1 Introduction
Zr-based bulk metallic glasses (BMGs) belong to the group of modern metallic materials. The fabrication of such materials,
i.e., obtaining an amorphous structure, requires sufficiently fast cooling rates during solidification to bypass the crystallization process. In comparison to their crystalline counterparts, BMGs are characterized by superior mechanical, physical and chemical properties.[
1] These characteristics make them a promising structural and functional material for many engineering applications,
e.g., sport goods, high-performance springs, micro-geared motor parts, pressure sensors, biomedical application,
etc.[
2,
3]
Unfortunately, the fabrication of Zr-based BMGs is very expensive, due to the necessity of using high purity constituent elements and strict processing conditions. Special attention must be paid to oxygen content, as this element dramatically reduces the alloy’s glass forming ability (GFA) and deteriorates its mechanical properties.[
4‐
8] Such a dramatic drop in the GFA is caused by the formation of oxides or other oxygen-induced inclusions, acting as perfect heterogeneous nucleation sites during solidification.[
9‐
12] The ZrO
2 oxide is regarded as a nucleation catalyst in Zr-based alloys. This oxide exists in three crystallographic forms: monoclinic α ZrO
2, which is thermodynamically stable up to 1478 K, tetragonal β ZrO
2 stable between 1475 K and 2650 K, and finally cubic γ ZrO
2 (CaF
2—type) phase, which is stable up to the melting point, 2983 K.[
13]
In our previous work, we studied the Zr
50Cu
40Al
10 alloy with two oxygen levels, 194 ± 26 wt ppm and 918 ± 72 wt ppm.[
4] The critical diameter D
c (the maximum amorphous thickness) of this alloy was determined to be 8 mm and below 3 mm for the alloy with low and high oxygen contents, respectively. If the sample diameter exceeded the critical size, the metastable CuZr phase, with a small amount of the
τ3 (Zr
51Cu
28Al
21) phase, were observed for the low oxygen variant. In comparison, only the
τ3 phase was present in the high oxygen level sample. According to the ternary phase diagram,[
14]
τ3 is the first solidifying phase for this chemical composition. This shows that oxygen impurities facilitate the
τ3 phase formation by providing heterogeneous nucleation sites during solidification.
It is not possible to completely eliminate oxygen from raw elements, due to the high chemical affinity of zirconium and oxygen. Moreover, high purity and Hf-free zirconium is not commercially available, as it exhibits a very low neutron capture cross-section, which made it a strategic material for nuclear power plant applications.[
15] Commercially available Zr-sponge can contain more than 1000 wt ppm of oxygen.
Therefore, an idea of doping low-purity (high oxygen) alloys with rare-earth elements (REEs) was proposed by Zhang et al.[
16] Such elements, with higher affinities to oxygen than zirconium, can act as an oxygen scavenger and form a less harmful oxide. So far, various glass forming systems doped with different rare-earth elements have been investigated.[
16‐
24] Significant attention was paid to yttrium, which increases the GFA of Zr-Al-Ni-Cu,[
20] Zr-Co-Al,[
21] Zr-Ti-Cu-Ni-Be,[
16] Cu-Zr-Al[
22] alloys. However, yttrium additions and oxygen concentrations were not correlated. Additionally, yttrium was changed monotonically (1, 2, 3 pct,
etc.), meaning that the total effect of yttrium is a sum of scavenging and alloying effects. As REEs exhibit strong affinities with oxygen, their scavenging effect should be studied with respect to the known, to be bound, oxygen content in the alloy. More detailed investigations were carried out by Kündig et al. [
19] where the effects of Sc and La additions on the GFA were studied in the Zr
52.5Cu
17.9Ni
14.6Al
10Ti
5 alloy that could be cast into 4 mm glassy sample without the necessity of doping. Small Sc additions, preceded by careful oxygen measurements, to the undoped alloy significantly increased the D
c. The highest GFA was obtained for 0.03 to 0.06 pct of Sc, reaching D
c of almost 12 mm. This Sc contents range corresponds to the stoichiometric and double stoichiometric level required to form the Sc
2O
3 oxide.
In this paper, we systematically studied the effect of yttrium additions to the Zr
50Cu
40Al
10 alloy, with high oxygen content, on its glass-forming ability. Yttrium is generally considered a heavy REE, due to its similar ionic radius to Dy.[
25] Similarly to other REEs, the Y
2O
3 sesquioxide is the most stable type of oxide. The Gibbs-free energy of yttrium oxide formation is significantly lower in comparison to zirconium oxide within the entire temperature range: Y
2O
3: − 1816.6 kJ/mol and ZrO
2: − 1042.8 kJ/mol at 298 K; Y
2O
3: − 1392.3 kJ/mol and − 763.7 kJ/mol at 1800 K.[
26] The crystallographic structure of Y
2O
3 will play a key role in the formation of undesirable phases during solidification. During heating, these sesquioxides polymorphically transform from a cubic (
\( Ia\bar{3} \) space group
) to a high-temperature hexagonal (
P63/
mmc space group) structure at 2600 ± 30 K, becoming stable until reaching its melting point (2712 K).[
27] Two other forms of Y
2O
3 can be found at higher pressures: monoclinic above 13 GPa and hexagonal above 24.5 GPa.[
28] As these two phases do not occur at ambient pressure, they are not considered in the present study.
2 Material and Methods
Based on our previous work,[
4] the oxygen level in the undoped Zr
50Cu
40Al
10 alloy (918 ± 72 wt ppm) was recalculated into atomic percentage (4213 at ppm) and this amount of oxygen is intended to be bound by yttrium. The stoichiometric yttrium concentration to bind all oxygen into the Y
2O
3 oxide (herein referred to as B2/3) was calculated to be 0.281 at. pct. Moreover, half (B1/3) and double stoichiometric (B4/3) levels of yttrium with respect to oxygen content were studied. The chemical compositions of the investigated alloys and sample designations with respect to the Y-to-oxygen ratios are presented in Table
I. The alloys were synthesized by means of arc melting and suction-casting into copper mold cavities in a Ti-gettered argon atmosphere (6 N) using low-purity Zr (99.8 pct) and high purity Cu (99.99 pct), Al (99.999 pct) and Y (99.9 pct). First, cone-shaped samples were produced to find the approximate D
c, which was determined using a threshold of 5 pct crystalline material.[
19] As the critical diameter obtained from the parallel-edge molds may differ from the conical ones,[
4,
29] a series of rod samples with a gradually increasing diameter were cast and characterized by microstructure observations and X-ray diffraction (XRD, Panalytical Empyrean diffractometer) with Cu K
α radiation. The rod sample with the highest diameter, in the absence of sharp Bragg peaks on XRD, was assumed to be amorphous, and this diameter was taken as the critical value. Critical diameter values determined on conical and rod samples are designated as
\( D_{c}^{c} \) and
\( D_{c}^{r} \), respectively.
Table I
Chemical Composition of Investigated Alloys
B | — | 50 | 40 | 10 | — |
B1/3 | 1:3 | 49.930 | 39.944 | 9.986 | 0.140 |
B2/3 | 2:3 | 49.860 | 39.888 | 9.972 | 0.281 |
B4/3 | 4:3 | 49.719 | 39.775 | 9.944 | 0.562 |
All prepared samples were cut (cones—longitudinally, rods—crosswise), grinded (SiC paper up to 7000-grit), polished (mixture of colloidal silica and hydrogen peroxide), etched (25 ml H2O + 22.5 ml HNO3 + 5 drops HF), and then studied using optical microscope (Nikon ECLIPSE LV150N). Scanning electron microscopy (SEM) observations were carried out using Versa 3D (FEI) microscope equipped with an energy dispersive spectrometer (EDS). A high-resolution scanning transmission electron microscope (FEI, Titan Cubed G2 60-300) equipped with EDX ChemiSTEM was used for detailed observations of small precipitates. Lamella for STEM study was extracted using the SEM-FIB technique (ZEISS NEON CrossBeam 40EsB).
The GFA indicators were determined based on differential thermal analysis (DTA, Setaram Labsys). The calorimeter was carefully calibrated before measurements. After resetting the correction coefficients in the DTA software, new melting measurements of the reference elements (Sn, Zn, Al, Cu, and Ni) were performed at different heating rates (10, 20, and 40 K/min). Based on the registered onset melting points and thermal effects, new correction coefficients were entered in the analyzer software. DTA measurements were conducted on thin slices cut from the middle of the as-cast samples using 100 µl alumina pans at a constant heating rate of 20 K/min and protective argon (5 N purity) gas flow with 20 ml/min.
All characteristic temperatures (
Tg-glass transition,
Tx-onset of crystallization,
Tp-peak of crystallization,
Ts and
Tf-start and finish of the eutectoid transformation,
Tm-solidus,
Tl-liquidus) were determined by the DTA software.
Tg temperatures were estimated as the inflection points on the DTA curves. Based on these values, several GFA indicators were evaluated: Δ
Txg =
Tx −
Tg,[
30]
T”
rg =
Tg/
Tl,[
31]
γ =
Tx/(
Tg +
Tl),[
32]
γm = (2
Tx −
Tg)/
Tl,[
33]
ω4 = (2
Tx −
Tg)/(
Tl +
Tx) [
34] and the
χ = [(
Tx −
Tg)/(
Tl −
Tx)]·[
Tx/(
Tl −
Tx)]
1.47.[
35]
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