HIGH-STRENGTH ALUMINUM-BASED ALLOYS
HARDENED BY QUASICRYSTALLINE NANOPARTICLES
Milman Yu.V., Lotsko D.V., Sirko O.I., Chugunova S.I.
I.M.Frantsevych Institute for Problems of Material Science, Kiev,
Ukraine
e-mail: [email protected]
Traditional strengthening mechanisms for Al alloys developed for last
eight decades [1] permitted to obtain the upper limit of tensile
strength s[f] to 660 MPa in sheets [2]. In our investi-gations this
level was increased to 800 MPa in rods due to alloying with Sc. Last
decade by a group of Japanese scientists headed by Prof. A.Inoue there
were proposed new high-strength Al-based alloys with s[f ]to about
1500 MPa that are described in reviews [1, 3]. For hardening these
alloys the nonequilibrium phase effects (the creation of nonperiodic
structures in particular) were used. The latter included amorphous and
quasicrystalline icosahedral phases (I-phases). Maximum s[f] was
obtained in melt-spun ribbons. In bulk materials with I-phase produced
from argon-atomized powders s[f ]was to 850 MPa in combination with
the elongation d ť 10 %. Last two years some works of German [4] and
Romanian [5] scientists in this subject were published.
High strength and good ductility in Al alloys with I-phase are
expected, when the I-phase is in the state of uniformly distributed
nanosize particles of spherical morphology. It is caused by a special
deformation mechanism of the I-phase [3]: a room temperature
deformation is connected with the formation of approximant crystalline
phases on a subnanoscale that takes place in phason defects. The
formation of these phases is as a rule accompanied by changing the
chemical composi-tion provided by diffusion from a-Al matrix. The
refinement of I-phase particles to nanoscale size in Al-rich alloys
introduces a high density of phason defects to the I-phase and
facilitates alloy ductility.
High-strength Al+I alloys were produced on the base of the systems
Al-R-EM and Al-R-LM, where R = rare-earth metals, EM = Ti, Zr, Hf, Nb,
Ta, Cr, Mo, W, and LM = Mn, Fe, Co, Ni, Cu, in Al-rich composition
range above 92 at. % Al. Elevated temperature alloys were developed on
the base of Al-Fe-Ti-M systems with M = Cr, V, Mn [1, 3, 6]. An alloy
Al[93]Fe[3]Cr[2]Ti[2] appeared the best. Its s[f] at 300 C was 350
MPa, which exceeds the air-force goal level and did not change after
annealing the sample at 300 C for 1000 h [1]. Hardening by I-pase
nanoparticles was found also in maraging steels [7].
We studied structure and mechanical properties of rapidly quenched
alloys of Al-Fe-Cr-Ti system with additional alloying with Zr and Sc.
The investigation was carried out with melt-spun ribbons and powder
alloys. Powders were produced by an original water-atomization
technique developed in the IPMS. In ribbons Al[93]Fe[3]Cr[2]Ti[2] of
25 mm in thickness and Al[92.8]Fe[3]Cr[2]Ti[2]Sc[0.2] of 45 mm in
thickness we obtained I-phase particles of 15-50 nm in size.
In addition we manufactured the ribbon Al[84.2]Fe[7]Cr[6.3]Ti[2.5]
that according to [6] shall be almost completely quasicrystalline.
Experiments with ribbon annealing have shown that ribbon hardness
remains practically unchanged after annealing at 400 C. A drop of
hardness after annealing at higher temperatures is caused by vanishing
the I-phase and appearing crystalline intermetallics (Al[23]Ti[9] and
Al[13](Cr,Fe)[2]) accompanied by their coagulation.
X-ray investigation confirmed a high level of phonon and phason
distortions in small I-phase particles that were higher than in purely
quasicrystalline ribbon 5 and 2 times respectively. It provided about
1 % of plastic strain while ribbon bending. In the ribbon
Al[95]Fe[0.7]Cr[4.3] (ribbon 4) I-phase particles were of 100-700 nm
in size, and its hardness was as low as 1200 MPa.
For producing powder alloys we used powder fractions 1: (- 63) mm and
2: (63-100) mm. For powders of Al[93]Fe[3]Cr[2]Ti[2 ]and
Al[92.8]Fe[3]Cr[2]Ti[2]Sc[0.2 ]alloys in both of them together with
I-phase a large amount of Al[23]Ti[9] intermetallic was revealed, a
small changing of composition permitted to obtain powders with only
icosahedral second phase (alloys #3-8 in Table). To preserve I-phase
it was necessary to carry out powder consolidation at temperatures not
exceeding 400 C. In alloys except 6 powder billets were degassed and
vacuum forged at 400 C, but their extrusion to rods of 9 mm in
diameter (extrusion ratio 7.7) was possible only after heating the die
to 500 C and the billet to 400 C. Alloy 6 was extruded in an
evacuated aluminum capsule without forging with heating to 420 C both
die and billet. The rod 9 was produced by consolidation of crushed
ribbon. Samples from rods with gauge diameter of 3 mm were tested in
an Instron-type machine at a strain rate of 10-3 s-1.
I-phase was preserved only in the rod 6 (of 70-200 nm in size). In
other rods including rod 9 crystalline intermetallics were formed to
400 nm in size. Rods were also distinguished by very small grain size
of a-Al matrix - less than 1 mm.
Evidently, the existence of small I-phase particles in powders
facilitated the formation of small uniformly distributed intermetallic
particles while thermomechanical treatment at temperatures exceeding
the temperature of quasicrystal stability.
Thus, Al alloys from water-atomized powders hardened by fine
quasicrystalline particles or intermetallic particles formed on their
base can have strength higher than 300 MPa at 300 C. Note that Sc and
Zr increased the elevated temperature strength of powder alloys.
Table. Composition and tensile mechanical properties of rods from
powder alloys
#
Composition
T,
C
s[0.2],
MPa
s[U],
MPa
d,
%
1
2
3
4
5
6
7
8
9
Al[93]Fe[3]Cr[2]Ti[2]
Al[92.8]Fe[3]Cr[2]Ti[2]Sc[0.2]
Al[93.4]Fe[2.6]Cr[2.8]Ti[1.2]
Al[93.7]Fe[2.6]Cr[2.8]Ti[0.6]Zr[0..3]
- " - fraction 2
- " - fraction 2, extrusion in capsule
Al[93]Fe[2.6]Cr[2.8]Ti[1.2]Zr[0.4]
Al[94.7]Fe[2.6]Cr[2.7]
Al[93]Fe[3]Cr[2]Ti[2], extruded ribbon
20
538
520
469
-
-
618
-
398
-
574
567
561
536
539
649
627
459
449
1.0
0.7
5.1
0.1
0.2
0.6
0.1
11
0
1
2
3
4
5
6
8
9
Al[93]Fe[3]Cr[2]Ti[2]
Al[92.8]Fe[3]Cr[2]Ti[2]Sc[0.2]
Al[93.4]Fe[2.6]Cr[2.8]Ti[1.2]
Al[93.7]Fe[2.6]Cr[2.8]Ti[0.6]Zr[0..3]
- " - fraction 2
- " - fraction 2, extrusion in capsule
Al[94.7]Fe[2.6]Cr[2.7]
Al[93]Fe[3]Cr[2]Ti[2], extruded ribbon
300
255
271
269
313
297
274
221
321
294
315
313
343
328
303
247
360
2.5
2.9
2.7
1.5
1.5
1.9
3.1
1.7
1. Inoue A., Kimura H.M. // Proc. Mater. Res. Soc. Symp., Warrendale,
PA, 1999. - 553. - P. 495.
2. Fridlyander I.N. // Mater. Sci. Forum. - 2000. - 331-337. - P.
921.
3. Inoue A. // Progr. Mater. Sci. - 1998. - 43. - P. 365.
4. Manaila R., Popescu R., Jianu A. et al. // J. Mater. Res. - 2000. -
15, No. 1. - P. 56.
5. Schurack F., Eckert J., Schultz L. // Acta mater. - 2001. - 49. -
P. 1351.
6. Kimura H.M., Sasamori K., Inoue A. // J. Mater. Res. - 2000. - 15,
No. 12. - P. 2737.
7. Nilsson J.-O., Liu P., Dzugutov M. // Proc. Mater. Res. Soc. Symp.,
Warrendale, PA, 1999. - 553. - P. 513.
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