Materials Transactions, Vol. 49, No. 8 (2008) pp. 1818 to 1821
#2008 The Japan Institute of Metals
Effect of T5 and T6 Tempers on a Hot-Rolled WE43 Magnesium Alloy
Kun Yu* , Wenxian Li, Richu Wang, Bo Wang and Chao Li
School of Materials Science and Engineering, Central South University, Changsha 410083, P.R.China
The influence of T5 and T6 tempers at 523 K on the mechanical properties and microstructure of hot-rolled WE43 alloy has been studied.
The microstructure of the alloy under various conditions was studied using an optical microscope, a transmission electron microscope, and a xray diffractometer. The mechanical properties were measured on a tensile testing system. The results show that the hot-rolled WE43 alloy has a
substantially higher hardness and strength improvement than the cast WE43 alloy. The hot-rolled WE43 alloy with a T5 temper showed better
properties than the alloy with a T6 temper because both strain hardening and age hardening increased the strength of the alloy. In addition, the
size of the precipitates after a T5 temper was smaller than that after a T6 temper. The yield strength of the alloy parallel to the rolling direction
was higher than that perpendicular to the rolling direction. [doi:10.2320/matertrans.MRA2008602]
(Received January 18, 2008; Accepted April 25, 2008; Published July 25, 2008)
Keywords: WE43 magnesium alloy, hot-rolling, heat treatment, rare earth element
1.
Introduction
Recently, magnesium alloys have become attractive for
many structural applications owing to their good specific
strength.1) Despite such properties, the use of magnesium
alloys is still relatively limited because of their low ductility
and low strength at elevated temperatures over 393 K.2)
Among commercial magnesium alloys, the WE43 alloy has
some of the best mechanical properties at high temperatures
and exhibits good corrosion resistance. It is a candidate
lightweight material for use in aerospace and automobile
applications at high temperatures.3,4) At the present, commercial products of WE43 alloy are always produced by
casting due to its low deformability. Some research on creep
and cracking in WE43 alloy and on structural aspects of that
alloy has focused on the cast material and not on deformed
alloy plate.5–7) But it can be anticipated that rolled plates of
this alloy will have more applications for industrial purpose
but research on deformed WE43 alloy plate is seldom
reported.8–10) Furthermore, the heat treatment of such
deformed WE43 alloy plate is also worth investigating
because this alloy belongs to heat treatment strengthen
magnesium alloy.2,11) Therefore, in the work described here,
the influence of two typical heat treatment methods, the T5
and T6 tempers, on the mechanical properties of hot-rolled
WE43 alloy plate were studied in order to develop a
technology suitable for producing high-strength plate. The
evolution of the microstructure of WE43 alloy during the
casting, hot-rolling and heat-treatment process was analyzed
in order to describe the variation of the properties of the alloy.
were hot rolled at 753–793 K with a pass reduction of about
20%–30% before reheating. The rolling had to be performed
at over 673 K, otherwise the plate would have cracked. The
hot-rolled experimental plates with a thickness of 2.6 mm
were subsequently heat treated in accordance with the T6 and
T5 tempers. In the case of the T6 temper, the plates were
solution treated at 798 K for 6 h, quenched in water at room
temperature, and then immediately artificially aged at 523 K
for 16 h. In the case of the T5 temper, the hot-rolled plates
were merely aged at 523 K for 16 h and then they were cooled
in the air at room temperature. In order to compared the
mechanical properties of as-cast and hot-rolled WE43 alloys,
the heat treatment temperatures and times were chosen
following those used for the as-cast WE43 alloy.2,11)
The tensile properties of the experimental alloy samples
(with a test size of 10 mm in width and 2.5 mm in thickness)
were measured by using a CSS-41000 testing system with
computerized data acquisition at a strain rate of 0.05 mm/s.
Hardness measurements of alloy specimens in various states
were made at room temperature using a Shimadzu HMV-2
Vickers hardness tester with a load of 9.8 N. The microstructure of the alloy was observed using a LEICA REICHERT Polyvar-MET optical microscope and a FEI Tecnai G2
20 transmission electron microscope (TEM). TEM foils were
prepared by standard twin-jet electropolishing in a solution
of 97% methanol and 3% nitric acid at 16 V and 253 K.
The phases in the alloy were analyzed by means of a Rigaku
D-MAX2000 x-ray diffractometer with a Cu target for
40 kV voltages and an operating current of 200 mA.
3.
2.
Results and Discussion
Experimental Methods
The experimental WE43 alloy was made using 99.9%Mg
and Mg–26.9%Y, Mg–31.5%Nd, and Mg–24.8%Zr master
alloys by melting at 1053–1083 K. The composition of the
experimental alloy was Mg–3.9%Y–2.8%Nd–0.6%Zr. The
cast billets, with a thickness of 30 mm, were homogenized at
793 K for 12 h to improve their deformability, and then they
*Corresponding
author, E-mail: [email protected]
3.1 Mechanical properties of alloy in various states
The hardness of the alloy specimens under various
conditions is shown in Fig. 1. It can be clearly seen that the
hardness values change with different conditions. In the ascast state, the Vickers hardness value of alloy after a T6
treatment is greater than that without heat treatment. The
hardness value of the hot-rolled wrought WE43 alloy was
greatly increased compared with that of the as-cast alloy. The
hardness of the alloy varied according to the subsequent
Effect of T5 and T6 Tempers on a Hot-Rolled WE43 Magnesium Alloy
Fig. 1
Hardness of WE43 alloy at different conditions.
Fig. 2 Tensile properties of WE43 alloy at different conditions.
C: cast; C+T6: cast+T6; R: hot-rolling; R+T6: hot-rolling+T6; R+T5:
hot-rolling+T5
heat treatment method after hot-rolling,. The hardness of the
hot-rolled alloy plate decreased greatly with a T6 temper, but
change was slightly that with a T5 temper. Therefore, the
alloy specimens have the highest hardness values (about
70 Hv) in a T5 temper following hot-rolling.
The yield tensile strength (0:2 ) and the ultimate tensile
strength (b ) show almost the same variation as the hardness.
The hot-rolled alloy samples with a T5 temper have the
maximum tensile strength under the same conditions as when
they have the maximum hardness value, as shown in Fig. 2.
On the other hand, hot rolling improves the yield strength of
the alloy substantially so that the ratio of 0:2 to b is higher
for the hot-rolled WE43 alloy than for the cast alloy. The hotrolled WE43 alloy also has an acceptable value of the
elongation (). The T6 temper is the favorite treatment for
improving tensile properties in the case of cast WE43 alloy,
but a similar situation does not apply in the case of hot-rolled
WE43 alloy. In hot-rolled Mg alloys, the main mechanical
properties are obtained through strain hardening. The
strength of hot-rolled WE43 alloy plate, especially the yield
strength, increases, although with some reduction of ductility. A subsequent heat treatment consisting of aging by
1819
Fig. 3 Yield strength of WE43 alloy parallel and perpendicular to the
rolling direction.
R: hot-rolling; R+T6: hot-rolling+T6; R+T5: hot-rolling+T5
means of a T5 temper combines strain hardening and
artificial age hardening. Therefore, the T5 temper WE43
alloy has good tensile strength. In contrast, a T6 temper may
improve the solid solution hardening of the alloy but will
relieve the strain hardening. Also, such a high solutiontreatment temperature would make the alloy grain recrystallize. Therefore, the strength of the hot-rolled alloy with a T6
temper will decrease to a level similar to that of the cast
alloy with a T6 temper.
It is important to investigate the variation of the yield
strength of hot-rolled WE43 alloy plate with direction. As
shown in Fig. 3, the yield strength of the sample parallel to
the rolling direction is higher than that perpendicular to the
rolling direction. The directionality of the mechanical
properties is more obvious than that of other wrought Mg
sheet alloys such as the AZ31 alloy.12) This phenomenon is
due to the orientation of second phases and deformed grains
in the WE43 alloy. The second phases in the alloy will be
dispersed along the rolling direction and will influence the
alloy strength substantially.
3.2 Microstructure of alloy under various conditions
The phases in the WE43 alloy were analyzed by using
x-ray diffraction. Three main compounds were identified in
the Mg matrix; these were Mg41 Nd5 , Mg12 Nd and, Mg22 Y5
(Fig. 4). The morphologies of these phases in various states
are shown in Fig. 5. All of these phases were located mainly
in the interdendritic boundaries (Fig. 5(a)). After homogenization, they were redissolved in the Mg matrix (Fig. 5(b)).
The microstructure in the as-cast alloy with a T6 temper is
shown in Fig. 5(c). It can be seen that there are many
dispersed second-phase precipitates in the matrix. Such small
precipitates will improve the alloy strength because these
second phase can act as obstacles for plastic flow. So the cast
alloy with a T6 temper will have a higher strength than that
at as-cast state. The hot-rolled microstructures are shown in
Fig. 5(d). The orientation of the second phases in the
experimental alloy parallel to the rolling direction is obvious
and thus is perhaps one of the main reasons that the strength
has different values in different directions in the plate in
Fig. 3. On the other hand, the pan-cake shape grains
K. Yu, W. Li, R. Wang, B. Wang and C. Li
Intensity /counts
1820
2θ/ °
Fig. 4
X-ray analysis of phases in WE43 alloy.
and 6(e)). Furthermore, the size of the second phases is larger
than in the T5 state because the precipitated phase will be
coarse on the high temperature during the solid solution
process at T6 temper. All of these changes are due to the
high-temperature solution treatment of the wrought plate.
The alloy will recrystallize and the second phase will coarsen
on such treatment. Recent investigations of the decomposition of WE alloys at 523 K have revealed the precipitation of
second phases such as phases in the Mg–Nd sequence, and
the main hardening phase in that case is the metastable 0
(fcc) phase.13,14) The yield strength of the alloy depends on
the temperature, which influences the precipitates of such
phases. Therefore, in accordance with the microstructure of
the alloy, a hot-rolled WE43 alloy with artificial aging and
without prior solution treatment will show an increase in
tensile strength.
Fig. 5 Microstructures of WE43 alloy at different conditions.
(a) as-cast condition; (b) homogenization state; (c) cast+T6 state; (d) hotrolling state;
elongated to rolling direction after hot-rolling are also
devoted to the increasing of strength.
The hot-rolled alloy retains the wrought morphology after
a T5 temper, as shown in Fig. 6(a); for instance, it retains
deformed grains along the rolling direction and deformed
twins in those grains. This microstructure shows that strain
hardening still influences the strength of the alloy. By TEM
observation (Fig. 6(b)), it was found that both deformed
dislocations and dispersed precipitates exist in the matrix.
The second phases are small and therefore they can pin the
dislocations (Fig. 6(c)). Therefore, both a strain-hardening
and a second-phase hardening mechanism influence the
high strength of the alloy under this condition. However, after
a T6 treatment, the alloy has recrystallized. The equiaxed
grains have no deformed twins or dislocations (Figs. 6(d)
4.
Conclusions
Hot-rolled WE43 alloy showed hardness and strength
substantially higher of than those of the cast alloy. Also, a
hot-rolled WE43 alloy with a T5 temper at 523 K had better
tensile properties than one with a T6 temper at 523 K because
both strain hardening and artificial age hardening increase the
strength of the alloy. In addition, the size of the precipitates
was smaller in the alloy with a T5 temper than in the alloy
with a T6 temper. The yield strength of hot-rolled WE43
alloy which was prepared parallel to the rolling direction was
higher than that perpendicular to the rolling direction. The
second phase was dispersed along the rolling direction and
influenced the variation of the strength of the alloy with
direction substantially. The alloy after a T5 temper retains
deformed twins and dislocations, but the alloy is recrystallized completely with a coarse regular grains after a T6
temper.
Effect of T5 and T6 Tempers on a Hot-Rolled WE43 Magnesium Alloy
(c)
(b)
(a)
50 µ m
(d)
1821
0.2µm
(e)
50 µ m
0.5µm
Fig. 6 Microstructures of WE43 alloy plates at T5 and T6 tempering.
(a) OM of alloy at T5 temper, (b) Twins and dislocations of alloy at T5 temper (TEM), (c) dislocations and second phase at T5 temper
(TEM), (d) OM of alloy at T6 temper and (e) TEM of second phase at T6 temper
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