stress, and the corresponding process on the stress—strain curve is the yield phenomenon.
5 Conclusions
1) Mg−6Zn−4Y alloy is mainly compod of α-Mg and W-pha, and the area fraction of W-pha is about 6.7%. The alloy extruded at 300 °C is not completely
