SOLID STATE AMORPHIZATION OF MgNi MULTILAYERS

Abstract

We report for the first time, the formation of amorphous MgNi alloys during cold-rolling of crystalline Mg-Ni multilayers. Amorphization of AB multilayers by solid state reaction usually occurs in systems with largely negative heats of mixing AHmix and with diffusivities DB >> DA in the amorphising A mamces. While the heat of formation of MgNi of the order of 10 % of that of ZrNi, Ni diffuses interstitiaIly in Mg. We prepared MgNi multilayer composites with nominal atomic compositions MgxNil., corresponding to X = 0.5,0.6,0.7 and 0.8 using codeformation of Mg and Ni foil sandwiches by cold-rolling at room temperature. The samples were examined by X-ray diffraction, SEM and DSc. Amorphous phase formation was detected during the cold rolling in each of the fom multilayers and amorphisation kinetics seem to follow a linear t-law. After several hundred rolling passes, Mg0.6Ni0.4 was found to be predominantly amorphous. Since the early work of Schwarz and Johnson /l/, amorphous alloys have been synthesized by solid-state reaction (SSR) in AuLa 121, ZrNi I3 I, Cu-Zr 141, NiNb, AI-Pt, FeTi /5,6,7/ multilayers and by mechanical alloying in NiTi 181, MnTi, CuTi, MnZr, FeZr 191, NbgSn, Nb3Ge /10/ systems. The most important criteria for spontaneous amorphisation by solid state reaction are a large negative heat of mixing, AHmix, for alloy formation and fast diffusion of one component in the other. Sommer et al. /l l / obtained amorphous alloys by rapid quenching of MgNi liquid alloys with compositions in the range of 8 to 25 at % Ni. Although the heat of mixing, AHmix, for MgNi estimated by the method of Miedema 112,131 is only about 6k.Tlg.at, Ni is known to exhibit interstitial fast diffusion in the matrix of Mg /14/.Furthermore, heavy deformation such as in ball-milling is known to facilitate amorphous phase formation even in systems with only slightly negative heats of mixing of the order of that of MgNi, such as in NbgSn 1101. We have therefore attemped and obtained amorphisation by cold-rolling in Mg-Ni multilayers. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990427 COLLOQUE DE PHYSIQUE Amorphisation by cold rolling had previously been shown to occur in the NiZr I31 Cu-Zr and AI-Pt multilayers /6,7/. The samples used in this study were prepared from about 20 pm thick elemental foils of magnesium and nickel with a total impurity content of 0,02 at %. ABAB-type sandwiches are repeatedly cold-rolled each time after folding the foil sandwiches as described elsewhere 1151. The samples are cold-rolled indirectly between two steel sheets.The average layer thickness is estimated from scanning electron microscopic observations (SEM) or by measuring the width at half-maximum of the Bragg peaks 1161, calorimetric measurements were performed on a Perkin-Elmer DSc-11. The scanning electron micrographs of figure 1 show cross-sections of Mg0.52Ni0.48 multilayers after 100 and 200 rolling passes (R.P.), where the individual lamellae are from 90 nm to 200 nm thick. We do not show scanning electron micrographs of Mg0.52Ni0.48 multilayer after 300 R.P. because amorphisation reduces electron density contrast and the thin layers are not clearly perceptible. Fig. 1 Scanning electron micrographs of the cross-section of Mg0.52Ni0.48 after 100 (left) and 200 rolling passes (right ) . The X-ray diffraction patterns of Figures 2, 3, 4 and 5 show the effect of number of R.P. and appearance of halos due to amorphous phase formation in multilayers with global compositions Mg0.52Ni0.48 , Mg0.6Ni0.4 , Mg0.7Ni0.3 and Mgo.gNi0.2 respectively. The intensities of the Bragg peaks of pure Mg and Ni decrease up on reaction and a broad maximum appears indicating the growth of an amorphous phase from 100 to 600 R.P.(Fig2).The evolution of the X-ray diffraction spectra during cold-rolling at near ambiant temperatures of Mg0.6Ni0.4 shows complete disappearance of pure Mg and simultaneous growth of the amorphous phase while a small quantity of pure Ni remain after extensive deformation (see fig. 3).