Wettability Of Metals And Ceramics By Zr₅₅Cu₃₀Al₁₀Ni₅ BMG-forming Liquid And Their Interfacial Characteristics

In recent decades, bulk metallic glasses (BMGs) have attracted great attention due to their outstanding properties such as high yield strength, high elastic strain limit, and good corrosion resistance. However, their low room-temperature plasticity can be a drawback in the application as engineering materials. In order to improve their plasticity, great efforts have been devoted to the development of the BMG matrix composites. The second phase in the glass matrix can act as a"crack-stopper"by adding impediments to the shear band propagation and thus considerable plasticity has been achieved. Therefore, the development of BMG-matrix composites is a promising way to broaden their applications.On the other hand, the traditional Ag-Cu-Ti solder can not meet all the needs of research and applications with the development of modern welding technology. In recent years, bulk metallic glass alloys have been developed rapidly. BMG alloys are an ideal brazing material because of outstanding characteristics such as direct forming and homogeneous chemical composition and structure. The metastable phases of BMG alloys are easy to link. In particular, Cu-based or Zr-based amorphous solder containing active Ti, Zr elements, which has good glass-forming ability, is likely to replace the widely used but most expensive Ag-Cu-Ti filler metal.It is well known that the wettability of ceramics by molten metals plays a crucial role in the fabrication of metal matrix composites using a liquid casting or infiltration route and in the joining of metal and ceramic. The wettability and reactivity also, to a large extent, determine the bonding quality of the components in the composites. Therefore, it is of vital importance to understand and further control the wettability and interfacial chemistry between the matrix and the reinforcement. However, only limited work has so far been concerned with these important issues in the BMG matrix composites. In the present study, the main goal is to study the wettability and interfacial characteristics of metals, oxides and carbides by molten Zr₅₅Cu₃₀Al₁₀Ni₅ amorphous alloy and to reveal the law of wettability. On the other hand, we can provide a standard for the choice of the Zr-based amorphous reinforcement from a viewpoint of wettability and chemical stability. The main conclusions are given as follows.(1) The present work reveals common rules of wetting between Zr₅₅Cu₃₀Al₁₀Ni₅ BMG-forming liquid and metals (Nb, Ta, W, Mo and Fe), oxides (Al₂O₃, SiO₂, ZrO₂, Y₂O₃) and carbides (B₄C, SiC, WC, ZrC), respectively, namely, the adsorption and accumulation of active element Zr at the interfaces, especially at the triple line, is a key factor in controlling the wettability. Although the interfacial chemical reaction and dissolution of the substrate can promote the wetting to some extent by reducing the solid-liquid interfacial energy, they consumed Zr or decreased the adsorptive concentration of Zr at the interface, which reduced or even suppressed the formation of precursor film at the triple line, thereby weakening the wettability of the system.(2) It has been found that the formation of the precursor film leads to excellent wettability. The formation mechanism of the precursor film can be characterized by rapid adsorption and accumulation of the active atoms and the subsequent formation of the film overflow. The main components in the precursor film are Zr and Cu. It is believed that the latter accompanied with the former when moving to the front of the triple line.(3) The molten Zr₅₅Cu₃₀Al₁₀Ni₅ alloy on different metal substrates(Nb,Ta,W,Mo and Fe) exhibits good wetting behavior. From the viewpoint of contact angle and spreading rate, the wettability of the Zr₅₅Cu₃₀Al₁₀Ni₅-Ta system (complete wetting) is the best among the five systems, followed by Nb, W, and then Mo, and the worst is Fe (the equilibrium contact angle is about 20o). The spreading behaviors of the five systems are well fitted with the molecular dynamic model, that is to say, the adsorption of the active element Zr at the triple line is indeed a key factor in controlling the wettability. Comparatively, the dissolution system is superior to the chemical reaction system in the wettability. Generally speaking, during the competitions among adsorption, dissolution and reaction, the weaker the dissolution or reaction, the more obvious the effect of adsorption, and hence the better the wettability of the system.(4) The wetting of molten Zr₅₅Cu₃₀Al₁₀Ni₅ metallic glass alloy on the oxides substrates (α-Al₂O₃,SiO₂,ZrO₂ and Y₂O₃) belongs to the reactive wetting. From the viewpoint of final contact angle, ?α-Al₂O₃ has the best wettability, followed by SiO₂, and then Y₂O₃. The worst is ZrO₂. The adsorption of the active element Zr is the main factor in promoting the wettability. In the systems that non-stoichiometric phases (ZrO₂ and Y₂O₃) formed easily, the anomalous dependence of wettability on the temperature can be ascribed to the competition between adsorption and accumulation of active element Zr at the interface, and interfacial chemical reaction as well as the pinning of the triple line caused by the releasing oxygen from ZrO₂. The structure transformation of ZrO₂ from monoclinic to tetragonal created more oxygen vacancies, which greatly enhanced the adsorption of Zr and improved the wettability significantly. However, the large amount of releasing oxygen will oxidize the component and pin the triple line, and hence weaken the effect of the subsequent interfacial reaction.(5) The wetting of molten Zr₅₅Cu₃₀Al₁₀Ni₅ metallic glass alloy on the carbides substrates (B₄C,SiC,WC and ZrC) belongs to the reactive wetting. From the viewpoint of final contact angle, the Zr₅₅Cu₃₀Al₁₀Ni₅-ZrC has the best wettability, followed by SiC, and then WC, while the worst is B₄C. From the viewpoint of spreading rate, at the same temperature, the Zr₅₅Cu₃₀Al₁₀Ni₅-WC has the fastest spreading rate, followed by SiC, and then ZrC, while the worst is B₄C. The strong interfacial reaction between Zr in molten Zr₅₅Cu₃₀Al₁₀Ni₅ and the covalent carbides (B₄C and SiC) reduced the adsorptive concentration of Zr at the interface. Thus the spreading behavior changes at different stages: the rate is first fast and then becomes slow; the role of interfacial reaction increases gradually. In metal-like carbides systems (ZrC and WC), interface reaction is weak, and the adsorption of Zr is more significant. Thus its wetting is better than that of the covalent carbides systems.(6) In view of the reasonable wettability and chemical stability, the present paper offers some guidance for preparing the Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass matrix composites and for joining of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass and oxide and carbides as follows:â‘ In order to avoid substrate dissolution and interfacial reaction and to achieve good wettability in the preparation of the W fibre or particles reinforced Zr-base BMG matrix composites, the processing temperature must be precisely controlled. For the Ta, Nb, W and Mo, the optimum temperature is suggested to be 1133±10 K, and the holding time is 0.5-3 minutes. Because of the strong interface reaction and the relatively slow spreading rate, Fe and Fe-based materials are not suitable as the reinforcement for the Zr-base BMG matrix composites.â‘¡Among carbides, WC and ZrC are the excellent reinforcements for the Zr-based amorphous materials, which can achieve good wetting in an appropriate temperature during a relatively short period of time, and also the interface reaction is weak. To fabricate the WC-reinforced Zr-base BMG matrix composites, the optimum temperature is suggested to be 1133±10 K. The optimum temperature for ZrC is suggested to be 1133-1253K. B₄C and SiC are not ideal reinforcements, because of the strong interface reaction, but there is a potential for preparing the Zr-base bulk metallic glass matrix composites reinforced by in-situ ZrC or ZrC-ZrB2 hybrid ceramic particulates using SiC and B₄C as a reaction agent by way of an infiltration synthesis technique.â‘¢Because of Al₂O₃, WC and ZrC have good wetting and interfacial bond strength; they are expected to use Zr-based amorphous brazing to realize joining. Although the wettability of Y₂O₃ and ZrO₂ are weaker, but they have good physical compatibility and interfacial adhesion, so it is expected to realize joining by choosing appropriate parameters (such as temperature) and controlling the cooling rate. However, because of the strong interfacial reaction of melt on SiO₂, SiC and B₄C ceramics and the large thermal stress, it is not suitable for brazing using Zr-based amorphous brazing.In a word, this paper demonstrates the similarities and differences between the above systems by comparing the relationship of the various systems. In addition, we attempted to provide a standard for the choice of the Zr-based amorphous reinforcement and the theoretical basis for their preparation and application from a viewpoint of the wettability and chemical stability.