Superplastic Behavior of an Extruded Mg-Zn-Y

Materials Science Forum Vols. 546-549 (2007) pp. 337-341
online at http://www.scientific.net
© (2007) Trans Tech Publications, Switzerland
Superplastic Behavior of an Extruded Mg-Zn-Y-Zr Alloy
Weineng Tang, Daokui Xu, Rongshi Chena, En-hou Hanb
Environmental Corrosion Center, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
a
[email protected], [email protected]
Keywords: Mg-Zn-Y-Zr alloy; Mechanical behavior, Superplasticity; Deformation mechanisms,
Abstract. A conventional extrusion has been carried out to induce significant grain refinement in
Mg-Zn-Y-Zr alloy. The results showed that good superplasticity have obtained in this extruded alloy.
The simple and rapid processing route may allow it to be put into successfully practical use in
industry. The effects of temperature and strain rate on superplastic deformation of the extruded alloy
were studied. The optimum condition with the elongation of more than 450% was found to be at
450°C and 3.3×10-4s-1. Jump tests were carried out at 300-450°C and 8.3×10-5 ~ 1.7×10-2s-1. The
activation energy for superplastic deformation at 300-450°C is 106kJ/mol and the stress exponent is
about 2.8. The superplasticity observed in this studied condition may be attributed to mechanisms of
dislocation creep mainly within large grains and grain boundary sliding (GBS) of small grains.
Introduction
Magnesium alloys have high potential as lightweight structural materials because of their low
density and recyclable ability. To date, most structural Mg products have been fabricated by casting
methods. In order to increase structural applications of Mg alloys, development of wrought alloys is
desirable [1]. However, Mg alloys generally exhibit poor workability because of its hcp structure. In
order to exploit the benefits of magnesium alloy, it is important to develop some processes, which can
effectively produce complex engineering components directly from wrought products. Therefore, it is
required to improve poor workability of Mg alloys for development of plastic forming technology. It
has been demonstrated that many magnesium alloys exhibit some extent superplasticity [1-3.]
Application of superplasticity is expected for the plastic forming of the hard-to-form magnesium
alloys [2].
Wrought Mg-Zn-Y-Zr alloys recently attracted wide attention because they have both high strength
and good ductility either at room temperature or at elevated temperatures [4]. An icosahedral phase
(I-phase) of composition Mg3Zn6Y usually appears in this alloy [5]. Since I-phase has many good
properties such as high hardness, thermal stability, low coefficient of friction, low interfacial energy
[5], its particles can effectively prohibit grain growth and cavitation during high temperature
deformation [4].
The aim of this paper is to systematically study the high temperature deformation behavior of the
extruded fine-grained Mg-Zn-Y-Zr alloy. The dependence of temperature and strain rate on
superplastic behavior was investigated in order to reveal the high temperature deformation
mechanisms of this alloy.
Experimental Procedure
The chemical composition of Mg-Zn-Y- Zr alloy used in this study is 6.5%Zn, 1.0%Y, and 0.9%Zr
(wt.%). The alloy was fabricated with pure magnesium (99.9%), pure Zn (99.99%), Mg-25%Y
master alloy, and Mg-33%Zr master alloy ingots by resistance melting under SF6 and CO2
atmosphere. The melting was poured into a cylindrical metal model with a diameter of 100 mm. The
ingots were homogenized at 350°C for 5h and then hot extruded into plates at 350°C with section of
5mm in thickness and 60mm in width. The extrusion ratio is 25:1. The extruded plates were annealed
at 350°C for 1h, and then were machined along the extruded direction into tensile specimens with the
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Light Metals, Aerospace Materials and Superconductors
section of 4mm × 3mm and gauge length 10mm. Tensile test was carried out at a constant cross-head
speed at temperature of 300°-450°C and strain rate of 8.3x10-4s-1 to 3.33x10-2s-1. Before tensile tests,
samples were hold in the furnace for about 15mins at the test temperature. Jump tests were also
carried out through instantaneously changing the crosshead speed by a factor of 2 or 2.5. The
microstructure and fracture surface of the deformed samples were observed with Philip XL30 Field
Emission Scanning Electron Microscope (FESEM) operated at 10 kV.
Results
Microstructure and Mechanical Behavior. The average grain size of the extruded alloy after
annealing at 350°C for 1h is about 10μm. It can be seen from Fig.1(a) that the microstructure is rather
heterogeneous. A small number of large grains of approximately 20 µm in diameter are embedded in
a matrix of small-recrystallized grains about 5 µm in diameter. There are some second phases in the
alloy, which are I-phase with the formula of Mg3Zn6Y.
Fig. 1. Microstructure of the extruded alloy after annealing at 350°C for 1h. Extrusion direction is
horizontal; (b) is the magnification of the region marked with A in (a).
Fig. 2. (a) .Stress-strain curves of the samples tested at 1.7×10-3s-1 and different temperatures;.(b).
samples before and after tensile tests.
Tensile tests at a constant strain rate of 1.7 × 10-3 s-1 and different temperatures are shown in Fig. 2.
The strain hardening gradually weakens with the tested temperature increasing. The dependence of
the elongation to failure on the strain rate and temperature are shown in Fig. 3. It is shown that the
elongation is better at intermediate strain rate of 1.7 × 10-3 s-1 and intermediate temperature of 400°C
than that in others conditions, except at 450°C and 3.3×10-4s-1, where the optimum superplastic
elongation of 450% in the tested temperatures and strain rates range was obtained. The fracture
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surfaces of the samples tested at different temperatures are partly shown in Fig. 4. The fracture
appears to be typical superplastic fractural surface.
Fig. 3. The dependence of the elongation to failure on: (a) strain rate and (b) temperature.
Fig. 4. The SEM images of fracture surface of samples tested at 450°C and 1.7 × 10-3 s-1. (b) the
magnification of the position marked with black rectangle in (a).
Deformation Mechanism. At constant strain (ε) and temperature (T), the strain rate sensitivity, m,
can be calculated according to m= ∂ (lnσ)/ ∂ (ln ε& ) [6], which can be obtained from the slopes in the
plot of lnσ vs. ln ε& (Fig. 5(a)). The m value is 0.33 for the sample tested at 300°C and about 0.36, 0.36
and 0.39 for the sample tested at 350, 400 and 450°C, respectively.
The superplastic deformation can be considered as a simple thermal activated process, which can
be expressed by an Arrhenius-type rate equation. According to the data in Fig. 5(a), the stress
exponent n can be assumed as a constant, and then be under a constant stress at these test
temperatures, the apparent activity energy can be evaluated according to the equation [7]:
∂ (ln ε& )
Qa = − R
(1)
σ = cons tan t .
∂ (1 / T )
Where Qa is the apparent activation energy for the rate controlling process, and R is the gas
constant (R=8.31J/Kmol). In the plot of ln ε& vs. 1/T, Fig. 5(b), Qa is calculated to be 106kJmol-1. The
stress exponent, n, can be obtained from the slope in the plot of ln[ ε& exp(Q/RT)] vs. lnσ. The value of
n is estimated to be around 2.8 as shown in Fig. 5(c).
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Light Metals, Aerospace Materials and Superconductors
4.5
4.0
-4
(a)
σ = 18 MPa,
-5
σ=18MPa
3.5
Q = 106 kJ/mol
-1
-6
2.5
lnε, s
lnσ, MPa
3.0
.
2.0
-7
0
.
300 C
0
350 C
0
400 C
0
450 C
1.5
1.0
3
lnε=-Q/RT= -12.779X(1/TX10 )
-8
-9
0.5
-10
-9
-8
-7
-6
-5
-4
1.3
-3
1.4
.
lnε
9
10
8
10
7
10
6
10
5
10
4
10
3
10
2
εexp(Q/RT), s
-1
10
1.5
1.6
3
1.7
1.8
-1
10 /T, K
0
300 C
0
350 C
0
400 C
0
450 C
2.8
.
1
1
10
100
σ, MPa
Fig. 5. Data obtained from jump tests performed at 300, 350, 400 and 450°C: (a) Strain rate
dependence of stress. (b) Activation energy for the deformation. (c) Relationship between the steady
state stress and the normalized strain rate over the temperature range of 300-450°C.
Discussion
It is well known that the value of strain rate sensitivity, m, of metals is usually low at room
temperature but it increases with temperature rising, especially at temperatures above half of the
absolute melting point. High strain rate sensitivity, usually m> 0.33, is a characteristic of superplastic
metals and alloys [6]. The m for the present alloy tested at 300 to 450°C are all around or more than
0.33, which also indicates that superplasticity should appear in this temperature and strain rate range.
As the experimental result show, in which over 450% elongation is obtained at 450°C and 3.3×10-4s-1.
Many attempts have been made to develop theories for predicting both the mechanical and
topological features of superplastic deformation [6]. One of the main mechanisms for superplasticity
in terms of microstructural features is grain boundary sliding, which has been accepted as the
dominant deformation process of superplastic flow [8, 9]. The grain compatibility during grain
boundary sliding is also required to maintain by some accommodation mechanisms [10].
Obvious grain boundary sliding characters can be found in Fig. 4 on fracture surfaces of the
deformed specimens. Superplastic deformation process dominated by a GBS mechanism usually
leads to a stress exponent equal to 2, while stress exponent of 3–7 is usually obtained for dislocation
creep controlled deformation processes [6,11]. The stress exponent, n, calculated based on the
activation energy is about 2.8 for present alloy, which is exactly the reciprocal of strain rate
sensitivity (m =0.36) calculated above. The n value of 2.8 also suggests that the mechanism of
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superplastic deformation in present alloy is mainly controlled by GBS assisted by dislocation creep
mechanism. The apparent activation energy for superplastic deformation in present alloy is calculated
to be about 106 kJmol-1, which is between the activation energy for magnesium lattice diffusion
(135kJmol-1) and that for grain boundary diffusion of magnesium (92kJmol-1) [3]. Higashi et al [12]
has pointed out that both lattice and grain boundary diffusion likely play an important role during
superplastic deformation of Mg alloys and an effective diffusion coefficient, Deff, must be considered.
This coefficient is a combination of lattice diffusion and grain boundary diffusion coefficients.
Moreover, it is also well known that grain boundary sliding is usually considered as a major possible
plastic deformation mechanism in fine grains materials, and dislocation activity in fine grains (less
than 5μm) is presumably very limited [6]. Therefore, in present alloy, the dislocation creep may
mainly control the plastic deformation within large grains. The large elongation observed in this
extruded alloy could be a result of combination of these two major deformation mechanisms. The
exact contribution of each mechanism is, however, still not clearly established.
Summary
Superplastic deformation behavior was investigated in the extruded Mg-Zn-Y-Zr alloy with mixture
microstructure of small-grained (~5μm) and large grains (~20μm) in order to understand the
dominant deformation mechanisms at 300-450°C. It was found that the material exhibited
superplasticity with maximum elongations-to-failure over 450% at 450°C and 3.3×10-4s-1. The
activation energy of the alloy for superplastic deformation at temperature of 300-450°C is about 106
kJ/mol, and the stress exponent is about 2.8. The mechanisms for superplasticity in present alloy may
be a combination of dislocation creep within large grains and GBS of small grains.
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