Formation Of Icosahedral Phase Research Articles

Structure and reflective properties of Al-Cu-Fe quasicrystalline thin film prepared by laser induced arc method

Al-Cu-Fe thin films were prepared by laser induced arc (laser-arc) method from a single source—Al63Cu25Fe12 alloy, which was proved to consist of quasicrystalline phase together with approximant phase. The composition of the deposited films meets the requirement for formation of icosahedral symmetry phase. Quasicrystalline phase was obtained after annealing the amorphous as-deposit film samples. The optical properties of the samples were investigated. Thin film samples of Al, Cu and Fe deposited under the same condition were employed for comparison. The results showed specific reflective properties of Al-Cu-Fe quasicrystal thin film in some wavelength range. The optical conductivity of the films exhibited a negative peak, centered about 440 nm in range of 190 to 800 nm. The Al-Cu-Fe quasicrystal thin films could absorb almost all the ray in the wavelength range from 420nm to 450 nm. The ratio of absorption was greater than 99%.

Evidence of icosahedral short-range order in Zr70Cu30 and Zr70Cu29Pd1 metallic glasses

Change in local atomic environment during crystallization of Zr-based glassy alloys was studied by extended x-ray absorption fine structure (EXAFS) spectroscopy. The formation of icosahedral quasicrystalline phase followed by crystallization of tetragonal CuZr2 has been observed in the Zr70Cu29Pd1 glassy alloy during annealing up to 850 K. On the other hand, the binary Zr70Cu30 alloy shows a single glassy to crystalline CuZr2 phase transformation. The local atomic environment of as-quenched Zr70Cu30 alloy is matched to an icosahedral local atomic configuration, which is similar to that of the as-quenched Zr70Cu29Pd1 alloy and the alloy annealed at 593 K containing icosahedral phase. Considering that the supercooled liquid region appears prior to crystallization in the Zr70Cu30 glassy alloy, the observed results support the theory claiming a strong correlation between the existence of local icosahedral short-range order and stability of the supercooled liquid state.

Nano icosahedral quasicrystals in Zr-based glassy alloys

A nanoscale icosahedral quasicrystalline phase formation was found by the slight deviation from the three-component rule for high glass-forming ability in a Zr65Al7.5Ni10Cu17.5 glassy alloy. Since the primary phase consists of fcc Zr2Ni and I-phases in the Zr65Al7.5Ni10Cu16.5(Ag, Pd, Au or Pt)1 glassy alloys, where an icosahedron is contained in both the phases, it is suggested that the local icosahedral atomic configuration exists in the glassy state and it stabilizes the supercooled liquid state. We also clarified that an I-phase precipitates in the Zr70Cu29Pd1 and Zr70Ni20Al9Pd1 glassy alloys and no I-phase formation is observed in the Zr70Ni29Pd1 amorphous alloy. Since the supercooled liquid region is confirmed in the Zr70Cu30 and Zr70Ni20Al10 glassy alloys and is not observed in the Zr70Ni30 amorphous alloy, it is concluded that the stability of the supercooled liquid state is strongly correlated with the existence of the local icosahedral order.

Stability of glassy state in Zr-based glassy alloys correlated with nano icosahedral phase formation

We investigated the stability of the glassy state by examination of transformation behaviour and structural analysis in Zr-based binary and ternary glassy alloys. It is well known that the melt-spun Zr-Cu binary alloys have a distinct glass transition prior to crystallization in a wide composition region of 20 to 70 at% Cu, while no supercooled liquid state appears in the melt-spun Zr-Ni binary alloys. It is expected that the local structure of the disordered state in the two alloy systems is different. The icosahedral quasicrystalline phase (I-phase) is formed as a primary phase in the Zr 70Cu 30 binary glassy alloy by substitution of Pd for Cu. The minimum Pd concentration for I-phase formation is 0.5 at%. In contrast, the primary phase is a tetragonal Zr 2Ni phase in a Zr 70Ni 30 amorphous alloy by substitution of Pd by less than 10 at% for the Ni element. The radial distribution function (RDF) of the Zr 70Cu 30 glassy alloy is different from that of the Zr 70Ni 30 amorphous alloy. The coordination numbers around Zr in the melt-spun Zr 70(Cu or Ni) 30 binary alloys are 12.4 and 10.7, respectively, indicating the possibility of existence of the icosahedral local atomic configuration in the Zr 70Cu 30 glassy alloy. It is also realized that a tetragonal Zr 2Ni-like local atomic configuration is formed in the Zr 70Cu 30 amorphous alloy. Considering that the I-phase is also formed in the Zr 60Ni 25Al 15 glassy alloy by addition of 3 at% Pd, where the distinct glass transition is observed, the appearance of the supercooled liquid region is attributed to the existence of the icosahedral local atomic configurations consisting of Zr-Cu or Zr-Ni-Al. These results show that the icosahedral short- and medium-range orders exist in the supercooled liquid as well as in the glassy phase and they stabilize the glassy state in the Zr-based alloys.

Icosahedral quasicrystalline phase formation in Zr–Al–Ni–Cu glassy alloys by the addition of V, Nb and Ta

The formation of an icosahedral quasicrystalline phase as a primary phase was found in Zr 65Al 7.5Ni 10Cu 12.5X 5 (X=V, Nb or Ta) glassy alloys. The icosahedral phase was precipitated during the first exothermic reaction in a two-stage crystallization process after a distinct glass transition. The diameters of the icosahedral particles are as small as 10–50 nm, similar to those found in Zr-based glassy alloys made with the addition of noble metals. Based on a Johnson–Mehl–Avrami analysis, we suggest that nano-quasicrystallization proceeds by a diffusion-controlled growth mode. The icosahedral phase in the Zr 65Al 7.5Ni 10Cu 17.5− x Ta x ( x=0–4) glassy alloys was formed at a Ta concentration of as little as 1 at.%, the primary crystallization phases were the fcc Zr 2Ni and icosahedral phases. For Ta concentrations above 3 at.%, a nearly single icosahedral phase was formed as the primary phase. The formation of a nano-scale icosahedral phase in Zr–Al–Ni–Cu glassy alloys with the addition of V, Nb or Ta, as well as noble metals, indicates that an increasing nucleation rate contributes to the precipitation of the icosahedral phase, leading to the concept that icosahedral short- or medium-range order exists in the glassy state of the Zr–Al–Ni–Cu alloy.

Icosahedral-phase formation in as-cast Ti-Zr-Ni alloys

Previously, the formation of the thermodynamically stable icosahedral Ti-Zr- Ni i-(Ti-Zr-Ni) phase has been observed in rapidly quenched alloys, or by a low-temperature solid-state transformation from crystal phases that form at higher temperatures. Here we report the nucleation and growth of the icosahedral (i) phase directly from the liquid in as-cast Ti 33 Zr 46 Ni 21 and Ti 37 Zr 42 Ni 21 alloys. Diffraction patterns obtained by transmission electron microscopy and X-ray diffraction confirm the phase identity. Differential scanning calorimetry measurements show an endothermic transformation from the i-phase to a phase mixture of a C14 Laves phase and a solid-solution phase, demonstrating that this i-phase is also stable. The short time that the liquid remains in the Laves-phase-forming field and the higher nucleation rate for the i-phase, owing to strong similarities in the local structures of the i-phase and liquid, allow i-phase nucleation and growth directly from the liquid.

Nanostructured icosahedral phase formation in Al70Cu20Fe10 by mechanical alloying: Comprehensive study

A systematic study on the formation and the stability of Al70Cu20Fe10 quasicrystalline alloy has been carried out through mechanical alloying. The influence of the milling parameters, sequence of additive elements, and heat treatment on the formation of icosahedral phase has been investigated through x-ray diffraction analysis and transmission electron microscopy. A combination of milling and heat treatment of Al70Cu20Fe10 composition resulted in the formation of a single icosahedral phase (i-phase) quasicrystal with face-centered structure. We present evidence that suggests the i phase is of a nanoquasicrystalline nature. From the present investigations, we propose an i-phase formation mechanism in Al–Cu–Fe system.

Enhancement of the quasicrystal-forming ability in Al-based alloys by Be-addition

The influence of beryllium (Be) addition on the quasicrystal-forming ability (QFA) in Al–Cu–Fe and Al–Mn systems has been investigated using conventional solidification technique. For a series of as-cast (Al 62− x Be x )Cu 25.5Fe 12.5 ( x=0,1,3,5,7) alloys, the Be addition modified the icosahedral phase formation mechanism from peritectic reaction to primary solidification and resulted in the increase in the volume fraction of the i-phase from 40% for x=0 to 90% for x=7. Such an enhancement of the QFA with the increase of Be content can be represented by using the reduced quasicrystal transition temperature proposed in the present paper. In Al–Mn alloy system, microstructural changes due to the addition of Be were examined in detail by comparing as-cast Al 90Mn 2.5Be 7.5 alloy with Al 97.5Mn 2.5 bulk ingot. For this alloy system, the substitution of Al by Be was found to reduce substantially the critical Mn-content and cooling rate necessary for the formation of the i-phase.

Icosahedral Phase Formation Domain in Al–Cu–Fe System by Mechanical Alloying

A systematic composition dependence study on icosahedral phase (i-phase) formation in the Al–Cu–Fe system has been carried out. Structural evolution during mechanical alloying and on subsequent heat treatment has been investigated by x-ray diffraction and transmission electron microscopy. The i-phase is observed to evolve from the reaction between the Al2Cu and b phases. The influence of the Cu to Fe ratio (RCuFe), Al to transition metal ratio (RAlTM), and the electron to atom ratio (e/a) on the volume fraction of the i-phase has been studied. The analysis of the present results and those published earlier indicates that quasicrystal-forming ability correlates better with the RAlTM and e/a ratios. The volume fraction of the i-phase is maximum when the RAlTM ˜ 2.3 and e/a ratio ˜ 2.0. Formation of the i-phase in Al65Cu25Fe10 by mechanical alloying is reported for the first time.

Effects of Nb addition on icosahedral quasicrystalline phase formation and glass-forming ability of Zr–Ni–Cu–Al metallic glasses

This work shows that the crystallization process of Zr–Ni–Cu–Al metallic glass is greatly influenced by adding Nb as an alloying element. Based on the results of the differential scanning calorimetry experiments for metallic glasses Zr69−xNbxNi10Cu12Al9 (x=0–15 at. %), the crystallization process takes place through two individual stages. For Zr69Ni10Cu12Al9 (x=0), metastable hexagonal ω-Zr and a small fraction of tetragonal Zr2Cu are precipitated upon completion of the first exothermic reaction. Contrary to this alloy, the precipitation of a nanoquasicrystalline phase is detected when 5–10 at. % Nb is added. Furthermore, the crystallization temperature Tx, supercooled liquid region ΔTx and reduced temperature Tg/TL (Tg is the glass transition temperature, TL the liquidus temperature) increase with increasing Nb content. These results indicate that adding Nb content to Zr–Ni–Cu–Al metallic glasses not only induces quasicrystalline phase formation, but also enhances glass-forming ability.

Effect of Fe Substitution on the Stabilization of the Icosahedral Phase in Al-Cu-V alloys

The effect of Fe substitution on the formation and stabilization of icosahedral phase in Al-Cu-V alloys has been investigated by employing transmission electron microscope and X-ray diffraction. In the as quenched Al75-x Cu15+x Fe5V5 alloys with x = 0, 5 and 10 it has been found that grain size of icosahedral phase increases from 156nm to 0.53μm with increasing x. All the three alloys exhibit the formation of α-Al and θ-Al2Cu phases along with the i-phase. The nanoquasicrystalline and α-Al phases in Al75Cu15Fe5V5 alloy transform to Al7Cu2Fe phase by annealing at 550°C for 4 hrs by peritectic type of reaction. A curious type of pseudo-twelve fold electron diffraction pattern devoid of any diffuse scattering has been observed in some local regions of Al65Cu25Fe5V5 alloy. The angles and details of twelve sectors are varying alternatively. A plausible explanation in terms of microtwin domains of θ-Al2Cu structure in [10] orientation has been given to explain the characteristic features of the pseudo-twelve fold pattern.

Nanoquasicrystallization of Zr-based metallic glasses

Nanoquasicrystallization occurs in a number of Zr based metallic glasses. This paper reviews the influence of various elements on the icosahedral phase formation in Zr-based metallic glasses in order to understand the necessary conditions for nanoquasicrystallization. Icosahedral phase is stabilized in Zr 65− x Cu 27.5Al 7.5O x alloys beyond a critical oxygen level, suggesting that oxygen is a factor to stabilize icosahedral phase formation in Zr–Cu(–Ni)–Al based metallic glasses. On the other hand, Zr-based alloys containing Pd do not require oxygen for icosahedral phase formation. In the presence of Pd, even binary Zr 100− x Pd x ( x=30 and 35) alloys show nanoquasicrystallizaiton in the course of amorphous to crystal transformation. Based on our recent results together with other published investigations, the necessary conditions for quasicrystallization are discussed.

Nano icosahedral phase formation by crystallization of Zr-based ternary glassy alloys

Nano icosahedral phase formation by crystallization of Zr-based ternary glassy alloys

Stabilization of icosahedral phase in Si substituted version of Al 65Cu 20Cr 15 alloy

In order to find out the effect of Si on quasicrystalline (QC) and related phases, Si has been substituted in place of Al in Al–Cu–Cr system. In the present investigation, various alloys corresponding to Al 65− x Cu 20Cr 15Si x ( x=1, 3,5,7 and 10 ) composition have been synthesized and characterized. The investigations reveal that at 1 at.% of Si both QC and crystalline (b.c.c., a=8.90 A ̊ ) phases are present but after annealing, the alloy becomes QC. The alloys having 3 at.% of Si has been found to show the formation of icosahedral phase which exhibits high degree of disorder. At 7 at.% Si, the dissociation of the icosahedral phase commences. The dissociated phases have been found to be icosahedral and disordered diamond cubic phase ( a=7.87 A ̊ ). At the highest silicon concentration (10 at.%) another crystalline phase: primitive cubic phase ( a≈10.90 A ̊ ) results along with the icosahedral phase.

APFIM and TEM study of the oxygen behavior during crystallization of Zr 65Cu 27.5Al 7.5 metallic glass

The influence of oxygen on the crystallization behavior of Zr 65− x Cu 27.5Al 7.5O x ( x=0.14, 0.43 and 0.82%) metallic glasses has been studied. The supercooled liquid regime (Δ T x) decreases with increase in oxygen. At low oxygen level (0.14%), the alloy crystallizes in a single stage by the formation of Zr 2(Cu, Al). At higher oxygen content, the amorphous phase crystallizes in two stages by the precipitation of a quasicrystalline phase first and subsequently the stable Zr 2(Cu, Al). The present paper is the first evidence of icosahedral phase formation in ternary Zr–Cu–Al system. The temperature and time window for the formation of quasicrystalline phase is quite narrow. Three-dimensional atom probe studies have shown that the quasicrystalline phase has a composition of Zr 65Cu 27.5Al 7.5 suggesting that it is rich in Zr and O and is depleted in Cu.

Formation of nano icosahedral quasicrystalline phase in Zr-Ni-M (M=Pd, Au, Pt) ternary glassy alloys

A nano icosahedral quasicrystalline phase was confirmed as a primary precipitation phase in the melt-spun Zr70Ni10M20 (M=Pd, Au and Pt) ternary glassy alloys. It is clarified that the precipitation of the icosahedral phase takes place by a diffusion-controlled growth mode with increasing nucleation rate. The formation of the nano scale icosahedral phase also indicates the existence of short-range order correlated with an icosahedral structure in the glassy state.

Structural Change from Amorphous to Nano Icosahedral Quasicrystalline Phase with Quenching Rate in Zr-Pt Binary Alloy

Structural change with quenching rate in the Zr 80 Pt 20 alloy was studied by controlling the roll speed in the melt-spinning technique. The as-quenched ribbon had an amorphous structure at a roll speed of 50 ms -1 . An icosahedral quasicrystalline phase was directly formed at a roll speed in the range of 40 to 30 ms -1 . The critical roll speed for the formation of the icosahedral phase was 25 ms -1 . where the volume fraction of the icosahedral phase was very small. Several crystalline phases precipitated in the melt-spun ribbon at roll speeds below 20 ms -1 . The DSC curve of the sample prepared at 50 ms -1 clearly exhibited two exothermic peaks. The first exothermic peak at 717 K, corresponded to the transformation from the amorphous to the icosahedral phase and the second peak was due to the decomposition reaction of the icosahedral phase. The size of the icosahedral phase in the as-quenched and annealed samples lay in the diameter range of 5 to 20 nm and the particles were e distributed homogeneously. Thus, it was clarified that the nano icosahedral phase can be formed in the as-quenched state from the melt and in the annealed state from the amorphous phase in the Zr 80 Pt 20 binary alloy. The icosahedral phase transformed to Zr, ZrPt, Zr 9 Pt 1 and Zr 5 Pt 3 phases at a temperature of approximately 900 K. The formation of the nano scale icosahedral phase indicated the possibility that icosahedral short-range order exists in the melted state of the Zr-Pt binary alloy. The strong chemical affinity between Zr and Pt probably contributed to the stabilization of the icosahedral short-range order and restraining the long-range rearrangement of constitutional elements to form a stable crystalline phase, which is an important factor in the formation of an icosahedral phase.

Nano Icosahedral Quasicrystalline Phase in Zr<SUB>65</SUB>Al<SUB>7.5</SUB>Ni<SUB>10</SUB>Ag<SUB>17.5</SUB> Quaternary Glassy Alloy

A nano icosahedral quasicrystalline phase was confirmed as the primary precipitation phase in the melt-spun Zr 65 Al 7.5 Ni 10 Ag 17.5 quaternary glassy alloy with a two-stage crystallization process. The onset temperature of the first exothermic peak is 704 K at a heating rate of 0.67 Ks -1 . Although no significant X-ray diffraction peaks were observed in the sample annealed at 705 K, we confirmed the formation of the icosahedral phase by nano beam electron diffraction using FE-TEM. The icosahedral particles have an extremely fine grain size in the diameter range of 2 to 4 nm. In addition to the icosahedral phase, the nano crystalline particles corresponding to Zr 2 Ag and/or fcc-Zr 2 Ni structures are also observed in the annealed sample. The second exothermic peak denotes the transition from the nano icosahedral to the crystalline phase of Zr 2 Ag, fcc-Zr 2 Ni and Zr 4 Al 3 . The formation of the nano icosahedral phase as the primary phase is due to a high nucleation rate and strongly implies the existence of an icosahedral short-range order in the glassy state.

Nanocrystalline Icosahedral Phase Formation in Melt Spun Ti-Zr-Ni Alloys

The paper reports Icosahedral phase (i-phase) formation in the nanocrystalline state in Ti 55 Zr 20 Ni 25 and Zr 55 Ti 25 Ni 20 alloys. While nanocrystalline i-phase forms directly from the melt during melt spinning at low wheel velocity of 20 m/s, nanoquasicrystallization of amorphous phase has been observed on subsequent annealing after melt spinning at higher wheel velocities of 30 and 40 m/s. The i-phase is stable up to 873 K in Ti-rich alloy, while it transforms to crystalline phases in Zr-rich alloy below this temperature. The i-phase formed both directly from the melt and from the amorphous phase is much finer in Zr-rich alloy compared to Ti-rich alloy.

Formation of nano-scale icosahedral quasicrystalline phase in amorphous Zr60Ni25Ti15 alloy

The precipitation of icosahedral quasicrystalline phases (I-phase) from Zr-based amorphous alloys was reported initially by Koster et al. [1]. Recently, much attention has been paid to amorphous Zr–Al–TM– M (TM = Co, Ni, Cu, M = Ag, Pd, Au, or Pt) system [2–4] and Zr–TM–M system [5–7] for the formation of I-phases from amorphous phases by crystallization. A typical alloy composition for the Zr–Al–TM–M system is Zr65Al7.5Ni10Cu12.5M5. In these alloys, noble metals (Pd, Pt, Au and Ag) are added to the wellknown good metallic glass former Zr65Al7.5Ni10Cu17.5 [8] in order to stabilize I-phases. For the Zr–TM– M system, the noble metals also play an important role in the formation of I-phases. It is noticed that most of the above amorphous alloys exhibit a large supercooled liquid region, Tx(=Tx − Tg), where Tx and Tg are the crystallization and glass transition temperature, respectively. Such a large supercooled region means good glass-forming ability for an amorphous alloy. It also seems that the addition of noble metals is essential for the formation of I-phases in these Zr-based amorphous alloys. For example, no icosahedral phase was observed in the Zr70(Ni, Cu)30 amorphous alloy, while I-phases could be acquired in the quaternary, ternary, and even binary Zr70(Ni, Cu, Pd)30 amorphous alloys [7]. In order to determine the possibility of getting an I-phase from a Zr-based amorphous alloy without a significant supercooled liquid region and noble metals, a study was undertaken on the formation of a nano-scale I-phase in amorphous Zr60Ni25Ti15 alloy. The Zr60Ni25Ti15 alloy ingot was prepared by arc melting a mixture of pure Zr, Ni and Ti. From the alloy ingot, a ribbon with a cross section of 0.03 × 1 mm2 was produced by a single roller melt-spinning method in an argon atmosphere. Annealing of the specimen was performed in a differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s. The microstructure was examined by a transmission electron microscope (TEM). The sample for TEM observation was prepared by the ion milling technique. The XRD pattern of the melt-spun Zr60Ni25Ti15 ribbon is shown in Fig. 1. The existence of a broad peak indicates the formation of an amorphous state. It is very clear that the as-quenched ribbon only contains a single amorphous phase. The DSC curve of the melt-spun Zr60Ni25Ti15 ribbon is shown in Fig. 2. We can see from this figure that the crystallization proceeds through two Figure 1 XRD patterns of the Zr60Ni25Ti15 ribbon: (a) melt-spun, (b) annealed to 685 K, and (c) annealed to 900 K at a heating rate of 0.67 K/s and then cooled rapidly in the DSC cell.