Materials Transactions, Vol. 49, No. 11 (2008) pp. 2728 to 2731
#2008 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Room Temperature Aging Characteristic of MgLiAlZn Alloy
Chang-Chan Hsu1 , Jian-Yih Wang2; * and Shyong Lee1
1
2
Department of Mechanical Engineering, National Central University, Chungli 320-01, Taiwan, R.O.China
Department of Materials Science, National Dong Hwa University, Hualien 974-01, Taiwan, R.O.China
Making alloys of Mg and Li is quite a task as their individual melting temperatures are vastly different. However, it is thought paid off
considering their extreme lightness. Basically, low melting temperature Li could cause Mg-Li alloys’ microstructures change at room
temperature (R.T.). Recrystallization and aging of Li-rich solid solution phase () can proceed under R.T. Previously, R.T. aging phenomenon
was mentioned without in-depth analysis. This paper fills up the blank by working on Mg-11.2%Li-0.95%Al-0.43%Zn (mass%), mainly due to
the TEM work brought in. With this apparatus for analysis, a transition precipitate (tentatively called pre- phase) is revealed. This pre-
precipitation is responsible for the distinctive strengthening in the initial stage of R.T. aging. However, it is unstable and subsequently transforms
to or phase. Both of these later precipitates are little effective in producing aging strengthening. [doi:10.2320/matertrans.MER2008225]
(Received July 24, 2008; Accepted August 21, 2008; Published October 25, 2008)
Keywords: magnesium-lithium alloy, aging, precipitation, mechanical property
1.
Introduction
Magnesium alloy is the lightest metal that can be
employed for structural use. With another advantage of
being recyclable, Mg alloy is welcome1,2) considering
environmental appeal. However, general Mg alloys possess
poor formability because of their hexagonal close packed
structure. To overcome this shortcoming and further reduce
weight, alloying magnesium with lithium of extremely low
density, 0.534 g/cm3 , can achieve both goals.3) For example,
a Mg alloy with 12 mass% Li content weighs 3=4 the
general Mg alloys, so NASA pioneered in developing Mg-Li
alloy and successfully applied the LAZ141 to aero application.4) However, this specific success has not been transferred
to consumer industry due to strength insufficiency. After
intensive applications of various kinds of regular Mg alloys
in recent years, a re-thrust in developing Mg-Li alloy is
coming back.5–12) The critical issue is how to enhance the
strength; the major strategies are ‘‘alloying additional
element(s)’’ and ‘‘severe plastic deformation’’.5,6,8–11,13–20)
The former method adds elements like Al, Zn, Mn,5) Y, Be8)
or Al, Be, Ca18) followed by proper heat treatment to activate
solid-solution or precipitation strengthening mechanism.
The latter one employs cold rolling plus annealing19) or
ECAP5,9,11) to refine grain size such that the associated
mechanical properties are improved.
The category of Mg-Li alloy is not so widely studied that
a few proposed mechanisms or postulations responsible for
its strengthening have not been clarified or unified. An
interesting and uncommon phenomenon, room temperature
aging, was observed. Alamo et al.21) used XRD to analyze
its associated precipitating process and proposed its
sequence to be ! þ þ , where , and is Li-rich
solid solution phase, MgLi2 Al phase and Mg-rich solid
solution phase respectively. However, morphologies of
these phases were not verified by TEM analysis. It is this
uncertain aspect that this paper is aimed for, since we will
facilitate the observation and analysis with TEM together
with XRD.
*Corresponding
author, E-mail: [email protected]
2.
Experimental
High purity of Mg, Al, and Zn were mixed and melt in a
vacuum induction furnace first. Then a specified amount
of Li was added in the melt. The well melt liquid was cast
into a steel cylindrical mold and 203.2 mm diameter and
450 mm length ingot was fabricated. The chemical composition (Mg-11.2 mass%Li-0.95 mass%Al-0.43 mass%Zn) of
solidified alloy was analyzed with ICP-OES and Spark-OES.
The ingot was homogenized at 623 K, then extruded at 483 K
to obtain a 10 mm thickness by 110 mm width sheet, which
represents a extrusion ratio of 29:4 : 1. The extruded specimen was subjected to solution treatment with or without
aging at room temperature. Three types of specimens-barely
extruded, extruded plus solution treated and additionally
aged were analyzed for their microstructures with optical
microscope (OM), transmission electron microscope (TEM)
and X-ray diffraction (XRD).
Various treated specimens were mechanically tested for
hardness and tensile strength. For the Micro Vickers hardness
test, 100 g load is applied and each plotted datum is obtained
by averaging ten measured values. The tensile test was done
with the SHIMADZU AG-I type machine. Testing specimens
were prepared according to ASTM specification E8M with
gage length and cross head speed of 25 mm and 0.1 mm/min.
respectively.
3.
Results and Discussion
3.1 Microstructure analysis
Figure 1 shows the optical microstructure of the asextruded specimen, in which a single phase of an average
grain size 52 mm dominates the whole metallographic
domain. However, there is another phase in small particle
form (0.1–0.2 mm wide, 1–10 mm long, as shown by arrows)
lining up along the extrusion direction, and its volume
fraction accounts for only 2.3%. This second phase belongs
to Mg-rich solid solution which is not supposed to show up
based on the amount of Li content as referring to the Mg-Li
equilibrium phase diagram. An explanation according to
Song et al.,18) is that the Li content quantity is near the ð þ
Room Temperature Aging Characteristic of MgLiAlZn Alloy
Extrusion Direction
Fig. 1 Optical micrograph of the as-extruded showing the average grain
size to be 52 mm, with embedded few fine particles.
Fig. 2 Microstructure of the solution treated subsequent to extrusion,
showing grain growth to 83 mm.
Þ= boundary such that solidification-induced segregation
results in a ( þ ) micro-structure. TEM bright image and
the associated SADP verify the particles to be structured
sizing between 100 and 200 nm.
The extruded specimen was heated up to 673 K for 30 min.,
followed by quenching in water. This procedure is substantially a solution treatment to dissolve the pre-existing phase, however, it also causes grain growth to 83 um, which
is about 1.6 times that prior to the heat treatment (Fig. 2). The
above OM observation is verified by XRD as depicted in
Fig. 3 in which the solution-treated contains only the phase
while is identified in the as-extruded (pre-solution-treated)
specimen. Besides, a third phase is detected, which is (MgLi2 Al) and its existence was mentioned in Song’s
paper.18) It is not seen under OM, probably because of its
minute quantity and small size. Under XRD, the diffraction
angles of crystallographic plane (200) of the two specimens
differ by a very small amount. It is due to the change of
lattice parameters from 3.518 to 3.533 A as a result of solid
solution effect. This supersaturated solid solution bears
2729
0.45% lattice strain which can raise the strength as comparing
with the strain released specimen after over aging (Fig. 4).
This figure presents the strength as a function of aging time,
showing that peak strength occurs at 48 h and descends down
to a plateau in 840 hrs.
A series of optical micrographs showing a progressive
increasing of precipitate were observed with aging time
ranging from 48 up to 1176 h. During the first two time
periods of 48 and 168 h, the supersaturated solid solution
does not precipitate. At the time of 336 h, 6.3 vol% of precipitates in needle form both inside grains and on
boundaries. For the following time period, the amount of keeps increasing till 840 h. Each of these states is examined
by XRD as recorded in Fig. 5. In the initial three states
(0, 48 and 168 h aging), there is no precipitate but aging
strengthening does exist as the mechanical property demonstrates (Fig. 4). The phase starts to come out at 336 h and
becomes obvious after 504 hrs, however, the strength keeps
decreasing even below the level of the solid-solution treated.
So, the initial strengthening can not be attributed to precipitation. Actually, it causes softening as also claimed
by Alamo et al.21) A postulation is that the strengthening
phenomenon around 48 h aging is due to some other very
fine metastable precipitate which cannot be detected by
XRD, so TEM work is needed. In Fig. 6, the TEM micrograph does exhibit fine pallet shape particles of 2–3 nm wide
and 10 nm long as shown by arrows in matrix. Its SADP
was also mentioned by Song22) without further analysis,
and we identify it as cubic structure with lattice constant,
a ¼ 3:62 A. This figure is very different from that of the phase (MgLi2 Al) which has a ¼ 6:36 A. Actually, it is close
to the lattice constant of phase of 3.518 A. This akin
phase is capable of strengthening but unstable, and soon
transforms, probably to phase. We tentatively assign it
as pre- phase, and further identification work is needed
for the justification.
The specimen over-aged for 840 h is examined by TEM
(Fig. 7). In the bright field image, there exist particles of
100 nm, which is identified by SADP as phase. This
particle size is much larger than that of pre- phase such that
there is little strengthening effect.
3.2 Mechanical properties
The as-extruded specimen possesses 123.2 MPa tensile
strength together with 58.2 HV hardness and 96% elongation. The subsequent solution treatment largely enhances
the strength to 205 MPa and hardness to 90 HV, accompanied by 39% elongation. This 1.6 times enhancement
(205/123.2) is solely resulted from solid solution strengthening mechanism which is concurrently offset partially by
grain coarsening as depicted above. According to the
Hall-Petch relation, strength is inversely proportional to
the square root of grain size. Solid solution strengthening
is very prominent such that when it is relieved by over
aging precipitation, the strength level drops by 25% (150
vs. 200 MPa as referring to Fig. 4). The maximum strength
of 226 MPa is obtained after aging for 48 h when a metastable pre- phase is precipitated. Another later precipitated
phase after 168 h does not produce any impressive
strengthening.
C.-C. Hsu, J.-Y. Wang and S. Lee
(200)
2730
(a)
20
30
40
50
60
(220)
(422)
(112)
(201)
(103)
(110)
(400)
(102)
(311)
(002)
(220)
(111)
(100)
(101)
Intensity
(211)
(110)
α
β
θ
70
80
(200)
2θ
(b)
(110)
(211)
Intensity
β
20
30
40
50
60
70
80
2θ
Fig. 3 XRD patterns of the as-extruded (a) and solution treated (b), The insert close-up differentiates their 2 diffraction angles of the
(200) crystallographic plane.
4.
Conclusions
A Mg-11.2 mass%Li-0.95 mass%Al-0.43 mass%Zn alloy
is studied, mainly on its room temperature aging behavior
and associated mechanical properties. It is confirmed that this
alloy can undergo room temperature aging, and a previously
undiscovered precipitate, tentatively called pre- phase, is
identified by TEM. Microstructural evolution is that solution
treatment can produce supersaturated solid solution .
Subsequent room temperature aging would let pre- phase
precipitated, then the final stable co-existing phases are
þ þ . The highest tensile strength obtained for this
MgLiAlZn is 226 MPa.
Acknowledgements
Fig. 4
Strength (UTS) as a function of R. T. aging time.
The authors are pleased to acknowledge the financial
support of this research by the National Science Council,
ROC, under Grant No. NSC 96-2622-E-259-004-CC3.
Room Temperature Aging Characteristic of MgLiAlZn Alloy
2731
Fig. 5 XRD patterns of each specimen after quenched and aged for various times.
10
200
110
Fig. 6 TEM bright-field image and SADP of the specimen subjected to
peak aging, showing pre- phase.
REFERENCES
1) J. Y. Wang, W. P. Hong, P. C. Hsu, C. C. Hsu and S. Lee: Mater. Sci.
Forum 419–422 (2003) 65.
2) Y. Kojima and S. Kamado: Mater. Sci. Forum 488/489 (2005) 9.
3) A. A. Nayeb-Hashemi, J. B. Clark and A. D. Pelton: Phases Diagrams
of Binary Magnesium Alloys ed. by A. A. Nayeb-Hashemi, J. B. Clark
(ASM 237 International, Ohio, USA, 1998) pp. 184.
4) T. G. Byrer, E. L. White and P. D. Frost: NASA Contractor Report,
(Battelle Memorial Institute, Columbus, OH), 1963.
5) T. C. Chang, J. Y. Wang, C. L. Chu and S. Lee: Mater. Lett. 60 (2006)
3272–3267.
6) C. C. Chiu, J. Y. Wang and H. Y. Wu: Mater. Sci. Forum 546–549
(2007) 229.
7) H. Zhong, P. Liu, T. Zhou and H. Li: J. University of Science and
Technology Beijing, V.12, N.2, April (2005) 182.
8) D. K. Xu, L. Liu, Y. B. Xu and E. H. Han: Scr. Mater. 57 (2007) 285.
9) J. Y. Wang, T. C. Chang, L. Z. Chang and S. Lee: Mater. Trans. 47
(2006) 971.
10) C. H. Chiu, J. Y. Wang and H. Y. Wu: Mater. Trans. 47 (2006) 966.
Fig. 7 TEM bright-field image and SADP of the specimen subjected to
over aging, showing phase.
11) T. Liu, S. D. Wu, S. X. Li and P. J. Li: Mater. Sci. Eng. A 460–461
(2007) 499.
12) C. H. Chiu, H. Y. Wu, J. Y. Wang and S. Lee: J. Alloy. Compd. 2007
(In press, Available online 2 June 2007).
13) G. V. Raynor and J. R. Kench: J. Inst. Met. 88 (1959–1960) 209.
14) J. C. McDonald: J. Inst. Met. 97 (1969) 353.
15) Y. Kojima, M. Inoue and O. Tanno: J. Jpn. Inst. Met. 54 (1990) 354.
16) S. Hori and W. Fujitani: J. Jpn. Inst. Light Met. 40 (1990) 285.
17) K. Matsuzawa, T. Koshihara, S. Ochiai and Y. Kojima: J. Jpn. Inst.
Light Met. 40 (1990) 659.
18) G. S. Song, M. Staiger and M. Kral: Mater. Sci. Eng. A 371 (2004) 371.
19) P. Crawford, R. Barrosa, J. Mendez, J. Foyos and O. S. Es-Said: J.
Mater. Proc. Technol. 56 (1996) 108.
20) N. Saito, M. Mabuchi, M. Nakanishi, K. Kubota and K. Higashi: Scr.
Mater. 36 (1997) 551.
21) A. Alamo and A. D. Banchik: J. Mater. Sci. 15 (1980) 222.
22) G. S. Song, M. Staiger and M. Kral: Magnesium Technology 2003, ed.
H. I. Kaplan, (TMS, 2003) p. 77.