Quasicrystal Formation in the system Zr75-xTixCu25-yNiy

During the crystallization study of the bulk metallic glass Zr46.8Ti8.2Cu7.5Ni10Be27.5 (Vit4) we have demonstrated that the primary crystals appearing during isothermal annealing in the glass region, above Tg and before Tx, were of quasicrystalline nature and beryllium free. The formula of the quasicrystalline phase so obtained and determined by electron techniques (EDX and EELS) was Zr63Ti14.4Ni12.4Cu10.2. We have synthesized phases in the system Zr75-xTixCu25-yNiy by melt spinning and obtained either amorphous or quasicrystalline phases. For the amorphous phases, we have studied the crystallization by in-situ neutron diffraction and obtained quasicrystals as primary crystals. Neutron and X-ray diffraction, DSC studies are presented. Introduction Bulk metallic glasses and quasicrystalline phases have attracted a lot of attention this last decade, and among them the family ZrTiCuNiBe presents the best performances in matter of glass forming ability over cost [1]. However these phases are metastable and several studies deal with the inherent instability of the amorphous state, as this instability modifies the mechanical properties. In particular when these alloys are heated in the supercooled liquid region, decomposition appears and is different for each alloy composition. For the alloy Zr46.25Ti8.25Cu7.5Ni10Be27.5 (called Vit4), no phase separation appears prior to crystallization and Wanderka et al. [2,3] reported that a quasicrystalline phase is formed in the first step of crystallization, followed by a decomposition of this phase at higher temperatures. Formation of quasicrystals during devitrification of metallic glasses has been also observed in rapidly quenched alloys Pd60U20Si20 [4], Ti45Zr38Ni17 [5], or bulk metallic glasses Zr65Ni10Ag10Al7.5Ni7.5 [6], ZrNiCuAl [7,8] and ZrTiCuNi [9]. During the crystallization study of the bulk metallic glass Zr46.8Ti8.2Cu7.5Ni10Be27.5 (Vit4) [10,11] we have demonstrated that the primary crystals appearing during isothermal annealing in the glass region, above Tg and before Tx, were of quasicrystalline nature and beryllium free. The formula of the quasicrystalline phase so obtained and determined by electron techniques (EDX and EELS) was Zr63Ti14.4Ni12.4Cu10.2. So we have extended this study of beryllium free phases to the system Zr75– xTixCu25-yNiy in order to study the stability domain of these new quasicrystalline phases. Experimental details Metallic alloys Zr75-xTixCu25-yNiy samples were prepared from a mixture of the pure elements by induction melting in a water-cooled copper crucible under argon gas. The ingots were remelted by electromagnetic levitation several times. Ribbons were obtained by the melt-spinning technique at a constant copper wheel speed of 21 ms, injected under argon by a cordierite rectangular nose of 7 by 0.4 mm and producing ribbons with a thickness of 50 μm and a width of 7 mm. The endothermic and exothermic reactions associated with glass transition, crystallization and melting processes were examined by DSC at different heating rates on a Netzsch DSC 404S calorimeter under a flowing argon atmosphere. Neutron diffraction experiments were performed on the D1B diffractometer of ILL-Grenoble which is equipped with a 400 cells position sensitive detector, covering an 80° 2-theta range, and allowing recording a pattern every 5 minutes. The experiments were performed in a dedicated furnace with a vanadium resistor under a secondary vacuum. The wavelength used was 0.2524 nm. The data presented are corrected of the background, sample container and furnace contributions. Results and discussion The first ribbon produced and analyzed was the composition determined during our study of the decomposition of the Vit4 bulk metallic glass, Zr63Ti14.4Ni12.4Cu10.2[10]. It is amorphous by X-ray and neutron diffraction. The DSC experiment performed at 2K/min is reported in figure 1a, it shows three exothermic peaks at 576, 609 and 656 K reported as Tx1, Tx2 and Tx3 on the figure. We have performed an in-situ neutron diffraction experiment by heating the ribbon between 300 and 670 K. The three dimensional plot is reported in figure 1b. a) b) Figure 1: a) DSC scan performed at 2K/min on the Zr63Ti14.4Ni12.4Cu10.2 alloy. b) 3D plot of the in-situ neutron diffraction experiment of the Zr63Ti14.4Ni12.4Cu10.2 alloy heated at 2K/min. At room temperature the sample is characterized by an amorphous bump centered around d = 0.232 nm, then at 610 K a series of peaks appears with two main contributions at d1 = 0.252 nm and d2 = 0.239 nm, this series of peaks disappears at 655 K to give new peaks that are stable up to 670 K. The phase formed at 610 K is very close to the one observed in Vit4 during the crystallization experiment [10], confirming that composition determined by chemical analysis is a composition that allows preparing quasicrystals. No diffraction peak is associated to the first event Tx1 in the DSC experiment. We have then produced ribbons by increasing the content in titanium in the system Zr75–xTixNi15Cu10 from x=10 to 60. All the ribbons have been produced in the same conditions at a constant speed of 21 ms. The X-ray diffraction patterns of the samples so obtained are reported in figure 2. By increasing the titanium content, the amorphous nature of the ribbons decreases up to x=25 where peaks are appearing at positions of the quasicrystal lines observed in the previous crystallization study. By increasing the titanium content, there is a superposition of an amorphous bump and of the lines due to the quasicrystalline particles. For the composition x=60 there is no more amorphous bump and the peak position have been indexed in the figure 3a with the six indices used for icosahedral phases [12]. It is also possible to produce the quasicrystalline phase by crystallization of the amorphous phase as seen in the in-situ neutron diffraction study. As an example we report in figure 3b the X-ray diffraction pattern of an amorphous sample as produced by melt spinning and crystallized at 583 K during 360 minutes, but longer exposition, for example 720 minutes, or higher temperature, for example 603 K, leads to crystallization of other phases (Zr2Cu). Figure 2: X-ray diffraction patterns of the phases formed directly by melt spinning in the system Zr75-xTixNi15Cu15 from x=20 to 60 a) b) Figure 3: X-ray diffraction patterns of: a) the phase formed directly by melt spinning for the alloy Zr15Ti60Ni15Cu15. b) The alloy Zr60Ti15Ni15Cu15 after annealing at 583K for 360 minutes. On figure 2, it is possible to note that the peak positions shift to higher angles when the titanium increases, meaning that the cell parameter a6D of the quasicrystalline lattice is decreasing when the titanium content is increasing. The lattice parameter a6D in six-dimensional (6D) reciprocal hypercubic space, is related to the absolute diffraction vector value Q by the expression: