Phase separation in metallic glasses

With the rapid improvement of computer performance, tremendous heat generation in the chip becomes a major serious concern for thermal management. Meanwhile, CPU chips are becoming smaller and smaller with almost no room for the heat to escape. The total power-dissipation levels now reside on the order of 100 W with a peak power density of 400–500 W/cm2, and are still steadily climbing. As a result, it is extremely hard to attain higher performance and reliability. Because the conventional conduction and forcedair convection techniques are becoming incapable in providing adequate cooling for sophisticated electronic systems, new solutions such as liquid cooling, thermoelectric cooling, heat pipes, vapor chambers, etc. are being studied. Recently, it was realized that using a liquid metal or its alloys with a low melting point as coolant could significantly lower the chip temperature. This new generation heat transfer enhancement method raised many important fundamentals and practical issues to be solved. To accommodate to the coming endeavor in this area, this paper is dedicated to presenting an overall review on chip cooling using liquid metals or their alloys as coolant. Much more attention will be paid to the thermal properties of liquid metals with low melting points or their alloys and their potential applications in the chip cooling. Meanwhile, principles of several typical pumping methods such as mechanical, electromagnetic or peristaltic pumps will be illustrated. Some new advancement in making a liquid metal cooling device will be discussed. The liquid metal cooling is expected to open a new world for computer chip cooling because of its evident merits over traditional coolant.

Liquid metal cooling in thermal management of computer chips

Nanocrystallization of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 metallic glass

Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC). In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu46Zr45Al7Y2 bulk metallic glass is 361 kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson–Mehl–Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484 kJ/mol in isothermal transformation for Cu46Zr45Al7Y2 bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu46Zr45Al7Y2 bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions.

Crystallization kinetics in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC)

An anomaly in differential scanning calorimetry has been reported in a number of metallic glass materials in which a broad exothermal peak was observed between the glass and crystallization temperatures. The mystery surrounding this calorimetric anomaly is epitomized by four decades long studies of Pd-Ni-P metallic glasses, arguably the best glass-forming alloys. Here we show, using a suite of in situ experimental techniques, that Pd-Ni-P alloys have a hidden amorphous phase in the supercooled liquid region. The anomalous exothermal peak is the consequence of a polyamorphous phase transition between two supercooled liquids, involving a change in the packing of atomic clusters over medium-range length scales as large as 18 A. With further temperature increase, the alloy reenters the supercooled liquid phase, which forms the room-temperature glass phase on quenching. The outcome of this study raises a possibility to manipulate the structure and hence the stability of metallic glasses through heat treatment. An anomalous exothermal calorimetric peak far below crystallization temperatures in prototypical Pd-Ni-P glasses has been recognized for four decades. Here authors use neutron and high-energy X-ray diffraction to find evidence for a polyamorphous phase transition where medium-range order undergoes large changes while short-range order changes little.

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Hidden amorphous phase and reentrant supercooled liquid in Pd-Ni-P metallic glasses

Crystallization behavior of ZrAlNiCu bulk metallic glass with wide supercooled liquid region

Nanocrystallization of Zr–Ti–Cu–Ni–Be bulk metallic glass

Hypereutectic Al-Si-Fe-Cu-Mg alloys manufactured by rapid solidification and powder metallurgy (RS/PM) process contain high volume fraction and finely dispersed Si particles and Al-Si-Fe intermetallic compounds (1–10). As a result, the unfavorable effects of coarse primary Si and Al-Si-Fe intermetallic compound caused by conventional ingot metallurgy (IM) process on mechanical properties of the alloy are remarkably eliminated (2, 7). In addition, the sizes of the matrix grain and the other second phases are refined and the solid solubility limit of alloying elements is increased by this process, thus the properties of the alloy are much more improved. In this paper, the microstructure and phase morphology of a RS/PM Al-Si-Fe-Cu-Mg alloy with high silicon content were reported. The nominal composition of the alloy was designed Al-17Si-6Fe-4.5Cu-0.5Mg(wt%). The powders were prepared by means of ultrasonic N2-gas atomization process at pressure of 2 MPa and temperature of 900 ◦C. Consolidation of the alloy powders was carried out by canning, degassing and hot extrusion with a reduction ratio 14:1 after holding at 450 ◦C for 1 h. The phases of the extruded alloy were identified by a D/max-rB X-ray diffractometer (XRD) using Cu-Kα1 radiation. The microstructure was analyzed by S-570 scanning electron microscope (SEM). The SEM specimens were cut directly from the extrudate, followed by grinding, polishing and etching with Killer’s reagent. The size and distribution of the second phase particles in the extruded alloy were measured by image analysis technique. The analysis of the phase morphology was performed using Philips-EM420 transmission electron microscope (TEM) attached with energy dispersive spectroscopy (EDS), operating at an accelerating voltage of 100 KV. Thinning of TEM samples after mechanical grinding of the extrudate was performed by ion beam milling. As shown in Fig. 1, the microstructure of the alloy was primarily consisted of Al, Si, Al5FeSi, Al7Cu2Fe and Al4Cu2Mg8Si7 phases. In contrast to the alloy powder [11], new phases Al5FeSi and Al7Cu2Fe were formed in the microstructure of the extruded alloy. By comparing the microstructures between the powder and extruded alloy, it can be concluded that in the course of vacuum degassing or heating before extrusion, Al5FeSi was formed by phase transformation, regarding Al9FeSi3 as its master phase and Al7Cu2Fe was precipitated from the matrix. Fig. 2 illustrated the microstructure of the extruded alloy under SEM. A large amount of fine particle phases were uniformly distributed in aluminum matrix. The results of image analysis of the particle phases were shown in Fig. 3. It was shown that the average size of these particles was only 0.5 μm and maximum size was less than 2 μm. Because of their small size, the chemical compositions of the particle phases could not be ascertained by EDS. It was presumed that these phases were mixture of Si, Ai5FeSi, Al7Cu2Fe and Al4Cu2Mg8Si7. According to the previous research results [11], there was a great amount of fine needle-shaped intermetallic compound Al9FeSi3 in the powder alloy, yet herein we found it disappeared after hot extrusion. It was

Bide Zhou

Papers

Crystallization behavior of ZrAlNiCu bulk metallic glass with wide supercooled liquid region

Nanocrystallization of Zr–Ti–Cu–Ni–Be bulk metallic glass

Microstructure characteristics of a hypereutectic Al-Si alloy manufactured by rapid solidification/powder metallurgy process

Superplastic behavior in pseudo-eutectic NiAl–9Mo alloy

Characteristics and Microstructure of a Hypereutectic Al-Si Alloy Powder by Ultrasonic Gas Atomization Process