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Technical progress requires strong and light materials. Al-based amorphous
alloys possess high tensile strength as well as high specific strength due to
density of Al and hence have attracted increasing interest for a large number of
technical applications as light and high-strength structural materials. The
most prominent group of Al-based amorphous alloys includes binary Al-RE (RE:
rare earth metals) and ternary Al-RE-TM (TM: transition metals) system alloys
that can be produced by the melt-spinning technique. Among these amorphous
alloys, alloys in ternary Al-Y-TM system were found to possess high strength and
good bending ductility. Moreover, Al-Y-Ni-Co amorphous alloys exhibits
different devitrification behavior above and below the glass-transition
temperature. In the present paper, we examined the influence of Sc addition on
the formation, thermal stability and mechanical properties of Al-Y-Ni-Co
amorphous alloys.
Alloy
ingots with compositions of (Al0.84Y0.09Ni0.05Co0.02)100-xScx
and Al85Y8-xScxNi5Co2
alloys were prepared by Ar arc-melting the mixtures of Al, Y, Ni, Co and Sc.
From these ingots, ribbon samples of about 0.015 mm in thickness and 0.5 mm in
width were prepared by melt-spinning technique with single copper roller. The
microstructure of the ribbon samples was examined by X-ray diffractometry (XRD)
and transmission electron microscopy (TEM). The phase-transformation
temperatures and heat effects during transformations were examined by
differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s.
Mechanical properties were examined by Vickers micro-hardness indentation and
tensile test. Deformation and fracture behavior were also examined by means of
scanning electron microscopy (SEM).
Based
on the XRD and TEM experimental results, it is concluded that an amorphous
single phase is formed in the Sc concentration range up to 7 % of the rapidly
solidified (Al0.84Y0.09Ni0.05Co0.02)100-xScx
and Al85Y8-xScxNi5Co2
alloys. Fig. 1 shows a high-resolution TEM image and a selected-area
electron diffraction pattern of the (Al0.84Y0.09Ni0.05Co0.02)95Sc5
alloy and one can admit a possible existence of medium-range ordered zones with
a size below 1 nm. Fig. 2 shows DSC curves of the samples subjected to
continuous heating at a rate of 0.67 K/s. The small addition of Sc causes an
expansion of the supercooled liquid region, while the glass transition
phenomenon in the (Al0.84Y0.09Ni0.05Co0.02)100-xScx
alloys becomes undetectable with further increase in Sc content.
The
increase in Sc content leads to an increase in Vickers hardness and tensile
strength values (Fig. 3). However, alloys containing 6 at.%Sc or more
exhibit embrittlement. The samples containing up to 5 at.%Sc maintain excellent
bend ductility, shown by the ability to be bent through 180 degree without
fracture. The tensile test mechanical properties of the (Al0.84Y0.09Ni0.05Co0.02)95Sc5
and (Al0.84Y0.08Ni0.05Co0.02)100-xScx
alloys are summarized in Table I. The exceptionally remarkable values
are the ultrahigh tensile strength of 1505 MPa and Young's modulus of 78 GPa for
the (Al0.84Y0.09Ni0.05Co0.02)95Sc5
alloy. The low density values also indicate that these alloys exhibit high
specific tensile fracture strength values of 4.45 x 105 and 4.42 x 105 Nm/kg
which are higher than for Al-based crystalline and amorphous alloys those
reported up to date.
The
tensile fracture plane makes an angle of 50 degree with a uniaxial tensile
stress axis that deviates from the maximum shear plane declined by 45 degree
from the stress axis (Fig. 4). It indicates that the fracture behavior
does not follow completely the von Mises criterion and the fracture of this
alloy takes place as a result of combination of normal and shear stress. The
fracture surface of the (Al0.84Y0.09Ni0.05Co0.02)95Sc5
amorphous alloy consists of vein pattern and smooth regions typical for
amorphous alloys with good ductility. A number of multiple deformation bands
generated on the ribbon surface are also observed in Fig. 4(b). slip
marks can be also observed on the bent tip after a bend test over 180 degree as
shown in Fig. 4(c). Ultra-strong and ductile Al-based amorphous alloy
has been obtained at 20 at.% of the solute content even though all of the strong
and ductile Al-based amorphous alloys reported previously have about 15 at.% of
the solute content.
It is seen that the ultra-high
strength (Al0.84Y0.09Ni0.05Co0.02)95Sc5
alloy has an amorphous structure without any trace of a crystalline phase.
Therefore, its strength should be determined by the interatomic constraint
force. Sc is an element with highly negative enthalpy of mixing with Al, Ni and
Co and it may be the reason for the increase in the interatomic constraint
force. At the same time, Sc is completely miscible in Y and thus can substitute
it without prejudice to the properties.
New
ultrahigh strength amorphous A-Y-Ni-Co-Sc alloys developed in the present work
demonstrate excellent mechanical properties superior to those of other Al-based
fully crystalline and amorphous alloys reported up do date. The fracture
surface of the (Al0.84Y0.09Ni0.05Co0.02)95Sc5
amorphous alloy consisted of vein and smooth pattern regions typical for
amorphous alloys with good ductility. The addition of Sc also led to an
extension of the supercooled liquid region to 38 K at an optimum concentration
of 1.2 at.%, which allows various viscose flow working treatments within the
supercooled liquid region as similar to other glass alloy.
[Published in Journal of Materials Research, Vol. 19, No. 5
(2004), pp. 1539-1543] |
