Big Cube Phase Research Articles

CuZr-based bulk metallic glass and glass matrix composites fabricated by selective laser melting

Monolithic bulk metallic glass and glass matrix composites with a relative density above 98 % were produced by processing Cu46Zr46Al8 (at.%) via selective laser melting (SLM). Their microstructures and mechanical properties were systematically examined. B2 CuZr nanocrystals (30–100 nm in diameter) are uniformly dispersed in the glassy matrix when SLM is conducted at an intermediate energy input. These B2 CuZr nanocrystals nucleate the oxygen-stabilized big cube phase during a remelting step. The presence of these nanocrystals increases the structural heterogeneity as indirectly revealed by mircrohardness and nanoindentation measurements. The corresponding maps in combination with calorimetric data indicate that the glassy phase is altered by the processing conditions. Despite the formation of crystals and a high overall free volume content, all additively manufactured samples fail at lower stress than the as-cast glass and without any plastic strain. The inherent brittleness is attributed to the presence of relatively large pores and the increased oxygen content after selective laser melting.

Processing a glass-forming Zr-based alloy by selective laser melting

Compact specimens were produced by selective laser melting (SLM) from glass-forming Zr52.5Cu17.9Ni14.6Al10Ti5 powder. Below a critical energy density of Ecrit=15J/mm3 fully glassy samples were obtained. When the energy density is higher, the metastable big cube phase, stabilized by oxygen, precipitates next to other crystalline phases. We introduce density contour maps to identify the process window, which allows obtaining fully glassy samples with a high relative density of up to 98.5%. The systematic parameter variation illustrates the importance of laser power for obtaining dense SLM parts. Moreover, the distribution of pores in the present alloy is rather sensitive to the scanning strategy. The compressive strength of the SLM samples is comparable to values measured for as-cast glasses and even a distinct plastic strain is found. Finally, scaffolds were produced and high-energy X-ray diffraction proves that they are fully glassy irrespective of differences in the local cooling rates. Our experiments contribute to better understanding the behaviour of Zr52.5Cu17.9Ni14.6Al10Ti5 during SLM and for making glass-forming alloys progressively more accessible to laser-based additive manufacturing in order to overcome the intrinsic size limitation of bulk metallic glasses.

Influence of Ag and Co additions on glass-forming ability, thermal and mechanical properties of Cu–Zr–Al bulk metallic glasses

In this study, the effect different amounts of Ag and Co have on thermal stability, phase formation and mechanical properties of the glass-forming alloys (Cu0.5Zr0.5)95−x−yAl5AgxCoy (2≤x≤5 and 0≤y≤2) and Cu47.5−yZr47.5Al5Coy (0≤y≤2) was investigated, respectively. The addition of Ag increases the glass-forming ability (GFA) of the alloys by enhancing the stability of the undercooled melt and by simultaneously reducing the stability of the B2 CuZr high-temperature phase. Co instead, has the opposite effect. It leads to an increased thermal stability of the B2 phase, as well as a decreased stability of the undercooled melt and, consequently, lowers the GFA of the alloys. Moreover, a metastable big cube phase precipitates in some alloys stabilized by oxygen, which is introduced during casting. There is a strong interdependence between the phase formation and the tendency of the present alloys to vitrify. This correlation can be captured by the K-parameter (Song et al. 2011 [1]) calculated from the respective transformation temperatures. The plastic strain of the present ternary, quaternary and quinary alloys reflects the GFA as well: the higher the tendency to form a glass, the smaller the plastic strain. Once B2 crystals precipitate in the glass, the plasticity is enhanced significantly.

Amorphous-i phase-big cube-amorphous cyclic phase transformation of mechanically alloyed Zr75Ni20Al5 system

Cyclic glassy-metastable-glassy phase transformations were investigated during mechanical alloying of Zr75Ni20Al5 powders, using high-energy ball milling technique. After 20 h of continuous milling, a single glassy phase was obtained. The synthesized glassy powder, which is homogeneous in structure and composition, showed a glass transition temperature lied at 510 °C and a large supercooled liquid region extended to 86.5 °C. This glassy phase, which crystallized through a single sharp exothermic peak at 596.5 °C, tended to transformed into icosahedral (i-phase) nano-quasicrystalline grains upon further milling to 30 h. This i-phase showed serious instability against the mechanical deformations generated by the milling tools, and hence transformed into big cube phase with increasing the milling time to 45 h. In a cyclic phase transformation the obtained big cube phase subsequently transformed into the same glassy phase after 60 h of milling. This observed cyclic phase transformation was observed twice upon further milling to 90 h without serious compositional changes. The iron contamination contents in the ball-milled powders had constant values of about 0.16 wt.% and did not change with changing the structure of the powders. The present work shows that this phenomenon does not have any obvious analogues with the periodic redox reactions or with diffusive-reactive phenomena known in chemistry. On the basis of our results, the destabilizing effect of the defects created by the milling media (balls), which leads to the cyclic transformations, depends on the milling time.

A study on the formation of crystalline phases during solidification and crystallisation in the bulk metallic glass of Zr53Cu21Al10Ni8Ti8 composition

The present paper considers the formation of crystalline phases during solidification and crystallisation of the Zr53Cu21Al10Ni8Ti8 alloy. Solidification was carried out by a copper mould casting technique, which yielded a partially crystalline microstructure comprising a ‘big cube phase’ in a dendritic morphology and a bct Zr2Ni phase. Detailed high-resolution microscopy was carried out to determine possible mechanisms for the formation of the crystalline phases. Based on microstructural examinations, it was established that the dendrites grew by the attachment of atomistic ledges. The bct Zr2Ni phase, formed during solidification and crystallisation, showed various types of faults depending on the crystallite size, and its crystallography was examined in detail. It has been shown that the presence of these faults could be explained by anti-site occupancy in the bct lattice of the Zr2Ni phase.

Neutron in situ crystallization studies of Ti-based BMGs

The phase evolution upon heating of Ti 40Zr 25Cu 9Ni 8Be 18 BMG was investigated by in situ neutron diffraction studies and compared to Differential scanning calorimetry measurements performed at the same heating rate. When the experiment is performed at the constant speed of 2 K/min, the amorphous to crystallization process proceeds by two complicated crystallization reactions: firstly, the precipitation of a cubic Ti(Zr)Be 2 phase at 608 K ( a = 6.603 Å), this phase then disappears at 810 K, secondly, by the formation at 749 K of two phases which have been determined by neutron diffraction to be a C14 hexagonal Laves phase and a big cube (Zr,Ti) 2Ni phase. When the sample is heated at 2 K/min, then hold at T g + 158 = 713 K (before T x2 = 749 K) the (Ti,Zr)Be 2 phase develops. After 60 min, the big cube phase starts to appear, an important amorphous bump being still present. These in situ diffraction experiments and the different phases formed are presented and discussed.

Mechanically induced cyclic metastable phase transformations ofZr2Nialloys

Cyclic crystalline-glassy-crystalline phase transformations have been investigated during high-energy ball milling of tetragonal-${\mathrm{Zr}}_{2}\mathrm{Ni}$ alloy powders using mechanical disordering (MD) method. The results show that the metastable $\mathrm{fcc}\text{\ensuremath{-}}{\mathrm{Zr}}_{2}\mathrm{Ni}$ alloy powders which is obtained after $173\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$ of MD time, transforms into a new metastable phase of nanocrystalline big cube-${\mathrm{Zr}}_{2}\mathrm{Ni}$ upon milling for $360\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$. The lattice parameters ${a}_{0}$ for the metastable-fcc and big-cube phases were calculated and found to be $0.451\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $1.226\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, respectively. The obtained big-cube phase subsequently transforms to a single glassy phase after $540\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$ of MD time. This obtained glassy phase, however, devitrificated into the same metastable big-cube phase upon increasing the MD time to $720\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$. As the MD time increases $(806--900\phantom{\rule{0.3em}{0ex}}\mathrm{ks})$, the obtained nanocrystalline big-cube phase transforms into the pervious metastable $\mathrm{fcc}\text{\ensuremath{-}}{\mathrm{Zr}}_{2}\mathrm{Ni}$ phase, indicating a cyclic-phase transformations between the three obtained metastable phases. Such cyclic phase transformations are observed several times during the milling procedure. On the basis of our results, the destabilizing effect of the defects created by the milling media (balls) is responsible for such mechanically induced solid-state devitrification that leads to the cyclic transformations.

Early crystallization stages in a Zr–Cu–Ni–Al–Ti metallic glass

The early crystallization stages of Zr 52.5Cu 17.9Ni 14.6Al 10Ti 5 metallic glass with different impurity oxygen levels were investigated with differential scanning calorimetry (DSC), high resolution electron microscopy (HREM), three-dimensional atom probe (3DAP) and small angle X-ray scattering (SAXS). While the low oxygen sample showed two peaks, the high oxygen sample showed one peak in isothermal DSC slightly above the glass transition temperatures. No evidence for the previously reported phase separation was detected in the supercooled liquid by 3DAP; compositional contrasts in SAXS and 3DAP were detected only when nanocrystals were confirmed by HREM. The low oxygen sample forms fine Ti-enriched nanocrystals and then crystallizes into Ti-poor NiZr 2-phase, the high oxygen sample crystallizes into very fine NiTi 2-type big cube phase.

APFIM/TEM/SAXS studies of early stage crystallization of a Zr52.5Cu17.9Ni14.6Al10Ti5 metallic glass

The early stage of the crystallization process of a Zr52.5Cu17.9Ni14.6Al10Ti5 metallic glass has been investigated by three-dimensional atom probe (3DAP), high-resolution electron microscopy (HREM) and small angle X-ray scattering (SAXS) technique. HREM observation of the alloys annealed near Tg always showed the dispersion of nanocrystalline fcc big cube phase dispersed in the amorphous matrix whenever concentration fluctuations were detected by SAXS and 3DAP. The distribution of impurity oxygen was uniform in the amorphous state, while it was rejected from the big cube phase to the remaining amorphous phase after the primary crystallization. The time evolutions of primary crystals followed t0.5, indicating the grain growth was controlled by the volume diffusion of solute atoms.

Influence of alloying on the formation of Zr-based quasicrystals

Glassy Zr–Cu–Ni–Al precursor alloys exhibit quasicrystal forming ability in a narrow concentration range via isothermal heating. The formation of quasicrystals has been explained by various arguments. Often, they are considered as Hume-Rothery phases, atomic size or bond strength dependent structures, or a hybrid structure built from Al 2Cu- and MoSi 2-type structural units. The aim of the present investigation is to find out the influence of alloying on quasicrystal formation using transmission electron microscopy and X-ray diffraction. Small addition of elements such as Sn or Mo was found to change the phase selection during the early stages of crystallization significantly. Substitution with Sn, having the same oxidation state, leads to the formation of the icosahedral phase. On the other hand, substitution with Mo, having a higher oxidation state, leads to the formation of the fcc big cube phase. However, in the present investigation substitution of atoms bigger than Zr such as Y or Ce leads to very easy formation of quasicrystals even at lower temperatures. Si, on the other hand, which is smaller than Zr leads to the formation of the tetragonal Zr 2Ni phase.

Room-temperature mechanically induced solid-state devitrifications of glassy Zr65Al7.5Ni10Cu12.5Pd5 alloy powders

The high-energy ball milling technique was employed for synthesizing a single phase of glassy Zr65Al7.5Ni10Cu12.5Pd5 alloy powder, using a room-temperature mechanical alloying method. Whereas the glass transition temperature of the obtained glassy alloy is 683 K, the crystallization temperature is 783 K. The mechanically alloyed Zr65Al7.5Ni10Cu12.5Pd5 glassy powders maintain their unique disordered structure through a large supercooled liquid region (100 K). The possibility of devitrification of the synthetic glassy phase upon increasing the ball milling time was investigated. The results have shown that the glassy powder that is obtained after 173 ks of milling is subjected to numerous lattice imperfections and tends to transform into a metastable big-cube phase after further ball milling (259–432 ks). After 540 ks of milling, a complete glassy–metastable phase transformation is achieved and the end-product of this stage of milling is nanocrystalline big-cube powder that has a lattice constant of 1.2293 nm. As the milling time increases (720 ks), the obtained big-cube phase can no longer withstand the shear and impact stresses that are generated by the milling media and is transformed into a new metastable phase of nanocrystalline fcc-Zr65Al7.5Ni10Cu12.5Pd5. The fcc-metastable phase transforms into a mixture of Zr2Cu and Zr6NiAl2 crystalline phases at rather high temperature, as high as 993 K.

Mechanically induced solid-state devitrifications of Zr70Pd20Ni10 glassy alloy powders

A single glassy phase of Zr70Pd20Ni10 alloy powder was synthesized by mechanical alloying the elemental powders for 48 hours, using a high-energy ball-milling technique. The obtained glassy phase transformed into a metastable big-cube phase upon increasing the ball-milling time (100 hours). After 150 hours of milling, a complete glass-metastable-phase transformation was achieved, and the end product was nanocrystalline big-cube powder, which has a lattice constant of 1.23 nm. As the ball-milling time was further increased the big-cube phase could no longer withstand the mechanical deformation that was generated by the milling media and transformed into a new metastable phase of nanocrystalline fcc Zr70Pd20Ni10. The lattice constant of this metastable phase was calculated to be 0.455 nm. The reported metastable phases here are new and have never been, so far as we know, reported for the ternary Zr-Pd-Ni system, or its binary-phase relations.

Structural and calorimetric evolutions of mechanically-induced solid-state devitrificated Zr 60Ni 25Al 15 glassy alloy powder

A single phase of glassy Zr 60Ni 25Al 15 alloy powder was synthesized by mechanically induced solid-state reaction (MISSR) technique. The MISSR was performed in a room-temperature high-energy ball mill, using a mechanical alloying method. Whereas the glass transition temperature of the obtained glassy alloy is 681 K, the melting and liquidus temperatures are 1179 K and 1256 K, respectively. The mechanically alloyed Zr 60Ni 25Al 15 glassy powders maintain its unique disordered structure through a large supercooled liquid region (99 K). It, however, transforms into a mixture of Zr 5Ni 4Al and Zr 6NiAl 2 crystalline phases at 780 K with a large enthalpy change of crystallization of −81.8 J/g. The possibility of devitrification of the synthetic glassy phase upon increasing the ball milling time was investigated. The results have shown that the glassy powder obtained after 173 ks of milling is subject to heavy lattice imperfections and tends to transform into a metastable-big cube phase after further ball milling times (216–259 ks). After 446 ks of milling, a complete glassy-metastable phase transformation is achieved and the end-product of this stage of milling is nanocrystalline big-cube powders which have a lattice constant of 1.2282 nm. The big-cube Zr 60Ni 25Al 15 phase transforms into the same crystalline mixture of Zr 5Ni 4Al and Zr 6NiAl 2 phases at 799 K with an enthalpy change of transformation of −73.7 J/g. As the milling time increases (720 ks), the obtained big-cube phase can no longer withstand against the shear and impact stresses that are generated by the milling media and surprisingly transformed into a new metastable phase of nanocrystalline fcc- Zr 60Ni 25Al 15. The lattice constant of this metastable phase was calculated and found to be 0.45449 nm. The fcc-metastable phase transforms into a mixture of Zr 5Ni 4Al and Zr 6NiAl 2 crystalline phases at rather a high temperature, as high as 901 K with a heat change of transformation of −24.4 J/g. The reported metastable phase here is new and has never been, so far as we know, reported for ternary Zr-Ni-Al system, or its binary phase relations.

Mechanically induced devitrifications of ball-milled Zr70Pd20Ni10 glassy alloy powders

Mechanical alloying using a high-energy ball milling technique was used to fabricate a single glassy phase of Zr70Pd20Ni10 alloy powders after 100 h milling time. Annealing the glassy powders at a temperature just below the crystallization onset temperature led to thermally enhanced devitrification and the formation of a metastable big-cube phase with a lattice constant of 1.2289 nm. The same metastable phase was obtained upon subjecting the end product of the glassy powders to further ball milling time (150 h). This metastable big-cube phase could no longer withstand the shear and impact stresses generated by the milling media and transformed into a new metastable phase of face-centered cubic Zr70Pd20Ni10. The lattice constant of this metastable phase was calculated to be 0.56838 nm. These metastable phases are new and have never been, so far as we know, reported for the ternary Zr–Pd–Ni system or its binary phase relations.

“Big cube” phase formation in Zr-based metallic glasses

The formation of a metastable fcc NiTi 2-type intermetallic compound (big cube phase) from Zr-based metallic glasses have been studied in this work. This phase was found in partially crystalline Zr 52.5Cu 17.9Ni 14.6Al 10Ti 5 amorphous ribbons. Its nucleation could be avoided by adding 2% of Y, which acts as an oxygen scavenger. After annealing a fully amorphous ribbon, nanocrystals of the big cube phase have been observed. The Zr 55Cu 30Ni 5Al 10 alloy has been partially amorphised by copper moulding and arc melting. Large agglomerates of crystals of the big cube phase have been found embedded in the amorphous matrix. Also for this alloy, nanocrystal of the same phase has been found after crystallisation. From the results, it appears clear that the big cube phase nucleates first from the undercooled liquid. Its formation is hindered if the oxygen content in the undercooled liquid alloy is significantly reduced. After solidification, coarse grains were found, whereas nanocrystals were produced by crystallisation. The role of nucleation on the crystallisation process is briefly discussed.