Structural evolution and property changes in Nd60Al10Fe20Co10 bulk metallic glass during crystallization

Abstract

The structural evolution and property changes in Nd 60 Al 10 Fe 20 Co 10 bulk metallic glass ͑BMG͒ upon crystallization are investigated by the ultrasonic method, x-ray diffraction, density measurement, and differential scanning calorimetry. The elastic constants and Debye temperature of the BMG are obtained as a function of annealing temperature. Anomalous changes in ultrasonic velocities, elastic constants, and density are observed between 600-750 K, corresponding to the formation of metastable phases as an intermediate product in the crystallization process. The changes in acoustic velocities, elastic constants, density, and Debye temperature of the BMG relative to its fully crystallized state are much smaller, compared with those of other known BMGs, the differences being attributed to the microstructural feature of the BMG. Recently, many glass forming alloys were discovered. 1,2Previous studies show that the crystallization in various bulk metallic glasses ͑BMGs͒ is characterized by the appearance of intermediate metastable phases. The formation of icosahedral phases as an intermediate product of the crystallization process in BMGs indicates that there is a structural relationship between the BMGs and quasicrystalline.3,4 The Ndbased BMGs have also evoked intensive interests due to their unique magnetic properties and anomalous crystallization behavior. 1,[5][6][7] It is found that the microstructural change induced by relaxation and crystallization has a very sensitive effect on the thermal and magnetic properties of the BMGs. This implies that the hard magnetic property of the BMGs may be related to the formed intermediate phases prior to the crystallization. In this letter, the microstructural and property changes in the Nd 60 Al 10 Fe 20 Co 10 BMG upon crystallization are investigated by using ultrasonic method and density measurement, which are effective and sensitive tools for studying the structural and vibrational characteristics of BMGs. 8-11The structural and property features, prior to and in the crystallization process, are connected.Nd 60 Al 10 Fe 20 Co 10 BMG was prepared by the die casting method. 6 The BMG rod was cut to a length of about 10 mm and its ends were polished flat and parallel. Afterward, the sample was stepwise isothermally annealed at various temperatures for 1.0 h in a vacuum of 10 Ϫ3 Pa, respectively. After each annealing, the rod was cooled to room temperature, and the acoustic velocities and density were measured. The acoustic velocities were measured by a pulse echo overlap method using a MATEC 6600 ultrasonic system with a 10 MHz frequency. 8,9 The density was measured by the Archimedean technique and the accuracy is 0.1%. Elastic constants ͑e.g., Young's modulus E, shear modulus G, bulk modulus K, and Poisson's ratio ͒ and Debye temperature ⌰ D of the BMG were derived from the acoustic velocities and densities. 12 The structure of the samples was characterized by x-ray diffraction ͑XRD͒ using a MAC M03 diffractometer with Cu K␣ radiation. Differential scanning calorimet...

Melting and crystallization of Nd60Al10Fe20Co10bulk metallic glass under high pressure

“…The first precipitated crystalline phase during HP annealing can be well identified as hexagonal Nd as indicated in figure 1. This is in agreement with that of the BMG crystallized at ambient pressure [5]. At higher temperature, the intensities of the crystalline reflections change.…”

“…It is also found that the Nd-based BMG can actually be regarded as a type of clustered amorphous material [2,5]. The direct evidence is that the HRTEM image of as-cast BMG exhibits a large numbers of nano-scale ordered regions dispersed in the glassy matrix, and the volume fraction is as large as 50% [5]. Clustered BMG exhibits hard magnetism while a ribbon sample without clustered structure is magnetically soft [3,5].…”

“…The crystallization and melting process are also changed under high pressure. The features of the crystallization and melting under high pressure are discussed.Nd-based bulk metallic glasses (BMGs) with hard magnetic properties and the anomalous crystallization behaviour [1][2][3][4][5] have evoked intensive interest. Previous works show that the magnetic property of a BMG is very sensitive to the microstructure of the alloy, and crystallization can cause markedly changes of the thermal and magnetic properties [1][2][3][4][5].…”

“…The BMG is fully crystallized above 753 K and the melting temperature, T m , is about 793 K [5]. Figure 1 shows the XRD patterns of the BMG in situ measured under 5.0 GPa at various temperatures.…”

Crystallization of metallic glasses under the influence of high density dc currents

“…The crystallization and phase separation processes of several bulk metallic glasses ͑BMGs͒ have been investigated recently by various techniques including differential scanning calorimetry ͑DSC͒, [1][2][3] x-ray diffraction ͑XRD͒, 1-3 electron microscopy, 1,3,4 and small-angle neutron scattering ͑SANS͒. 4 -6 The role of diffusion in the decomposition and/or nucleation and growth phenomena near the glass transition temperature, T g , has been characterized and models have been suggested.…”