The Role of Transition Metals in Crystallization of Amorphous Al–Ni–Co–Yb Alloys

Abstract

Al86Ni4Co4Yb6 and Al86Ni6Co2Yb6 metallic ribbons have been obtained by a standard planar f low method. According to X-ray diffraction data, the ribbons are amorphous. Their crystallization kinetics at different heating rates and resistivity has been determined. These alloys remain amorphous in a wide range of compositions, and the crystallization process depends significantly on transition metal concentration. DOI: 10.1134/S1063784219100190 INTRODUCTION Aluminum-based metallic glasses offer unique metallic and corrosion properties and are widely used in the aerospace industry. To obtain these alloys in an amorphous state, 3d transition metals (Ni or Co) and rare-earth elements (REE) are usually applied as admixtures [1–5]. Ternary nickel-containing alloys (Al–Ni–REE) offer a high tensile strength (up to 1000 MPa) [1], whereas cobalt-containing alloys (Al– Co–REE) are characterized by a high corrosion resistance [6]. It was shown [7] that ytterbium used as an admixture to Al–Ni alloy raises the microhardness to a record high value of 4 GPa. It can be supposed that amorphous Al–Ni–Co–Yb alloys with different concentrations of transition metals may exhibit both good mechanical performance and high corrosion resistance. In this paper, we report data on the crystallization kinetics of Al86Ni4Co4Yb6 and Al86Ni6Co2Yb6 alloys obtained using differential scanning calorimetry (DSC) and measuring their resistivity. EXPERIMENTAL Al86Ni4Co4Yb6 and Al86Ni6Co2Yb6 preforms to produce amorphous ribbons were obtained by melting initial components in an argon-filled induction furnace for 30 min at 1823 K. The chemical composition of prepared alloys was determined using an atomic adsorption spectrometer. Amorphous ribbons of the above compositions (2 mm in width, 36–45 μm in thickness) were produced by a standard planar f low method in a controlled argon atmosphere after heating melts to 1500–1523 K and subsequently injecting them on a rotating water-cooled copper wheel [8−10]. The structure of ribbons was examined by the method of X-ray diffraction using a Bruker D8 Advance diffractometer (СuKα radiation). DSC analysis was carried out on a PerkinElmer DSC 7 instrument with a heating rate of 10, 20, and 40 K/min. The resistivity of ribbons was measured by the standard four probe method at a heating rate of 10 K/min. RESULTS AND DISCUSSION According to X-ray analysis data, the ribbons are amorphous (Fig. 1): no Bragg peaks were observed in diffraction patterns. The main diffraction peak of ribbons was detected in the interval 2θ = 38°–39° irrespective of their chemical composition. A pre-peak (swelling on the left shoulder of the main diffraction peak), which is typical of Al–Ni–REE alloys [11], was absent in our experiments. It seemed to shift toward smaller angles. By means of DSC, we studied the crystallization kinetics of amorphous ribbons. Typical curves taken at different heating rates are shown in Fig. 2. For the alloy containing 4 at % Ni and 4 at % Co, which was obtained at a heating rate of 10 K/min, two exothermal peaks are observed. When the heating rate is increased to 20 and 40 K/min, one more peak with a moderate thermal effect appears in between the first and second ones. At the same time, the curve for the alloy with 2 at % Co has four exothermal peaks even for a heating rate of 10 K/min. The temperatures of the peaks are given in Table 1. It was found that with an increase in the Ni content to 6 at %, the temperatures of all crystallization stages